temperature fluctuations, ecto and endotherms, homeotherms, heterotherms and thermo-conferms

Temperature fluctuations:

They can occupy a place in the environment where the temperature remains constant and compatible with their physiological processes. Physiological processes may have adapted to the range of temperature in which the animals are capable of living or they can generate and trap heat internally to maintain constant body temperature, despite fluctuation in the temperature of external environment.

Ectotherms:

They derive most of their body heat from the environment rather than from their own metabolism. They have low rates of metabolism and are poorly insulated. In general, reptiles, amphibians, fishes and invertebrates are ectotherms, although few reptiles, insects and fishes can raise their internal temperature. Eclotherms tend to move about the environment and find places that minimize heat or cold stress to their bodies.

Endotherms:

Birds and Mammals are endotherms because they obtain their body heat from cellular processes. A constant source of internal heat allows them to maintain nearly constant core temperature, despite the fluctuating environmental temperature. Core means body’s internal temperature as opposed to the temperature near its surface. Most endotherms have bodies insulated by fur of feathers or hairs and large amount of fat. This insulation enables them to retain heat more efficiently and to maintain high core temperature. Endothermy allows animals to stabilize their core temperature so that biochemical processes and nervous system functions can proceed at steady high levels. Endothermy allows some animals to colonize habitats derived to ectotherms.

Homeotherms and heterotherms:

Most endotherms are homeotherms (maintain constant body temperature) and most ectoderms are heterotherms (have variable body temperature) there are many exceptions. Some endotherms vary their body temperatures seasonally (e.g. hibernation), others vary it on daily basis. For example some birds (e.g. humming birds) and mammals (e.g. Shrews) can only maintain high body temperature for a body mass so small that they cannot generate enough heat to compensate for the heat lost across their large surface area. 

Humming birds must devote much of the day to locating and sipping nector (a very high calorie food source) as a constant energy source for metabolism. When not feeding, humming birds rapidly run out of energy unless their metabolic rates decrease considerably. At night humming birds enter a sleep like state called daily torpor and their body temperature approaches that of cooler surroundings, some bats also undergo daily torpor to conserve energy. Some ectotherms like some reptiles that can maintain fairly constant body temperatures by changing position and location during the day to equalize heat gain and loss.

Thermoconformers:

Many invertebrates have low metabolic rates and have no thermoregulatory mechanisms, thus, they passively confirm to the temperature of their external environment. These invertebrates are termed thermoconformers. Zoologists know that many arthropods such as insects, crustaceans and horse shoe crab (limulus) can sense thermal variation e.g. ticks of warm blooded vertebrates can sense the ‘warmth of a nearby meal’ and drop on the vertebrate most.

osmoregulation in terrestrial and aquatic animals

Mechanism of osmoregulation is very important for all groups of animals whether inhibting land or water. Unlike plant cells, animal cells when placed in hypotonic solution burst due to the continuous absorption of water. On the contrary they would shrink and die if constantly placed in hypertonic solution. Normally uptake and loss of water are in balance for proper survival of cell.

(1)        Osmoregulation in terrestrial animals:

Terrestrial animals are more likely to loose water by evaporation through their permeable surfaces exposed to amsophere. Among various animal groups only Arthropods and vertebrates became the most successful land dwellers. They have developed number of strategies to maintain osmoregulation of their body fluid.

(a) Water proof external coverings: To prevent water loss through external surfaces, vertebrates like reptiles, birds and mammals have water proof keratinized epidermis. Similarly the insects have developed external water proof layer called cuticle.

(b) Storing and excretion of solid wastes: Reptiles, birds and insects excrete uric acid as nitrogenous waste, which is insoluble in water. It is stored temporarily in cloaca where water is reabsorbed from it before its removal from the body in semi fluid form.

(c) Use of metabolic water: Some mammals like camel, kangaroo, rate etc make use of water production during the breakdown of body fats.

(d) Storing the wastes: Mammals do retain some urea in their kidneys where it helps in re-absorption of wastes.

(2)        Osmoregulation in freshwater animals:

(a) Osmoregulation by contractile vacuole: Fresh water protests like Amoeba, Paranecium etc bear one or more membrane bound tiny sac called contractile vacuole. Since such freshwater protests have higher osmotic pressure than their surrounding water, so the water constantly comes in by osmosis. If it is not regulated, the organism would burst. Therefore the excess water is stored in contractile vacuole. After it is completely filled, water is discharged out of the cell through a pore into the surrounding water.

(b) Osmoregulation by producing dilute urine: Freshwater animals like fishes have a hypertonic body fluids than the surrounding water. Thus they remove excess water by passing large quantity of very dilute urine.

During the excretion process, they lose some essential ions as well. This is over come by actively absorbing selected ions from outside.

Osmoregulation in marine animals:

Marine bony fishes have hypotonic internal environment; so they are liable to lose water. Thus in order to conserve water, they constantly drink water. The salts taken in along with water are actively excreted by special excretory cells in the gills. Moreover, the filtration rate in their kidneys in very low, so small quantity of concentrated urine in excreted.

Unlike marine bony fishes, sharks and rays maintain relatively slightly hypertonic osmotic pressure of body fluids than their surroundings by storing high concentration of urea in their bodies. Thus they do not have problem of water loss. Excess salts are removed by special glands in their rectum. Marine invertebrates as well as hag fishes have isotonic body fluids, so they do not have osmoregulation Mechanism. Such animals are termed as osmoconformers.

The Digestive System of Human

Man has gastro intestinal tract which runs from mouth to the anus. It begins with mouth and buccal cavity followed by pharynx, oesophagus, stomach, small intestine bearing duodenum, jejunum and ileum. Large intestine consist of caecum bearing appendix, colon and rectum terminating at the anus oral cavity has three pairs of salivary glands. Saliva contains 95% water, some mucus, and amylase and lysozyme enzymes. Mucus moistens and lubricates the food, semi solid food is passed into oesophagus in the mass of bolus. 

Oesophagus is narrow muscular tuber about 25 cm long and conveys food from pharynx to stomach. Stomach is muscular bag lying below diaphragm on left side of abdominal cavity. It performs three functions: storage of food, mechanical digestion by peristalsis and chemical digestion of food by enzymes, which is reduced to creamy paste called chyme. Stomach has cardiac region, middle region the fundus and posterior pyloric region. It opens into duodenum through pyloric sphincter or pylorus. Gastric juice contains pepsin, HCl which kills bacteria, pepsin converts proteins into polypeptides.

Next to stomach is small intestine about 6 metres long. It is in coiled loops and is divided into duodenum, jejunum and ileum. Duodenum is 30 cm long and runs parallel to stomach. It receives bile duct from liver and pancreatic duct from pancreas. Chyme meets with juices of liver and pancreas. Bile salts neutralize the acid of gastric juice and make chime alkaline. Bile pigments bilirubin red, and biliverdin (green) are excretory products formed by breakdown of haemoglobin of worm out RBC in liver. Pancreatic juice contains four enzymes trypsin (protease). Chymotrypsin, amylase and lipase. Trypsin acts on polypeptides and proteins and convert them into polypeptides. 

Chymotrypsin converts casein (milk protein) into short chain amino acids. Amylase converts starch and glycogen into maltose and lipase converts emulsified fats into fatty acids and glycerol which are soluble products of fat. Thus digestion of fat is completed in the duodenum. Duodenum passes into jejunum which is about 2.4 meter long. Digestion of food is completed within the jejunum by enzymes like maltase, sucrase, lactase and peptidase. End products are monosaccharides and amino acids are liberated into the lumen of small intestine. Jejunum passes into ileum which is about 36 meter long. It receives much diluted food chyle containing digested food in true solution form. It has finger like villi which can contract and relax. This increases the absorptive surface area. 

Monosaccharides and amino acids are absorbed into the blood capillaries by diffusion or active transport, while the fatty acids and glycerol enter the epithelial cells of villi. Here they are reconverted into simple fats which then enter the lacteals and pass into blood stream.

The blood capillaries converge to form hepatic portal vein which delivers absorbed food to liver where it is stored and is distributed to all the cells of the body.

Small intestine opens into large intestine bearing short caecum, a long colon and a terminal rectum. Caecum gives off a blind tube about 18 cm long from its lower portion known as vermiform appendix which is vestigial organ. Colon is longest part; vitamins and water are absorbed here. Rectum is last portion of large intestine, undigested food in passed from rectum and out by anus in the form of faeces. Liver and pancreas are glands present behind diaphragm. Liver has two main lobes a right and a left. Left lobe is further divided into two lobes. Liver is redish brown in colour. Both produce juices to digest food.

How small intestine and large intestine is main site of digestion of food in Humans

Small intestine:

Human small intestine is about 4 cm in diameter and 7 to 8 meter in length. It is intermediate in length between small intestines of typical carnivores and herbivores of similar size. It reflects the human’s omnivorous eating habits. The length of small intestine directly releases to the total surface area available for absorbing nutrients as determined by many circular folds and minute projections of the inner gut surface on the circular folds, thousands of finger like projections called villi project from each square centimetre of mucosa. 

These minute projections are so dense that the inner wall of human small intestine has total surface area of approximately 300 mm² – the size of a tennis court. First part of small intestine called duodenum functions primarily in digestion. Next part is jejunum and the last part is ileum. Both function in nutrient absorption. The duodenum contains many digestive enzymes that intestinal glands in the duodenal mucosa secrete. The Pancreas secretes other enzymes. In duodenum digestion of carbohydrates and proteins is completed and most lipids are digested. 

The jejunum and ileum absorb the end products of digestion by active transport (amino acids, simple sugars, fatty acids, glycerol, nucleotides, and water). Sugars and amino acids are absorbed into the capillaries of villi, where as free fatty acids enter the epithelial cells of the villi and recombine with glycerol to form small droplets called chylomicrons which enter the lacteals of the villi. From lacteals chylomicrons move into the lymphatics and eventually into the blood stream for transport throughout the body. Small intestine absorbs water and dissolved mineral ions. Small intestine absorbs about 9 litres of water per day and large intestine absorbs the rest.

Large intestine:

Large intestine has small surface area. Small intestine joins large intestine near blind ended sac, the caecum, Human caecum and its extension, the appendix are storage sites and possibly represent the evolutionary remains of large functional caecum such as is found in herbivores. Appendix contains an abundance of lymphoid tissues and many function as part of immune system.

Working:

Functions of large intestine include re-absorption of water and minerals and the formation and storage of faeces. As peristaltic waves move food residue along, minerals diffuse or are actively transported from the residue across the epithelial surface of large intestine into the blood stream water follows osmotically and returns to the lymphatic system and blood stream. When water re-absorption is insufficient diarrhea results. If two much water is reabsorbed, faecal matter becomes too thick resulting in constipation.

Many bacteria and fungi exist symbiotically in large intestine. They feed on the food residue and further breakdown its organic molecules to waste products, in turn, they secrete amino acids and vitamin K, which the hosts’ gut absorbs. What remains faeces is a mixture of bacteria, fungi, undigested plant fibre, sloughed off intestinal cells and other waste products.

Liver, gall bladder and pancreas of Man and their function

The Pancreas lies ventral to the stomach and has both endocrime and exocrime function. Exocrime cells in the pancreas secrete digestive enzymes into pancreatic duct, which merges with hepatic duct from the liver to form common bile duct that enters the duodenum. Pancreatic enzymes complete the digestion of carbohydrates and proteins and initiate the digestion of lipids.

Trypsin, carboxypeptidaze and chymotrypsin digest protein into smaller peptide and amino acids.

Lipases convert triglycerides into smaller glycerol and free fatty acids. Amylase converts polysaccharides into disaccharides and monosaccharides.

The pancreas also secretes bicarbonate (HCO-3) ions that help neutralize the acidic food residue coming from the stomach. Bicarbonate raises the pH from 2 to 7 for pancreatic enzymes to work.

Liver and gall bladder:

The liver, the largest organ in mammalian body is just under the diaphragm. In the liver millions of specialized cells called hepatocytes take up nutrients absorbed from the intestines and release them into bloodstream. Hepatocytes also manufacture blood protein prothrombin and albumin.

Metabolic functions:

There is removal of amino acids from organic compounds. There is urea formation from proteins and conversion of excess amino acids into urea to decrease body levels of ammonia. There is manufacture of most of plasma proteins, formation of fetal erythrocytes, and destruction of worm out erhythrocytes and synthesis of blood clotting agents prothrombin and Fibrinogin from amino acids. There is synthesis of non essential amino acids.

There is conversion of galactose and fructose to glucose. There is oxidation of fatty acids. There is formation of lipoproteins, cholesterol and phospholipids (essential cell membrane components). There is conversion of carbohydrates and proteins into fat. There is modification of waste products, toxic drugs and poisons (detoxification). There is synthesis of vitamin A from carotene and with the kidneys, participation in the activation of vitamin D.

There is maintenance of stable body temperature by raising the temperature of the blood passing through it. Its many metabolic activities make the liver the major heat produces in mammal’s body.

There is manufacture of salts which are used in the small intestine for emulsification and absorption of simple fats, lipid and lipoprotein.

Liver stores glucose in the form of glycogen and with the help of insulin and enzymes, converts glycogen back into glucose as the body needs it. Liver also stores fat soluble vitamins (A, D, E and K) and minerals such as iron from the diet. Liver can also store fats and amino acids and convert them into usable glucose as required.

Gall bladder is a small organ near the liver. Gall bladder stores greenish fluid called bile that the liver cells continuously produce. Bile is very alkaline and contains pigments, cholesterol, lecithin, mucin, bilirubin and bile salts that act as deterents to emulsify fats from them into droplets suspended in water and aid in fat digestion and absorption. Bile salts also combine with the end products of fat digestion to form micelles. Micelles are lipid aggregates (fatty acids and glycerol) with surface coat of bile salts.

Structure and function of oral cavity, pharynx, esophagus and stomach in Mammals

Oral cavity:

A pair of lips projects the oral cavity (mouth). The lips are lightly vascularized, skeletal muscle tissue with an abundance of sensory nerve endings. Lips help retain food as it is being chewed and play a role in phonation (the modification of sound). The oral cavity contains tongue and teeth. Mammals can mechanically process a wide range of food because their teeth are covered with enamel (hardest material in the body) and because their jaws and teeth exert a strong force. The oral cavity is continuously bathed by saliva secreted by three pairs of salivary glands. Saliva moistens food, binds it with nucius (glycoproteins) and forms moist mass called bolus. Saliva also contains bicarbonate ions (HCO_3-) which buffer chemicals in mouth and thiocynate ions (SCN_-) and the enzyme lysozyme, which kill micro-organisms. It also contributes enzyme (amylase) necessary for initiation of carbohydrate digestion.

Pharynx and esophagus:

Air and swallowed foods and liquids pass from the mouth into the pharynx. The epiglottis temporarily seals off the opening (glottis) to the trachea so that swallowed food does not enter the trachea. Initiation of swallowing reflex can be voluntary but most of the time it is involuntary. When swallowing begins, sequential, involuntary contractions of smooth muscles in the walls of esophagus propel the bolus or liquid to the stomach.

Stomach:

Mammalian stomach is distensible sac with three main functions. It stores and mixes the food bolus received from the esophagus. It secretes substances enzymes, mucus and Hydrochloric acid (HCl) that start the digestion of proteins and helps control the rate at which food moves into small intestine via pyloric sphincter.

Structure:

Stomach is made up of inner mucous membrane containing thousands of gastric glands. Three types of cells are present:

(1) Parietal cells: Parietal cells secrete a solution containing HCl.

(2) Chief cells: Chief cells secrete pepsinogen the precursor of the enzyme pepsin. Both of the cells are in the pits of the gastric glands. The surface of the mucus membrane at the openings of the glands contain mucous cells that secrete mucus that coat the surface of stomach and protect it from HCl and digestive enzymes. The surfaces of esophagus and mouth have much thinner mucous cell layer than the stomach which is why vomiting can cause burning sensation in the esophagus or mouth.

(3) Endocrime cells: Endocrime cells in one part of the stomach mucosa release the hormone gastrin which travels to target cells in the gastric glands, further stimulating them.

Working:

When the bolus of food enters the stomach, it distends the walls of the stomach. This distention as well as the act of eating, causes the gastric pits to secrete HCl (as H⁺ and Cl^-) and pepsinogen.

H⁺ions cause pepsinogen to be converted into active pepsin. As pepsin, mucus and HCl mix with and begin to break down proteins, smooth mucosal muscles contract and vigorously churn and mix the food bolus. About 3 – 4 hours after a meal, the stomach contents have been sufficiently mixed and are semi-liquid mass called chyme.

The pyloric sphincter regulates the release of chyme into small intestine.

Evolution of Caeca, Pancreas, intestine, liver and gall bladder

Caeca:

Macro-organisms attack the food of ruminants before gastric digestion but in the typical non ruminant herbivore, microbial action on cellulose occurs after digestion. Rabbits, horses and rats digest cellulose by maintaining a population of micro-organisms in their usually large caecum, adding to this efficiency, a few non ruminant herbivores such as mice and rabbits, eat some of their own faeces to process the remaining material in them like vitamins.

Pancreas:

Every vertebrate has Pancreas. In lampreys and lung fishes it is embedded in the wall of intestine and is not a visible organ. Both endocrime and exocrime tissues are present but the cell composition varies. Pancreatic fluid containing many enzymes empties into the small intestine viz the pancreatic duct.

Intestines:

The configuration and divisions of small and large intestine vary greatly among vertebrates. Intestines are closely related to animals’ type of food, body size and levels of activity e.g. cyclostomes, chordrichthyes fishes and primitive bony fishes have short nearly straight intestines that extend from the stomach to the anus. In more advanced bony fishes the intestine increases in length and begins to coil. The intestines are moderately long in most amphibians and reptiles. In birds and mammals the intestines are longer and have more surface area than those of other tetrapodes. Birds have two caeca, and mammals have single caecum at the beginning of large intestine. Large intestine is much longer in mammals than in birds and it empties into the cloaca in most vertebrates.

Livers and gall bladder:

In vertebrates with a gall bladder it is closely associated with the liver. The liver manufactures bile pigments. Bile salts emulsify dietary fat to ease the enzyme lipase function. Bile pigments result from phagocytosin of red blood cells in the spleen, liver and red bone marrow. Phagocytosis cleaves the haemoglobin molecule, releasing iron and the remainder of the molecule is converted into pigments that enter the circulation. These pigments are extracted from the circulation in the liver and excreted in the bile as bilirubin (green bile). As bile helps in fat digestion, the gall bladder is large is carnivores and vertebrates in which fat is an important part of the diet. It is much reduced or absent in blood suckers, lamprey and herbivores like Teleosts, many birds and rats.

Diversity in vertebrate tongue, salivary glands esophagus, girzzard and stomach

Tongue:

A tongue develops in the floor of oral cavity in many vertebrates e.g. lamprey has a prototractible tongue with horny teeth that rasp its prey flesh. Fishes may have a non-muscular tongue that bears teeth that help hold prey. Tetrapods have evolved mobile tongues for gathering food. Frogs and salamanders and some lizards can rapidly project part of their tongue from the mouth to capture an insect. A wood pecker has a long spiny tongue for gathering insects and grubs. Ant and termite eating mammals also gather food with long, sticky tongues, spiny papillae on the tongues of cats and other carnivores help these animals rasp flesh from a bone.

Salivary glands:

Most fishes lack salivary glands in the head region. Lampreys are an exception because they have a pair of glands that secrete an anticoagulant needed to keep their preys blood flowing as they feed. Modified salivary glands of some snakes produce venom that is injected through fangs to immobilize prey.

In amphibians or reptiles and must birds lack salivary glands while all mammals have them.

Esophagus:

Esophagus is short in fishes and amphibians but much longer in amniotes due to their longer necks. Grain and seed eating birds have a crop that develops from the caudal portion of the esophagus. Storing food in the crop ensures continuous supply of food to the stomach and intestine for digestion. This structure allows these birds to reduce the frequency of feeding and still maintain high metabolic rate.

Stomach:

Stomach is an ancestral vertebrate structure that evolved as vertebrates began to feed on longer organisms that were caught at less frequent intervals and required storage. The gastric glands and their production of Hydrochloric acid (HCl) evolved to kill bacteria and helping preserve food. The enzyme pepsinogen has evolved later because the stomach is not essential for digestion.

Gizzards:

Some fishes, crocodilians and all birds have a gizzard for grinding up food. The bird’s gizzard develops from the posterior part of stomach called the ventriculus. Pebbles that have eating birds and facilitate the grinding process.

Intracellular digestion, extra cellular digestion and feeding strategy adopted by animals

Intracellular digestion:

In simple animals (protitsts and sponges) some cells take in whole food particles directly from the environment by diffusion, active transport and endocytosis and break them down with enzymes to obtain nutrients. It is called intracellular (within the cell) digestion. It circumstances the need for the mechanical breakdown of food or for a gut or other cavity in which to chemically digest food. It limits animal’s size and complexity only very small pieces of food can be used. It provides all or some of the nutrients in protozoa, sponges, cnidarians, platyheliminthes, rotifers, bivalve molluscs and primitive chordates.

Extra cellular digestion:

Larger animals have evolved structures and mechanisms for extra cellular digestion; the enzymatic breakdown of larger pieces of food into small molecules usually in special organ or cavity. Digested food then passes into body cells lining the organ or cavity and can take part in energy metabolism or biosynthesis.

Suspension feeders:

It is removal of suspended food particles from the surrounding water by capturing trapping or filtration structure. It involves transport of water past the feeding structure. It involves in removal of nutrients from water and transport of nutrients to the mouth of the digestive system.

Sponges, ascidians, branchiopods, entoprocts, phoronids, most bivalves and many crustaceans, are suspension feeders.

Deposit feeders:

It involves primarily omnivorous animals. These animals obtain their nutrients from sediments of soft bottom habitats (mud and sand) or terrestrial soils. Direct deposit feeders simply swallow large quantity of sediment mud, sand, organic matter; the usable nutrients and digested and the remains pass out the anus. E.g. polychaete annelids, some snails, some sea urchins and in most earthworms. Other direct deposit feeders utilize tentacle like structures to consume sediment e.g. sea cucumbers, most sipanculans, curlain clams and several types of polychaetes.

Herbivory:

It is consumption of macroscopic plants. It requires the ability to bite and chew large pieces of plant matter (macroherbivory). Biting and chewing mechanisms evolved in a number of invertebrate lineages are often characterized by the development of hard surface (e.g. teeth) that powerful muscles manipulate e.g. Molluscs, polychaete worms, arthropods and sea urchins.

Many molluscs have a radula. A radula is a muscularized belt like rasp armed with chitinous teeth. Molluscs use the radula to scrape algae of rocks or to tear the leaves of terrestrial plants. Polychaets have sets of large chitnous teeth on eversible proboscis on pharynx that is used to scrape off algae. This toothed pharynx is also suitable or carnivory when plants material is scarce.

Macroherbivory is found in almost every group of Arthropods e.g. insects and crustaceans have large powerful, mandibles of biting off plant material and grinding and chewing it.

Macronutrients and Micronutrients

Macronutrients:

With a few exceptions, heterotrophs require organic molecules such as carbohydrates, lipids and proteins in their diets. Enzymes breakdown these molecules into components that can be used for energy production or as sources for the “binding blocks” of life.

Carbohydrates: Carbon and energy from sugars and starches. The main dietary source of energy for heterotrophs is carbohydrates. Most carbohydrates come from plant sources. Various sugars (mosachharides) can meet this dietary need. Carbohydrates also are a major carbon source for use in organic compounds. Many plants also supply cellulose (polysaccharide) that human and other animals (except herbivores) cannot digest. Cellulose is called dietary fibre. It assists in the passage of food through the alimentary canal of mammals. Cellulose may also reduce the risk of cancer of the colon, because the mutogen and reduced if faecal elimination is more frequent.

Lipids: Highly compact energy – storage nutrients. Neutral lipids (fats) or triacyl glycerols are contained in fats and oils meat and dairy products, nuts and some fruits and vegetables high in fats such as avocados. Lipids are the good source of food energy. They produce about 9 calories (K. Cal) of usable energy per gram more than twice the energy available from equal mass of carbohydrate or protein.

Many heterotrophs have dietary requirement for lipids e.g. many animals require unsaturated fatty acids (e.g. linoleic acid, limolenic acid and arachidonic acid). These fatty acids act as precursor molecules for synthesis of steroids e.g. cholesterol. The steriols are also required for the synthesis of steroid hormones and cholesterol, which is a part of cell membranes. Other lipids insulate the bodies of some vertebrates and help maintain constant temperature proteins. Basic to the structure and function of cells. Animal sources of protein are other animals and milk. Plant sources are beans, peas and nuts. Proteins are needed for their amino acids which heterotrophs use to build their own body proteins.

Micronutrients:

They are usually small ions, organic vitamins, inorganic minerals and molecules that are used repeatedly in enzymatic reactions or as parts of certain proteins (e.g. copper in hemocyanin and iron in haemoglobin). Animals cannot synthesize them rapidly. Thus they must be oblaived from the diet.

Minerals: Some minerals are needed in large amount and are called essential minerals or macro-minerals. E.g. sodium and potassium are vital to the working of nerve and muscle in animal’s body. Animals lose large sodium, in the urine every day. Animals that sweat to help regulate body temperature lose sodium in their sweat. Daily supply of calcium is needed for muscular activity and with phosphorus for bone formation.

Other nutrients are known as trace minerals or micronutrients. Animals need these in only very small amount for various enzymatic functions.

Vitamins: Normal metabolic activity depends on small amount of more than a dozen organic substances that occur in many foods in small amount and necessary for normal metabolic functioning.

Vitamins may be water soluble or fat soluble. Most water soluble vitamins such as B vitamins and vitamin C are coenzymes needed in metabolism. The fat soluble vitamins have various functions. The dietary need for vitamin C and the fat soluble vitamins (A, D, E and K) tends to be limited to the vertebrates vitamin requirement vary in different vertebrates.

What is evolution in digestive structures in invertebrates

Protozoa:

Protozoa may be autotrophic, saprozoic or heterotrophic (ingest food pasticles). Ciliated Protozoa are good example for protests that utilize heterotrophic nutrition. Ciliary action directs food from the environment into the buccal cavity and cytostome. The cytostome opens into cytopharynx which enlarges as food enters and pinches off food containing vacuole. The detached food vacuole then moves through the cytoplasm. During this movement excess water is removed from the vacuole; the contents are acidified and then made alkaline and a lysosome adds digestive enzymes. The food particles are then digested within the vacuole and the nutrients are absorbed into the cytoplasm. The residual vacuole excretes its waste products via cytopyge.

Bivalve Molluscs:

Many bivalve molluscs feed and ingest small food particles. The digestive tract has a short oesophagus opening into stomach, midgut and hindgut. The stomach contains a crystalline style gastric shield and diverticulated region. These diverticulate are blind ending sacs that increase the surface area for absorption and intercellular digestion. The midgut, hindgut and rectum function in extra cellular digestion and absorption.

Digestion is a coordination of three cycles (1) feeding (2) extra cellular digestion and (3) intracellular digestion. The mechanical and enzymatic breakdown of food during feeding provides small particles for intracellular digestion. Intracellular digestion releases nutrients into the blood and produces fragmentation spherules that both excrete wastes and lower the p for optimal extra cellular digestion. These three cycles are linked to the tidal immersion and emersion of molluscs.

Insects:

Grass hopper is an insect with complete digestive and extra cellular digestion. During feeding the mandibles and maxillae first break up (masticate) the food which is then taken into mouth and passed to the crop via oesophagus. During mastication, the salivary gland, add saliva to the food to lubricate it. Saliva also contains the enzyme amylase which digests carbohydrates. This digestion continues during food storage in the crop. The midgut secretes other enzymes (carbohydrases, lipases, proteases) that enter the crop. Food passes slowly from the crop to the stomach, where it is mechanically reduced and the nutrient particles sorted. 

Large particles are returned to the crop for further processing; small particles enter the gastric caecae where extra cellular digestion is completed. Most nutrient absorption then occurs in the intestine. Undigested food is moved along the intestine and passes into the rectum where water and ions are absorbed. Solid fecal pallets that form the pass out of the animal via anus.

During this entire feeding process, the nervous system, the endocrine system and the presence of food extent considerable control over enzyme production at various points in the digestive tract.

What are antigens, antibodies, humoral and cell mediated immune response

Antigens:

A recognition system allows the mammalian immune system to distinguish ‘self’ from ‘non self’. Prior to birth, the body inventories the proteins and various other large molecules present in the body (“self”) and inactivates most of the genetic programming for making antibodies to self molecules.

The body can distinguish self molecules from foreign non self substances and lymphocytes can produce specific immunological reactions against the foreign material leading to its removal. Foreign (“non self”) substances to which lymphocytes respond are called antigens (antibody generator). Most antigens are large proteins or other complex molecules with a molecular weight generally greater than 10,000.

Antibodies:

Plasma cells manufacture antibodies (immunoglobulins) a group of recognition glycoproteins present in the blood and tissue fluids of birds and mammals. All antibody molecules have a basic Y structure composed of four chains of polypeptides connected by disulphide bonds. The arms of Y contain binding sites or fragments (F×b) for specific invaders (i.e. antigens). The tail of Y can activate the complement system (20 distinct defensive proteins in serum) or bind to receptors on phagocytic cells.

Antibody-Mediated (Humoral):

Immune Response: B cells are of great importance in fighting invading organisms because they produce antibodies that identify the antigens for destruction. B cells carry some of their particular antibodies on their plasma membrane. When an antigen comes into contact with B cell whose antibodies recognize the antigen, B cell binds to the antigen when stimulated by one kind of T cell (a helper T cell). B cell divides many times, producing plasma cells that produce and secrete more of this particular antibody. These antibody molecules are carried through the circulation. If they encounter antigens, the antibodies bind to the antigen molecules and thus mark them for destruction of other parts of human system (e.g. macrophages).

Cell-Mediated Immune Response: T cells are involved directly in destroying invading cells, as well as in regulating other parts of immune system. T cell responses are called the cell mediated immune response.

Natural Killer (NK) cells, also called cytotoxic T cells, recognize cell surface changes on cancer cells, virus infected cells fungi, protozoa or Helminth parasites.

The action of NK cells in why tissue and organ transplants are rejected in birds or mammals.

Rejection mechanism: In rejection mechanism NK cells enter the transplanted tissue through the blood vessels, recognize it as foreign, attach to the tissue and destroy it. The body tolerates tissue and organ transplants between identical twins that have identical sets of DNA and hence identical self recognition marker regions.

T cells also regulate the activity of other parts of the immune system. For example, suppose several bacteria penetrate the first line of defense (the skin) through an abrasion or cut. Inflammation occurs and macrophages phagocytize the bacteria. Macrophages destroy most of antigens (bacteria) but some of the antigens are moved to the surface of the plasma membrane of both macrophages and B cells, where they are displayed alongside the self recognition marker. It is this specific combination of self recognition marker and antigen that helper T cells recognize.

When the macrophage reacts with a helper T cell, it releases interleukin – 1 (IL – 1). Interleukin I stimulates other helper T cells to secrete interleukin – 2 (IL – 2), which stimulates their (helper T cells) growth and cell division. Some of the interleukin – 2 that helper T cells produce acts as sensitized B cells. Sensitized B cells have recognized and processed the antigen onto their plasma membrane alongside their self recognition makes. These stimulated B cells mature and divide into differentiated plasma cells.

Respiratory system of Human

It consists of paired lungs and air passage ways. Lungs lie in thoracic cavity walls of which are formed of intercostals muscles attached with bony cage formed by 12 pairs of ribs, vertebral column and sternum bone. Thoracic cavity is separated from abdomen by a muscular partition called diaphragm.

Air enters into lungs by a pair of openings called external nares or nostrils which lead into pharynx. Pharynx leads air into larynx by opening called glottis guarded by flap of tissue called epiglottis. Larynx leads air into trachea. Trachea leads into two bronchi. Each bronchus leads into each lung and subdivides into bronchioles. Each bronchiole ends into alveoli or air sacs.

Alveoli are respiratory surfaces of lungs. In alveoli exchange of gases takes place.

Breathing:

It is the process of taking in (inspiration or inhalation) air and giving out air (expiration or exhalation) from atmosphere.

Inspiration (inhalation): Inspiration or process of taking in of air, in which volume of thoracic cavity is increased due to concentration of intercostal muscles and diaphragm. Thoracic cavity in larges and negative pressure is developed inside the thoracic cavity and in lungs. So the air through respiratory passage rushes into the lungs up to alveoli where exchange of gases occurs.

Expiration (exhalation): Expiration is giving out of air. It is passive process which takes due to increased pressure in thoracic cavity as well as lungs. It is caused by relaxation of external intercostal muscles and the contraction of internal intercostal muscles. Which move ribs as well as sternum inward and downward. Diaphragm also relaxes which makes it dome shaped thus reducing the volume of thoracic cavity. As a consequence lungs are compressed as the air along with water vapour is exhaled outside through respiratory passage.

What is vertebrate respiratory system

Gas exchange through gills:

Gas exchange across internal gill surfaces is extremely efficient. It occurs as blood and water move in opposite direction on either side of lamellar epithelium. For example the water that passes over a gill first encounters vessels that are transporting blood with low oxygen partial pressure into the body.

Thus oxygen diffuses into the blood water than passes over the vessels carrying blood high in oxygen. More oxygen diffuses inward because this blood still has less oxygen than the surrounding water. Carbon dioxide also diffuses into water because its pressure is higher in the blood than in water. This counter current exchange mechanism provides efficient gas exchange by maintaining a concentration gradient between blood and water over the length of capillary bed.

Respiratory organs of frog:

Frog can live in water as well as on land. Its larval stages respire by gills, the adult has to develop some special respiratory organs adapted for terrestrial mode of life like other terrestrial vertebrates frog has evolved vascularized paired outgrowths from the lower part of the pharynx known as lungs. They are located inside the body and are simple sac like structures with shallow internal folds that increase the inner surface to form many chambers called alveoli. These are separated from each other through septa. The inner surface of alveoli is attached with blood capillaries. Alveoli are site of exchange of gases. From each lung arises a tube or bronchus. Both bronchi open into larynx or sound box which leads into the buccal cavity through glottis.

Like all other amphibians, in frog, ventilation is a single, two way path. Frog uses positive pressure i.e. it pushes the air into buccal cavity by lowering its bucco pharyngeal floor. During this process it opens the nares and closes the glottis. Then with nostrils closed and glottis opened. Air is pushed into lungs. This is called incomplete ventilation. Air forced into lungs mixes with air already present in lungs and deleted in oxygen. On land this exchange of gases is called pulmonary respiration.

Cirtaneous respiration: When frog goes into water or buries itself in mud, it exchanges gases by its moist and highly vascularized thin skin. This is known as cirtaneous respiration. It can also exchange gasses through its thin vascularized lining of buccal cavity. It is called bucco pharyngeal respiration.

Respiratory system of Bird:

Birds are lung breathers. The lungs of a bird are internally subdivided into numerous small, highly vascularized thin membranous channels called parabrochi. In addition to a pair of lungs, a bird has 8 to 9 thin walled non-muscular nonvascular sacs that penetrate the abdomen, neck and even the wings. Air sacs work as bellows that ensure unidirectional flow of air or complete ventilation. Thus a bird must take two breathes to move air completely through the system of air sacs and lungs. First breathe draws fresh air into posterior air sacs of the lungs. The second breathe pushes the first breathe into anterior air sacs and then out of the body. Thus one way flow of air enables a bird to fly at very high attitude without any shortage of oxygen as air coming in lungs is always oxygen rich.

Air exchange in human lungs: Air normally enters and leaves this system through either nasal or oral cavities. From these cavities air moves into the pharynx which is common area for respiratory and digestive tracts. During inhalation air from the larynx moves into the trachea (wind pipe) which branches into right and left bronchus. After each bronchus enters the lungs, it branches into smaller tubes called bronchioles which are part of gas exchange portion of respiratory system. During exhalation intercostals muscles and diaphragm relax allowing the thoracic cavity to return to its original smaller size and increasing the pressure in the thoracic cavity. Abdominal muscles contract pushing the abdominal organs against the diaphragm, further increasing the pressure within the thoracic cavity. The action causes the elastic lungs to contract and compress the air in the alveoli. With this compression alveolar pressure becomes greater than atmospheric pressure, causing air to be expelled from the lungs.

Significance Of The Concept Of Time Value Of Money

Time value of money is a widely used concept in literature of finance. Financial decision models based on finance theories basically deal with maximization of economic welfare of shareholders. The concept of time value of money contributes to this aspect to a greater extent. The significance of the concept of time value of money could be stated as below:

Investment Decision

Investment decision is concerned with the allocation of capital into long-term investment projects. The cash flow from long-term investment occur at different point in time in the future. They are not comparable to each other and against the cost of the project spent at present. To make them comparable, the future cash flows are discounted back to present value.

The concept of time value of money is useful to securities investors. They use valuation models while making investment in securities such as stock and bonds. These security valuation models consider time value of cash flows from securities.

Financing Decision

Financing decision is concerned with designing optimum capital structure and raising funds from least cost sources. The concept of time value of money is equally useful in financing decision, especially when we deal with comparing the cost of different sources of financing. The effective rate of interest of each source of financing is calculated based on time value of money concept. Similarly, in leasing versus buying decision, we calculate the present value of cost of leasing and cost of buying. The present value of costs of two alternatives are compared against each other to decide on appropriate source of financing.

Besides, the concept of time value of money is also used in evaluating proposed credit policies and the firm's efficiency in managing cash collection under current assets management.

Concept Of Time Value Of Money

The concept of tome value of money suggests that the money received at different point of time has different value. The financial manager must appreciate this fact and understand why they are different and how they are made comparable.

Time value of money is a concept to understand the value of cash flows occurred at different point of time. If we are given the alternatives whether to accept $ 100 today or one year fro now, then we certainly accept $ 100 today. It is because there is a time value to money. Every sum of money received earlier has reinvestment opportunity. For example, if we deposited $ 100 in saving account at 5% annual rate of interest, it will increase to $ 105 at the end of one year. Money received at present is preferred even if we do not have reinvestment opportunity. The reason is that the money that we receive in future has less purchasing power than the money that we have at present due to the inflation. What happens if there is no inflation? Still, money received at present is preferred, it is because most of us have a fundamental behavior to prefer current consumption to future consumption. Thus, i) The reinvestment opportunity or the earning power of the money, ii) the risk of inflation, and iii) an individual's preference for current consumption to future consumption are the reasons for the time value of money.

The concept of time value of money is useful in addressing our real life problems relating to planning for future family expenditure. For instance, if we need $ 50,000 after the retirement from job in 15 years, the amount we need to deposit at interest every year from now until the retirement is conveniently determined by using the time value of money concept.

Many financial decisions of the firm require a consideration regarding time value of money. The corporate manager must always concentrate on maximizing shareholders wealth. Maximizing shareholders wealth, to a larger extent, depends on the timing of cash flows from investment alternatives. In this regard, time value of money concept deserves serious considerations on all financial decisions.

What is Immunity and define immune system and immunization

Animal body is always exposed to the invasion of countless infectious microorganisms such as virus, bacteria. Due to the defense mechanisms evolved by the animals, such invasions in number of cases are overcome. The ability of the body to resist microorganisms, their toxins if any, foreign cells and abnormal cells of the body is termed as immunity.

Immune system:

Immunity is conferred to animals through the activities of the immune system which combats infectious agents. The study of functioning and disorders of the immune system is termed as immunology.

Immune system is a collection of cells and proteins that work to protect the body from potentially harmful, infectious microorganisms. It also plays role in the control of cancer, allergy, hypersensitivity and rejection problems when organs or tissues are transplanted. The immune system can be divided into two functional divisions.

(1) Innate immune system and (2) Adaptive immune system.

(1) Innate immune system: 

It is responsible for natural immunity which is non specific in nature since it combats all microorganisms. It consist of physical (e.g. skin mucous membrane) and chemical (e.g. lysozyme, gastric juice etc) barriers against infectious microorganisms. Skin and mucous membrane with their secretions act as first line of defense. The intact skin provides an impenetrable barrier to the vast majority of infectious agents, most of which can enter only through the mucus membranes that lines the digestive, respiratory and urinogenital tracts. These areas are protected by movement of mucus and secretions (e.g. lysozyme in tears) to destroy many microbes.

Most of the microorganisms present in food or trapped in swallowed mucus from the upper respiratory tract are destroyed by highly acidic gastric juice of stomach. If some how microorganisms are able to penetrate the outer layer of the skin or mucous membrane, they encounter second line of defense offered by the innate immune system. Phagocytes are certain type of WBC which can digest and destroy the particles including agents. Short lived phagocytic cells called neutrophils ingest bacteria actively. Another group of WBC, the natural killer cells (NK cells) destroy virally infected own cells of the body. They also attack abnormal cells (cancerous cells).

Inflammation (to set on fire) is body’s reaction to an injury or by the entry of microorganisms. A cascade of chemical reactions takes place during inflammatory response. It is characterized by redness, heat, swelling and pain in the injured tissue. When injured basophils and most cells release a substance called histamine which causes increase in the permeability of the adjacent capillaries, local vasodilation and also make capillaries leakier. Due to chemotaxis phagocytes and macrophages are attracted at the injured site. Thus phagocytes literally eat up microorganisms, dirt, cell debris etc forming pins.

In case of warm blooded animals a number of microorganisms who escape away from the inflammatory response to infect some large part of the body, trigger fever. It is usually caused by certain WBC that release substance called pyrogen. It sets the temperature of the body higher than the normal. It inhibits the growth of some microorganisms, facilitates phagocytosis, increases the production of interferon and may speed up repair of damaged tissue.

(2) Adaptive immune system:

It is externally complex. It produces specific immune response against a range of different invading organisms, toxins, transplanted tissues and tumour cells. This is the third live of defense which comes into play simultaneously with the second line of non specific defense.

The responses of the adaptive immune system are provided chiefly by two types of lymphocytes called B cells and T cells. Depending upon their migration and maturity during early development in either bone marrow or thymus, they are designated as B and T cells. Although B cells and T cells play quite different roles in the immune system, yet they share the basic key features of the immune response.

In Order to develop immune response, the immune system must recognize the invading organisms or foreign proteins from its self tissues and proteins. A foreign substance that elicits immune response is called antigen. The immune system responds to an antigen by activating lymphocytes and producing specific, soluble proteins called antibodies. The antibody combines with the antigen and helps to eliminate it from the body. The immune system of a vertebrate has virtually unlimited capacity to generate different antibodies which recognize and bind millions of potential antigens or foreign molecules.

The immune system has also the ability to memorize antigens it has encountered. Thus upon subsequent response to the same pathogen, it respond, very quickly and effectively. The adaptive immune system mounts two types of attacks termed as humoral immunity and cell mediated immunity (CMT) on invading microorganisms.

Humoral immunity:

Immunity provided by antibodies secreted in circulatory system by B cells is termed as humoral immunity. It is helpful in bacterial invasion.

Cell mediated immunity (CMI): It is contributed by the second family of lymphocytes called T cells which do not secrete antibodies. They mediate immunity by killing infected cells and aiding in inflammation.

Active immunity: Immunity acquired by own immune response is called active immunity. It is a consequence of natural infection it is said to be Natural Active Immunity, and is acquired by artificial active immunity.

Passive immunity: It depends upon the antibodies transported from another person or even an animal. It could be natural passive immunity.

Passive immunity can be transferred artificially by introducing antibodies derived from animals or human beings who are already immune to that disease. This is termed as Artificial Passive Immunity.

Immunization: It is the process of introducing immunity as a preventive measure against certain infectious diseases. The incidence of number of diseases e.g. diphtheria measles etc has declined dramatically since the introduction of effective immunization programmes once thought to be the dreadful diseases like tuberculosis etc is now under control through immunization and treatment.

what is lymphatic system

lymphatic system:

A system of blind vessels i.e. lymphatics that drains lymph from all over the body into the blood stream is called lymphatic system. In addition to lymphatics and lymph it consists of lymph nodes, splan, thymus, tonsils and some of the patches of tissues in vermiform appendix and small intestine.

Lymph vascular system starts at capillary bed, where tissue fluid (intestinal fluid surrounding the cells of the tissues) enters the lymph capillaries which are closed towards the tissue sinuses. These are thin walled anastomosing microscopic vessels which form a network in every organ except the nervous system. The lymph capillaries merge into lymph vessels which have large diameter. These vessels contain smooth muscles in their walls as well as internal valves to prevent backward flow of lymph. The lymph circulates through the lymph vessels by the contraction of surrounding skeletal muscles in one direction, towards the heart. These vessels converge into collecting ducts that drain into veins in the lower neck.

All body tissues are bathed in a watery fluid derived from the blood stream. This inter cellular or tissue fluid is formed when blood passes through the capillaries. The capillary walls are permeable to all components of blood except RBC and blood protein. The fluid passes from the capillary into the intercellular spaces as the intercellular or tissue fluid. About 85% fluid returns into the blood at the venous end of the capillary. The rest 15% of the tissue fluid drains into blindly ending lymphatic capillaries as lymph along with WBC cell debris and microorganisms like bacteria are transported back to the heart through lymphatic system. Thus lymph can be defined as a colourless body fluid that contains lymphocytes (agranular WBC), small proteins and fats. Lymph takes fluid substances from cells of tissues and intercellular spaces which cannot penetrate the blood capillaries. It is a medium of exchange between blood and body cells.

Throughout the course of lymphatics lie lymph nodes through which lymph flows. Lymph nodes vary considerably in size from microscopic to about one inch in diameter. Each node consists of thin, fibrous outer capsule and an inner mass of lymphoid tissue. Penetrating the capsule are several small, lymphatic vessels which carry lymph into lymph node, while single large vessel carries it out. The lymphoid tissue contains anti bodies, lymphocytes and macrophages.

These nodes act as filters that trap microorganisms and other foreign bodies in the lymph. The lymphocytes and macrophages present here, neutralize and engulf the microorganisms.

Function of lymphatic system:

Drainage System: Lymphatic vessels act as drainage channels for water and plasma proteins that have leaked away from blood at capillary bed and that must be delivered back to blood circulation, without which death can occur in 24 hours.

Defense of the body: Microorganisms, foreign cells, cellular debris in the lymph are removed by macrophages residing in the lymph nodes. These are also sight for differentiation of B cells into antibody secreting cells.

Absorption and delivery of fats: Lymph capillaries called lacteals penetrate the villi of small intestine where fats are absorbed and delivered to blood circulatory system.

Edema: Whenever the tissue fluid accumulates rather than being drained into blood by lymphatic system, tissues and body cavities become swollen. This condition is known as edema.

What is invertebrate respiratory system

Invertebrate Respiratory System:

In single celled Protists such a Protozoa diffusion across the Plasma membrane moves gases into and out of the organism. In that worms all body cells are relatively close to the body surface or are thin walled and hallow. (e.g. Hydra). Again gases diffuse into and out of the animal. Forth worms live in moist environments and have capillary network just under their integument and they exchange gases with the air spaces among soil particles. Most aquatic invertebrates carry out gas exchange with gills e.g. sea stars. Marine and annelid worms have parapodia that are richly supplied with blood vessels and function as gills.

Crustaceans and Molluscs have gills that are compact and protected with hard covering devices. Such gills divide into highly branched structures to minimize the gas exchange. Some terrestrial invertebrate like insects, centipedes and some mites, ticks and spiders have tracheal systems consisting of highly branched chitin lined tubes called tracheae. Trachea opens to the outside of the body through spiracles; most gas exchange occurs by diffusion though preumostome. Arachnids possess tracheae, book lungs or both. Book lungs are paired invaginations of ventral body wall. Air enters the book lung through a spiracle and circulates between lamellae. Respiratory gases diffuse between the hemolymph moving along the lamellae and the air in the air chamber.

Some ventilations also result from the contraction of a muscle attached to the dorsal side of a muscle attached to the dorsal side of the air chamber. This contraction dilates the chamber and opens the spiracle. Most gas exchange is still by diffusion. In the snails and slugs gas exchange organ is a pulmonate lung that opens to the outside via preumostome. Thus lung is derived from the mantle cavity. Primitive pulmonate snails are aquatic and close the preumostome during submergence. When the snail surfaces to breathe air, the preumostome opens. Most of the higher pulmonaters are terrestrial and rely on their lungs for gas exchange. The lung may be ventilated by arching and then flat.

Structure of Human Heart

Human heart moves blood into the body. It pumps its entire blood volume about five litres every minute; about 8000 litres of blood move through 96000 km; of blood vessels every day.

The heart of average adult beats about 70 times per minute. Most of the human heart is composed of cardiac muscle tissue called myocardium (myo=muscle). Outer protective covering of heart is fibrous connective tissue called pericardium. Connective tissue and endothelium from the inside of the heart, the endocardium.

Left and right halves of heart are two separate pumps each containing two chambers. In each half blood first flows into a thin walled atrium then into thick walled ventricle.

Valves:

Tricuspid valve is between right atrium and right ventricle. Bicuspid valve is between left atrium and left ventricle. They are collectively called Atrio-vascular values. Pulmonary semilunar valve is at the exit of right ventricle. The aortic semilunar valve is at the exit of left ventricle (collectively these are referred to as semilunar valves). All of these valves open and close due to blood pressure changes when the heart contracts during each heart beat. Heart valves prevent backflow of blood.

Heart cycle:

The heart beat is a sequence of muscle contractions and relaxation called cardiac cycle.

A ‘pacemaker’, a small mass of tissue called sinoatrial node (SA node) at the entrance to the right atrium initiates each heart beat. The SA node initiates the cardiac cycle by producing an action potential that spreads over both atria, causing them to contract simultaneously and eject blood into the pulmonary and systemic circulations. The action potential moving over the surface of the heart causes current flow which can be recorded. 

During each cycle the atria and ventricles go through a phase of contraction called systole and a phase of relaxation called diastole. Specifically while the atria are relaxing and filling with blood, the ventricles are also relaxed. As more and more blood accumulates in the atria, blood pressure rises and the atria contract, forcing AV values open and causing blood to rush into the ventricles. When the ventricles contract the AV values close and the semilunar values open, allowing blood to be pumped into the pulmonary arteries and aorta. After the blood has been ejected from the ventricles they relax and start the cycle again.

Components of vertebrate blood

Plasma:

Plasma (any thing formed or molded) is the straw coloured liquid part of the blood. In mammals plasma is about 90 percent water and provides the solvent for dissolving and transporting nutrients.

A group of proteins (albumen, fibrinogen and globulin) comprises another 7 percent of the plasma. The concentration of these plasma proteins influences the distribution of water between the blood and extra cellular fluid. Albumen is about 60 percent of the total plasma proteins and it plays important role with respect to water movement. Fibrinogen is necessary for blood coagulation (clotting). Globulins include immuno globulins and various metal binding proteins. Serum is plasma from which the protein involved in blood clotting has been removed.

Gamma globulin portion functions in the immune response because it consists mostly of antibodies. Remaining 3 percent of plasma is composed of electrolytes, amino acids, glucose and other nutrients, various enzymes, hormones, metabolic wastes and traces of many inorganic and organic molecules.

Formed elements:

Cellular component of vertebrate blood consists of erythrocytes (red blood cells i.e. RBC), leucocytes (white blood cells i.e. WBC) and platelets (thrombocytes). White blood cells are present lower number than are red blood cells, 1 to 2 percent of blood by volume. White blood cells are divided into agranulocytes and granulocytes. Two types of agranulocytes are lymphocytes and monocytes.

Three types of granulocytes are eosinophils, basophils and neutrophils. Fragmented cells are called platelets (thrombocytes).

Red blood cells:

Erythrocytes (Erythros=red cells) vary in size shape and number of different vertebrates.

Mammalians RBC are enucleated (without nucleus).

Some fishes and amphibians also have enucleated RBC. Salamander (Amphiuma) has largest RBC.

Avian RBC is oval shaped nucleated and larger than mammalian RBC. Among birds ostrich has largest RBC. Most mammalian RBC is biconcave disks but camel and Llama have elliptical RBC. The shape of biconcave disk provides larger surface area for gas diffusion.

Lower vertebrates tend to have fewer but larger RBC than higher invertebrates. Entire mass of a RBC consists of hemoglobic (haeme=blood + globules=little globe) an iron-containing protein. Major function of an erythrocytes to pick up oxygen from the environment, bind it to haemoglobin to form oxy-haemoglobin is bright red. As oxygen diffuses into the tissue, blood becomes larger and blue when observed through the blood vessel wall. When this less oxygenated blood is exposed to oxygen (such as when a vein is cut and a mammal begins to bleed), it turns bright red. Haemoglobin also carries waste carbon dioxide (in the form of carbamino haemoglobin) from the tissues to the lungs (or gills) for removal from the body.

White blood cells:

White blood cells or leucocytes are scavengers that destroy the microorganisms at infection sites, remove foreign chemicals and remove debris that results from dead or injured cells. All WBC are derived from immature cells called stem cells in bone marrow by a process called haematopoiesis.

Among the gramulocytes is phagocyte and ingest foreign proteins and immature complexes rather than bacteria. In mammals cosinophils also release chemicals that counteract the effects of certain inflammatory chemicals released during allergic reactions.

Basophils are the least numerous WBC. When they react with a foreign substance, their granules release histamine and heparin. Histamine causes blood vessels to dilate and lead fluid at a site of inflammation and heparin prevents blood clotting.

Neutrophils are the most numerous of white blood cells. They are chemically attracted to sites of inflammation and are active phagocytes.

Two types of agranulocytes are monocytes and lymphocytes. Two types of lympocytes are B cells and T cells, both of which are central to the immune response. B cells originate in the bone marrow and colonized the lymphoid tissue where they mature, when B cells are activated. They divide and differentiate to produce plasma cells. T cells are associated with and influenced by thymus gland before they colonize lymphoid tissue and play their role in immune response.

Platyleles (Thrombocytes):

Platylets or thrmocotyes (thrombus=clot + cells) are dise shaped cells fragments that initiate blood clotting. When a blood vessel is injured, platelets immediately move to the site and clump, attaching themselves to the damaged area, and thereby beginning the process of blood coagulation.

Vertebrate blood vessels and evolution of heart and blood vessels of vertebrates

Vertebrate Blood Vessels:

Arteries: Arteries are elastic blood vessels that carry away blood from the heart to the organs and tissues of the body. Surrounding the lumen of an artery is a thick wall composed of three layers or tunicae or coverings. Outermost layer consists of connective tissue.

The middle layer has elastic and smooth muscle tissue.

Inner layer consists of single layer of smooth endothelial cells. Wall of artery is thicker than vein.

Veins: Most veins are relatively inelastic, large vessels carry blood from the body tissues to the heart. The wall of a vein contains same three layers (tunicae) as arterial walls but the middle layer is much thinner and one or more valves are present. The valves permit blood flow in only one direction which is important in returning the blood to the heart.

Capillaries: Arteries lead to terminal arterioles. The arterioles branch to form capillaries which connect to venules and the veins. Capillaries are generally composed of single layer of endothelial cells and are most numerous blood vessels in animals’ body. An abundance of capillaries makes an enormous surface area available for exchange of gases, fluids, nutrients and wastes between the blood and nearby cells.

Evolution of heart and blood vessels:

Fishes: Bony fish heart has two chambers, atrium and ventricle. Blood leaves the heart via ventral aorta which goes to the gills; blood becomes oxygenated, loses carbon dioxide and enters the dorsal aorta. Dorsal aorta distributes blood to all body organs and then blood returns to the heart via various system.

Since blood only passes through the heart once, this system is called single circulation circuit. This circuit has the advantage of circulating oxygenated blood from the gills to the systemic capillaries in all organs simultaneously. The circulation of blood through the gill capillaries, offers resistance to flow. Blood pressure and rates of flow to other organs are thus reduced.

Amphibians: A single ventricle pumps blood both to the lungs and to the rest of the body. Blood returning from the skin also contributes oxygenated blood to the ventricle. The blood pumped out to the rest of the body is thus highly oxygenated. The ventricle has incomplete partition.

Reptiles: In the heart of most reptiles, the ventricle is partially divided into right and left side oxygenated blood from lungs returns to left side of the heart via pulmonary vein and does not mix much with deoxygenated blood in the right side of the heart. When the ventricles contract, blood is pumped out aorta for distribution to the body as well as to lungs. The incomplete separation of ventricles is an adaptation for reptiles such as turtles because it allows to be diverted away from the pulmonary circulation during dividing and when the turtle is withdrawn into its shell. This conserves energy and diverts blood to vital organs during the time when lungs can not be ventilated.