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.
.jpg)