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.
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