The blood system of a vertebrate consists of four parts : the heart, arteries, capillaries and veins. The sole function of the heart is to pump the blood round the body. It lies in a cavity separated from other organs by a pericardium. The arteries convey blood away from the heart, branching into smaller vessels called arterioles which branch into the tiniest blood vessels, the capillaries. Blood being incompressible, when it is forced out of the heart the artery walls have to relax to accommodate it. Since this blood is under the greatest pressure in the body their walls have also to be muscular, making it difficult to see the blood through the artery wall, which therefore is pinkish in colour. The capillaries run between the cells of all the tissues, and possess extremely thin walls through which the substances in the blood can pass into the surrounding spaces between the cells. These spaces contain lymph. Veins convey blood towards the heart, receiving it from smaller tributaries, venules, which in their turn have received it from the capillaries. Some veins do not pour their blood into the heart, but into some other organ, notably the vein in all vertebrates conveying blood from the walls of the alimentary canal to the liver. Such a vessel begins and ends in capillaries, and the blood in it therefore has to pass through the gateway of another organ before reaching the heart, and is therefore termed a portal vein. The blood in a vein having passed through capillaries, which because of their small size offer a high resistance to its motion, is not at a high pressure like that in arteries, and so it drains back into the heart. The walls of veins are therefore thin, and are elastic to accommodate blood until it gets back to the heart. To prevent reflux or backfiow there are ’watch-pocket ’or ’semilunar ’valves along each vein at intervals. These flap back and shut up the cavity should the blood begin to flow in the wrong direction. Muscular movements assist the return of the blood to the heart by squeezing the veins.
The simplest type of vertebrate blood circulation is possessed by fish. The heart, lying in the pericardium, has four compartments : a sinus venosus, a thin-walled bag into which all blood collects from veins, leading to an auricle, a slightly muscular structure which beats gently, conveying blood into the next chamber, the ventricle, a muscular organ which contracts strongly to drive the blood within along a comis arteriosus. This passes blood along the artery immediately outside the pericardium, the ventral aorta. This divides into vessels which go to the gills where gaseous exchange occurs and which therefore are called afferent branchial vessels. The gill capillaries run together to form efferent branchial vessels which join to form a single vessel, the dorsal aorta. This gives branches forwards to the head and runs backwards, giving branches to all the organs. Blood from these gathers into large spaces called sinuses which empty blood into the sinus venosus. In one complete circulation the blood passes through the heart once, and the fish is therefore said to have a single circulation. To make the blood flow in one direction only there are flaps at the beginning of each chamber of the heart and in the conus arteriosus. These are normally directed the same way as the flow of blood, but should reflux or back pressure occur they move back with the blood and so close the aperture.
The heart, lying in the pericardium, consists of a sinus venosus, two auricles, a ventricle and a truncus arteriosus. Until recently it was believed that the heart of the frog functioned as follows : The left auricle receives oxygenated blood from the lungs by the pulmonary veins. At the same time the right auricle fills with deoxygenated blood from the sinus venosus, which has come from the rest of the body. Reflux is prevented by valves in both cases. Both auricles are slightly muscular and now contract, the blood passing into the ventricle, there being deoxygenated blood on the right side, oxygenated blood on the left side, and some mixed blood in the middle. Mixing is prevented to a great extent by the blood being held in the spongy walls ; also it is there for only a very short space of time. The ventricle has thick muscular walls and now contracts, the blood forcing up the auriculo-ventricular valves and shutting up the aperture leading back into the auricles, the chorda? tendinea; preventing the flaps from turning inside out. Blood thereforeby the only other aperture, along the truncus arteriosus. Deoxygenated blood, being on the right, is that which first to pass along via the pulmocutaneous arches to the lungs and skin The truncus is somewhat muscular and now begins to contract, pushing the spiral valve against the aperture of the pulmocutaneous arches. The mixed blood therefore flows along the systemic arches to all parts of the trunk and limbs, where it gives up its available oxygen, and along the lingual arteries to the tongue, where it becomes oxygenated. At this stage it cannot go along the internal carotid arteries as these begin in a capillary network which offers a high resistance. At the height of the beat of ventricle and truncus arteriosus, this resistance is overcome, and the remaining blood, which is most oxygenated, is pumped to the head. The valves at each end of the conus arteriosus prevent the blood from flowing backwards.
All the deoxygenated blood returns to three venas cavse which empty into the sinus venosus. Blood passes through the heart twice on one circulation. As the two kinds of blood can mix in the single ventricle, we say that the circulation is incompletely double.
Experimental work on the frog ’s heart has failed to establish the truth of the foregoing account of the circulation of the blood through the heart and truncus arteriosus. There is no evidence that a separation of the blood into deoxygenatcd, ’mixed ’and oxygenated streams actually occurs. Moreover it has been pointed out that a considerable volume of the blood entering the right auricle has come from the skin where it has been oxygenated. It has been suggested that the heart of the frog and the truncus with its valves is a ’relic. ’ The ancestors of the frog may have had scaly skins which could not function as respiratory organs and possibly in these the heart and truncus did operate as described.
In addition to the portal vein in a frog there are three other portal veins.
A branch of each femoral vein joins with the corresponding sciatic vein to form the renal portal vein going to a kidney. The other branch of the femoral joins its fellow to form the anterior abdominal vein which goes to the liver, and which is so clearly seen on removing the skin from the belly wall of a dead frog. Thus all blood coming up from the legs has to pass through either the kidneys or the liver before reaching the heart.
The heart, lying in the pericardium, consists of two auricles and two ventricles, having neither sinus venosus nor conus arteriosus as in the fish and frog.
Oxygenated blood enters the left auricle from each lung by a pulmonary vein, reflux being prevented by contraction of the veins when the auricle, which is slightly muscular, contracts, causing the blood to pass into the left ventricle. This chamber is the most muscular and has therefore the thickest walls, and it now contracts. Some of the blood forces up the flaps of the mitral valve, shutting the aperture leading back into the left auricle, and the blood leaves by the only other route, the aortic arch, to be distributed to all tissues except the lungs. Reflux is prevented by three well-defined semilunar valves at the beginning of the aorta, which fill up with blood and block the base of the aorta when the ventricle relaxes.
Deoxygenated blood from these tissues returns to the heart, in the rabbit, by way of the three venas cava?. These pour their blood into the right auricle, which is slightly muscular and which contracts to drive the blood into the right ventricle ; this has slightly less muscular walls than the left ventricle, although its cavity holds the same amount of blood. When it contracts, the aperture leading back into the right auricle is closed by blood pushing up flaps of the tricuspid valve, and the blood is forced to go along the pulmonary artery to the lungs, where it becomes oxygenated once more. Here also reflux is prevented by three semilunar valves at the beginning of the pulmonary artery when the ventricle relaxes.
The two auricles beat simultaneously, and when they relax the two ventricles beat together. The mitral and tricuspid valves are prevented from turning inside out by the chorda? tendinea?. At the height of the beat of the ventricles the walls of the aorta and pulmonary artery expand, otherwise their walls would burst. This sometimes happens with old people due to ’hardening of the arteries, ’ so that their walls are no longer elastic. At each beat of the ventricles, a shock wave travels along the arteries. This is known as the pulse and is most easily felt at the base of the thumb in the wrist.
We see here that oxygenated blood passes through the left side of the heart, and deoxygenated blood through the right side—the circulation being completely double. It should be noticed that although the pulmonary artery carries dcoxygenated blood, it is still called an artery, since it carries blood away from the heart. Similarly the pulmonary veins are called veins although they carry oxygenated blood, since they, like all veins, carry blood tozvards the heart.