Portal Circulation.—The portal system of veins includes four large trunks which collect the blood from the viscera of digestion, the superior and inferior mesenteric veins from the intestines, the splenic vein from the spleen, and the gastric from the stomach. These join together to form the portal vein, the only vein that breaks up into capillaries. This divides and ramifies through the liver, whence it emerges as the hepatic veins. The whole is known as the portal circulation.
Pulmonary Circulation.—Of the pulmonary circulation and its vessels a few words might also be said. The pulmonary artery, which carries the blood from the right ventricle to the lungs, is only about two inches long and divides into a right and a left pulmonary artery, which pierce the pericardium and go to their respective lungs. The right one is the larger and longer, for it has farther to go and gives off a branch to supply the third lobe of the right lung. The vessels finally divide and subdivide, terminating in the pulmonary capillaries. The venous capillaries then gather together to form a main vein in each lobule, these veins uniting into two trunks for each lung, the pulmonary veins, which empty into the left auricle.
Nerves of Heart.—The muscular fibers of the heart have the power of rhythmical contraction. Independent nerve centers or ganglia are also found in the muscular walls and influence the mechanism of the heart, especially the acceleratory mechanism. Thus, in some of the lower animals the heart can be removed from the body, and if placed in normal salt solution will go on beating for some time. The heart is controlled, however, by two nerves, the vagus or pneumogastric and the sympathetic. Of these the vagus is the inhibitory mechanism. It acts as a check and makes the heart’s action regular and rhythmic. If it is cut, the action of the heart becomes very rapid and irregular. The sympathetic is the acceleratory mechanism. When the vagus alone is stimulated, it first slows, then stops the heart, for it weakens the systole and prolongs diastole. Acceleration follows stimulation of the sympathetic, both the rapidity and the force of the beat being increased. When a person faints from a blow in the abdomen, it is because the pneumogastric is affected and inhibits the action of the heart. The work of the heart is very dependent upon its nervous condition and functional diseases of the heart are practically wholly due to nervous derangement.
Heart Sounds.—Through the stethoscope two heart sounds may be heard. They are known as the first and second sounds. The first is a soft, rushing sound, stronger and louder than the other, and is caused in part by the contraction of the muscle itself when the blood is forced out and in part by the closure of the auriculo-ventricular valves. The second sound is shorter and sharper, a snap, and is caused by the closure of the semilunar valves when the contraction of the ventricles ceases and they begin to refill. In certain diseased conditions, where the edges of the valves are roughened, they do not snap properly and the sound varies from the normal.
The Heart Beat.—The rate of the heart beat is proportionate to the size of the person and increases in rapidity as the size diminishes. If the ear is placed over the abdomen of a pregnant woman, the heart of the fetus can be heard beating very rapidly. In prolonged labor it may become more rapid or very faint and warn the doctor that something should be done. The usual rate of the pulse in the fetus is 140 to 150 times a minute, though it varies with size and sex. At birth it drops to 140 to 130; for the first year it is 130 to 115; for the second year 115 to 105; for the third year 105 to 95; from the seventh to the fourteenth years 80 to 90; from the fourteenth to the twenty-first years 75 to 80; from twenty-one to sixty 60 to 75. In old age it rises a little and is 75 to 80. The rate is higher in the average woman than in the average man and increases with exercise, with increase of temperature, and in high altitudes, where the atmospheric pressure is less.
At each beat of the heart from four to six ounces of blood are expelled into the pulmonary artery and the aorta, and in 22 or 23 beats all the blood in the body passes through the heart. The power exerted by the heart every minute in thus driving the blood upon its course has been estimated as sufficient to raise its own weight, three-quarters of a pound, the height of the Washington monument or 150 meters; for the ventricles have to force the blood into vessels already full.
Factors Affecting Circulation.—There are three main factors in the circulation: 1. the systole, which gives the blood its first impulse; 2. the peripheral resistance in the capillaries, which serves to hold it in check, slowing the circulation and doing away with its rhythmic character, and 3. the elasticity of the walls of the arteries.
If a ligature is tied about an artery, there is a swelling on the side toward the heart, while in the case of a vein, the swelling is on the side away from the heart, that is, the swelling is in either case on the side from which the blood comes. When an artery is cut, however, the blood comes out rhythmically in spurts, though from a cut vein it oozes slowly and regularly. For the blood is pumped out by the heart rhythmically and its rhythmic beating against the walls of the artery is felt in the pulse, which follows slightly after the beat of the heart itself. The pulse is due to the fact that the vessels into which the blood is forced are already full. This causes a local dilation at the beginning of the artery which passes with diminishing force along its entire length, the distention being due to the fact that more force is needed to drive the blood through the small arteries and capillaries than to stretch the elastic walls of the aorta and the large arteries. It is this elastic character of the arteries that makes the blood flow constant, for otherwise the blood would come intermittently in jets, as it is pumped from the heart. The elastic walls of the vessels, however, offer a certain resistance to the pumping of the fluid through them and at the same time, by relaxing between whiles, allow a certain amount of fluid to be retained in them, so that they continue full and the flow is more or less constant. The insufficient outlet also helps to make the flow constant.
By the time the blood reaches the veins its rhythmic character has been done away with, but though there are no elastic walls in the veins, it still has force enough after the slowing in the capillaries to return to the heart. In this it is aided to a certain extent by the valves and by the action of the skeletal muscles as they contract and expand, especially in the arms and legs, where the blood runs perpendicularly and there is a high column to be supported. There are also more veins than arteries, each large artery having two large veins, the venæ comites, to help get the blood back to the heart, and the veins anastomose freely. Thus, if the blood cannot get back by one channel it does by another. In parts like the brain, where it is very important that there should be no compression, since any disturbance of circulation would lead to serious results, the vessels are enclosed in thick walls, and in the liver, through which all the blood passes and where compression is sure to cause trouble, the veins are simply caverns carved out in the organ and have no walls. They lie open when the organ is opened. Varicose veins are the result of valves giving way through inherited weakness or disease so that others have an unduly large weight to support.
The Pulse.—The pulse wave is characterized by a quick rise and a slow fall, though this cannot ordinarily be distinguished by the finger. In some slow fevers, however, the fall is very long and distinct ripples can be felt. This is known as the dicrotic pulse. With age the arterial walls grow stiffer and more rigid and less adapted to their work. In certain cases of heart disease the heart does not transmit all the beats to the pulse and to get the true rate the heart must be listened to.