108 VASCULAR SYSTEM Volume of organs. Distribu tion of blood. Cranial. Hepatic. Pulmon ary. of blood in the body passes through the heart in about 30 pulsa tions. Taking the pulse beat at 72 per minute, it follows that the duration of the circulation is about 26 seconds. It has been satisfactorily proved by Mosso, Von Basch, Dogiel, and Francois Franck that there is a slight change in the volume of any distensible organ with each beat of the heart. Mosso devised the plethysmograph, 1 an instrument by which the following results have been obtained. (1) The volume of an organ is not fixed, but varies according to the amount of blood contained in it. (2) Its volume changes with each cardiac pulsation, increasing when blood is forced into it and diminishing by the emptying of the capillaries into the veins. (3) Variations in the volume of one or more organs, say, by compression, or by the application of cold, or by the internal administration of substances which affect the calibre of blood vessels, such as ergot, cause corresponding variations in the volume of other organs. (4) The pulsatile variations are very similar to the pulse-curve, and there are respiratory undulations correspond ing to similar variations in blood-pressure tracings. (5) Movements of the limb cause diminution in volume, as was shown by Glisson in 1677, in consequence of acceleration of the venous current. (6) Mental exercise and sleep cause a diminution in the volume of the limb (Mosso). (7) So delicately attuned is the organism that music has been observed to cause a rise and fall in the tracings (Dogiel). The blood is distributed throughout the body in varying propor tions, according to the requirement of any set of organs at a par ticular time. When any tissue or organ is active, there is a deter mination of blood towards it, the amount being increased from 30 to 50 per cent. : thus, during digestion the mucous membrane of the stomach and intestinal organs is richly supplied with blood. Increased muscular velocity is always accompanied by increased vascularity ; but, whilst this is the rule, there are organs, such as the heart, the muscles of respiration, and nervous centres like those in the medulla oblongata, in which there is a condition of continuous activity, and in which there is a uniform vascularity. Seeing that the activity of certain organs varies at different times, it follows that, whilst some organs are congested, others are at rest. In the child there appears to be a different distribution of blood from what obtains in the adult. The heart of a child is relatively small up to puberty, while the vessels are relatively large ; after puberty the reverse is the case. Arterial pressure is less in the child than in the adult, whilst the pressure in the pulmonary circulation is larger in the child than in the adult (Beneke). Attempts have been made to estimate the distribution of blood after death. Ranke states that one-fourth of the total blood is in the muscles, one-fourth in the liver, one-fourth in the heart and vessels, and the remaining fourth in the rest of the organs, Special Forms of Circulation. The cranial circulation has been already described under PHYSIO LOGY, vol. xix. pp. 42-43. The peculiarity of the portal circulation is that the blood passes through two sets of capillaries. Arterial blood is conveyed to the stomach, spleen, pancreas, and intestines by branches of the abdo minal aorta. These branches divide and subdivide, terminating in a capillary plexus in the various organs above enumerated. From this plexus the radicles of the various veins spring, and they unite with each other into larger and larger trunks, until by the conflu ence of the mesenteric veins with the splenic vein the portal vein is formed. The portal vein conveys the blood to the liver, where it divides into smaller and smaller branches constituting a plexxis in the lobules of the liver. From this plexus spring the roots of the hepatic vein, which conveys the blood from the liver to the inferior vena cava (see NUTRITION, vol. xvii. p. 678). There are thus in the portal circulation two sets of capillaries, one in the abdominal viscera and the other in the liver. Ligature of the portal vein causes distension of all the abdominal vessels and a highly congested state of the abdominal viscera, whilst the blood- pressure quickly falls, and the animal dies. So distensible are the abdominal vessels that they can contain nearly all the blood in the body. Blood from such congested vessels has toxic properties (Schiff and Lautenbach). The ventricular systole may send a pulse down the valveless inferior vena cava and cause a pulse in the liver. The liver swells with each systole and relaxes with each diastole of the heart. The pulmonary artery, carrying venous blood, divides and sub divides, and the smallest branches end in a plexus of capillaries on the walls of the air-cells of the lung. From this plexus the radicles of the pulmonary veins originate ; and finally the four portal veins, two from each lung, carry the arterialked blood to the left auricle. Considering the apparently small extent of the pulmonary as com pared with the systemic circulation, and the fact that the two ventricles, of about equal capacity, empty themselves simultane ously, it is clear that the pulmonary circulation presents many points of interest. In the first place, the pressure in the pulmon ary artery is considerably less than that of the aorta. In 1850 it was determined by Ludwig and Beutner to be in the dog equal to a 1 Figured in M Kendrick s I liijsivloijy, p. Mi>, fig. mercurial column of T17 inches, in the cat to one of 69 inch, and in the rabbit 47 inch, or about three times less in the dog, four times less in the rabbit, and five times less in the cat than the pressure in the aorta. Hering passed simultaneously a tube through the muscular walls of each ventricle of a calf, and the blood rose in the tube in the right ventricle 21 inches and in the left 33 4 inches (quoted by Landois and Stirling). Fick and Badoud found a pres sure of 3 54 in the pulmonary artery of the dog, whilst the carotid pressure at the same time was 4 17 inches. The ratio of pulmonary to aortic pressure has been stated as 1 to 3 (Beutner and Marey) and as 2 to 5 (Goltz and Gaule). Next, it is important to note the peculiar physical conditions in Influei the chest during respiration. As already shown (p. 102), the lungs of re- are distended in consequence of the positive pressure on their inner spiratii surfaces being greater than the negative pressure on their outer m pul- pleural surfaces. But when the lungs are distended by a full in- monar spiration they exert an elastic force (termed elastic recoil or " elastic circula traction ") amounting to about 1 18 inches of mercury. Outside the tion. lungs, in the cavity of the chest, the surface of the heart and vessels is subjected to a pressure which is the difference betM een atmo spheric pressure (29 92 inches) and the "elastic traction" (1 18 inches) or 28 74 inches. It is clear that the more the lungs are distended the greater is the elastic traction, and consequently the less the pressure on the outer surface of the vessels. The thin- walled pulmonary veins yield more during a deep inspiration, thus diminishing pressure, than the thicker-walled pulmonary artery, whereby the flow of blood from the capillaries of the lung by the pulmonary veins to the left auricle is favoured. On the other hand, expiration by increasing the pressure tends to retard the flow of blood. Further, the velocity of the stream of blood is accelerated in the pulmonary vessels by inspiration and retarded by expiration. As regards the influence of the movements of the lung on the calibre of the pulmonary capillaries and smaller vessels, experi ment has shown that the blood-vessels of the lungs containing air and distended are wider than those of collapsed lungs. Suppose an elastic bag having minute tubes in its walls to be dilated in a free space, the lumen of these tubes will be diminished ; but, if it be placed in a closed space, as in a wide glass bottle, and if the pres sure on its outer surface be diminished by removing air from the space between the bag and the side of the bottle, the bag will dis tend and the lumen of the tubes will be increased. Thus it is evident that inspiration, by increasing the calibre of the pulmonary vessels, draws blood towards the lungs, and the movements of the lungs become an effective force in carrying on the pulmonary circu lation. The velocity of the blood is greater in the pulmonary than in the systemic capillaries, and greater in the pulmonary veins than in the pulmonary arteries. The great degree of distensi- bility of the pulmonary vessels allows of frequent adjustments being made, so that, within limits, as much blood in a given time will pass through the pulmonary as through the systemic circula tion. This adjustment, however, may be readily disturbed. For example, violent muscular exertion hurries the blood along the veins to the right side of the heart, and by the right ventricle the blood is discharged into the pulmonary circulation. If more arrives than can be transmitted to the left auricle by the pulmonary veins in a given time, the pulmonary capillaries become engorged, breath ing becomes quick and possibly irregular, the right side of the heart becomes engorged, signs of venous congestion appear in the flushed face and turgid veins, and perhaps the pulmonary capillaries may rupture, causing haemorrhage from the lung. The weaker the muscular structure of the heart the more likely is this to occur. Hence the breathlessness in many cardiac affections, aggravated by muscular exertion, more especially in ascending a stair or hill. In the mature foetus the fluid brought from the placenta by the Fcetal. umbilical vein is partly conveyed at once to the vena cava ascendens by means of the ductus vcnosus and partly flows through two trunks that unite with the portal vein, returning the blood from the intes tines into the substance of the liver, thence to be carried back to the vena cava by the hepatic vein. Having thus been transmitted through the placenta and the liver, the blood that enters the vena cava is purely arterial in character ; but, being mixed in the vessels with the venous blood returned from the trunk and lower extremities, it loses this character in some degree by the time that it reaches the heart. In the right auricle, which it then enters, it would also be mixed with the venous blood brought down from the head and upper extremities by the descending vena cava were it not that a provision exists to impede (if it docs not entirely prevent) any further admixture. This consists in the arrangement of the Eusta- chian valve, which directs the arterial current (that flows upwards through the ascending vena cava) into the left side of the heart, through the foramen ovale, an opening in the septum between the auricles, whilst it directs the venous current (that is being returned by the superior vena cava) into the right ventricle. When the ventricles contract, the arterial blood contained in the left is pro pelled into the ascending aorta, and supplies the branches that proceed to the head and upper extremities before it undergoes any
further admixture, whilst the venous blood contained in the right