of measuring the capillary pressure. It and the venous pressure constantly vary from nothing to a positive amount with rest or movement of muscles, change of posture, &c.
The arterial pressure is raised during exertion by the more forcible beat of the heart—e.g. pressures of 140–190 mm. Hg have been observed immediately after a 3-mile race. It rapidly sinks to a lower level than usual after the exertion is over, e.g. 90 mm. Hg, owing to the quieter action of the heart and the persistence of the cutaneous dilatation of the blood vessels which is evoked by the rise of body temperature. The writer has observed in athletes rectal temperatures of 102–105° F. after long races. After meals there is an increase in cardiac force to maintain the flow through the dilated splanchnic vessels. Mental excitement raises the pressure—e.g. the writer’s pressure may be 110 mm. before and 125 mm. Hg after giving a lecture. The origin of the blood pressure in the arteries is the energy of the heart. The pressure gradient depends on the peripheral resistance. In the arterial the pressure is spent, and little of it reaches the capillaries. The return of the capillary blood to the veins and the pressure in the veins is due partly to the remainder of the cardiac force, but more largely to the contraction of the skeletal muscles and the viscera, to the action of gravity in changes of posture and to the respiratory pump.
The pulmonary artery, carrying venous blood, divides and subdivides, and the smallest branches end in a plexus of capillaries on the walls of the air-cells of the lung. From this plexus the blood is drained by the radicles of the four pulmonary veins which open into the left auricle. The pressure in the pulmonary artery is less than one-third the aorticThe pulmonary circulation. pressure, and the blood takes only one-third of the time to complete the pulmonary circuit that it takes to make the systemic. The four chief factors which influence the pulmonary circulation are: (1) the force and output of -the right ventricle; (2) the diastolic filling action of the left auricle and ventricle; (3) the diameter of the pulmonary capillaries, which varies with the respiratory expansion of the lungs; (4) the intrathoracic pressure.
In inspiration the lungs are distended in consequence of the greater positive pressure on the inner surfaces being greater than the negative pressure on their outer pleural surfaces. The negative pressure in the intrathoracic cavity results from the enlargement of the thorax by the inspiratory muscles. /Vhen the elastic lungs are distended by a full inspiration they exert an elastic traction amounting to about 15 mm. Hg. The heart and vessels within the thorax are submitted to this traction-that is, to the pressure of the atmosphere minus 15 mm. Hg-while the vascular system of the rest of the body bears the full atmospheric pressure. The thin walled auricles and veins yield more to this elastic traction than the thick-walled ventricles and arteries. Thus inspiration exerts a suction action, which furthers the filling of the veins and auricles. This action is assisted by the positive pressure exerted by the descending diaphragm on the contents of the abdomen. Blood is thus both pushed and sucked into the heart in increased amount during inspiration.
Experiment has shown that the blood vessels of the lungs when distended are wider than those of collapsed lungs. Suppose an elastic bag having minute tubes in its walls be dilated by blowing into it, the lumina of the tubes will be lessened, and the same occurs in the lungs if they are artificially infiated with air; but if the bag be placed in a glass bottle, and the pressure on its outer surface be diminished by removing air from the space between the bag and the side of the bottle, the bag will distend and the lumina of the tubes be increased. Thus it is evident that inspiration, by increasing the calibre of the pulmonary vessels, draws blood into the lungs, and the movements of the lungs become an effective force in carrying on the pulmonary circulation. It has been estimated that there is about one-twelfth of the whole blood quantum in the lungs during inspiration, and one-fifteenth during expiration. The great degree of distensibility of the pulmonary vessels allows of frequent adjustments being made, so that within wide limits, as much blood in a given time will pass through the pulmonary as through the systemic system. The limits of their adjustment may, however, be exceeded during violent muscular exertion. The compressive action of the skeletal muscles returns the blood to the venous cistern, and if more arrives than can be transmitted through the lungs in a given time, the right heart becomes engorged, breathlessness occurs, and, signs of venous congestion appear in the flushed face and turgid veins. The weaker the musculature of the heart the more likely is this to occur; hence the breathlessness on exertion which characterizes cardiac affections. The training of an athlete consists largely in developing and adjusting his heart to meet this strain. Similarly the weak heart may be trained and improved by carefully adjusted exercise. Rhythmic compression of the thorax is the proper method of resuscitation from suffocation, for this not only aerates the lungs, but produces a circulation of blood. By compressing the abdomen to fill the heart, and then compressing the thorax to empty it, the valves meanwhile directing the flow, a pressure of blood can be maintained in the aorta even when the heart has ceased to beat, and this if patiently continued may lead to renewal of the heart-beat. There is no certain evidence that the pulmonary arteries are controlled by vaso-motor nerves. In the intact animal it is difficult to determine whether a rise of pressure in the pulmonary artery is induced really by constriction of the pulmonary system, or by changes in the output of the heart; hence different observers have reached conflicting conclusions. In the case of lungs which have been supplied with an artificial circulation and a constant head of pressure to eliminate the action of the heart, no diminution in outflow has been observed in exciting the branches of the vagus or sympathetic nerves which supply the lungs, or by the injection of adrenalin (Sir Benjamin C. Brodie (1783–1862), and Dixon, Burton-Spitz).
The portal circulation is peculiar in that the blood passes through two sets of capillaries. Arterial blood is conveyed to the capillary networks of the stomach, spleen, pancreas and intestines by branches of the abdominal aorta. The portal vein is formed by the confluence of the mesenteric veins with the splenic vein, which together drain these capillaries. The The portal circulation. portal blood breaks up into a second plexus of capillaries within the substance of the liver. The hepatic veins carry the blood from this plexus into the inferior vena cava. Ligation of the portal vein causes intense congestion of the abdominal vessels, and so dis tensile are these that they can hold nearly all the blood in the body: thus the arterial pressure quickly falls, and the animal dies just as if it had been bled to death. The portal circulation is largely maintained by the action of the respiratory pump, the peristaltic movements of the intestine and the rhythmic contractions of the spleen; these agencies help to drive the blood through the second set of capillaries in the liver. The systole of the heart may tell back on the liver and cause it to swell, for there are no valves between it and the inferior vena cava. Obstruction in the right heart or pulmonary circulation at once tells back on the liver. The increased respiration which results from muscular exercise greatly furthers the hepatic circulation, while it increases the consumption of food material. Thus exercise relieves the over-fed man. The liver is so vascular and extensile that it may hold one-quarter of the blood in the body.
The circulation of the brain is somewhat peculiar, since this organ is enclosed in a rigid bony covering. The limbs, glands and viscera can expand considerably when the blood pressure rises, but the expansion of the brain is confined. By the expression of venous blood from the veins and sinuses the brain can receive a larger supply of arterial blood at The cerebral circulation. each pulse. Increase in arterial pressure increases the velocity of flow through the brain, the whole cerebral vascular system behaving like a system of rigid tubes when the limits of expansion have been reached. For as the pressure transmitted directly through the arteries to the capillary veins must always be greater than that transmitted through the elastic wall 'of the arteries to the brain tissue, the expansion of the arteries cannot obliterate the lumina of the veins. The pressure of the brain against. the skull wall is circulatory in odgin: in the infant's fontanelle the brain can be felt to pulse with each heart-beat and to expand with expiration. The expiratory impediment to the venous flow produces this expansion. A blood clot on the brain or depressed piece of bone raise the brain pressure by obliterating the capillaries in the compressed area and raising the pressure therein to the arterial pressure. The arterial supply to the brain by the two carotid and two vertebral arteries is so abundant, and so assured by the anastomosis of these vessels in the circle of Willis, that at least two of the arteries in the monkey can be tied without grave effect. Sudden compression of both carotids may render a man unconscious, but will not destroy life, for the centres of respiration, &c., are supplied by the vertebral arteries. The vertebral arteries in their passage to the brain are protected from compression by the cervical vertebrae.
Whether the muscular coat of the cerebral arteries is supplied with vaS0-motor nerves is uncertain. Hiirthle and others observed a rise of pressure in the peripheral end of the carotid artery on stimulating the cervical sympathetic nerve. The writer found this to be so only when the cervical sympathetic nerve was excited on the same side as the carotid pressure was recorded. If the circle of Willis was constricted, excitation of either nerve ought to have the effect; it is possible that the effect was produced by the vasoconstriction of the extra-cranial branches of the carotid. After establishing an artificial circulation of the brain Wiggins found that adding adrenalin to the nutritive fluid reduced the outflow, and it is supposed that adrenalin acts by stimulating the ends of the vasomotor nerves, rather than by stimulating the muscular coats of the arteries. The veins of the pia and dura mater have no middle muscular coat and no valves. The venous blood emerges from the skull in man mainly through the opening of the lateral sinuses into the internal jugular vein; there are communications between the cavernous sinuses and the ophthalmic veins of the facial system, and with the venous plexuses of the spinal cord. The points of emergence of the veins are well protected from closure by compression. The brain can regulate its own blood supply by means of the cardiac and vaso-motor centres. Deficient supply to these centres excites increased frequency of the heart and constriction of the arteries, especially those of the great splanchnic area. Cerebral j excitement has the same effect, so that the active brain is assured of a greater blood supply (Bayliss and L. Hill).
