Monary Circulation

There is, however, one substance which may possibly be pro duced in the organism under normal conditions, and which is known to have a powerful capillary dilator effect. This substance is histamine, a product of the amino-acid histidine, which is a normal constituent of most proteins. It has been suggested that the production of histamine plays an important part in giving rise to the symptoms of "surgical shock." In this condition there is a universal dilation of the capillaries, and therefore a fall of blood pressure and a diminution of the amount of blood in circulation which finally leads to death. This condition, which occurs after laceration especially of muscles, can easily be reproduced by injec tion of large doses of histamine.

The mechanism of adaptation of the circulation in various organs to their needs must at present be regarded as in part dependent on the delicate balance of the nervous vasodilator and vasoconstrictor influences, which modify the tone chiefly of the arterioles, and also on chemical substances produced locally as the result of activity, which modify the tone of the capillaries.

The Portal Circulation.—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 portal vein 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. The portal circulation is largely maintained by the action of the respiratory pump, the peristaltic movements of the intestine and the rhythmic contrac tions of the spleen ; these agencies help to drive the blood through the second set of capillaries in the liver. Obstruction in the right heart or pulmonary circulation at once affects the circulation in the liver. The increased respiration which results from muscular exercise greatly furthers the hepatic and portal circulation.

The Cerebral Circulation.

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 each pulse. Increase in arterial pressure increases the velocity of flow through the brain, the whole cerebral vascular system be having like a system of rigid tubes when the limits of expansion have been reached. The pressure of the brain against the skull

wall is circulatory in origin; in the infant's fontanelle the brain can be felt to pulse with each heart-beat and to expand with expiration. 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 can be tied without grave effect. Sudden com pression of both carotids may render a man unconscious, but will not destroy life, for the centres of respiration, etc., 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 vasomotor nerves is uncertain.

The Coronary Circulation.

Extremely peculiar conditions exist in the coronary circulation, i.e., the blood supply to the heart muscle itself. The coronary arteries originate at the root of the aorta. They turn back into the substance of the cardiac muscle and break up into a rich meshwork of capillaries which convey the blood into the coronary veins, the majority of which collect the blood into the coronary sinus, while some open on the inner surface of the chambers of the heart (Thebesian veins). During systole the heart muscle develops a pressure which must be higher than that in the aorta, as otherwise no ejection of blood from the ventricle into the aorta would be possible. The coronary arteries, which are at any time distended by the pressure prevailing in the aorta, are thus compressed, and during systole the inflow of blood into those of the left ventricle is stopped. The right ventricle by its weak contraction only affects the coronary circulation to a slight extent. While restraining the inflow of blood into the coro nary arteries, the contraction of the heart greatly facilitates the outflow from the veins by mechanically expressing their contents. It is interesting to note that adrenaline, which greatly increases the strength of the cardiac beat, also produces a great dilation of the coronary blood vessels, so that a larger amount of blood is provided to meet the increased expenditure of energy. The coro nary blood vessels are richly provided with vasodilator and vaso constrictor nerves ; the sympathetic supplies the vasodilator, and the vagus the vasoconstrictor fibres.