blood circulation The circulation of blood refers to its continual flow from the heart, through branching arteries, to reach and traverse the microscopic vessels in all parts of the body, reconverging in the veins and returning to the heart, to flow thence through the lungs and back to the heart to start the circuit again. This uninterrupted movement of the blood is necessary to maintain the supply of oxygen from the lungs and nutrients from the gut, as well as for the distribution of hormones, many other chemicals, water, and heat, and the delivery of waste for excretion. The 5 litres of blood contained in the blood vessels of a typical adult at rest complete the circuit in about one minute: the blood recirculates 1500 times each day even without any exercise to speed it up.

The beginning of the modern concept of the circulation of blood is attributed in Western society to William Harvey, who, in a treatise published in 1628, Exercitatio anatomica de motu cordis et sanguinis, presented convincing evidence that blood flowed in arteries out from the heart to the tissues and returned back along veins. He did not know how blood passed from arteries to veins, as this was long before Malpighi of Bologna discovered the microscopic capillaries which connect them, but he deduced that some sort of channels must exist. Harvey based his radical conclusions on experiments on a wide range of animals and then demonstrated his results and explanations to his colleagues.

Harvey's contribution was of startling significance. Before his time there had been no serious challenge to the teaching of Galen in the second century ad. Arguing from gross anatomy, Galen believed that blood passed from the right side to the left side of the heart through invisible pores in the muscular septum which separates the two ventricles. Somewhat paradoxically, Harvey dismissed the existence of these cardiac septum ‘pores’ which were widely accepted but had never been seen, whilst simultaneously postulating the existence of components — the capillaries — which he was likewise unable to see.

That blood flows away from the heart in the arteries is clear fom observation following injury, when a high pressure pulsatile jet of blood comes from the heart end of a cut artery and not from the other end. Flow in veins can readily be demonstrated to be only in the direction towards the heart, as illustrated by Harvey (Fig. 1). His experiment is readily repeated. An arm is congested (by letting it hang by the side or squeezing the upper arm) so that the veins stand out; a finger is pressed on a vein and held there; another finger is pressed just above the first and moved along the vein so as to expel the blood towards the heart, then released. Given that there is a valve in the segment of vein which was chosen, as long as the end furthest from the heart remains compressed, the vein remains empty between that point and the valve. It cannot fill from the heart end even when back pressure is applied due to the action of the valves.

The heart is not actually a single pump but, rather, two pumps in series (blood flows in sequence from the first pump round to the second pump and thence back round to the first). The two pumps — the right and left ventricles — are adjacent, sharing the muscular wall which partitions them. The right ventricle pumps blood to the lungs at relatively low pressure (pulmonary circulation). Blood returns from the lungs to the left atrium, enters the left ventricle to be pumped to the rest of the body (systemic circulation) at a much higher pressure, and returns to the right atrium. The various regions of the systemic circulation are perfused with blood through innumerable branching pathways, which are effectively arranged in parallel (see fig.).

Because the right and left heart pumps are in series, apart from transient changes their outputs must be identical. Even a minute difference between the outputs, if sustained, would very rapidly empty either the pulmonary or the systemic circulation. In fact, the rates of beating of the two sides of the heart are linked together electrically and the volumes pumped at each contraction are controlled such that each pump ejects at each stroke the volume which it has received, so that the output always matches the input (the Starling mechanism).

The importance of venous return

It is axiomatic that the heart, like any other pump, can only pump out the volume of blood that flows into it. If venous inflow is reduced, for example following haemorrhage, no matter how fast and hard the heart beats, it cannot restore flow to normal. When people are resting supine, the return of blood along the veins to the heart is largely a passive process. Sufficient pressure is transmitted from arteries through capillaries to veins to provide an adequate pressure for venous return. However, when we stand, blood distends dependent veins and the return is decreased. Physical exercise, particularly involving the legs, causes an increase in the flow of blood into the leg veins, acting as an auxiliary pump mechanism to enhance the return of blood to the heart.

There are actually two auxiliary pump mechanisms. Veins possess valves and many run deeply in the limbs, surrounded by muscles. Rhythmic movements, as when walking or running, cause alternate muscle tensing and relaxing. During the relaxing phase blood flows into the veins between the muscles, distending them. When the muscle then contracts the veins are compressed, so blood is forced along them. The valves ensure that blood can only move towards the heart.

The other auxiliary mechanism is due to breathing. Pressure in the chest is normally negative and that in the abdomen positive. As we breathe in, this pressure difference increases, and the downward movement of the diaphragm compresses the abdominal contents forcing blood into the chest through the inferior vena cava.

Regional circulations

Since the systemic and pulmonary circulations form one continuous circuit, the flow through the lungs is entirely dependent on that through the systemic circulation. Systemic flow is the sum of flow through the many parallel circuits supplying different organs and tissues. Systemic and pulmonary flow must clearly each be the same as the cardiac output — the volume pumped by each ventricle. At rest the cardiac output is typically about 5 litres/min. This can increase during exercise to as much as 25 litres/min in fit young people and 35 litres/min, or briefly even more, in elite endurance athletes. The quantity of blood flowing through each organ or region supplied by the systemic circulation is regulated according to its own particular physiological requirement.

Brain circulation.

About 15% of the resting cardiac output supplies the brain. This flow is vital as the brain cannot withstand more than a few seconds of interruption of flow without loss of consciousness, and longer interruptions cause irreversible damage. Overall brain blood flow remains relatively constant, although regional changes occur in response to changes in neuronal activity. For example, shining a light in the eye results in an increase in blood flow to the region of the brain concerned with vision. In the upright position, because of the effects of gravity, the blood pressure in the brain is lower than elsewhere, making it susceptible to low blood pressure. However the brain does show the phenomenon of ‘autoregulation’, whereby its blood flow is kept relatively stable over a quite wide range of blood pressure. Although the brain blood flow is determined to some extent by nervous control of the diameter of blood vessels, it is more importantly controlled by chemical factors, particularly the level of carbon dioxide which, when it increases, dilates the cerebral vessels and increases flow. Overbreathing lowers the level of carbon dioxide in the body and can cause dizziness due to decreasing brain blood flow.

Muscles.

A contracting muscle produces several chemicals which are the end products of its metabolic activity. These ‘metabolites’ act directly on the resistance vessels (arterioles), dilating them and thus regulating blood flow so that it is appropriate for the level of activity. Although sympathetic nerves do supply muscle blood vessels, they control only the resting blood flow and play no part in the response to exercise. At rest, flow in all muscles comprises only about 1 litre/min out of the cardiac output of 5 litres/min. During exercise, if cardiac output increases to 25 litres/min, 20 litres of this goes to the working muscles.

Heart (coronary) circulation.

The flow to the heart muscle via the coronary arteries takes about 5% of cardiac output both at rest and when it increases during exercise. The heart removes nearly all the oxygen from the coronary blood even at rest, so that, when heart work increases, the only way that more oxygen can be supplied is for blood flow to increase. If this cannot happen, as for example if coronary vessels are partially blocked, there is coronary insufficiency and heart pain (angina). Coronary flow is influenced by mechanical factors: left ventricular flow is low or absent during contraction (systole) and maximal during the relaxation phase (diastole). As for skeletal muscle, coronary flow is controlled by metabolites rather than directly by nerves.

Skin.

Blood flow to the skin is controlled by the mechanisms of temperature regulation. If local skin temperature or general body temperature rises, skin vessels, including special arterio–venous shunt vessels, dilate to increase skin blood flow and thereby increase skin temperature and facilitate cooling. Skin flow is controlled partly by nerves and partly by direct local temperature effects (warm hands are red). During very cold conditions blood flow to the entire skin is almost completely cut off. During extreme heat, flow may increase to the extent that most of the output of the heart flows through the skin.

Circulation to the gastrointestinal tract (splanchnic flow).

The anatomy of the circulation of the gut (splanchnic circulation) is unusual in that the venous blood does not return directly to the heart, but instead it flows in the portal vein to the liver, and only after this does it reach the hepatic vein and the inferior vena cava. Since the splanchnic circulation is concerned with the digestion and absorption of food, this means that the blood carrying products of digestion passes through the liver before reaching any other part of the body. Splanchnic vessels also serve a ‘reservoir’ function. Constriction of splanchnic arerioles — the resistance vessels — is under the control of the sympathetic nervous system and makes a major contribution to blood pressure control. The splanchnic veins also constrict in response to stimulation of the sympathetic nerves; this reduces their capacity and increases the flow of blood returning to the heart.

Kidneys.

Although the kidneys are relatively small organs they receive about 20% of cardiac output. Most of this blood flow is concerned with the filtration of the plasma and the subsequent concentration of the filtrate to form urine. The blood flow is normally autoregulated and is therefore relatively independent of blood pressure unless this falls to abnormally low levels.

Roger Hainsworth, and E. M. Tansey

Bibliography

Vogel, S. (1992). Vital circuits — on pumps, pipes, and the workings of circulatory systems. Oxford University Press, New York, Oxford.

See cardiovascular system.See also autonomic nervous system; blood pressure; blood vessels; heart.