Unit IV-Circulatory System

Chapter 12

Distribution of Blood Flow in the Arteries

1. Arterioles as Flow Regulators:

     The large arteries subdivide like the major branches of a tree. Their terminal branches, which like the major arterial branches still have muscle tissue, are called arterioles. After flowing through the arterioles, the blood enters vessels which have no muscle. These are called the capillaries.

     The manner in which the blood flow is distributed to the organs and tissues is determined by the state of their arterioles. We may look at the arterial circulation as a reservoir, whose pressure is determined by the output of the left ventricle and the resistance of the arterioles. The arterioles and smallest arteries are like faucets connected to the reservoir. Since the arterioles are not necessarily all open to the same degree at any time, some organs (those whose arterioles are open) will receive a large flow of blood. Others (whose arterioles are closed) will receive small amounts of blood.

     To illustrate how arterioles can control the manner in which the blood flow is distributed, consider a man, a dolphin, or a duck swimming under water. In such a case, the arterioles of the skin, the kidneys, and the digestive organs constrict. The arterioles of the brain, muscles, and the heart itself dilate (open). As a result, the blood flow to the skin, kidneys, and digestive organs is reduced; while the blood flow to the brain, muscles, and heart is increased. The flow of blood is, therefore, changed so that it perfuses the active organs and is directed away from the inactive ones.

     Another example, which most people believe, though some doubts have been raised about its truth, has to do with the redistribution of blood after eating. The general belief is that the arterioles of the digestive organs dilate; those of the brain, heart, and muscles constrict. If this were true, it would lead to a useful result--the blood would go to the digestive organs which are most active rather than to those organs whose activity at the time was less. (Actually, there is evidence that, during digestion, the arterioles of all organs dilate, not just those of the digestive organs, so that the blood flow is increased everywhere equally.)

     Measurements have actually been made in dolphins and ducks; the results are probably also true in man. In any case, the arterioles of most organs function independently of those of other organs, so that the manner in which blood flow is distributed is subject to precise control. In this chapter, we will consider the factors that influence the arterioles.

2. Control of Arterioles:

     The factors which control the arterioles are physical, chemical and nervous, in order of increasing importance.

     High temperatures cause arteriolar dilation. In a hot room, this may be seen quite easily in the skin. The flushing of the face is due to increased blood in the face. Working muscles become hot. This leads to arteriolar dilation in those muscles. Thus, the working muscle (which needs more blood) has arteriolar dilation (which obtains more blood). Cold, if not extreme, produces the opposite effect. In moderate cold, the face becomes pale. In extreme cold, on the other hand, the face reddens again. This reddening in extreme cold may not be the direct effect of cold as such, but rather the effect of pain, working through nervous mechanisms. This will be taken up in more detail later.

     Light pressure tends to cause constriction of the arterioles, as least in the skin. This is easily seen by stroking the skin lightly with a pencil point. There is a momentary blanching of the skin which makes out the area stroked. Contraction of veins also plays a role in this blanching.

     The regulation of the arterioles is brought about by many chemical substances. Some of them have the same effect on all arterioles and act locally; these will be considered first. Others are carried by the blood stream and may have different effects at different locations. These are the circulatory hormones and will be considered second.

     All tissues, when working, consume extra oxygen and produce extra carbon dioxide. Many tissues also produce acids when their activity is increased. Some other substances are also produced.

     It is a general rule that oxygen lack, carbon dioxide excess, acid excess, and/or excesses of the other substances produced in activity cause dilation of the arterioles in the area where they occur, but not in other areas. This is an obviously useful mechanism for increasing the blood supply in active areas.

     The arterioles of the brain seem to be extremely sensitive to carbon dioxide excess; those of the heart are unusually responsive to oxygen lack. In other areas the chemical substances predominantly involved in arteriolar dilation have been less positively identified. Skin and muscle arterioles may respond better to some unidentified product of activity than to oxygen lack or carbon dioxide excess. In other organs, the picture is even less clear. Some chemical alteration which occurs in activity causes dilation of arterioles, but exactly what the substance or substances are has not been established.

     Adrenaline and nor-adrenaline: Catecholamines from the adrenal medulla or sympathetic terminals have profound effects on the arterioles when released into the blood. Some arterioles constrict, for example, those of the skin and kidney; others dilate, particularly those of the heart and the brain. The blood vessels of voluntary muscle are dilated weakly.

     The renin-angiotensin system: In certain circumstances, the kidney releases a material called renin (pronounced ree-nin) into the blood. This acts on a protein in the blood to produce a substance called angiotensin. Angiotensin is a very powerful constrictor of arterioles in most areas; it is dilator in the heart and brain. High blood pressure may, in a few cases, be due to an overproduction of renin by the kidney. Many attempts have been made to determine whether there are abnormalities in the renin-angiotensin system in all cases of high blood pressure. So far, the evidence is against this idea. It would be important to determine which causes of high blood pressure are produced by abnormal renin production, particularly if this results from disease of one kidney, for in such people, removal of the diseased kidney may cure the high blood pressure.

     Vasopresin: Extracts of the pituitary gland (posterior lobe) appear to constrict all arterioles. It is doubtful whether this effect is of physiological significance, since the amount of the active material required to affect arterioles is about a thousand times larger than that which is required to produce the other physiological effect of this material on the kidney's handling of water (See also Chapter 22)

     Bradykinin: This is a material which can be extracted from many organs. It has been suggested that it specifically dilates the arterioles of the salivary glands and perhaps also those of the pancreas and sweat glands. Present evidence suggests that this is not so. The biological importance of bradykinin is unknown.

     Other Substances: At one time or another, almost every organ in the body has been suspected as the source of a material or materials which have specific arteriolar effects. A good deal of the interest in finding such substances has resulted from two puzzling disorders of the circulation. In high blood pressure, there is generalized arteriolar constriction. In surgical (wound) shock, there appears to be generalized arteriolar dilation. In seeking explanations of these conditions, many substances which produce arteriolar constriction and dilation have been found. Unfortunately, though materials which produce the effects can be prepared by extracting different organs in different ways, it has not been possible to show that these materials are really involved in either shock or high blood pressure. Among these substances are VEM (Vaso Excitatory Material), VDM (Vaso Depressor Material), histamine, and 5-hydroxytryptamine (serotonin). At the present time, none of these materials has been proved to play a significant role in the control of the arterioles.

     In all probability, the arterioles are controlled more by nervous influences than by either physical or chemical factors. The nerve supply is autonomic and probably almost entirely sympathetic rather than parasympathetic.

     The primary effect of sympathetic stimulation in the arterioles is to produce arteriolar constriction, especially in the kidneys, spleen and skin. There are important exceptions-the heart, bronchial circulation and the brain. Here, sympathetic stimulation produces arteriolar dilation. The different responses are believed to be due to different types of receptors in the arterioles. Alpha receptors respond by constriction; beta receptors by dilation. The blood vessels of the kidneys, spleen and skin have only alpha receptors; those of the heart and brain have beta receptors. In other organs both are present.

     Just as increased sympathetic activity produces constriction of arterioles, so too, does decreased sympathetic activity produce dilation. Again, the arterioles of the heart, lung, and brain work oppositely.

     Some sympathetic fibers, when stimulated, cause dilation of the arterioles rather than constriction. This has been especially well known for skeletal muscle and the skin. This system is probably of importance in the regulation of the blood flow to muscle in extreme circumstances. In muscle, it may play a role in producing fainting after hemorrhage, and perhaps in preparation for very violent exercise. The sympathetic vasodilator system appears to originate in the hypothalamus. Its anatomical course is not certainly known. These fibers are cholinergic.

     At one time the parasympathetic nervous system was believed to have the effects on arterioles opposite to those of the sympathetic. This view is no longer held. Parasympathetic nerves produce dilation of the arterioles of the salivary glands, and of the genital organs. It is possible, but not proved, that the parasympathetic nervous system constricts the arterioles of the heart. Most of the vasodilator effects once attributed to the parasympathetic nervous system are now believed to be due to the sympathetic vasodilator system.

     A type of nervous control of arterioles of the skin familiar to everyone is the dilation of arterioles which follows injury to the skin, as, say, after a mosquito bite. The nerves in the injured area, which carry sensory information to the brain, have branches called collaterals. One or more of these branches goes to arterioles in the area. Impulses carried through these arteriolar branches produce dilation of the arterioles concerned. An interesting aspect of this response is that it occurs even if the connections of the sensory nerve to the central nervous system are cut, so that it is quite independent of the central nervous system. The reddening of the skin which occurs around a mosquito bite (or after a surgical incision or a burn) is due to this reflex.

     Recent work suggests that the arterioles of active muscle may dilate by a mechanism similar to that described above. Working muscle may stimulate its sensory nerve fibers and branches of these may send dilator impulses via the arteries to the arterioles of the muscle. It is still too early to judge whether this reflex is significant in controlling muscle blood flow.

     It should be obvious from the preceding that the question of the mechanism of arteriolar regulation has not yet been answered. It does, however, seem quite clear that nervous, chemical, and physical factors are all involved, and that, in most circumstances, through the interplay of these factors on the arterioles, blood will be directed toward those areas which are active and away from areas which are inactive.

3. Disorders of the Arterioles:

     Disorders of the arterioles may have very serious consequences--high blood pressure and shock are only the most dramatic two of the many examples that could be cited. Further research into the normal mechanism of regulation and into its disturbances in high blood pressure and shock is urgently required (See also Chapter 16).

Continue to Chapter 13.