An Address ON THE PERIPHERAL CIRCULATION AND ITS TREATMENT: Delivered before the Annual Meeting of the British Balneological and Climatological Society, 1909
BMJ (Clinical Research Edition)
THE arterioles and capillaries and venules pervade every tissue and organ of the body, and specialists in this line can never be accused of poaching on anotber man's preserves. The importance of -the capillaries has been aptly described by Leonard Hill, who says: The blood is brought into intimate relation with the tissues by diffusing through the endothelial wall of the capillaries, and this wall is of great tenuity; thereby takes place that change of material which maintains the combustion of
... s the combustion of the body and the fire of life. It is the resistance in -the arterioles and capillaries which maintains the mean or diastolic pressure in the aorta and large arteries, and any increase in this resistance has to be overcome 'by greater work on the part of the heart. The arterioles constitute the first line of defenace, and, as I said in my Toronto address,' it is exceedingly fortunate that there is this first line of defence created by the action of the vasomotor nerves in the sxiall arteries and arterioles, because if this were wanting, as at present constituted we would either have to go about on all fours or constantly run the risk of fatal -syncope. Moreover, the blood would gravitate into the most dependent parts, the cooling surface would be enormous, the capillary velocity would be diminished, the blood would become surcharged with 002, and we would become cold-blooded animals. When the vasomotor nerves of a rabbit are paralysed it appears all right until you suspend it by the ears, and then it immediately dies. The arteriolar resistance -saves us from such risks. When you get vasomotor paralysis the resistance is transferred to the capillaries, and you get a more gradual fall in the pressure gradient, the velocity of the brood in the capillaries is diminished, the pressure increased, and transudaition readily takes place. In severe cases of Addisonis disease in which the vasomotor nerves are deprived of their usual stimul as, the patient may not be able to assume the erect posture. The wonderful regulative mechanism !of the vasomotor system enables the blood to be driven from the high-pressure main, the aorta and its principal branches, to the organs or systems that immediately require it. Witness the great supply to the extremities d'uring exertion, and to the abdominal viscera during digestion. Increase of pressure within an arbery improves its tone and -raises its coefficient of elasticity. It is probably oving to these facts, as well as to the intervention of the vasomotor nerves, that the arteries of the lower extremities often contract when they are dependent and dilate when they are raised. This, of course, happens most readily when the arteries are healthy and the arterial blood pressure high. Under such conditions the common carotid can be felt contracted when the body is inverted. The arteries of the upper extremities obey the usual law and dilate under increased pressure. This increased tone under heightened pressure is probably a determining factor in the production of claudication of the arteries-a condition almost confined to the lower extremities, and one which is very difficult to remedy when once established. Perhaps other causal factors are excessive amount of lime salts, overaction of ithe suprarenal glands, and defective action of the thyroid. 'ThE ARTERIOLES. It would be difficult or impossible to estimate the evervarying capacity and sectional area of the arterioles and the velocity the blood passing through them. What we do know is that on an average a similar quantity of blood passes through a section of the arterioles to that which passes through a section of the aorta in a unit of time. The resistance in the arterioles is directly as their length, and inversely as the squaro of their crs-sections or the fourth power of the. diameter, and drety as the square of the velocity. The total tion ae of the arterioles is perhaps considerably less than tht of the small arteries, and the velocity of the blood in -them .corre. spondingly greater. In cases of local syncope the arteri. oles shut down, and therefore the pressure and velociy in these particular vessels fall to zero. When the artrioles are contracted we know that the energy of the blood is to a large extent converted into velocity with a correspondin diminution in the lateral pressure. If you te a col hand, there may be a difference of 50 mm. -of rcury between the laral prure in the ra-dial and that in a digital artery, but if the axterioles are dilated there is a much less abrupt fall in the ptessure gradient, Moreover, in post-mortem examinations there is very-selcom anything found wrong with the arterioles, except that thev may be hypertrophied, but this is evidence of overaction rather than of lateral strain. The general arterial blood pressure can only be raised by the contraction of a large tract of arterioles, such as those of the splanchnic area, the skin, or muscles, and generally in health when one set is contracted another is dilated, so that a uniform mean arterial pressure is maintained. As a rule, the more contracted the arterioles the higher the arterial pressure in the arteries; and, again, this high potential in the arteries drives the blood with great velocity through the arterioles, and, of course, the greater the velocity the less the lateral pressure. A small area of arterioles, such as that of the hand, ma,y be contracted without any effect on the general arterial pressure, and in this case the velocity of the blood in the contracted arterioles is not increased but diminished, owing to the increased friction and heightened viscosity of the blood. The 'blood ma,y almost come to a standstill, and then the energy is largely that of pressure. When the arterioles of any local area are dilated, as at the commencement of an inflammation, or after the removal of an Esmarch's tourniquet, the whole of the capillaries are opened up and engorged with blood, and with this increased mass the pressure is raised, but the velocity is also heighltened owing to the arterial potential remaining high and the resistance in the enlarged vessels beingg diminished. These little muscular vessels are well endowed with vasomotor nerves, and are subjected to postural variations in pressure. It is entirely due to their intervention ihat the ca,pillaries are protected from the full hydrostatic effect of the blood when the body is in the erect posture. The potential increases in the arteries of the limbs when they are dependent, and if the -arterioles are dilated from any cause the pressure in these little vessels and in the capillaries is much augmented, so that you may readily get petechial haemorrhages, but on the other hand, if the arterioles are much contracted, the potential in the arteries, is largely converted into kinetic energy in the arterioles and capillaries.