Patterns of Arterial Distribution

We have already mentioned certain more obvious features of arterial distribution: the increase in total cross-sectional area at each branching, the variation in the angle of branching, the preference for protected courses within the limbs, and the generosity of interarterial anastomoses (p.

FIG. 7.33 Baroreceptors and chemoreceptors at the origin of the internal carotid artery (a.). 1, Common carotid a.; 2, external carotid a.; 3, internal carotid a.; 4, carotid sinus branch of the glossopharyngeal nerve; 5, carotid sinus (baroreceptor); 6, carotid body (chemoreceptor).

Collateral Circulation

Few arteries of any size terminate in capillary beds without first detaching side or collateral branches. Most collateral branches, whether large or small, anastomose with their neighbors, connections that may not be apparent on dissection because so many are concealed within muscles and other organs (Fig. 7.34). The anastomoses enlarge when the bloodstream is diverted from its normal route by occlusion of a principal trunk; the widening initially is due to relaxation and stretching of the wall but later is due to reconstruction of the anastomotic links. Thus, provided that sufficient blood can pass in the meantime, tissues deprived of their usual sources of supply generally survive, though possible temporary loss of function of the ischemic parts may occur. Experiments have shown that healthy dogs have a fair chance of survival even if the aorta is ligated (caudal to the origin of the renal arteries). The ability to develop an adequate collateral circulation is increased when the obstruction develops slowly but is markedly lessened by sudden onset of obstruction, aging, or pathologic changes in the vessel wall.

FIG.

The blockage of end-arteries produces a cone-shaped infraction. By strict definition, the end­artery is a rarity, but "functional" end-arteries, in which the collateral connections are of insufficient caliber, are more common (Fig. 7.35). The adequacy of collateral circulation cannot be assessed from purely morphologic evidence. For example, intramuscular arteries appear to anastomose freely, but the occlusion of one frequently leads to local necrosis. The consequences of obstruction of the central artery of the retina and many small vessels within the brain that lack adequate anastomoses are immediate and catastrophic. This situation may be contrasted with the liberal anastomoses between the major arteries that join to form the arterial circle on the ventral surface of the brain. Although anastomoses between finer branches of the coronary arteries are also poor and usually incapable of maintaining an adequate collateral circulation, not all coronary emboli are fatal. Much of the outcome may depend on the size and specific site of the infarct and on immediate medical care.

Anastomoses between small arteries within the limbs are especially numerous in the regions of the joints and sometimes form visible networks or retia; a prominent example exists over the dorsal aspect of the carpus of the horse (rete carpi dorsale).

FIG. 7.35 True (1) and functional (2) end-arteries. Closure (double black lines) of an end-artery leads to necrosis of the tissue it supplies (1). In the case of a functional end-artery (2), a potential but inadequate alternative route exists (2).

The retia just described are not to be confused with the so-called retia mirabilia of more restricted occurrence. Retia mirabilia are found where a main trunk splits more or less at once into a leash of parallel vessels. The "bipolar" arrangement has parallel trunks reuniting later, as in the arteries to the brain in some species (Fig. 7.36/7) and, on a diminished scale, in the renal glomeruli (see Fig. 5.28). In the "unipolar" arrangement, the branches remain separate as found within the limbs of slow-moving arboreal creatures (sloths, lemurs) and in the thoracic cavity of whales and other diving mammals. No convincing explanations exist of the adaptive value of most of these arrangements; the renal glomeruli, however, are the obvious exception (p. 170).