This diagram illustrates the following:

• Arteries and Veins are, on an individual basis, larger in diameter than their smaller versions (arterioles, venules, capillaries)

• The Venous side of the circulatory system is the Capacitance side of the Circulatory System...i.e.it stores most of the blood but has limited control of vascular tone and therefore contributes relatively little to total peripheral resistance (TPR). ( Vessel size is, in some degree. affected by the compliance of the surrounding tissues.). The Arterial side, in contrast, is the Pressure side; variation in vessel wall strength, elasticity, and the magnitude of vessel smooth muscle activity on vascular tone determine the average vessel diameter...and thus the resistance (AND, therefore pressure). Just the arterioles and smaller arteries account for more than 50 % of the (TPR)! "How?", you ask...

1. Keep in mind that even tiny changes in the average arteriolar diameter (or radius, if you prefer) can markedly alter the TPR; the relationship of radius- (the term "r" in the following equation) and vessel resistance, is shown according to the following:

• Resistance = (Viscosity & Length Constants) x (r)^-4

...where "r" refers to the radius of the cross section of a vessel...Thus resistance increases as the inverse of the 4th power of the radius.

• ....therefore, a simple change of only 16% in the average radius of the pressure vessel can double (or halve) the TPR!!

• An interesting (though not directly related observation): In terms of Cross Sectional Area, the sum of the area of smaller vessels in contact with tissues is actually larger than the corresponding sum of areas of the larger vessels. This means exchange of gases and nutrients is, theoretically, greatest in the arterioles-capillaries-venules. (However, other factors, including vessel wall permeability factors favor capillaries for the bulk of tissue-blood gas and metabolite exchanges.)

Copyright© 12/97, Chick Newman, Newman Veterinary Medical Services®