Arteries are muscular, elastic vessels that carry blood away from the heart under high pressure.

When animals become active after a period of rest and feeding, blood flow must be increased to their muscles and reduced to their digestive organs (Fig. 39.18). Animals meet this challenge by changing the radius of vessels. Recall that resistance to flow is strongly affected by the vessel radius. Arterioles supplying regions of the body that need less blood become narrower by contracting the circular smooth muscle fibers in their walls. This change dramatically increases the resistance to flow and thereby reduces the rate of blood flow. For example, when you put your hands or feet into cold water, the arterioles constrict, reducing the loss of heat from blood flowing to your hands and feet.

image
FIG. 39.18 Changes in blood flow from resting to exercise as measured in humans. Numbers in parentheses indicate percent of total blood flow. Source: Data from C. B. Chapman and J. H. Mitchell, 1965, “The Physiology of Exercise,” Scientific American 212:88–96.

842

Arterioles supplying regions of the body that need more blood become wider by relaxing the smooth muscles lining their walls. The larger vessels offer less resistance, so more blood can flow through them. As the rate of cellular respiration in these cells increases, they release more CO2 as well as other metabolites, such as lactate and hydrogen ions. The increase of these molecules in the bloodstream acts as a signal to the smooth muscle in the arteriole walls to relax, allowing the arterioles to increase in diameter. In this way, blood flow increases to the capillaries to match O2 and nutrient delivery to the metabolic need of the cells.

843

A strong pulse of flow pushes against the arterial walls about every second in a human with each heartbeat that pumps blood through the arteries. This pulse of flow causes a momentary expansion of the arteries. Artery walls can withstand these repeated pressure pulses because they contain multiple elastic layers composed of two proteins: collagen and elastin. Collagen fibers are strong and resist the expansion of the arterial wall during these pressure pulses. Together with elastin fibers, the collagen provides an elastic rebound of the arterial wall once the pulse has passed, returning energy to help smooth out blood flow. The collagen and elastin resist overexpansion of the arterial wall. However, if an artery wall deteriorates, the collagen and elastin can become so thin that the artery bulges outward. This outward bulge, called an aneurysm, can lead to a life-threatening rupture.

Quick Check 5 What change in vessel shape most affects resistance to blood flow?

Quick Check 5 Answer

A change in a vessel’s radius affects blood flow most. For example, a twofold reduction in radius increases resistance to flow 16 times.