7.2: Blood Vessels, Blood Pressure

Arteries, veins, and capillaries circulate blood throughout the body. The distribution of blood is changed by constricting (narrowing) or dilating (widening) certain blood vessels. In a resting muscle, more than 95% of the capillaries are unfilled. When the muscle is exercised, blood flows through all its capillaries to provide needed oxygen and nutrients and to remove waste products. In a more familiar example, we look flushed when warm because more blood is diverted to the skin by filling its capillaries (as evaporating perspiration cools the skin, the underlying blood is cooled as well).

The heart begins circulation by pumping oxygenated blood into arteries (Figure 7.1). This creates high pressure in the arteries, which have thick, muscular walls and can be thought of as high-pressure tubing. Rupturing an artery is often lethal; abnormally high blood pressure raises the risk of damage or rupture.

Blood pressure is routinely measured at the arm, and the numbers (e.g., 120/80) tell how high (in millimeters) the pressure raises a calibrated column of mercury. The top number is the [systolic] pressure in the arteries when the heart contracts (beats); the bottom number is the [diastolic] pressure when the heart rests between contractions. Both pressures are important in terms of stress on the blood vessels.

To illustrate, imagine using a pump to pulse air into a high-pressure rubber air hose. How long the hose lasts depends not only on how much pressure is exerted with each pulse, but also on how much steady pressure there is between pulses. A certain amount of pressure is needed to get the job done, whether with the air hose or with the circulatory system, but excess pressure can cause damage. Obviously, a defective artery doesn’t last as long as one that’s in good shape.

Blood pressure goes up with age, in part because the arteries become more rigid and, don’t easily expand slightly as they should with each heartbeat.* A rubber air hose also becomes less flexible as it ages. When air is pulsed in, the hose doesn’t expand as much in response, resulting in higher pressure. If the passageway in the hose has also narrowed due to damage and accumulated debris, the steady pressure between pulses will also be higher. This happens with narrowed arteries as well.

Arteries branch into smaller and smaller vessels. The smallest are capillaries that go to every living cell in the body (note that you bleed wherever you prick yourself). Capillary walls are very thin, and some capillaries are so narrow that red blood cells can go through only in a single file.

As arteries branch, blood pressure falls from high pressure in the large arteries to the very low pressure in the capillaries. The low pressure allows the exchange of substances between tissue cells and capillary blood.

To complete the circle (Figure 7.3), blood goes back to the heart via veins. Compared to the high-pressure/thick-walled arteries and the low-pressure/thin-walled capillaries, veins are intermediate in pressure, diameter, thickness, and muscularity. Unlike arteries and capillaries, veins have valves that direct blood back to the heart and help prevent pooling of blood in the veins.

Figure 7.3: Circulatory Route of Blood Vessels Through the Body

The return of blood back to the heart is helped by muscle action—particularly in the legs, where gravity impedes the return. Walking squeezes the leg veins and helps return blood to the heart. Blood can pool in leg veins when a person stands or sits still, thereby reducing circulation. Someone who stands in a crowd for hours while waiting for a parade may faint from insufficient blood going to the brain.** In contrast, the strong and fast muscle actions of running increase circulation.

^{*In relatively isolated rural populations in some low-income countries, blood pressure doesn’t go up, or goes up only minimally with age. These people typically have a low-salt diet, aren’t overweight, and are physically active at all ages.
**A somewhat different example is a weight lifter who feels dizzy when straining to lift weights. A deep breath is held during the lift, causing a big rise in pressure in the chest cavity. This compresses the vein that passes through this area, obstructing the return of blood back to the heart and to the brain}