Info

Figure

Sites where an arterial pulse is most easily detected (a. stands for artery).

Vessel

Type of Wall

Function

Artery

Thick, strong wall with three layers—an endothelial lining, a middle layer of smooth muscle and elastic tissue, and an outer layer of connective tissue

Carries blood under relatively high pressure from the heart to arterioles

Arteriole

Thinner wall than an artery but with three layers; smaller arterioles have an endothelial lining, some smooth muscle tissue, and a small amount of connective tissue

Connects an artery to a capillary, helps control the blood flow into a capillary by vasoconstricting or vasodilating

Capillary

Single layer of squamous epithelium

Provides a membrane through which nutrients, gases, and wastes are exchanged between the blood and tissue fluid; connects an arteriole to a venule

Venule

Thinner wall, less smooth muscle and elastic tissue than in an arteriole

Connects a capillary to a vein

Vein

Thinner wall than an artery but with similar layers; the middle layer is more poorly developed; some with flaplike valves

Carries blood under relatively low pressure from a venule to the heart; valves prevent a backflow of blood; serves as blood reservoir

Shier-Butler-Lewis: IV. Transport 15. Cardiovascular System © The McGraw-Hill

Human Anatomy and Companies, 2001

Physiology, Ninth Edition

Blood volume increases

Heart rate increases

Stroke volume increases

Blood pressure Increases

Figure 15.35

Some of the factors that influence arterial blood pressure.

Figure 15.35

Some of the factors that influence arterial blood pressure.

initially rises. Conversely, if the stroke volume or the heart rate decreases, the cardiac output decreases and blood pressure also initially decreases.

Blood Volume

Blood volume equals the sum of the formed elements and plasma volumes in the vascular system. Although the blood volume varies somewhat with age, body size, and sex, it is usually about 5 liters for adults or 8% of body weight in kilograms.

Blood volume can be determined by injecting a known volume of an indicator, such as radioactive iodine, into the blood. After a time that allows for thorough mixing, a blood sample is withdrawn, and the concentration of the indicator measured. The total blood volume is calculated using the formula: blood volume = amount of indicator injected/concentration of indicator in blood sample.

Blood pressure is normally directly proportional to the volume of the blood within the cardiovascular system. Thus, any changes in the blood volume can initially alter the blood pressure. For example, if a hemorrhage reduces blood volume, blood pressure initially drops. If a transfusion restores normal blood volume, normal pressure may be reestablished. Blood volume can also fall if the fluid balance is upset, as happens in dehydration. Fluid replacement can reestablish normal blood volume and pressure. Clinical Application 15.5 describes how the unusual conditions of microgravity in outer space affect the distribution of blood volume and control of blood pressure.

Peripheral Resistance

Friction between blood and the walls of the blood vessels produces a force called peripheral resistance (pe-rif'er-al re-zis'tans), which hinders blood flow. Blood pressure must overcome this force if the blood is to continue flow ing. Therefore, factors that alter the peripheral resistance change blood pressure. For example, contraction of smooth muscles in the walls of contracting arterioles increases the peripheral resistance by constricting these vessels. Blood tends to back up into the arteries supplying the arterioles, and the arterial pressure rises. Dilation of the arterioles has the opposite effect—peripheral resistance lessens, and the arterial blood pressure drops in response (fig. 15.36).

Because arterial walls are quite elastic, when the ventricles discharge a surge of blood, arteries swell. Almost immediately, the elastic tissues recoil, and the vessel walls press against the blood inside. This action helps force the blood onward against the peripheral resistance in arterioles and capillaries. It is this recoiling of the arteries that maintains blood pressure during diastole. If there were no elasticity in the arterial walls, blood pressure would fall to zero between ventricular contractions. Elastic recoil also converts the intermittent flow of blood, which is characteristic of the arterial system, into a more continuous movement through the capillaries.

Viscosity

The viscosity of a fluid is a physical property that derives from the ease with which its molecules flow past one another. The greater the viscosity, the greater the resistance to flow.

Blood cells and some plasma proteins increase blood viscosity. Since the greater the blood's resistance to flowing, the greater the force needed to move it through the vascular system, it is not surprising that blood pressure rises as blood viscosity increases and drops as blood viscosity decreases.

Although the viscosity of blood normally remains stable, any condition that alters the concentrations of blood cells or specific plasma proteins may alter blood viscosity. For example, anemia may decrease viscosity and consequently lower blood pressure. Excess red blood cells increase viscosity and blood pressure.

99 How is cardiac output calculated?

^9 What is the relationship between cardiac output and blood pressure?

^9 How does blood volume affect blood pressure?

□ What is the relationship between peripheral resistance and blood pressure? Between blood viscosity and blood pressure?

Essentials of Human Physiology

Essentials of Human Physiology

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