Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Cardiovascular System
Blood Vessels of the Human Body
Blood Flow through Vessels

Blood circulates continuously through a closed Vascular System in a specific direction due to the rhythmic contractions of The Heart—a living muscular pump that drives blood from the Veins into the Arteries. In a healthy individual, the volume of blood entering the heart equals the volume leaving it. Blood flow velocity varies across arteries, capillaries, and veins, depending on the lumen width of these vessels. In the capillaries of the systemic Circulation, blood flows slowly—at a rate of 0.5 mm per 1 s. This sluggish capillary flow facilitates Metabolic exchange between the blood and the surrounding Tissues. Such exchanges occur across a vast surface area of 6,300 m², which represents the total surface area of the capillary walls in The Human Body.

Blood moves fastest in the aorta—at 50 cm per 1 s, which is 1,000 times faster than in the capillaries. The velocity of BLOOD FLOW IN the veins is half that in the arteries, because the total cross-sectional lumen of the veins is twice as large as that of the arteries.

Oxygen, nutrients, and Hormones pass from the blood into the tissues. Metabolic waste products are eliminated from the tissues into the blood through the thin capillary walls. In addition to filtration, metabolic exchange between blood and tissues is driven by osmosis and diffusion, processes in which substances move from an area of higher concentration to an area of lower concentration. Oxygen and other nutrients enter the tissues thanks to high blood pressure at the arterial end of the capillaries (up to 30 mm Hg). At the venous end of the capillary bed, blood pressure drops (to about 15 mm Hg), allowing metabolic waste products—such as carbon dioxide, urea, and other substances—to pass out of the tissues and into the bloodstream.

Blood pressure refers to the pressure exerted by blood on the walls of Blood Vessels. It depends on the force with which blood is ejected into the aorta during ventricular systole and the resistance offered by small vessels (arterioles, capillaries) to blood flow. A vital condition for Blood Circulation is the pressure gradient between veins and arteries (blood pressure in the aorta is 120 mm Hg, whereas in the veins it is 3–8 mm Hg). Blood naturally flows from a region of higher pressure to a region of lower pressure.

With each systole of the left ventricle, 60–70 ml of blood is pumped into the aorta. Nevertheless, blood flows through the Circulatory system in a continuous stream. This continuity is maintained by the resistance encountered as blood passes through fine vessels (capillaries), as well as by the elasticity of the walls of the aorta and other large arteries. During ventricular systole, the aorta expands slightly, returning to its initial state during diastole. During diastole, the elastic walls of the aorta exert pressure on the blood, continuing to propel it forward from the arteries into the capillaries. The greater the constriction of the small arteries and capillaries, and the stronger the heart's contraction, the higher the blood pressure will be.

Due to the rhythmic action of the heart, ARTERIAL BLOOD PRESSURE fluctuates. During ventricular systole and the ejection of blood into the aorta, arterial pressure rises, dropping during diastole. The peak pressure during ventricular systole is called systolic pressure, and the lowest pressure during diastole is called diastolic pressure. In healthy adults, maximum (systolic) pressure ranges from 110–120 mm Hg, while minimum (diastolic) pressure is 70–80 mm Hg. In children, blood pressure is lower than in adults due to the greater elasticity of their arterial walls. In middle-aged and elderly individuals, pressure increases as vessel wall elasticity declines. The difference between the maximum and minimum pressure is known as pulse pressure, which normally ranges from 40 to 50 mm Hg.

Arterial blood pressure can be measured by wrapping a rubber cuff around the upper arm. By adjusting the cuff's pressure on the Tissues of the arm—and consequently on the brachial artery—a manometer can be used to determine the maximum and minimum pressure within the brachial artery.

The pulse is the rhythmic throbbing of arterial walls caused by the passage of blood. These oscillations stem from heart contractions (60–70 beats per 1 min). During left ventricular systole, blood is forcefully ejected into the aorta, stretching its walls. During diastole, the elastic and resilient walls of the aorta snap back to their original position. These successive expansions and contractions generate the rhythmic pulsations of the arterial walls.

The pulse is most commonly palpated at the radial artery in the lower forearm near the wrist, or at the dorsalis pedis artery on the dorsal surface of the FOOT at the level of the ankle joint.

Blood flow in the veins. Blood returns to the heart via the veins. Venous return is driven not by the force of the heart contractions, but by other factors. The blood pressure generated by the heart drops significantly in the initial segments of the Venous system (the venules), down to just 10–15 mm Hg. Consequently, the movement of blood through thin-walled veins toward the heart is facilitated by: 1) the contraction of neighboring skeletal Muscles, which compress the veins and propel the blood upward; 2) the presence of venous Valves, which prevent backflow and permit blood to travel exclusively toward the heart; and 3) the negative pressure within the thoracic cavity during respiratory movements, which exerts a suction effect, assisting the venous return of blood to the heart.