Physical Characteristics of the Circulation

The circulation, shown in Figure 14-1, is divided into the systemic circulation and the pulmonary circulation. Because the systemic circulation supplies blood flow to all the tissues of the body except the lungs, it is also called the greater circulation or peripheral circulation.

Functional Parts of the Circulation. Before discussing the details of circulatory function, it is important to understand the role of each part of the circulation.

The function of the arteries is to transport blood under high pressure to the tissues. For this reason, the arteries have strong vascular walls, and blood flows at a high velocity in the arteries.

The arterioles are the last small branches of the arterial system; they act as control conduits through which blood is released into the capillaries. The arte-riole has a strong muscular wall that can close the arteriole completely or can, by relaxing, dilate it severalfold, thus having the capability of vastly altering blood flow in each tissue bed in response to the need of the tissue.

The function of the capillaries is to exchange fluid, nutrients, electrolytes, hormones, and other substances between the blood and the interstitial fluid. To serve this role, the capillary walls are very thin and have numerous minute capillary pores permeable to water and other small molecular substances.

The venules collect blood from the capillaries, and they gradually coalesce into progressively larger veins.

The veins function as conduits for transport of blood from the venules back to the heart; equally important, they serve as a major reservoir of extra blood. Because the pressure in the venous system is very low, the venous walls are thin. Even so, they are muscular enough to contract or expand and thereby act as a controllable reservoir for the extra blood, either a small or a large amount, depending on the needs of the circulation.

Volumes of Blood in the Different Parts of the Circulation. Figure 14-1 gives an overview of the circulation and lists the percentage of the total blood volume

Pulmonary circulation-9%

Superior -vena cava

Inferior-vena cava

Veins, venules, and venous sinuses-64%

Pulmonary circulation-9%

Superior -vena cava

Inferior-vena cava

Veins, venules, and venous sinuses-64%

Aorta

Heart-7%

-Arteries-13%

-Arterioles and

-capillaries-7%

Aorta

Heart-7%

-Arteries-13%

-Arterioles and

-capillaries-7%

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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