The circulatory system carries out many important functions that contribute to homeostasis. It obtains oxygen from the lungs; nutrients from the gastrointestinal tract; and hormones from the endocrine glands; and it delivers these substances to the tissues that need them. Furthermore, it removes metabolic waste products, such as carbon dioxide, lactic acid, and urea, from the tissues. Finally, it contributes to the actions of the immune system by transporting antibodies and leukocytes to areas of infection. Overall, the circulatory system plays a vital role in maintenance of optimal conditions for cell and tissue function.

All tissues are perfused, that is, all tissues receive blood flow. The amount of blood that flows through each tissue, however, depends upon that tissue's function. For example, many tissues, such as the heart, brain, and skeletal muscles, receive blood flow sufficient to supply their metabolic needs. When metabolic activity increases, as it does during exercise, blood flow to these tissues increases accordingly. Other tissues, however, receive blood flow in significant excess of their metabolic needs. These tissues, including the kidneys, organs of the digestive system, and skin, have important homeostatic functions. Among other vital activities, kidneys filter the blood and remove waste products; the organs of the digestive system absorb nutrients into the blood; and thermoregulation involves control of blood flow to the body surface where heat can be eliminated.

These functions are carried out most effectively and efficiently when the involved tissues receive an abundant blood flow. Under normal resting conditions, the kidneys, which account for only 1% of the body's weight, receive 20% of the cardiac output (CO); the gastrointestinal tract receives approximately 27% of the CO; and the skin receives 6 to 15% of the blood pumped by the heart per minute. Because these tissues receive more blood than they need to support metabolic activity, they can easily tolerate a sustained decrease in blood flow. During exercise, when the metabolic demand of the working skeletal muscles and the heart increases substantially, blood flow is directed away from the kidneys and organs of the digestive system and toward the skeletal and cardiac muscles.

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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