The circulatory system consists of two subdivisions: the cardiovascular system and the lymphatic system. The cardiovascular system consists of the heart and blood vessels, and the lymphatic system consists of lymphatic vessels and lymphoid tissues within the spleen, thymus, tonsils, and lymph nodes.
The heart is a four-chambered double pump. Its pumping action creates the pressure head needed to push blood through the vessels to the lungs and body cells. At rest, the heart of an adult pumps about 5 liters of blood per minute. At this rate, it takes about 1 minute for blood to be circulated to the most distal extremity and back to the heart.
Blood vessels form a tubular network that permits blood to flow from the heart to all the living cells of the body and then back to the heart. Arteries carry blood away from the heart, whereas veins return blood to the heart. Arteries and veins are continuous with each other through smaller blood vessels.
Arteries branch extensively to form a "tree" of progressively smaller vessels. The smallest of the arteries are called arterioles. Blood passes from the arterial to the venous system in microscopic capillaries, which are the thinnest and most numerous of the blood vessels. All exchanges of fluid, nutrients, and wastes between the blood and tissues occur across the walls of capillaries. Blood flows through capillaries into microscopic veins called venules, which deliver blood into progressively larger veins that eventually return the blood to the heart.
As blood plasma (the fluid portion of the blood) passes through capillaries, the hydrostatic pressure of the blood forces some of this fluid out of the capillary walls. Fluid derived from plasma that passes out of capillary walls into the surrounding tissues is called tissue fluid, or interstitial fluid. Some of this fluid returns directly to capillaries, and some enters into lymphatic vessels located in the connective tissues around the blood vessels. Fluid in lymphatic vessels is called lymph. This fluid is returned to the venous blood at specific sites. Lymph nodes, positioned along the way, cleanse the lymph prior to its return to the venous blood. The lymphatic system is thus considered a part of the circulatory system and is discussed at the end of this chapter.
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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.