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Capillary, one of the minute blood vessels that form the connection between the arteries and the veins. These tiny vessels vary in diameter from 0.0127 to about 0.2032 mm (0.0005 to about 0.008 in) and are present in great numbers throughout the entire body. The walls of capillaries are exceedingly thin and readily permeable. They are surrounded by lymph, and there is a constant interchange between the substances in the blood within the capillaries and the waste products in the body tissues and lymph outside. This interchange facilitates the processes of nutrition and elimination and enables the exchange of oxygen and carbon dioxide to take place. Lymph capillaries assist the blood capillaries in this process.
Blood vessels circulate blood through the body. The three major types of blood vessels are the arteries, veins, and capillaries. Arteries carry blood away from the heart, while veins carry blood toward the heart. Capillaries form at the junction between arteries and veins and they are the only blood vessel to come into direct contact with tissue cells. Oxygen, nutrients, and other substances transported in blood through the arteries pass through thin capillary walls into tissue cells. Wastes and other products of cellular metabolism pass from cells back through the capillary walls and into veins. The walls of arteries and veins are composed of three tunics (layers) that surround a central opening called a lumen, through which blood flows. The innermost layer is the tunica intima, composed of endothelial cells that form a slick lining that minimizes friction as blood moves through the lumen. The middle layer is the tunica media. Composed of smooth muscle cells and sheets of elastic tissue, this layer enables the lumen to narrow or widen to regulate blood flow in the body. The tunica adventitia forms a protective outer layer of the blood vessel wall and it also anchors the blood vessel to surrounding structures.
HISTORY OF HOSPITALS:
Today the United States is home to 6,021 hospitals that contain over 1 million hospital beds. U.S.hospitals annually admit some 34 million patients who are assigned a bed and receive medical or surgical treatment as inpatients. Hospitals also provide outpatient treatment in clinics or other walk-in, or ambulatory, settings for an additional 483 million patients every year.
Hospitals in the United States are classified by the services they provide (general or specialized), the length of stay they offer patients (short stay or long-term care), and by their ownership (not-for-profit, proprietary, or government owned). Although most U.S. hospitals are classified as not-for-profit, any one hospital will fall into several of the above categories. For example, Methodist Hospital in Houston, Texas, with more than 300,000 sq m (3 million sq ft) of space, is one of the largest short-stay, not-for-profit, general hospitals in the country.
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HISTORY OF HOSPITALS
