Functions of Blood
Blood, a liquid connective tissue, has three general functions:
Transportation. Blood transports oxygen from the lungs to cells throughout the body and carbon dioxide from body cells to the lungs for exhalation. It also carries nutrients from the gastrointestinal tract to body cells, hormones from endocrine glands to other body cells, and heat and waste products away from cells to various organs for elimination from the body.
Regulation. Circulating blood helps maintain homeostasis of all body fluids. Blood helps regulate pH through the use of buffers (chemicals that convert strong acids or bases into weak ones). It also helps adjust body temperature through the heat-absorbing and coolant properties of the water in blood and blood's variable rate of flow through the skin, where excess heat can be lost from the blood to the environment. Also, blood osmotic pressure influences the water content of cells, mainly through interactions of dissolved ions and proteins.
Protection. Blood can clot (become gel-like) in response to an injury, which protects against its excessive loss from the cardiovascular system. Blood's white blood cells protect against disease by carrying on phagocytosis. Several types of blood proteins, including antibodies, interferons, and complement, also help protect against disease.
Blood is a connective tissue that consists of plasma (liquid) plus formed elements (red blood cells, white blood cells, and platelets).
Physical Characteristics of Blood
Blood is denser and more viscous (thicker) than water, which is part of the reason it flows more slowly than water. The temperature of blood is (), about higher than oral or rectal body temperature, and it has a slightly alkaline pH ranging from 7.35 to 7.45. The color of blood varies with its oxygen content. When saturated with oxygen, it is bright red. When blood has a low oxygen content, it is dark red. Blood constitutes about 8 percent of the total body weight. The blood volume is 5 to 6 liters (1.5 gal) in an average-sized adult male and 4 to 5 liters (1.2 gal) in an average-sized adult female. The difference in volume is due to differences in body size. Several hormones, regulated by negative feedback, ensure that blood volume and osmotic pressure remain relatively constant.
Components of Blood
Whole blood is composed of plasma, a liquid extracellular fluid that contains dissolved substances, and formed elements, which include cells and cell fragments. If a sample of blood is centrifuged (spun at high speed) in a small glass tube, the formed elements (which are more dense) sink to the bottom of the tube, and the plasma (which is less dense) forms a layer on top (Figure 18.1a). Blood is about 45 percent formed elements and about 55 percent plasma. Normally, more than 99 percent of the formed elements are cells named for their red color—red blood cells (RBCs). Pale, colorless white blood cells (WBCs) and platelets comprise less than 1 percent of the formed elements. Because they are less dense than red blood cells but denser than plasma, WBCs and platelets form a very thin layer, called the buffy coat, between the packed RBCs and plasma in centrifuged blood.
FUNCTIONS OF BLOOD
Transports oxygen, carbon dioxide, nutrients, hormones, heat, and wastes.
Regulates pH, body temperature, and water content of cells.
Protects against blood loss through clotting and against disease through phagocytic white blood cells and antibodies.
When the formed elements are removed from blood, the straw-colored plasma remains. Plasma is about 91.5 percent water and 8.5 percent solutes, most of which are proteins. Some of the proteins in plasma are also found elsewhere in the body, but those confined to blood, called plasma proteins, are synthesized mainly by liver cells. The most plentiful plasma proteins are the albumins (al′-BŪ-mins), which account for about 54 percent of all plasma proteins. Among