324 Anatomy & Physiology Essentials Copyright Goodheart-Willcox Co., Inc. the blood helps the body fight infection through the action of white blood cells in a process known as phagocytosis. Circulating blood also carries antibodies and proteins that attack foreign bodies to help protect against disease. SELF CHECK 1. What two gases does the blood transport throughout the body? 2. In what ways does the blood help protect the body? Composition of Blood The blood has two basic types of components: the liquid component, called plasma, and the solid components, collectively referred to as the formed elements (Figure 11.2). The formed elements consist of red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes). Red blood cells carry oxygen to tissue, white blood cells protect the body from infection, and platelets play a vital role in blood clotting. The formed elements make up approximately 45% of the blood in adults, and plasma comprises the remaining 55%. The components of blood can be separated by spinning a tube containing a blood specimen in a centrifuge. A centrifuge is a machine that spins rapidly, generating a centrifugal force that separates elements in the blood by weight. The centrifugal force separates the blood into three layers: liquid plasma rises to the top of the tube, red blood cells settle at the bottom of the tube, and a thin layer called the buffy coat settles between the red blood cells and plasma. The buffy coat contains white blood cells and platelets (Figure 11.3). Separating these blood elements allows health pro- fessionals to determine a person’s hematocrit: the proportion of the total blood volume that is composed of red blood cells. This proportion is expressed as a percentage of total blood volume. Hematocrit values range from 42% to 54% of total blood volume in men and from 38% to 46% of total blood volume in women. Each person’s hematocrit value is fairly constant, but dehydration can cause hematocrit to increase due to loss of blood plasma, or decreased blood volume. It can also be lowered due to a decrease in red blood cells, which is common in people who are anemic. However, if a person lives at a high altitude, his or her hematocrit may increase to 60% or 65% of total blood volume because chronic altitude exposure increases red blood cell production. This is due to an increased secretion of the hormone erythropoietin by the kidneys. Physical Properties Blood is a thick fluid that makes up roughly 8% of total body weight. Generally, blood volume ranges from 5 to 6 liters (L) in men and from 4 to 5 L in women. However, blood volume depends on body size, muscle mass, and physical fitness. Athletes, for example, have higher blood volumes than other people. Red blood cells Platelets Blood vessel White blood cells Plasma © Body Scientifi c International Figure 11.2 Blood has both liquid (plasma) and solid (formed) components.