填空题

Blood is vital to maintaining a stable body temperature; in humans, body temperature normally fluctuates within a degree of 37.0℃. Heat production and heat (36) in various parts of the body are balanced out by heat transfer (37) tile bloodstream. This is accomplished by varying the (38) of blood vessels in the skin. When a person becomes (39) , the vessels dilate(膨胀) and an increased volume of blood flows through the skin. Heat dissipates(消散) through the skin, effectively (40) the body temperature. The increased flow of blood in the skin makes the skin appear (41) or flushed. When a person is cold, the skin may become pale as the (42) narrow, diverting blood from the skin and reducing heat loss. About 55 percent of the blood is (43) of a liquid known as plasma(血浆). (44) : red blood cells, white blood cells, and platelets. Blood, vital fluid found in humans and other animals that provides important nourishment to all body organs and tissues and carries away waste materials. Sometimes referred to as "the river of life," (45) An adult human has about 5 to 6 liters of blood, which is roughly 7 to 8 percent of total body weight. (46) .The volume of blood in an individual fluctuates.