HUMAN BEING

(species Homo sapiens), a bipedal primate mammal that is anatomically related to the great apes but is distinguished by a more highly developed brain, with a resultant capacity for articulate speech and abstract reasoning, and by a marked erectness of body carriage that frees the hands for use as manipulative members. Humans occur in a number of freely interbreeding races and are the sole recent representatives of the family Hominidae. (See hominid.) The term man has traditionally referred to the human race in general, or mankind, though in the term's modern and more limited usage, a man is simply an adult male human. The idea of man is treated in a number of articles. For a philosophical treatment of the subject, see philosophical anthropology. For a physical anthropological treatment, see human evolution. For an examination of human culture, see culture. For other related articles, see collective behaviour; death; emotion; family; human behaviour; human rights; intelligence; kinship; language; learning theory; mind, philosophy of; motivation; perception; personality; population; sexual behaviour, human; social structure; Stone Age; and thought. Figure 4: Major sources of natural background radiation and their respective contributions to the exposure of the average person. From the National Radiological Protection Board, Living with Radiation, 3rd ed., Reading, Eng., 1986 human blood liquid medium (plasma) containing several types of specialized cells in suspension. The circulatory system provides the mechanism by which the blood transports substances to and from the organs and tissues. The circulating blood continuously supplies oxygen, nutrient substances, and other materials necessary for the viability and activity of all the cells of the body and carries away cell products, including carbon dioxide and other waste materials. If blood flow ceases, death occurs within minutes because of the effects of an unfavourable environment on highly susceptible cells. The constancy of the composition of the blood is made possible by the circulation, which conveys blood through the organs that regulate the concentrations of its components. In the lungs blood acquires oxygen and releases carbon dioxide transported from the tissues. The kidneys remove excess water and dissolved waste products. Nutrient substances derived from food reach the bloodstream after absorption by the intestinal tract. Endocrine glands release their secretions into the blood, which transports these hormones to the tissues in which they exert their effects. Many substances are recycled through the blood; for example, iron released during the destruction of old red cells is conveyed by the plasma to sites of new red cell production where it is reused. Each of the numerous components of the blood is kept within appropriate concentration limits by an effective regulatory mechanism. In many instances feedback control systems are operative; thus a declining level of blood sugar leads to accelerated release of sugar into the blood so that a potentially hazardous depletion of blood sugar does not occur. Unicellular organisms, primitive multicellular animals, and the early embryos of higher forms of life lack a circulatory system, and exchange of substances between cell and environment is accomplished by simple diffusion. In larger and more complex animals transport of adequate amounts of oxygen and other substances requires some type of blood circulation. The diffusion process then occurs between the body cells and the fluid derived from the blood, which by its constant motion maintains the constancy of the internal environment. Some simple animals, including small worms and mollusks, have blood that lacks an oxygen-binding substance analogous to hemoglobin; others are provided with pigments capable of transporting relatively large amounts of oxygen. In many invertebrates the blood pigment is dissolved in the plasma. Hemocyanin, a copper-containing protein chemically unlike hemoglobin, is found in certain crabs and other lower animals. Hemocyanin is blue in colour when oxygenated and colourless when oxygen is removed. Some invertebrates have hemoglobin in solution in the plasma. In almost all vertebrates, including humans, hemoglobin is contained exclusively within the red cells (erythrocytes) of the blood. The red cells of the lower vertebrates (e.g., birds) have a nucleus, whereas mammalian red cells lack a nucleus. Red cells vary markedly in size among mammals; those of the goat are much smaller than those of humans, but the goat compensates by having many more red cells per unit volume of blood. The concentration of hemoglobin inside the red cell varies little between species. For additional information on blood in general and comparison of the blood and lymph of diverse organisms, see tissues and fluids: The tissues and fluids of animals: Blood and lymph and circulation.