Human body

Human body, complex organism consisting of some 50 trillion cells, organized into tissues, organs, and structures. The body can best be studied as a complex of systems.

(1) The skeletal system, the body's framework, consists of more than 200 bones tied together with strong yet elastic ligaments. It accounts for about 18% of the body's weight.

(2) The muscular system consists of more than 650 distinct muscles that, by pulling on bones and bending joints, cause the body to move. Also part of this system are the visceral muscles, which cause movement in internal organs. The muscular system accounts for about 40% of body weight.

(3) The skin covers the whole body, allowing the maintenance of a constant internal environment. The sweat glands play an important part in excretion. Nerve endings in the skin provide the sense of touch and warn the body of external danger. The skin serves as a barrier against destructive organisms and foreign bodies.

(4) The circulatory system, consisting of the heart and thousands of miles of blood vessels and lymphatics, carries blood to all parts of the body, supplying every cell with oxygen and food and carrying from it carbon dioxide, water, and other wastes. The blood also carries hormones (the body's chemical messengers), which attack invading microorganisms, and help repair wounds by clotting.

(5) The respiratory system-including the lungs and the respiratory passages-allows the interchange of oxygen and carbon dioxide with the blood.

(6) The digestive system takes in food and breaks it down into a usable form that can be absorbed into the bloodstream. Basically, it consists of a tube extending from mouth to anus, with various organs, such as the stomach and accessory glands (including liver and pancreas) attached.

(7) The main organs of the urinary system are the kidneys, which remove waste from the blood, and the bladder, which stores and then expels that waste in the form of urine. (8) The reproductive system ensures the continuation of the species and the passing along of traits that may be necessary for survival.

(9) The nervous system consists of the central nervous system (CNS), comprising the brain and spinal cord, and the peripheral nervous system, the nerves that carry messages to and from the CNS.

(10) The endocrine system, consisting of the hormone-producing glands, responds to nervous commands or to other hormones and along with the nervous system, coordinates the activities of other organs and adjusts their functioning in relation to the external and internal environment.

(11) The body possesses a number of specialized senses, including sight, hearing, balance, and the visceral sensations such as hunger.