Human anatomy divides the body into the following distinct functional systems: cutaneous, muscular, skeletal, circulatory, nervous, digestive, urinary, endocrine, respiratory, and reproductive. This division helps the student understand the organs, their relationships, and the relations of individual organs to the body as a whole.
The cutaneous system consists of the integument—the covering of the body, including the skin, hair, and nails. The skin is the largest organ in the body, and its most important function is to act as a barrier between the body and the outside world. The skin's minute openings (pores) also provide an outlet for sweat, which regulates the body temperature. Melanin, a dark pigment found in the skin, provides protection from sunburn. The skin also contains oil-producing cells.
The muscles of the muscular system enable the body to move and provide power to the hands and fingers. There are two basic types of muscles. Voluntary (skeletal) muscles enable movements under conscious direction (e.g., to walk, move an arm, or smile). Involuntary (smooth) muscles are not consciously controlled, and operate independent of conscious direction. For example, they play an important role in digestion. The third type of muscle, cardiac muscle is involuntary, but also is striated, as in skeletal muscles. Because cardiac muscle is self-contractile it allows the heart to pumps blood throughout the body, without pause, from early in embryogenesis to death.
The skeletal system, or the skeleton, is the general supportive structure of the body. In addition, the skeletal system is the site of many important and complex physiological and immunological processes. The skeletal frame provides the support that muscles need in order to function. Of the 206 bones in the human body, the largest is the femur, or thigh bone. The smallest are the tiny ear ossicles, three in each ear, named the hammer (malleus), anvil (incus), and stirrup (stapes). Often included in the skeletal system are the ligaments, which connect bone to bone; the joints, which allow the connected bones to move; and the tendons, which connect muscle to bone.
The circulatory system comprises the heart, arteries, veins, capillaries, blood and blood-forming organs, and the lymphatic sub-system. The four chambers of the heart allow the heart to act as a dual pump to propel blood to the lungs for oxygenation (pulmonary system) and to pump blood throughout the body (systemic circulation). From the heart, the blood circulates through arteries. The blood is distributed through smaller and smaller tubes until it passes into the microscopic capillaries which bathe every cell. The veins collect the "used" blood from the capillaries and return it to the heart.
The nervous system consists of the brain, the spinal cord, and the sensory organs that provide information to them. For example, our eyes, ears, nose, tongue, and skin receive stimuli and send signals that travel both electrically and chemically to the brain. The brain is an intricate system of complicated neurons (nerve cells) that allow us to process sensory information, visceral signals (e.g. regulating breathing, body temperature, etc.), and perform cognitive thought.
The digestive system is essentially a long tube extending from the mouth to the anus. Food entering the mouth is conducted through the stomach, small intestine, and large intestine, where accessory organs contribute digestive juices to break down the food, extracting the molecules that can be used to nourish the body. The unusable parts of the ingested food are expelled through the anus as fecal matter. The salivary glands (in the mouth), the liver, and the pancreas are the primary digestive glands.
The urinary system consists of the kidneys, the bladder, and the connecting tubules. The kidneys filter water and waste products from the blood and pass them into the bladder. At intervals, the bladder is emptied through the urinary tract, ridding the body of unneeded waste.
The endocrine system consists of ductless (endocrine) glands that produce hormones that regulate various bodily functions. The pancreas secretes insulin to regulate sugar metabolism, for example. The pituitary gland in the brain is the principal or "master" gland that regulates many other glands and endocrine functions.
The respiratory system includes the lungs, the diaphragm, and the tubes that connect them to the outside atmosphere. Respiration is the process whereby an organism absorbs oxygen from the air and returns carbon dioxide. The diaphragm is the muscle that enables the lungs to work.
Finally, the reproductive system enables sperm and egg to unite and the egg to remain in the uterus or womb to develop into a functional human.