Physiology is the science of the functioning of living organisms. Like otheranimals, humans move, respond to environmental stimuli, breathe, eat and digest food, excrete wastes, reproduce, and grow. Multiple organ systems in the body perform these various functions. Each organ system is characterized by aspecialized function and thus, division of labor is a basic principle underlying physiology. Another striking feature is the complex organization of an organ system. Several structurally and functionally distinct organs make up anorgan system, and each organ contributes to the specialized function of the system as a whole. An organ is further made up of two or more related tissuesthat determine the specific function of the organ. Based on function, tissuesin the human body have been classified into four primary types: epithelial,connective, muscle, and nervous tissues. A tissue is composed of functionallysimilar cells. Cells are the smallest living (distinct and functional) unitof an organism. Regardless of their differences, all cells are capable of synthesizing carbohydrates, proteins, lipids, and nucleic acids (the building blocks of the cell), producing chemical energy by breakdown of nutrients, secreting wastes, reproduction, and growth. It is the proper functioning of the individual cells that underlies the normal functioning of the organ system as awhole. Thus, within an organ system, there is both structural and functionalhierarchy.
The human body consists of 11 organ systems: The integumentary system (skin,hair) covers the body externally and acts primarily as a boundary between theinternal and external environments. The musculoskeletal system is really twosystems: the skeletal system (bones, cartilage, joints) and the muscular systems (skeletal muscles); these provide support and control movement, respectively. The nervous system (brain, sensory receptors, spinal cord, nerves) is the fast-acting response system of the human body; it responds to changes within and outside the body. The function of the respiratory system (nasal cavity, pharynx, larynx, trachea, bronchus, lungs) is to exchange oxygen for carbondioxide from the atmosphere. Food is digested by the digestive system (oralcavity, esophagus, stomach, small and large intestines, rectum, anus). Nutrients, oxygen, and wastes are transported within the body by the cardiovascularsystem (heart, arteries, veins). Nitrogen-containing wastes are excreted bythe urinary system (kidneys, ureter, urinary bladder, urethra). The lymphaticsystem (lymph nodes, thoracic duct, lymphatic vessels) houses the white blood cells involved in immunity. Reproduction is carried out by the reproductivesystems of males (seminal vesicles, prostate gland, vas deferens, testis, scrotum, penis) and females (ovary, fallopian tube, uterus, vagina), which primarily contribute the sperm and egg, respectively; fusion of the egg and spermand subsequent development of the resulting embryo occurs in the female reproductive system. A number of processes such as growth, development, etc. areregulated by chemicals called hormones that are produced by a variety of glands that constitute the endocrine system (pineal, pituitary, thyroid, parathyroid, thymus, adrenal, pancreas, testis, ovary); hormones are transported through out the body by the blood and affect target cell function by binding to receptors present in the target tissue.
Although each organ system has a specific role to play, a number of human functions involve the interaction of more than one organ system--e.g., movementrequires both the muscular and the skeletal systems. Given this level of complexity in the functioning of the human body, the tight regulation of the various organ systems is crucial for maintaining relatively stable internal conditions. Such a state is referred to as homeostasis. Homeostasis in the human body is sustained by both the nervous and endocrine systems, as they are ableto access all parts of the body. These two systems act via electrical impulses (quick acting) and hormones (slow acting), respectively, to control the activity of all the organ systems (including themselves). Disruption of homeostatic control mechanisms in the body lead to altered physiology and, thus, disease.