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Physiology is the study of how living organisms function, including their physical and chemical processes.

Organ systems

In physiology the various organs are grouped into systems based on their function. This makes it easier to understand how an organism works. More and more, however, we learn that the organs interface with one another in ways outside of the classic definitions of the systems. For example, the kidneys, generally thought of as being in the urinary system, are involved in producing hormones (endocrine system) that regulate blood pressure (circulatory system). The systems are: Circulatory, Immune/Lymphatic, Skeletal, Muscular, Digestive/Excretory, Endocrine, Respiratory, Reproductive, Nervous/Sensory, Integumentary, and Urinary. Not every physician or scientist uses the same groupings. For example, skeletal and muscular may be together, as the musculoskeletal system, and the reproductive and urinary system may be together, as the genitourinary system.


The circulatory system interface with the respiratory system to pump oxygenated blood throughout the body, thus supplying the organs with nutrients. The circulatory system consists of the heart and blood vessels. The heart is a special muscle which constantly and automatically pumps blood through the blodo vessels. The blood vessels consist of arteries, arterioles (smaller arteries), capillaries, venules (small veins), and veins. Arteries carry blood away from the heart, whereas veins carry blood to the heart. Normally arteries carry oxygenated blood and the veins normally carry oxygen-poor blood, but this is not always the case, as with the pulmonary arteries and veins which carry blood back and forth between the heart and lungs. Capillaries are the smallest blood vessels and are where oxygen exchange between blood and organs takes place.

Immune and Lymphatic


Bones (206 in all); joints, tendons, and cartilage.


Consists of over 600 named muscles.[1] Muscles are classified into three types: smooth or involuntary, striated or voluntary, and cardiac.

  • Smooth muscle is found in the linings of arteries, in the lining of the digestive system, and in various organs. Smooth muscle is also called "involuntary" muscle.

Most smooth muscle, such as the lining of the stomach, is controlled by the sympathetic and parasympathetic nervous system. Some smooth muscles, such as the myometrium (smooth muscle of the uterus), act mostly in response to hormones.

Digestive and Excretory

  • Mouth—digestion starts in the mouth, as that is where the food is first broken down
  • Esophagus—carries food and fluids from mouth to stomach
  • Stomach—digests food
  • Small intestine—allows fluids and nutrients to be absorbed into the system, by means of villi
  • Cecum or caecum—absorbs more fluid into the system
  • Colon or large intestine—concentrates waste for elimination
  • Rectum—expels waste from the body


Glands including the thyroid, pituitary, pineal, gonads, thymus, pancreas, and adrenals. They produce hormones to facilitate metabolic, reproductive, and circadian cycles.


Consists of the lungs, bronchi, trachea (windpipe), larynx (voice box), epiglottis, nose, and sinuses.


The reproductive system facilitates procreation, and is closely associated with the endocrine system.

  • Male reproductive organs
  • Testicles—Produce spermatozoa; also produce hormones and thus are also classified as endocrine organs. Homologous to the ovaries in the female.
  • Prostate—Produces fluids to supply nutrients to the spermatozoa; also closes the bladder.
  • Bulbourethral gland—An exocrine gland that produces mucus to lubricate the ducts of the reproductive system.

Each male reproductive organ is homologous, or directly corresponding, to a particular female reproductive organ.


  • Nervous system: brain, nerves, nerve endings, and spinal cord
  • Sensory organs: optical (eyes and optic nerve), olfactory (nose, sinuses, olfactory bulb), gustative (taste buds), and auditory (ears, ossicles, eustachian tubes)


Consists of the skin and its glands (eccrine, apocrine, and sebaceous), and the hair follicles. The female mammary gland is also often considered part of the integumentary system, because it is structurally a modified form of the eccrine gland.


  • Kidneys—filter selected compounds such as urea from the blood, generating urine in the process
  • Ureters—carry off urine, draining the kidneys
  • Bladder—contractable organ that stores urine; its wall is composed of muscle which expands to allow room for urine, or contracts to expel it from the body (micturition)
  • Urethra—passageway through which urine exits the bladder during urination


  1. Stoppard, 1988.