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Human physiology is the science of the mechanical, physical, and biochemical functions of normal humans or human tissues or organs. The principal level of focus of physiology is at the level of organs and systems. Most aspects of human physiology are closely homologous to corresponding aspects of animal physiology, and animal experimentation has provided much of the foundation of physiological knowledge. Human physiology is one of the basic sciences of medical study, and as such is most often applied as medical care.
Many physiologic variables (such as blood glucose (sugar) level, body temperature, blood pH, and so on) must be maintained within narrow limits for proper health. An overriding theme in physiology is that of homeostasis, maintaining a stable internal environment despite external fluctuations. Indeed, the primary functions of many organ systems are to maintain homeostasis. For instance, the urinary system helps to control water levels as well as that of blood pH and various waste products, and the circulatory system ensures a constant supply of oxygen and nutrients to tissues and the removal of waste products.
Traditionally, the academic discipline of physiology views the body as a collection of interacting systems, each with its own combination of functions and purposes.
- The nervous system consists of the central nervous system (which is the brain and spinal cord) and peripheral nervous system. The brain is the organ of thought, emotion, and sensory processing, and serves many aspects of communication and control of various other systems and functions. The study of the nervous system is termed neuroscience; with a focus on disease, it is termed neurology. The branch of medicine that diagnoses, treats, and studies mental illness and behavioral conditions, is psychiatry.
- The musculoskeletal system consists of the human skeleton (which includes bones, ligaments, tendons, and cartilage) and attached muscles. It gives the body basic structure and the ability for movement. In addition to their structural role, the larger bones in the body contain bone marrow, the site of production of blood cells. Also, all bones are major storage sites for calcium and phosphate. The study of the skeleton is termed osteology; with a focus on bone disorders it is termed orthopedics.
- The circulatory system consists of the heart and blood vessels (arteries, veins, capillaries). The heart propels the circulation of the blood, which serves as a "transportation system" to transfer oxygen, fuel, nutrients, waste products, immune cells, and signalling molecules (i.e., hormones) from one part of the body to another. The study of the circulatory system is termed cardiovascular physiology; with a focus on disease it is cardiology.
- The blood consists of fluid that carries cells in the circulation, including some that move from tissue to blood vessels and back, as well as the spleen and bone marrow. The cells include red blood cells that carry oxygen, white blood cells that mediate our response to infection and foreign materials, and platelets with complementing plasma proteins that promote clotting and wound healing. The study of blood is termed hematology.
- The gastrointestinal system consists of the mouth, esophagus, stomach, gut (small and large intestines), and rectum, as well as the liver, pancreas, gallbladder, and salivary glands. It converts food into small, nutritional, non-toxic molecules for distribution by the circulation to all tissues of the body, and excretes the unused residue. The study of this system is termed gastroenterology.
- The respiratory system consists of the nose, nasopharynx, trachea, and lungs. It brings oxygen from the air and excretes carbon dioxide and water back into the air. The study of this system is termed respiratory physiology. Its clinical counterpart is pulmonology.
- The urinary system consists of the kidneys, ureters, bladder, and urethra. It removes water from the blood to produce urine, which carries a variety of waste molecules and excess ions and water out of the body. The study of the function of the urinary system is termed nephrology or renal physiology; with a focus on structural disease it is urology.
- The immune system consists of the white blood cells, the thymus, lymph nodes and lymph channels, which are also part of the lymphatic system. The immune system provides a mechanism for the body to distinguish its own cells and tissues from alien cells and substances and to neutralize or destroy the latter by using specialized proteins such as antibodies, cytokines, and toll-like receptors, among many others. The study of the immune system is termed immunology.
- The endocrine system consists of the principal endocrine glands: the pituitary, thyroid, adrenals, pancreas, parathyroids, and gonads, but nearly all organs and tissues produce specific endocrine hormones as well. The endocrine hormones serve as signals from one body system to another regarding an enormous array of conditions, and resulting in variety of changes of function. The study of this system is termed endocrinology.
- The reproductive system consists of the gonads and the internal and external sex organs. The reproductive system produces gametes in each sex, a mechanism for their combination, and a nurturing environment for the first 9 months of development of the offspring. The study of the physical function of this system is termed reproductive physiology; when applied to the disorders of reproduction it is termed gynecology or andrology. The study of the behavioral aspects is sexology and when applied to the developmental aspects is termed embryology.
- The integumentary system consists of the covering of the body (the skin), including hair and nails as well as other functionally important structures such as the sweat glands and sebaceous glands. The skin provides containment, structure, and protection for other organs, but it also serves as a major sensory interface with the outside world. The study of skin is dermatology.
- The adipose tissue consists of several types of body fat, below the skin and surrounding the internal organs. It serves principally as storage for fuel, as insulation, and provides signals about nutritional status to other systems. The study of the functions of this system is new enough that there is not a widely used name for the discipline.
The traditional divisions by system are somewhat arbitrary. Many body parts participate in more than one system, and systems might be organized by function, by embryological origin, or other categorizations. In particular, is the "neuroendocrine system", the complex interactions of the neurological and endocrinological systems which together regulate physiology. Furthermore, many aspects of physiology are not as easily included in the traditional organ system categories.
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