Human physiology

Human physiology is the science of the mechanical, physical, bioelectrical, and biochemical functions of humans in good health, their organs, and the cells of which they are composed. Physiology focuses principally 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. Anatomy and physiology are closely related fields of study: anatomy, the study of form, and physiology, the study of function, are intrinsically related and are studied in tandem as part of a medical curriculum.

The concept of homeostasis

The term "homeostasis" refers to the maintenance of overall inner resistance in the body. Homeostasis stabilizes the body by regulating the internal environment. This is required for the body to function sufficiently. The Homeostatic process is essential for the survival of each cell, tissue, and body system. Homeostasis in a general sense refers to stability, balance or equilibrium. Maintaining a stable internal environment requires constant monitoring, mostly by the brain and nervous system. The brain receives information from the body and responds appropriately through the release of various substances like neurotransmitters, catecholamines, and hormones. Individual organ physiology furthermore facilitates the maintenance of homeostasis of the whole body e.g. Blood pressure regulation: the release of renin by the kidneys allow blood pressure to be stabilized (Renin, Angiotensinogen, Aldosterone System), though the brain helps regulate blood pressure by the Pituitary releasing Anti-Diuretic Hormone (ADH). Thus, homeostasis is maintained within the body as a whole, dependent upon its parts.

Systems

Traditionally, the academic discipline of physiology views the body as a collection of interacting systems, each with its own combination of functions and purposes. Each body system contributes to the homeostasis of other systems and of the entire organism. No system of the body works in isolation, and the well-being of the person depends upon the well-being of all the interacting body systems.

	System	Clinical study	Physiology
	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, memory, and sensory processing, and serves many aspects of communication and control of various other systems and functions. The special senses consist of vision, hearing, taste, and smell. The eyes, ears, tongue, and nose gather information about the body's environment.	neuroscience, neurology (disease), psychiatry (behavioral), ophthalmology (vision), otolaryngology (hearing, taste, smell)	neurophysiology
	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.	osteology (skeleton), orthopedics (bone disorders)	cell physiology, musculoskeletal physiology
	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 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.	cardiology (heart), hematology (blood)	cardiovascular physiology
	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.	pulmonology.	respiratory physiology
	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.	gastroenterology	gastrointestinal physiology
	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.	dermatology	cell physiology, skin physiology
	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.	nephrology (function), urology (structural disease)	renal physiology
File:Male anatomy.png	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.	gynecology (women), andrology (men), sexology (behavioral aspects) embryology (developmental aspects)	reproductive physiology
	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.	immunology	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. There is also the exocrine system.	endocrinology	endocrinology

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.

The study of how physiology is altered in disease is pathophysiology.

History

The study of human physiology dates back to at least 420 B.C. and the time of Hippocrates, the father of medicine. The critical thinking of Aristotle and his emphasis on the relationship between structure and function marked the beginning of physiology in Ancient Greece, while Claudius Galenus (c. 126-199 A.D.), known as Galen, was the first to use experiments to probe the function of the body. Galen was the founder of experimental physiology. The medical world moved on from Galenism only with the appearance of Andreas Vesalius and William Harvey.

During the Middle Ages, the ancient Greek and Indian medical traditions were further developed by Muslim physicians. Notable work in this period was done by Avicenna (980-1037), author of the The Canon of Medicine, and Ibn al-Nafis (1213–1288), among others.

Portrait of Vesalius from his De humani corporis fabrica (1543).

Following from the Middle Ages, the Renaissance brought an increase of physiological research in the Western world that triggered the modern study of anatomy and physiology. Andreas Vesalius was an author of one of the most influential books on human anatomy, De humani corporis fabrica. Vesalius is often referred to as the founder of modern human anatomy. Anatomist William Harvey described the circulatory system in the 17th century, demonstrating the fruitful combination of close observations and careful experiments to learn about the functions of the body, which was fundamental to the development of experimental physiology. Herman Boerhaave is sometimes referred to as a father of physiology due to his exemplary teaching in Leiden and textbook Institutiones medicae (1708).

In the 18th century, important works in this field were by Pierre Cabanis, a French doctor and physiologist.

In the 19th century, physiological knowledge began to accumulate at a rapid rate, in particular with the 1838 appearance of the Cell theory of Matthias Schleiden and Theodor Schwann. It radically stated that organisms are made up of units called cells. Claude Bernard's (1813–1878) further discoveries ultimately led to his concept of milieu interieur (internal environment), which would later be taken up and championed as "homeostasis" by American physiologist Walter Cannon (1871–1945).

In the 20th century, biologists also became interested in how organisms other than human beings function, eventually spawning the fields of comparative physiology and ecophysiology. Major figures in these fields include Knut Schmidt-Nielsen and George Bartholomew. Most recently, evolutionary physiology has become a distinct subdiscipline.

The biological basis of the study of physiology, integration refers to the overlap of many functions of the systems of the human body, as well as its accompanied form. It is achieved through communication that occurs in a variety of ways, both electrical and chemical.

In terms of the human body, the endocrine and nervous systems play major roles in the reception and transmission of signals that integrate function. Homeostasis is a major aspect with regard to the interactions within an organism, humans included.