Human body

The human body is the entire structure of a human being and comprises a head, neck, trunk (which includes the thorax and abdomen), two arms and hands and two legs and feet. Every part of the body is composed of various types of cell. At maturity, the estimated number of cells in the body is given as 37.2 trillion. This number is stated to be of partial data and to be used as a starting point for further calculations. The number given is arrived at by totalling the cell numbers of all the organs of the body and cell types. The composition of the human body shows it to be composed of a number of certain elements in different proportions.

The study of the human body involves anatomy and physiology. The human body can show anatomical non-pathological anomalies which need to be able to be recognised. Physiology focuses on the systems and their organs of the human body and their functions. Many systems and mechanisms interact in order to maintain homeostasis.

Structure
Skeletal structure frames the overall shape of the body and does not alter much over a lifetime. General body shape (and female body shape) is influenced by the distribution of muscle and fat tissue and also affected by various hormones. The average height of an adult male human (in developed countries) is about 1.7–1.8 m (5'7" to 5'11") and the adult female is about 1.6–1.7 m (5'2" to 5'7"). Height is largely determined by genes and diet. Body type and composition are influenced by factors such as genetics, diet, and exercise.

Composition
The average adult body contains between 5 and 5½ litres of blood. The composition of the human body can be referred to in terms of its water content, elements content, tissue types or material types. The adult human body contains approximately 60% water, and so makes up a significant proportion of the body, both in terms of weight and volume. Water content can vary from a high 75% in a newborn infant to a lower 45% in an obese person. (These figures are necessarily statistical averages).

Composition by tissue type can refer to the cells of the human body where the vast majority of cells are not human but bacterial cells and also archaea cells, particularly methanogens such as Methanobrevibacter smithii. The whole population of microbiota include microorganisms of the skin and other body parts and these in total are also termed the human microbiome the largest proportion of these form the gut flora.

The proportions of the elements of the body can be referred to in terms of the main elements, minor ones and trace elements. Material type may also be referred to as including water, protein, connective tissue, fats, carbohydrates and bone.

Human anatomy
Human anatomy (gr. ἀνατομία, "dissection", from ἀνά, "up", and τέμνειν, "cut") is primarily the scientific study of the morphology of the human body. Anatomy is subdivided into gross anatomy and microscopic anatomy (histology) Gross anatomy (also called topographical anatomy, regional anatomy, or anthropotomy) is the study of anatomical structures that can be seen by the naked eye. Microscopic anatomy involves the use of microscopes to study minute anatomical structures, and is the field of histology which studies the organization of tissues at all levels, from cell biology (previously called cytology), to organs. Anatomy, human physiology (the study of function), and biochemistry (the study of the chemistry of living structures) are complementary basic medical sciences that are generally taught together (or in tandem) to students studying medical sciences.

In some of its facets human anatomy is closely related to embryology, comparative anatomy and comparative embryology, through common roots in evolution; for example, much of the human body maintains the ancient segmental pattern that is present in all vertebrates with basic units being repeated, which is particularly obvious in the vertebral column and in the ribcage, and which can be traced from the somitogenesis stage in very early embryos.

Generally, physicians, dentists, physiotherapists, nurses, paramedics, radiographers, and students of certain biological sciences, learn gross anatomy and microscopic anatomy from anatomical models, skeletons, textbooks, diagrams, photographs, lectures, and tutorials. The study of microscopic anatomy (or histology) can be aided by practical experience in examining histological preparations (or slides) under a microscope; and in addition, medical and dental students generally also learn anatomy with practical experience of dissection and inspection of cadavers (corpses). A thorough working knowledge of anatomy is required for all medical doctors, especially surgeons, and doctors working in some diagnostic specialities, such as histopathology and radiology.

Human anatomy, physiology, and biochemistry are basic medical sciences, which are generally taught to medical students in their first year at medical school. Human anatomy can be taught regionally or systemically; that is, respectively, studying anatomy by bodily regions such as the head and chest, or studying by specific systems, such as the nervous or respiratory systems. The major anatomy textbook, Gray's Anatomy, has recently been reorganized from a systems format to a regional format, in line with modern teaching.

Anatomical variations
In human anatomy, the term anatomical variation refers to a non-pathologic anatomic structure that is different from the norm. The possible anatomic variations in each organ and its arterial and venous supply must be known by physicians, such as surgeons or radiologists, in order to identify them. Unlike congenital anomalies, anatomic variations are typically inconsequential and do not constitute a disorder. Accessory muscles are rare anatomical duplicates of muscle that can occur and only require treatment where function is impaired. The accessory soleus muscle in the ankle, is one such variation and one which does not need to be rectified.

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 study of how physiology is altered in disease is pathophysiology.

Systems
The human body consists of many interacting systems. Each system contributes to the maintenance of homeostasis, of itself, other systems, and the entire body. A system consists of two or more organs, which are functional collections of tissue. Systems do not work in isolation, and the well-being of the person depends upon the well-being of all the interacting body systems. Some combining systems are referred to by their joint names such as the nervous system and the endocrine system known together as the neuroendocrine system.

Homeostasis
The term homeostasis refers to a system that regulates its internal environment and maintains a stable, relatively constant condition; such as maintaining an equal temperature or pH. This is required for the body to function sufficiently. Without a relatively constant pH, temperature, blood flow, and position, it would be impossible for the human to survive.

Many interacting systems and mechanisms act to maintain the human's internal environment. The nervous system receives information from the body and transmits this to the brain via neurotransmitters. The endocrine system may release hormones to help regular blood pressure and volume. Cell metabolism may help to maintain the blood's pH.

Depiction
Anatomy has become a key part of the visual arts. Basic concepts of how muscles and bones function and change with movement are vital in drawing, painting or animating a human figure. Many books (such as "Human Anatomy for Artists: The Elements of Form") have been written as guides to drawing the human body anatomically correctly. Leonardo da Vinci sought to improve his art through a better understanding of human anatomy. In the process he advanced both human anatomy and its representation in art.

Because the structure of living organisms is complex, anatomy is organized by levels, from the smallest components of cells to the largest organs and their relationship to others.

History of anatomy
The history of anatomy has been characterized, over a long period of time, by an ongoing, developing understanding of the functions of organs and structures in the human body. Methods have advanced dramatically, from the simple examination by dissection of animals and cadavers (corpses), to the development and use of the microscope, to the far more technological advances of the electron microscope and other complex techniques developed since the beginning of the 20th century. During the 19th and early 20th centuries it was the most prominent biological field of scientific study.

History of physiology
The study of human physiology dates back to at least 420 B.C. and the time of Hippocrates, the father of western 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 in Islamic medicine. 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.

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 in the body.