The main elements that compose the human body are shown from most abundant, by mass, to least abundant.
The average adult body contains between 5 and 5½ litres of blood and approximately 10 litres of interstitial fluid.
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).
The vast majority of cells in the human body are not human at all; rather they are of bacteria, archaea, and methanogens such as Methanobrevibacter smithii. The largest proportion of these form the gut flora. The whole population of microbiota include microorganisms of the skin and other body parts and this altogether is termed as the human microbiome.
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
Further information: Head and neck anatomy and Nail (anatomy)
Anatomical study by Leonardo da Vinci
Human anatomy 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.[3] 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 medicine.
Front view of viscera
In some of its facets human anatomy is closely related to embryology, comparative anatomy and comparative embryology,[3] 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, other health professionals, 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, 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
Further information: List of anatomical variations
In human anatomy, the term anatomical variation refers to a non-pathologic anatomic structure that is different from normal. 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 soles muscle in the ankle is one such variation and one which does not need to be rectified. Another more common variation found in around ten per cent of the population is the accessory spleen.
Human physiology
Main article: 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
List of systems of the human body
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 acid balance pH. This is required for the body to function properly. Without a relatively constant pH, temperature, blood flow, and position, survival would be impossible.
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 regulate blood pressure and volume. Cell metabolism may help to maintain the blood's pH.
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