Human body
The human body is the entire structure of a human being. It is composed of many different types of cells that together create tissues and subsequently organ systems. They ensure homeostasis and the viability of the human body.
It comprises a head, neck, trunk (which includes the thorax and abdomen), arms and hands, legs and feet.
The study of the human body involves anatomy, physiology, histology and embryology. The body varies anatomically in known ways. Physiology focuses on the systems and organs of the human body and their functions. Many systems and mechanisms interact in order to maintain homeostasis, with safe levels of substances such as sugar and oxygen in the blood.
The body is studied by health professionals, physiologists, anatomists, and by artists to assist them in their work.
Composition
The human body is composed of elements including hydrogen, oxygen, carbon, calcium and phosphorus.[1] These elements reside in trillions of cells and non-cellular components of the body.
The adult male body is about 60% water for a total water content of some 42 litres. This is made up of about 19 litres of extracellular fluid including about 3.2 litres of blood plasma and about 8.4 litres of interstitial fluid, and about 23 litres of fluid inside cells.[2] The content, acidity and composition of the water inside and outside cells is carefully maintained. The main electrolytes in body water outside cells are sodium and chloride, whereas within cells it is potassium and other phosphates.[3]
Cells
The body contains trillions of cells, the fundamental unit of life.[4] At maturity, there are roughly 30[5]–37[6] trillion cells in the body, an estimate arrived at by totalling the cell numbers of all the organs of the body and cell types. The body is also host to about the same number of non-human cells[5] as well as multicellular organisms which reside in the gastrointestinal tract and on the skin.[7] Not all parts of the body are made from cells. Cells sit in an extracellular matrix that consists of proteins such as collagen, surrounded by extracellular fluids. Of the 70 kg weight of an average human body, nearly 25 kg is non-human cells or non-cellular material such as bone and connective tissue.[5]
Cells in the body function because of DNA. DNA sits within the nucleus of a cell. Here, parts of DNA are copied and sent to the body of the cell via RNA.[8] The RNA is then used to create proteins which form the basis for cells, their activity, and their products. Proteins dictate cell function and gene expression, a cell is able to self-regulate by the amount of proteins produced.[9] However, not all cells have DNA – some cells such as mature red blood cells lose their nucleus as they mature.
Tissues
The body consists of many different types of tissue, defined as cells that act with a specialised function.[10] The study of tissues is called histology and often occurs with a microscope. The body consists of four main types of tissues – lining cells (epithelia), connective tissue, nervous tissue and muscle tissue.[11]
Cells that lie on surfaces exposed to the outside world or gastrointestinal tract (epithelia) or internal cavities (endothelium) come in numerous shapes and forms – from single layers of flat cells, to cells with small beating hair-like cilia in the lungs, to column-like cells that line the stomach. Endothelial cells are cells that line internal cavities including blood vessels and glands. Lining cells regulate what can and can't pass through them, protect internal structures, and function as sensory surfaces.[11]
Organs
See also: List of organs of the human body
Organs, structured collections of cells with a specific function,[12] sit within the body. Examples include the heart, lungs and liver. Many organs reside within cavities within the body. These cavities include the abdomen and pleura.
Systems
See also: List of systems of the human body

Circulatory system
Main article: Circulatory system
The circulatory system comprises the heart and blood vessels (arteries, veins and 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.[13][14][15]
