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The human body is the 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 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 artists to assist them in their work.

The human body is composed of elements including hydrogen, oxygen, carbon, calcium, and phosphorus. These elements reside in trillions of cells and non-cellular components of the body.

The adult male body is about 60% water for the total water content of some 42 liters. This is made up of about 19 liters of extracellular fluid including about 3.2 liters of blood plasma and about 8.4 liters of interstitial fluid, and about 23 liters of fluid inside cells. The content, acidity, and composition of the water inside and outside cells are carefully maintained. The main electrolytes in body water outside cells are sodium and chloride, whereas within cells it is potassium and other phosphates.

The body contains trillions of cells, the fundamental unit of life. At maturity, there are roughly 30–37 trillion cells in the body, an estimate arrived at by totaling the cell numbers of all the organs of the body and cell types. The body is also hosting to about the same number of non-human cells as well as multicellular organisms which reside in the gastrointestinal tract and on the skin. 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.

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. 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 can self-regulate by the number of proteins produced. However, not all cells have DNA; some cells such as mature red blood cells lose their nucleus as they mature.

The body consists of many different types of tissue, defined as cells that act with a specialized function. The study of tissues is called histology and often occurs with a microscope. The body consists of four main types of tissues. These are lining cells (epithelia), connective tissue, nerve tissue, and muscle tissue.

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.

Organs, structured collections of cells with a specific function, mostly sit within the body, except skin. Examples include the heart, lungs, and liver. Many organs reside within cavities within the body. These cavities include the abdomen (which contains the stomach, for example) and pleura, which contains the lungs.

The heart is an organ located in the thoracic cavity between the lungs and slightly to the left. It is surrounded by the pericardium which holds it in place in the mediastinum and serves to protect it from blunt trauma, and infection and helps lubricate the movement of the heart via pericardial fluid. The heart works by pumping blood around the body allowing oxygen, nutrients, waste, hormones, and white blood cells to be transported.

The heart is composed of two atrium and two ventricles. The primary purpose of the atrium is to allow uninterrupted venous blood flow to the heart during ventricular systole. This allows enough blood to get into the ventricles during atrial systole. A lack of the atrium would cause a decrease in cardiac output of 75%. The purpose of the ventricles is to pump blood to the lungs through the right ventricle and to the rest of the body through the left ventricle.

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