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 viability of human body. 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 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 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. The content, acidity and composition of the water inside and outside of cells is carefully maintained. The main electrolytes in body water outside of 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 37.2 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 also plays the role of host to trillions of cells which reside in the gastrointestinal tract and on the skin.[citation needed] 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. 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.[6] The RNA is then used to create proteins which form the basis for cells, their activity, and their products. 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 specialised function. 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. 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, 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.

The main systems of the human body are as follows:

Cardiovascular / Circulatory system:
Circulates blood around the body via the heart, arteries and veins, delivering oxygen and nutrients to organs and cells and carrying their waste products away.

Digestive system / Excretory system:
Mechanical and chemical processes that provide nutrients via the mouth, esophagus, stomach and intestines. Eliminates waste from the body.

Endocrine system:
Provides chemical communications within the body using hormones.

Integumentary system/ Exocrine system:
Skin, hair, nails, sweat and other exocrine glands.

Lymphatic system / Immune system:
The system comprising a network of lymphatic vessels that carry a clear fluid called lymph. Defends the body against disease-causing agents.

Muscular system/Skeletal system:
Enables the body to move using muscles. Bones supporting the body and its organs.

Nervous system:
Collects and processes information from the senses via nerves and the brain and tells the muscles to contract to cause physical actions.

Renal system / Urinary system:
The system where the kidneys filter blood.

Reproductive system:
The sex organs required for the production of offspring.

Respiratory system:
The lungs and the trachea that bring air into the body.

Vestibular system:
The sensory system that provides the leading contribution to the sense of balance and spatial orientation for the purpose of coordinating movement with balance.

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