Human anatomy

The human body is the structure of a. It is composed of many different types of that together create  and subsequently. They ensure and the  of the human body.

It comprises a head,, (which includes the  and ), arms and hands,  and feet.

The study of the human body involves, , and. The body 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 s, physiologists, anatomists, and by artists to assist them in their work.

Composition
The  including, , ,  and. These elements reside in trillions of cells and non-cellular components of the body.

The adult male body is about 60% 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, and about 23 litres of fluid inside cells. The content, acidity and composition of the water inside and outside cells is carefully maintained. The main electrolytes in body water outside cells are and, whereas within cells it is  and other s.

Cells
The body contains trillions of, the fundamental unit of life. At maturity, there are roughly 30–37 cells in the body, an estimate arrived at by totaling the cell numbers of all the  of the body and. The body is also host to about the same number of non-human cells as well as multicellular organisms which reside in the and on the skin. Not all parts of the body are made from cells. Cells sit in an that consists of proteins such as, 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 sits within the of a cell. Here, parts of DNA are and sent to the body of the cell via. The RNA is then used to s 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. However, not all cells have DNA – some cells such as mature lose their nucleus as they mature.

Tissues
The body consists of many different types of, defined as cells that act with a specialised function. The study of tissues is called and often occurs with a. The body consists of four main types of tissues – lining cells, , and.

Cells that lie on surfaces exposed to the outside world or gastrointestinal tract or internal cavities  come in numerous shapes and forms – from, to cells with small beating hair-like  in the lungs, to column-like cells that line the. 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 with a specific function, sit within the body. Examples include the, s and. Many organs reside within within the body. These cavities include the (which contains the stomach, for example) and, which contains the lungs.

Circulatory system
The consists of the  and s (, s and ). The heart propels the circulation of the blood, which serves as a "transportation system" to transfer, fuel, nutrients, waste products, immune cells and signalling molecules (i.e. ) from one part of the body to another. Paths of blood circulation within the human body can be divided into two circuits: the, which pumps blood to the lungs to receive and leave , and the systemic circuit, which carries blood from the heart off to the rest of the body. The blood consists of fluid that carries in the circulation, including some that move from tissue to blood vessels and back, as well as the  and.

Digestive system
The consists of the mouth including the  and, , , (,  and s, and ), as well as the , , , and s. It converts food into small, nutritional, non-toxic s for distribution and absorption into the body. These molecules take the form of s (which are broken down into s), s, s and s (the last of which are mainly ionic rather than molecular). After being, food moves through the by means of : the systematic expansion and contraction of muscles to push food from one area to the next.

Digestion begins in the, which food into smaller pieces for easier digestion. Then it is, and moves through the to the. In the stomach, food is mixed with s to allow the extraction of s. What is left is called ; this then moves into the, which absorbs the nutrients and water from the chyme. What remains passes on to the, where it is dried to form ; these are then stored in the until they are expelled through the.

Endocrine system
The consists of the principal endocrine glands: the, , s, , s, and s, but nearly all organs and tissues produce specific endocrine s as well. The endocrine hormones serve as signals from one body system to another regarding an enormous array of conditions, and resulting in variety of changes of function.

Immune system
The consists of the s, the, s and  channels, which are also part of the. The immune system provides a mechanism for the body to distinguish its own cells and tissues from outside cells and substances and to neutralize or destroy the latter by using specialized proteins such as, , and , among many others.

Integumentary system
The consists of the covering of the body (the skin), including hair and  as well as other functionally important structures such as the s and s. The skin provides containment, structure, and protection for other organs, and serves as a major sensory interface with the outside world.

Lymphatic system
The extracts, transports and metabolizes lymph, the fluid found in between cells. The lymphatic system is similar to the circulatory system in terms of both its structure and its most basic function, to carry a body fluid.

Musculoskeletal system
The consists of the  (which includes s, s, s, and ) and attached s. It gives the body basic structure and the ability for movement. In addition to their structural role, the larger bones in the body contain, the site of production of blood cells. Also, all bones are major storage sites for and. This system can be split up into the and the.

Nervous system
The consists of the body's s, which together form the  and  which in turn form the  and related structures. The brain is the organ of, emotion, , and ; it serves many aspects of communication and controls various systems and functions. The consist of, , , and. The, ears, , and nose gather information about the body's environment.

From a structural perspective, the is typically subdivided into two component parts: the  (CNS), composed of the brain and the ; and the  (PNS), composed of the nerves and  outside the brain and spinal cord. The CNS is mostly responsible for organizing, processing , thought, memory, and other such functions. It remains a matter of some debate whether the directly gives rise to. The (PNS) is mostly responsible for gathering information with s and directing body movements with s.

From a functional perspective, the nervous system is again typically divided into two component parts: the (SNS) and the  (ANS). The SNS is involved in voluntary functions like and. The ANS is involved in involuntary processes, such as and regulating.

The nervous system is subject to many different diseases. In, abnormal electrical activity in the brain can cause. In, the attacks the , damaging the nerves' ability to transmit signals. (ALS), also known as 's disease, is a disease which gradually reduces movement in patients. There are also many other diseases of the nervous system.

Reproductive system
The consists of the s and the internal and external s. The reproductive system produces s in each sex, a mechanism for their combination, and in the female a nurturing environment for the first 9 months of development of the infant.

Respiratory system
The consists of the nose,, , and. It brings oxygen from the air and excretes and water back into the air. First, is pulled through the trachea into the lungs by the  pushing down, which creates a. Air is briefly stored inside small sacs known as (sing.: alveolus) before being expelled from the lungs when the diaphragm contracts again. Each alveolus}} is surrounded by carrying deoxygenated blood, which absorbs oxygen out of the air and into the.

For the respiratory system to function properly, there need to be as few impediments as possible to the movement of air within the lungs. of the lungs and excess are common sources of breathing difficulties. In, the respiratory system is persistently inflamed, causing and/or. occurs through of the alveoli, and may be caused by. , commonly a result of, is caused by damage to connections between the alveoli.

Urinary system
The consists of the s, s,, and. It removes toxic materials from the blood to produce urine, which carries a variety of waste molecules and excess s and water out of the body.

Anatomy
Human is the study of the shape and form of the human body. The human body has four (two arms and two legs), a head and a  which connect to the. The body's shape is determined by a strong made of  and, surrounded by fat, muscle, connective tissue, organs, and other structures. The at the back of the skeleton contains the flexible  which surrounds the, which is a collection of nerve fibres connecting the brain to the rest of the body. s connect the spinal cord and brain to the rest of the body. All major bones, muscles, and nerves in the body are named, with the exception of s such as s and s.

Blood vessels carry blood throughout the body, which moves because of the beating of the. Venules and s collect blood low in oxygen from tissues throughout the body. These collect in progressively larger veins until they reach the body's two largest veins, the and, which drain blood into the right side of the heart. From here, the blood is pumped into the s where it receives oxygen and drains back into the left side of the heart. From here, it is pumped into the body's largest, the , and then progressively smaller arteries and arterioles until it reaches tissue. Here blood passes from small arteries into, then small veins and the process begins again. Blood carries, waste products, and s from one place in the body to another. Blood is filtered at the s and.

The body consists of a number of, separated areas which house different organ systems. The brain and reside in an area protected from the rest of the body by the. The lungs sit in the. The s,, and sit in the

Height, weight, and other  vary individually and with age and sex. Body shape is influenced by the distribution of and.

Physiology
Human is the study of how the human body functions. This includes the mechanical, physical,, and functions of humans in good health, from  to the  of which they are composed. The human body consists of many interacting systems of. These interact to maintain, keeping the body in a stable state with safe levels of substances such as sugar and oxygen in the blood.

Each system contributes to homeostasis, of itself, other systems, and the entire body. Some combined systems are referred to by joint names. For example, the nervous system and the endocrine system operate together as the. The nervous system receives information from the body, and transmits this to the brain via s and s. At the same time, the releases hormones, such as to help regulate  and volume. Together, these systems regulate the internal environment of the body, maintaining blood flow, posture, energy supply, temperature, and acid balance.

Development
Development of the human body is the process of growth to maturity. The process begins with fertilisation, where an egg released from the of a female is penetrated by. The egg then lodges in the, where an and later  develop until. Growth and development occur after birth, and include both physical and psychological development, influenced by genetic, hormonal, environmental and other factors. Development and growth continue throughout life, through, , and through to , and are referred to as the process of.