Human body

The human body is made of many different kinds of cells. These cells join together to form tissues, which then form organs and organ systems. All of these parts together make up the whole human body.

The outside of the human body includes the head, hair, neck, and torso. The torso holds the thorax and abdomen. The outside also includes the genitals, arms, hands, legs, and feet. Inside the body are organs, teeth, bones, muscles, tendons, and ligaments. There are also blood vessels, which carry blood, and lymphatic vessels, which carry lymph.

Scientists and doctors study the human body in many ways. This field includes anatomy, which is the study of body structure, and physiology, which looks at how the body's systems and organs work. It also includes histology and embryology. The body varies anatomically in known ways, so people can differ in certain predictable details.

Many parts of the body work together to keep things in balance. This is called homeostasis. For example, the blood keeps safe levels of substances such as sugar, iron, and oxygen.

The human body is studied by health professionals, physiologists, anatomists, and artists. They use this knowledge to help them in their work.

Composition

Main article: Composition of the human body

The human body is made up of elements such as hydrogen, oxygen, carbon, calcium and phosphorus. These elements are found in trillions of cells and in other parts of the body that are not cells.

About 60% of an adult man's body is total body water, which is about 42 litres. About 23 litres of this water is inside cells. About 19 litres is extracellular fluid, which is outside them. The body carefully keeps the water inside and outside cells in balance, including its acidity and makeup. The main electrolytes outside cells are sodium and chloride. Inside cells, the main ones are potassium and phosphates.

The body contains trillions of cells. A cell is the basic unit of life. When fully grown, a body has roughly 30 trillion cells and 38 trillion bacteria, counting the cells of all the organs. Cells sit in an extracellular matrix. This is made of proteins, such as collagen.

Cells form tissues, which are groups of cells with a special job. The body has four main types of tissue: epithelia, connective tissue, nerve tissue and muscle tissue.

Organs are groups of cells that have a specific function. Examples include the heart, lungs and liver. Most organs sit inside the body. The skin is the exception, because it covers the outside. Organs work together in organ systems, such as the circulatory system, digestive system, nervous system and respiratory system.

Anatomy

Main articles: Outline of human anatomy and Anatomy

Human anatomy is the study of the shape and form of the human body. The body has four limbs (two arms and two legs), a head, and a neck, all joined to the torso. Its shape comes from a strong skeleton made of bone and cartilage. Fat (adipose tissue), muscle, connective tissue, and organs surround the skeleton. The spine holds the flexible vertebral column, which surrounds the spinal cord. Nerves connect the spinal cord and brain to the rest of the body. Almost all major bones, muscles, and nerves have names, except for anatomical variations such as sesamoid bones and accessory muscles.

Blood vessels carry blood through the body, and the heart pumps it along. Venules and veins collect blood that is low in oxygen. This blood flows into the superior and inferior vena cava before entering the right side of the heart. From there, blood travels to the lungs to pick up oxygen. Then it returns to the left side of the heart and leaves through the artery, the aorta, and smaller arterioles that reach the tissues. Blood carries oxygen, waste products, and hormones. It is cleaned at the kidneys and liver. The body's body cavities hold different organ systems. The brain and central nervous system are protected by the blood brain barrier. The lungs sit in the pleural cavity, and the intestines, liver, and spleen sit in the abdominal cavity. Height, weight, shape, and other body proportions differ from person to person. They change with age and sex, and depend on the bones, muscles, and fat tissue.

Physiology

For broader coverage of this topic, see Physiology.

See also: Outline of physiology

Human physiology is the study of how the human body works. It looks at the mechanical, physical, bioelectrical, and biochemical functions of healthy people. It covers everything from organs down to the cells they are made of. The body has many systems of organs that work together to keep it in a stable state, called homeostasis. This keeps safe levels of things like sugar and oxygen in the blood.

Some systems work together and share joint names. For example, the nervous system and the endocrine system form the neuroendocrine system. The nervous system collects information from the body and sends it to the brain through nerve impulses and neurotransmitters. The endocrine system releases hormones, which help control blood pressure and volume. Together, these systems keep the inside of the body steady. This includes blood flow, posture, energy supply, temperature, and pH, which is the body's acid balance.

Development

Main article: Development of the human body

Development of the human body is the process of growing up to full maturity. It begins with fertilisation, when a sperm joins with an egg released from a female's ovary. The egg then settles in the uterus, where an embryo and later a fetus grow until birth.

Growth and development continue after birth. They include changes in the body and in the mind. These changes are shaped by genes, hormones, the environment, and other factors. This process continues through childhood, adolescence, and adulthood to old age. It is called aging.

Society and culture

Further information: History of anatomy, History of medicine, and History of physiology

Health professionals learn about the human body from pictures, models, and demonstrations. Medical and dental students also get hands-on experience, for example through dissection of bodies that have died. Human anatomy, physiology, and biochemistry are basic medical sciences. Medical students usually learn them in their first year. In Western societies, pictures of the human body are often made for information or art. Information includes science and education, such as anatomy drawings. Some images do not clearly fit these purposes, so people may misunderstand them, which can lead to arguments.

Main article: Depictions of nudity

Main article: History of anatomy

In Ancient Greece, the Hippocratic Corpus described the skeleton and muscles. Later, Galen of Pergamum collected what was known about anatomy into a text that was used for many centuries. During the Renaissance, Andreas Vesalius (1514–1564) helped start the modern study of human anatomy by dissection. The microscope later helped people study the tiny cell structure of tissues and organs. Modern tools such as magnetic resonance imaging, computed tomography, fluoroscopy and ultrasound imaging allow doctors and scientists to study the body in great detail. Physiology, the study of how the body works, began with Hippocrates in Ancient Greece. Aristotle (384–322 BCE) showed that the shape of a body part is linked to what it does. Galen was the first to use experiments to explore how the body works. The French doctor Jean Fernel (1497–1558) introduced the word physiology. In the 1600s, William Harvey (1578–1657) described the circulatory system. In 1838, Matthias Schleiden and Theodor Schwann proposed the cell theory, which says that living things are made up of cells. Claude Bernard (1813–1878) came up with the idea of the milieu interieur (internal environment). Walter Cannon (1871–1945) later said that this inner environment is kept steady through homeostasis. In the 1900s, Knut Schmidt-Nielsen and George Bartholomew extended these studies to comparative physiology and ecophysiology. More recently, evolutionary physiology has become a separate field of study.