Body, Human

The human body is a very complex set of systems that can withstand enormous challenges as well as accomplish dramatic feats. It can be described chemically, biologically, physically, and in terms of its functions. Chemically, the body is mostly water with additional elements including calcium, nitrogen, oxygen, sodium chloride, iron, phosphorus, potassium, trace minerals, and carbon. The human body is composed of a number of organic compounds that include carbohydrates, lipids, nucleic acids, and proteins.

The carbohydrates supply energy to the body's cells. The body is maintained by energy derived from chemical reactions with carbohydrates. Lipids are fats that store energy for future use. Some lipids are the material used by the body to make the living cells of the body. Nucleic acids supply cells with instructions to perform their jobs. Proteins also serve as building blocks for cells. Some proteins are enzymes, which perform a variety of functions such as speeding chemical reactions within the body.

Biologically, the human body is composed of cells, which is the basic unit of living things. Cells are mostly composed of proteins and nucleic acids in addition to water. The cells in the human body perform many functions such as providing food and oxygen, eliminating wastes, defending against disease organisms, and regulating body temperature. They stimulate growth and other activities as well. Each cell, such at blood cells, muscle cells, or brain cells, have unique features.

There are four major kinds of tissue cells. Connective tissue cells join together various parts of the body and also provide support. Most of the connective tissues, such as those attaching bones to muscles, are very strong and can withstand rugged use. The elastic connective tissues attach muscles to bones, or in the case of cartilage, support bones and act as support for motion.

Muscle tissue is fibrous and threadlike, and can contract or stretch in actions that enable a variety of motions, such as work or play. Nervous tissue transmits electro-chemical impulses that act as signals to the brain, muscles, sensory organs, and to other parts of the body. The epithelial tissue covers the body with skin and orifices such as the mouth and throat with a lining. The epithelial tissues protect the body from invasive organisms or from harmful substances.

Some of the tissues in the human body are organized into organs, which perform specialized functions. Groups of organs create networks to perform major functions in the body, such as the digestive system and the nervous system. Organs are composed of two or more types of tissue. The heart is an organ composed of muscle tissue, nervous tissue, and connective tissue.

The human body can be compared to a complex process organized into systems. The major systems include the skeletal, muscular, nervous, respiratory, circulatory, digestive, lymphatic, urinary, endocrine, reproductive, and epidermis (integumentary).

The skeletal system is the body's framework. It supports the body, protects its vital organs, and enables it to do activities. The skeletal system is composed of 206 bones. Marrow forms the inside of bones and serves as an intricate microstructure fed by the blood. Bones also make red blood cells.

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