Organ

views updated

Organ

An organ is a structural part of a plant or animal that carries out a certain function and is made up of two or more types of tissue. Each organ plays a key role in keeping the organism alive. A group of organs that work together to do a certain job is called an organ system.

Very often, a person's liver, heart, brain, and kidneys (among others) are referred to as internal organs, or vital organs. This is because they are usually inside the body (although some, like eyes, are not), and they are vital, or essential, to keeping us alive. The word vital can also mean living or alive. Plants also have many kinds of vital organs, such as leaves, stems, roots, and the various parts of a flower. However, whether in plants or animals, an organ is usually composed of two or more types of tissue that work together to do a particular job. For example, the heart is an organ whose job it is to pump blood throughout the body. It is made up of muscle tissue that contracts, nerve tissue that transmits impulses, connective tissue that binds it together, and epithelial tissue that lines its surfaces. Tissue is considered to be a group of similar cells that all do the same job. Some organs, like the heart, have only one job to do, while others, like the liver, may have many tasks to perform.

When a group of organs are linked together to perform a certain function, it is called an organ system. In large, complex animals, there are ten major organ systems that work together to make up the organism. The main organ systems of complex animals include the skeletal system, nervous system, circulatory system, respiratory system, muscular system, digestive system, excretory system, endocrine system, reproductive system, and integumentary system.

The skeletal system consists of bones and cartilage and helps support and protect the body. The nervous system consists of the brain, spinal cord, sense organs, and nerves. It collects, processes, and distributes information. The circulatory system includes the heart and blood vessels. It transports essential materials throughout the body. The respiratory system is made up of the lungs and air passageways. It supplies necessary oxygen while removing carbon dioxide (a major atmospheric gas). The muscular system consists of large, skeletal muscles that contract, as well as cardiac muscle and the smooth muscle. The digestive system includes the mouth, stomach, intestines and other organs that break down and absorb food. The excretory system consists of the kidneys, bladder, and other ducts, or tubes, that remove waste from the blood. The endocrine, or glandular, system consists of the thyroid, pituitary, mammary, and other glands. These glands release hormones (chemical messengers) into the circulatory system to regulate metabolic activities (all the body's chemical processes). The reproductive system is made up of the testes, ovaries, penis, vagina, and uterus. It passes on genes to its offspring. The integumentary system consists of the skin, hair, and nails that serve to protect the body and to regulate its temperature, as well as to receive stimuli.

The organs of a plant are its leaves, stems, roots, and the various parts of its flowers. A flower is actually an example of an organ system in a plant, since it is usually composed of stamens, pistils, petals, bracts, and receptacle. The stamens are its male reproductive organ, and are composed of anthers and filaments. The pistil is the female reproductive organ and consists of a stigma, style, and ovary.