Organ

An organ is a group of tissues working together to perform a specific function in the body.

Definition

An organ is a group of different tissues that work together to perform a specific function in the body. Organs are more complex than individual tissues: each organ contains several tissue types that contribute to its overall job. Organs themselves are grouped into organ systems — for example, the heart is part of the circulatory system, and the stomach is part of the digestive system. Together, the organ systems make up the complete organism.

The hierarchy of biological organisation is: cells → tissues → organs → organ systems → organism.

Key Characteristics

Made of two or more tissue types working together
Each organ has a specific function within the body
Organs are grouped into organ systems (e.g. digestive, circulatory, nervous)
The human body has around 78 recognised organs
Vital organs — such as the brain, heart, lungs, liver, and kidneys — are essential for survival
Organs can be studied at the level of their structure (anatomy) and their function (physiology)

Real-World Analogy

An organ is like a department in a large company. The heart is the "pump department," the lungs are the "gas exchange department," and the brain is the "control centre." Each department is made up of different workers (cells) with specialised roles, but they all cooperate to fulfil the department's goal. The company as a whole (the body) only works when every department functions properly.

Note

An organ is a group of tissues working together to carry out a specific body function. The human body contains around 78 organs organised into systems such as the circulatory, respiratory, digestive, nervous, and excretory systems. The heart, brain, lungs, liver, and kidneys are examples of vital organs.

Worked Examples

The heart as an organ

The heart is an organ made of several tissue types: cardiac muscle tissue (contracts rhythmically to pump blood), connective tissue (forms valves and the outer pericardium), and nervous tissue (the sinoatrial node generates the heartbeat signal). Together these tissues make the heart a self-contained pumping organ.

Organ transplantation

When a person's kidneys fail, doctors can perform a kidney transplant — replacing the damaged organ with a healthy one from a donor. This is only possible because an organ is a discrete, transplantable structure. A kidney filters blood and produces urine; no single tissue type alone can do this.

Practice Problems

Place the following in order from smallest to largest level of organisation: organ, organism, cell, organ system, tissue.

Show answer

Cell → tissue → organ → organ system → organism.

Name five human organs and state the organ system each belongs to.

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Heart — circulatory system; lungs — respiratory system; stomach — digestive system; brain — nervous system; kidneys — excretory (urinary) system. (Other correct answers accepted.)

Why must an organ contain more than one type of tissue?

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A single tissue type can only perform one function. To carry out a complex task — such as pumping blood or digesting food — an organ needs multiple tissue types that together provide structure, contraction, nerve signals, and blood supply.

The liver is described as a 'multifunctional organ'. Give three functions of the liver.

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Any three of: produces bile for fat digestion; detoxifies harmful substances (e.g. alcohol); stores glycogen; produces blood-clotting proteins; regulates blood glucose levels; breaks down old red blood cells.