Organ System: Definition And Examples

In biology, an organ system refers to a group of one or more organs arranged in a particular way that work together to perform some physiological function. The term “organ” refers to specialized collections of biological tissue that perform similar functions. Thus, an organ system can be considered any interacting network of specialized tissues that work together to perform a specific function.

According to the classical theory of biological organization, organ systems occupy the second-highest level of the organizational hierarchy, above that of the individual organs and below that of the whole organism.


“The human body is the most complex system ever created. The more we learn about it, the more appreciation we have about what a rich system it is.” — Bill Gates

Generally when one hears the term “organ,” one thinks primarily of the internal organs made out of fleshy smooth muscle that reside in the central abdominal cavity: things like the heart, lungs, liver, spleen, etc. Actually, the term “organ” refers to any specialized tissue that serves a particular function, so things like skin, muscles, and the skeleton count as organs that are part of organ systems. To be clear, an organ system is not just any collection of organs, but a collection of organs that are arranged a certain way to perform a certain function. Animals like humans have a number of different organ systems that each perform a function vital to life, such as the respiratory system, the circulatory system, and the nervous system. Plants too have organ systems, composed out of their roots, leaves, stamen, and seeds.

Examples Of Organ Systems

Circulatory System

The most obvious example of an organ system is the heart and the surrounding circulatory system. The circulatory system functions primarily to circulate blood to the various parts of the body. The primary components of the circulatory system are the heart, blood, and blood vessels such as arteries, veins, and capillaries. Humans have a closed circulatory system, meaning that their blood never leaves its network vessels, unlike the circulatory system of insects or mollusks. Through the pumping motion of the heart and vasoconstriction, blood is circulated from the lungs where it is oxygenated, to the various parts of the body where it is deoxygenated via diffusion, and back to the lungs where it is reoxygenated to start the process over again.

In addition to providing oxygen to the body, the circulatory system works as a sort of highway for hormones and other messenger chemicals of the endocrine system. Also, the circulatory system serves to keep the body warm as the circulation of blood circulates heat to the body. The circulatory system is closely related to the lymphatic system, which functions to fight infection and reintegrate plasma into the blood.


Respiratory System

“The human body is not a thing or substance, given, but a continuous creation. The human body is an energy system which is never a complete structure; never static; is in perpetual inner self-construction and self-destruction; we destroy in order to make it new.” — Norman O. Brown

The respiratory system refers to the collection of organs that facilitate gas exchange in animals and plants. In humans and most other mammals, the main constituents of the respiratory system are the lungs, trachea, bronchi, diaphragm, and alveoli. When the diaphragm contracts, the chest cavity expands causing the pressure inside the empty lungs to change. Air from outside rushes down the trachea to equalizes the thoracic pressure and is pulled into the lungs. Once in the lungs, inhaled air enters the alveoli, tiny sacs made of thin membranes surrounded by capillaries of the circulatory system. Oxygen in the inhaled air diffuses across the alveolar membrane into the capillaries of the circulatory system and into the blood. Carbon dioxide from the blood also diffuses into the air in the lungs. Then, the diaphragm is relaxed and the deoxygenated air is exhaled out of the lungs as the thoracic cavity contracts.

In plants, the main organs of the respiratory system are its leaves. On every leaf are tiny pores known as stomata that facilitate carbon dioxide exchange. Carbon dioxide from the atmosphere enters the plant through the stomata and is the main ingredient, along with sunlight,  in the process of photosynthesis. The plant then expels the waste product oxygen out of the same stomata.

Digestive System

The digestive system serves mainly to break down consumed food into nutrients for the body to absorb. The main organs implicated in the digestive system are the esophagus, stomach, small intestine, and large intestine. The process of digestion actually starts as soon as you put food into your mouth. The combination of chewing and saliva breaks down the food enough to be swallowed down the esophagus. Rhythmic contractions of the esophageal lining (known as “peristalsis”) transport food into the stomach, where it is exposed to numerous digestive acids. Mucus produces by stomach cells protect the inside of the stomach from gastric acids strong enough to dissolve stainless steel blades.

Once food has been processed in the stomach, it moves through the duodenum to the small intestines. While in the small intestine, the majority of nutrient absorption occurs. Tiny finger-like filaments on the inside of the intestinal wall called villi draw nutrients out of the digested food and the continued peristalsis pushes food further down the GI tract. Next, the digested material enters the large intestine where any water is absorbed and the remaining material is stored as feces which is later expelled through the rectum. The digestive system is the single longest organ system in the body as the small intestine alone is between 6-7 meters long; slightly longer than three average adult humans.


“Digestion is one of the most delicately balanced of all human and perhaps angelic functions.” — M. F. K. Fisher

There are a number of other organ systems found in humans such as the skeletal system meant for providing internal structure, the musculature system for locomotion and manipulation of the environment, the nervous system meant to let the brain communicate with the rest of the body, the endocrine system which sends messenger hormones to the body telling it how to behave, the reproductive system, and the integumentary system composed of skin, hair, fat, and nails.

In actuality, most organ systems do not have clearly defined boundaries and they all operate interdependently. The lymph system is extremely closely related to the circulatory system, and the activity of the respiratory system feeds directly into the circulatory system. None of the organ systems would work if the digestive system could not get energy from nutrients in food, and the digestive system would not be able to function if the nervous system could not send electrical signals from the brain to the intestines. So, the various organ systems of the human body form a complex interconnected network and cannot operate in isolation from each other. It is only when they are integrated into a complete biological organism do the organs systems perform their main functions

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