Body Systems

Body Systems Definition

Body systems are groups of organs and tissues that work together to perform important jobs for the body. Some organs may be part of more than one body system, if they serve more than one function.

All body systems are necessary for life to be sustained. In this article we’ll focus on the systems of the human body – similar systems are required by all animals, but the details of how they accomplish their tasks may vary.

Functions that must be performed by an animal to stay alive include:

  • Must be able to take in oxygen for use in cellular respiration, and excrete waste carbon dioxide.
  • Must be able to ingest and process food to obtain sugars and other nutrients.
  • Must be able to transport necessary substances, such as oxygen and nutrients, to all cells.
  • Must be able to clear toxic waste products from the body.
  • Must be able to respond to to the environment.
  • Must protect the body’s organs from the environment.
  • Must be able to fight infections.
  • For a species to survive, its individuals must be able to reproduce.

Below, we’ll see how our organs and tissues work together as body systems to accomplish these tasks.

List of Body Systems

Organ Systems

Organ Systems

  1. Respiratory System – Allows gas exchange between cells and the environment. Includes trachea and lungs.
  2. Digestive System/Excretory System – Ingests food and breaks it down into usable nutrients. Excretes solid waste products. Includes the mouth, esophagus, stomach, and intestines.
  3. Cardiovascular/Circulatory System – Moves materials between body systems, including oxygen, nutrients, hormones, and waste products. Includes the heart, arteries, and veins.
  4. Renal System/Urinary System – Cleans dissolved waste products from the blood and excretes them. Includes kidneys and bladder.
  5. Endocrine System – Secrets chemical signals that allow body systems to act cooperatively sa needed. Includes hormone-producing tissues of the pineal gland and pituitary gland in the brain; the thyroid gland; the adrenal glands; the pancreas; and the ovaries and testes.
  6. Nervous System – Allows perception, emotion, thought, and rapid response to the environment. Includes brain and nerves.
  7. Musculoskeletal system – Allows the body to move on command.
  8. Integumentary System/Exocrine System – Covers the body and regulates its exchange with the outside world. Includes skin, hair, nails, sweat, and other glands which secrete substances onto the skin.
  9. Lymphatic System/Immune System – Fights infection. Includes lymphatic vessels which permeate the body.
  10. Reproductive System – Allows the production of offspring. Includes ovaries, uterus, mammary glands (breasts), penis, and testes.

Functions of Body Systems

Respiratory System

The respiratory system takes oxygen from the environment and converts it into a form that cells can use.

In humans, that means that our lungs take in oxygen, and rapidly diffuse it into the blood. The lungs accomplish this by passing large amounts of blood over gas exchange membranes; the body’s whole blood volume passes over these membranes about once per minute!

It could be argued that the respiratory system is one of the body’s most important. Without oxygen to fuel cellular respiration, cells begin to die within minutes.

This is the real reason why heart attacks are deadly; although the heart is part of the circulatory system, not the respiratory system, it is responsible for transporting oxygen from lungs to our cells. When the circulatory system stops working, our tissues begin to die from lack of oxygen.

The lungs also expel carbon dioxide – a waste product of cellular respiration which could otherwise build up to toxic levels.

Digestive System/Excretory System

The digestive system takes in food and processes it to obtain useful nutrients.

One of the most important purposes of food is to serve as cellular fuel; carbohydrates, proteins, and fats can all be used by our cells to as sources of the energy they need to stay alive.

We can also get other important nutrients from food, such as essential amino acids (amino acids our bodies can’t make themselves), good fats, and vitamins and minerals that our cells need to keep their machinery in good working order.

When food enters the body, it is first chewed by the mouth to break it down into a mush that stomach acids can penetrate.

In the stomach, it is treated with acids and special enzymes that break the food’s components down into more useful forms.

Finally, it passes through the intestines: being squeezed through the huge surface area of the intestines’ narrow tubes ensures that as many useful nutrients are extracted from the food as possible.

The liver helps by releasing substances that assist the stomach and intestines in breaking down food, and by breaking down toxic substances in the blood.

Once these nutrients have been extracted from foods, they are distributed to the body’s cells by the circulatory system.

The digestive/excretory system also expels solid waste components of our food that our body can’t use in the form of fecal matter.

Cardiovascular/Circulatory System

The cardiovascular system is a highly efficient system for moving substances around the body. The body’s entire blood volume takes about a minute to circulate – making this a truly high-speed expressway for distributing oxygen, nutrients, messages, and removing waste.

The heart is the central pump of the circulatory system, sending blood to throughout the body at very high speeds. To ensure that we get enough oxygen, the heart even pumps blood through a special circuit to send large amounts of blood through the lungs quickly.

The arteries are the oxygen-delivery system that carry oxygenated blood through the body at high speeds and pressures. Arteries don’t merely contain the blood; they have walls of smooth muscle which contract to help the blood keep going, even far away from the heart. This is why injuries to arteries are so dangerous; if an artery is injured, the body’s whole blood volume can drain out through it very fast!

The veins return blood to the heart after its oxygen has been removed. The blood in veins moves a bit slower and at lower pressures.

At the finest level of the circulatory system, tiny blood vessels called capillaries carry blood all throughout the tissues. By passing blood flow close to every cell, the capillaries ensure efficient delivery of needed substances. Most bleeding from superficial cuts comes from blood seeping from these tiny, often microscopic, blood vessels.

In addition to oxygen and nutrients, the circulatory system also transports chemical messages, such as hormones, around the body. This allows the adrenal glands, for example, to send messages that cause our whole body to prepare for fight or flight.

Lastly, of course, the circulatory system performs the vital task of carrying waste products away from our cells. It delivers carbon dioxide to the lungs, and other toxins to the liver and kidneys to be destroyed or excreted.

Renal System/Urinary System

The renal/urinary system keeps our body healthy by removing dangerous waste products from our blood, and expelling them from our body in the form of urine.

All blood is passed through the kidneys, where special filters allow dangerous substances to pass out of the bloodstream, while keeping helpful substances in.

The waste liquid that’s filtered out by the kidneys is stored in the bladder until the body expels it.

Endocrine System

The endocrine system consists of a number of tissues that send out chemical messages – called ‘hormones’ – to the rest of the body. Each of these messages has its own unique purpose, to which the body’s other systems respond accordingly.

The endocrine system allows the body to respond to environmental changes, and to other types of survival changes, such as the need to reproduce. Some examples of messages sent by the endocrine system are:

  • Fight or flight – When a threat appears in the environment, the adrenal glands secrete adrenaline. In answer to this chemical message, the heart pumps blood faster; breathing deepens to take in more oxygen; and the nervous system sharpens perception and memory formation. Other changes also occur to make the body ready to fight or flee from a potential threat.
  • Reproductive signals – When the body is ready to reproduce, the ovaries or testes send chemical messages that affect other organs, including the brain. For the female reproductive system, preparing the uterus for pregnancy involves a complex cascade of chemical messages that repeat on a monthly cycle.
  • Hungry or full – When the body is hungry, your stomach releases a hormone that tells the brain and other organ systems to start seeking food. When the body is full, on the other hand, another set of hormone messengers go out telling the body it can stop eating.

Nervous System

The nervous system allows us to sense stimuli such as light, sound, smell, and touch from our environment. It also allows rapid communication of stimuli within our body, such as sensations of pain, illness, and wellness.

It also gives us the brain – a huge central processing unit that combines these stimuli into unified experiences, and performs tasks such as recording memories, producing emotional responses, and thinking.

The last important function of the nervous system is to allow our brain to send signals to back to our body, enabling us to respond to environmental stimuli.

The brain can be thought of as the control center that receives data, analyzes it, and then commands the body to respond.

The nervous system accomplishes all of this using highly specialized cells called neurons, which can transmit signals extremely fast by firing electrochemical potentials.

In order to fire these signals, neurons must use huge amounts of energy – as much as 25% of the calories we eat are used by the nervous system to allow us to perceive, feel, think, and respond!

Some scientists believe that human ancestors were not able to become smart until we were able to meet the huge energy needs of a big brain. Our ancestors were able to meet these needs by becoming good hunters, good cooks – which makes food easier to digest – and eventually developing agriculture.

Muscular System

The system of muscles throughout an organism operates to move the organism and stimulate the internal organs. There are several main types of muscles in mammal: smooth muscle, skeletal muscle, and cardiac muscle.

Cardiac muscle is the muscle surrounding the heart, and has the most important function within the circulatory system. Cardiac muscle is different from both smooth muscle and skeletal muscle, and is adapted to make contractions continuously. Smooth muscle is better at squeezing and holding, while skeletal muscle is best adapted to short bursts of strenuous exercise.

Smooth muscle covers many internal organs, and is responsible for holding certain passages shut, erecting hairs, and even moving food through the gut in an action called peristalsis. Smooth muscle is generally controlled by the subconscious, or autonomous nervous system. In a few cases, these muscles can be controlled voluntarily. In others, they are completely automatic. That is in direct contrast to skeletal muscles, which are almost entirely controlled by the somatic nervous system.

Skeletal muscles work by attaching to the skeleton and contracting or relaxing. When you make a fist, the muscles on the insides of your fingers are contracting, while the outside muscles relax and stretch out. When you fold your fingers out flat, the backside muscles are contracting. These opposite groups of muscles rely on the skeletal system to create forces. Thus, the systems are often combined and called the musculoskeletal system.

Skeletal System

The skeletal system of animals consists of either an endoskeleton, like mammals, or an exoskeleton, seen in insects and other arthropods. Some animals also use water-pressure as a form of skeleton, known as a hydrostatic skeleton. Whatever form of skeleton is used, the skeletal system has the same purpose, to provide support and attachment to for the muscles.

With endo and exoskeletons, the muscles attach directly to the skeleton, through tendons and other connective tissues. This allows the muscles to pull on the skeleton, creating opposing forces. These forces allow the limbs to move. While a crab and a monkey seem very different, their skeletons are working in similar ways. To move a limb, muscles on one side must be extended, while on the other side they are shortened. Thus, the crab and monkey move their limbs in the same way, even though the crab wears its skeleton on the outside of its body. With a hydrostatic skeleton, muscles attach to the skin and squeeze pockets of liquid to create movement. This is how many mollusks, like snails and octopi, move around.

Besides support and attachment for the muscles the skeletal system is also a very important protective measure. This is clearly seen in animals with exoskeletons. A crab or insect is always protected by their thick layer of armored skeleton. In animals with endoskeletons, the effect is the same but less obvious. Our cranium in a series of interconnected bones which serve to encase and protect our brain from damage. The rib cage is a series of bones which extends around the thoracic cavity to protect the heart and lungs. Therefore, our skeleton also protects our most vital organs.

Integumentary System/Exocrine System

Like our bones and muscles, the job of our skin may seem mundane – but it is very important! Skin keeps our other organs in, and everything else out.

Skin is our body’s first line of defense against bacteria, viruses, injuries, and more. It also controls how much heat and water our body loses to the environment, allowing us to sweat. Even goosebumps are part of our skin’s regulation system; the tightening of the skin raises our fine hairs upright, trapping warm air close to our skin.

The skin is a surprisingly complex material, which scientists have not been able to reproduce artificially. This is because it is a living tissue, which is constantly maintained by the nourishing circulatory system underneath; and by a number of glands on the outside of our skin, which secrete oils and other substances that keep our skin from drying and cracking.

Fun fact: skin is also the largest organ in the body. Or perhaps we should say, the largest organ on the body.

Lymphatic System/Immune System

Every living thing needs to be able to fight infection.

This is because for every organism that’s made of delicious carbohydrates, proteins, and lipids; there’s another organism that wants to eat it. Some of these organisms are big predators, but most are microscopic pathogens that can easily fit inside us, instead of fitting us inside of them.

In the case of animals, we have white blood cells that can specially target and destroy invading pathogens. These white blood cells are made in our bone marrow and stored in our blood and our lymphatic systems.

The lymphatic system is a circulatory system separate from the cardiovascular system that that carries water, white blood cells, and other substances. It does not have red blood cells or platelets. Because it is not the body’s main carrier of oxygen, lymph can move more slowly than the bloodstream – giving the white blood cells more time to find and attack invaders.

“Lymph nodes” are nexuses in the lymphatic system where white blood cells can cluster and attack invading pathogens. Sometimes when we’re sick, our lymph nodes – such as those behind the ears, under the jaw, and in the armpits and groin- can become painful and swollen as our immune systems fight the infections in these nodes.

Our immune systems are very, very good at their jobs. If you want to know how good, look at patients with severely compromised immune systems. People without functioning immune systems can contract fatal infections just from walking around in everyday environments without protection; prolonged lack of an immune system is usually fatal.

Reproductive System

The reproductive system is not essential to individual survival, but it is essential for the survival of the species. After all, a species whose members couldn’t reproduce would not last very long!

In humans, there are two very different reproductive systems: the male system, which is concerned primarily with producing sperm and finding mates; and the female system, which must prepare for pregnancy, childbirth, and baby care for reproduction to be successful.

The female reproductive system is a particularly fascinating study in the way body systems work together to ensure our survival. Throughout the course of a woman’s monthly cycle, her body uses four different hormones – most of which are produced by her ovaries – to decide when and whether her body should prepare for pregnancy.

The major effects of the reproductive hormones are on the reproductive organs themselves, which must bring eggs for maturity and prepare uterine lining, rich in blood vessels, to nurture a possible embryo.

But other effects are also seen in other organ systems. As a woman’s cycle progresses, her hormones may effect her body temperature; blood flow; and even her appetite and her attraction to the opposite sex, to ensure that all the right resources are in place at the right time.

Women in some parts of the world have been known to develop bizarre eating habits due to the demands of their reproductive cycle. In areas with poor nutrition, for example, the monthly shedding of blood-rich uterine lining can cause deficiencies of the minerals found in blood. As a result, women in these areas may actually eat clay from the ground to ensure these minerals are replenished! Their bodies, through some set of chemical signals, simply know what to do.

We may not see such clear examples of our body systems interacting on a daily basis; but the organs and tissues that make up our body systems are always communicating, and working together, to keep us and our species alive and healthy.


1. Which of the following is NOT a basic survival need of animals?
A. The need for oxygen to power cellular respiration.
B. The need for food to power cellular respiration, and keep cellular machinery in good working order.
C. The need to respond appropriately to environmental conditions.
D. None of the above.

Answer to Question #1
D is correct. All of the above are survival needs of animals.

2. Which of the following is NOT a major organ system?
A. Circulatory system
B. Respiratory system
C. Auditory system
D. Nervous system

Answer to Question #2
C is correct. The auditory system is the name for the part of the nervous system that processes sound. But it is not a major organ system and is not, strictly speaking, necessary for survival.

3. Which of the following organ systems could we survive without for the longest period of time?
A. Digestive system
B. Reproductive system
C. Respiratory system
D. Cardiovascular system

Answer to Question #3
B is correct. An individual without a reproductive system would be able to live until they died of natural causes. Of course, the human life expectancy of ~85 years is nothing in evolutionary terms. A population that could not reproduce would not last long!


  • (n.d.). Retrieved April 23, 2017, from[email protected]/Circulatory-Pathways
  • Pocock, Gillian (2006). Human Physiology (Third ed.). Oxford University Press. p. 382. ISBN 978-019-856878-0.
  • Vander, Arthur (2008). Vander’s Human Physiology: the mechanisms of body function. Boston: McGraw-Hill Higher Education. pp. 332–333.
  • Marieb, Elaine; Katja Hoehn (2007). Human Anatomy & Physiology (7th ed.). Pearson Benjamin Cummings. p. 142.
  • Sherwood, L., & Ward, C. (2015). Human physiology: from cells to systems. Toronto: Nelson Education.
  • “Nervous System”. Columbia Encyclopedia. Columbia University Press.
  • West, John B. Respiratory physiology– the essentials. Baltimore: Williams & Wilkins. pp.110. ISBN 0-683-08937-4.
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