139 16.2 Organs of Excretion

Created by CK-12 Foundation/Adapted by Christine Miller

16.2.1 Chimneys
Figure 16.2.1 Chimney view.

Getting Rid of Wastes

The many chimneys on these houses are one way that the inhabitants of the home get rid of the wastes they produce. The chimneys expel waste gases that are created when they burn fuel in their furnace or fireplace. Think about the other wastes that people create in their homes and how we dispose of them. Solid trash and recyclables may go to the curb in a trash can, or in a recycling bin for pick up and transport to a landfill or recycling centre. Wastewater from sinks, showers, toilets, and the washing machine goes into a main sewer pipe and out of the house to join the community’s sanitary sewer system.

Like a busy home, your body also produces a lot of wastes that must be eliminated. Like a home, the way your body gets rid of wastes depends on the nature of the waste products. Some human body wastes are gases, some are solids, and some are in a liquid state. Getting rid of body wastes is called excretion, and there are a number of different organs of excretion in the human body.

Excretion

 is the process of removing wastes and excess water from the body. It is an essential process in all living things, and it is one of the major ways the human body maintains . It also helps prevent damage to the body. Wastes include by-products of — some of which are toxic — and other non-useful materials, such as used up and broken down components. Some of the specific waste products that must be excreted from the body include carbon dioxide from , and  from protein catabolism, and  from nucleic acid catabolism.

Excretory Organs

Organs of excretion include the , , , , and (see Figure 16.2.2). Together, these organs make up the . They all excrete wastes, but they don’t work together in the same way that organs do in most other body systems. Each of the excretory organs “does its own thing” more-or-less independently of the others, but all are necessary to successfully excrete the full range of wastes from the human body.

Figure 16.2.2 Internal organs of excretion are identified in this illustration. They include the skin, liver, large intestine, lungs, and kidneys.

Skin

16.2.3 Sweating
Figure 16.2.3 The purpose of sweat production is mainly to cool the body and prevent overheating, but it also contributes to excretion.

The is part of the integumentary system, but it also plays a role in excretion through the production of by sweat glands in the dermis. Although the main role of sweat production is to cool the body and maintain temperature , sweating also eliminates excess water and salts, as well as a small amount of urea. When sweating is copious, as in Figure 16.2.3, ingestion of salts and water may be helpful to maintain homeostasis in the body.

Liver

16.2.4 LIver
Figure 16.2.4 The liver is an organ of excretion.

The liver (shown in Figure 16.2.4) has numerous major functions, including secreting for digestion of lipids, synthesizing many proteins and other compounds, storing and other substances, and secreting endocrine hormones. In addition to all of these functions, the liver is a very important organ of excretion. The liver breaks down many substances in the blood, including toxins. For example, the liver transforms — a poisonous by-product of protein — into , which is filtered from the blood by the kidneys and excreted in urine. The liver also excretes in its bile the protein , a byproduct of that forms when red blood cells die. Bile travels to the small intestine and is then excreted in by the .

Large Intestine

The is an important part of the digestive system and the final organ in the gastrointestinal tract. As an organ of excretion, its main function is to eliminate solid wastes that remain after the digestion of food and the extraction of water from indigestible matter in food waste. The large intestine also collects wastes from throughout the body. secreted into the gastrointestinal tract, for example, contains the waste product from the liver. Bilirubin is a brown pigment that gives human its characteristic brown colour.

Lungs

The lungs are part of the respiratory system (shown in Figure 16.2.5), but they are also important organs of excretion. They are responsible for the excretion of gaseous wastes from the body. The main waste gas excreted by the lungs is carbon dioxide, which is a waste product of in cells throughout the body. Carbon dioxide is diffused from the blood into the air in the tiny air sacs called in the lungs (shown in the inset diagram). By expelling carbon dioxide from the blood, the lungs help maintain acid-base . In fact, it is the pH of blood that controls the rate of breathing. Water vapor is also picked up from the lungs and other organs of the respiratory tract as the exhaled air passes over their moist linings, and the water vapor is excreted along with the carbon dioxide. Trace levels of some other waste gases are exhaled, as well.

16.2.5 Respiratory System
Figure 16.2.5 The alveoli are the functional structures in the lungs where gaseous wastes enter the air from the blood.

Kidneys

The paired kidneys are often considered the main organs of excretion. The primary function of the kidneys is the elimination of excess water and wastes from the bloodstream by the production of the liquid waste known as . The main structural and functional units of the kidneys are tiny structures called nephrons.  filter materials out of the blood, return to the blood what is needed, and excrete the rest as urine. As shown in Figure 16.2.6, the kidneys are organs of the urinary system, which also includes the ureters, bladder, and urethra — organs that transport, store, and eliminate urine, respectively.

16.2.6 Urinary System
Figure 16.2.6 The urinary system consists of two kidneys and the structures that transport and store urine.

By producing and excreting urine, the kidneys play vital roles in body-wide . They maintain the correct volume of extracellular fluid, which is all the fluid in the body outside of cells, including the blood and lymph. The kidneys also maintain the correct balance of salts and pH in extracellular fluid. In addition, the kidneys function as endocrine glands, secreting hormones into the blood that control other body processes. You can read much more about the kidneys in section 16.4 Kidneys.

16.2 Summary

  • is the process of removing wastes and excess water from the body. It is an essential process in all living things and a major way the human body maintains .
  • Organs of excretion include the skin, liver, large intestine, lungs, and kidneys. All of them excrete wastes, and together they make up the .
  • The plays a role in excretion through the production of sweat by sweat glands. Sweating eliminates excess water and salts, as well as a small amount of , a byproduct of protein catabolism.
  • The is a very important organ of excretion. The liver breaks down many substances in the blood, including toxins. The liver also excretes — a waste product of — in bile. Bile then travels to the , and is eventually excreted in by the .
  • The main excretory function of the large intestine is to eliminate solid waste that remains after food is digested and water is extracted from the indigestible matter. The large intestine also collects and excretes wastes from throughout the body, including bilirubin in .
  • The are responsible for the excretion of gaseous wastes, primarily carbon dioxide from  in cells throughout the body. Exhaled air also contains water vapor and trace levels of some other waste gases.
  • The paired are often considered the main organs of excretion. Their primary function is the elimination of excess water and wastes from the bloodstream by the production of urine. The kidneys contain tiny structures called  that filter materials out of the blood, return to the blood what is needed, and excrete the rest as . The kidneys are part of the urinary system, which also includes the ureters, urinary bladder, and urethra.

16.2 Review Questions

  1. What is excretion, and what is its significance?
  2. Describe the excretory functions of the liver.
  3. What are the main excretory functions of the large intestine?
  4. List organs of the urinary system.
  5. Describe the physical states in which the wastes from the human body are excreted.
  6. Give one example of why ridding the body of excess water is important.
  7. What gives feces its brown colour? Why is that substance produced?

16.2 Explore More

Why Can We Regrow A Liver (But Not A Limb)? MITK12Videos, 2015.

Are Sports Drinks Good For You? | Fit or Fiction, POPSUGAR Fitness, 2014.

Why do we sweat? – John Murnan, TED-Ed, 2018.

 

Attributions

Figure 16.2.1

Chimneys/ Kingston upon Hull, England [photo] by Angela Baker on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Figure 16.2.2

Figure 16.2.3

gym_room_fitness_equipment_cardiovascular_exercise_elliptical_bike_cardio_training_sports_equipment_bodybuilding-825364 from Pxhere is used under a CC0 1.0 Universal public domain dedication license (https://creativecommons.org/publicdomain/zero/1.0/).

Figure 16.2.4

Tags: Liver Organ Anatomy by zachvanstone8 on Pixabay is used under the Pixabay License (https://pixabay.com/service/license/).

Figure 16.2.5

Lung_and_diaphragm by Terese Winslow/ National Cancer Institute on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 16.2.6

512px-Urinary_System_(Female) by BruceBlaus on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.


References

MITK12Videos. (2015, June 4). Why can we regrow a liver (but not a limb)? https://www.youtube.com/watch?v=erMCADOJcHk&feature=youtu.be

POPSUGAR Fitness. (2014, February 7). Are sports drinks good for you? | Fit or Fiction. YouTube. https://www.youtube.com/watch?v=SeK0zFB9yHg&feature=youtu.be

TED-Ed. (2018, May 15). Why do we sweat? – John Murnan. YouTube. https://www.youtube.com/watch?v=fctH_1NuqCQ&feature=youtu.be

License

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Human Biology by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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