Created by CK-12 Foundation/Adapted by Christine Miller

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 homeostasis. It also helps prevent damage to the body. Wastes include by-products of metabolism — 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 cellular respiration, ammonia and urea from protein catabolism, and uric acid from nucleic acid catabolism.

Organs of excretion include the skin, liver, large intestine, lungs, and kidneys (see Figure 16.2.2). Together, these organs make up the excretory system. 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.

Skin

The skin is part of the integumentary system, but it also plays a role in excretion through the production of sweat by sweat glands in the dermis. Although the main role of sweat production is to cool the body and maintain temperature homeostasis, 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

The liver (shown in Figure 16.2.4) has numerous major functions, including secreting bile for digestion of lipids, synthesizing many proteins and other compounds, storing glycogen 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 ammonia — a poisonous by-product of protein catabolism — into urea, which is filtered from the blood by the kidneys and excreted in urine. The liver also excretes in its bile the protein bilirubin, a byproduct of hemoglobin catabolism that forms when red blood cells die. Bile travels to the small intestine and is then excreted in feces by the large intestine.

Large Intestine

The large intestine 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. Bile secreted into the gastrointestinal tract, for example, contains the waste product bilirubin from the liver. Bilirubin is a brown pigment that gives human feces 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 cellular respiration in cells throughout the body. Carbon dioxide is diffused from the blood into the air in the tiny air sacs called alveoli in the lungs (shown in the inset diagram). By expelling carbon dioxide from the blood, the lungs help maintain acid-base homeostasis. 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.

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 urine. The main structural and functional units of the kidneys are tiny structures called nephrons. 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.

By producing and excreting urine, the kidneys play vital roles in body-wide homeostasis. 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.

Figure 16.2.3