Excretory system

The excretory system is a system of organs that removes waste products from the body. When cells in the body break down proteins (large molecules that are essential to the structure and functioning of all living cells), they produce wastes such as urea (a chemical compound of carbon, hydrogen, nitrogen, and oxygen). When cells break down carbohydrates (compounds consisting of carbon, hydrogen, and oxygen and used as a food), they produce water and carbon dioxide as waste products. If these useless waste products are allowed to accumulate in the body, they would become dangerous to the body's health. The kidneys, considered the main excretory organs in humans, eliminate water, urea, and other waste products from the body in the form of urine.

Other systems and organs in the body also play a part in excretion. The respiratory system eliminates water vapor and carbon dioxide through exhalation (the process of breathing out). The digestive system removes feces, the solid undigested wastes of digestion, by a process called defecation or elimination. The skin also acts as an organ of excretion by removing water and small amounts of urea and salts (as sweat).

Urinary system

The kidneys are bean-shaped organs located at the small of the back near the spinal column. The left kidney sits slightly higher than the right one. The size of an adult kidney is approximately 4 inches (10 centimeters) long and 2 inches (5 centimeters) wide. To maintain human life, it is necessary for at least one of the kidneys to function properly.

Blood carries waste products to the kidneys via the renal artery. Inside each kidney, blood is transported to 1.2 million filtering units called nephrons (pronounced NEFF-rons). The cells in nephrons take in the liquid portion of the blood and filter out impurities (urea, mineral salts, and other toxins). Necessary substances such as certain salts, water, glucose (sugar), and other nutrients are returned to the blood stream via the renal vein.

Words to Know

Antidiuretic hormone: Chemical secreted by the pituitary gland that regulates the amount of water excreted by the kidneys.

Hemodialysis: Process of separating wastes from the blood by passage through a semipermeable membrane.

Nephron: Filtering unit of the kidney.

Urea: Chemical compound of carbon, hydrogen, nitrogen, and oxygen produced as waste by cells that break down protein.

Ureter: Tube that carries urine from a kidney to the urinary bladder.

Urethra: Duct leading from the urinary bladder to outside the body through which urine is eliminated.

The waste-containing fluid that remains in the nephrons is called urine. Urine is 95 percent water, in which the waste products are dissolved. A pair of tubes called ureters carry urine from the kidneys to the urinary bladder. Each ureter is about 16 to 18 inches (40 to 45 centimeters) long. The bladder is a hollow muscular sac located in the pelvis that is collapsed when empty, but pear-shaped and distended when full. The bladder in an adult can hold more than 2 cups (0.6 liters) of urine. The bladder empties urine into the urethra, a duct leading to outside the body. In males, the urethra is about 8 inches (20 centimeters) long. In females, it is less than 2 inches (5 centimeters) long. A sphincter muscle around the urethra at the base of the bladder controls the flow of urine between the two.

The volume of urine excreted is controlled by the antidiuretic hormone (ADH), which is released by the pituitary gland (a small gland lying at the base of the skull). If an individual perspires a lot or fails to drink enough water, special nerve cells in the hypothalamus (a region of the brain controlling body temperature, hunger, and thirst) detect the low water concentration in the blood. They then signal the pituitary gland to release ADH into the blood, where it travels to the kidneys. With ADH present, the kidneys reabsorb more water from the urine and return it to the blood. The volume of urine is thus reduced. On the other hand, if an individual takes in too much water, production of ADH decreases. The kidneys do not reabsorb as much water, and the volume of urine is increased. Alcohol inhibits ADH production and therefore increases the output of urine.

Urinary disorders

Disorders of the urinary tract (kidneys, ureters, bladder, and urethra) include urinary tract infections (UTI). An example is cystitis, a disease in which bacteria infect the urinary bladder, causing inflammation. Most UTIs are treated with antibiotics. Sometimes kidney stones, solid salt crystals, form in the urinary tract. Kidney stones can obstruct the urinary passages and cause severe pain and bleeding. If they do not pass out of the body naturally, a physician may remove them surgically or disintegrate them using shock waves.

Chronic renal failure is the permanent loss of kidney function. Hemodialysis and kidney transplant are two medical treatments for this condition. In hemodialysis, an artificial kidney device cleans the blood of wastes. During the procedure, blood is taken out of an artery in the patient's arm and passed through a tubing that is semipermeable (allows certain materials to pass through its sides). The tubing is immersed in a solution. As the blood passes through the tubing, wastes pass out of the tubing and into the surrounding solution. The cleansed blood then returns to the body. In a kidney transplant, a surgeon replaces a diseased kidney with a closely matched donor kidney. Although about 23,000 people in the United States wait for donor kidneys each year, fewer than 8,000 receive kidney transplants.

[See also Digestive system; Integumentary system; Respiratory system; Transplant, surgical ]

Excretory Systems

Most animals require some system to excrete the waste products of metabolism from the body fluids. Kidneys are the major organs of the excretory systems of humans and other vertebrates, but several other kinds of excretory organs occur in other kinds of animals.

Functions and Principles

Besides metabolic wastes, kidneys and other excretory organs also eliminate excess water, ions , or other substances that are taken in with food. Moreover, although they are referred to as excretory systems, what they retain in the body fluids is just as important as what they excrete. They are best viewed as systems that maintain a constant, or homeostatic, composition of the body fluids.

Water and ions are among the important components of the body fluids that must be maintained homeostatically. Cells will shrink if placed in a fluid that has too high an ion concentration because water will be drawn out of the cell by osmosis . Cells will swell in a fluid that is too dilute because water will be drawn into them by osmosis. Maintaining a balance of water and ions—osmoregulation—is another major homeostatic function of the excretory organs of most animals.

With few exceptions, the osmoregulatory and excretory functions are combined in the same organ, which generally works in two stages. First the organ filters blood or another body fluid into a tubule. Then as the filtrate passes through the tubule, needed molecules are pumped out of it and back into the body fluid. At the same time, metabolic wastes and excess water and other molecules in the body fluids are pumped into the filtrate by active transport. The resulting fluid, called urine, is eliminated through the open end of the tubule outside the body.

Variety in the Animal Kingdom

In flatworms and a few kinds of invertebrates the excretory/osmoregulatory organ is a protonephridium. By definition, the protonephridium has a tubule that is open only at the end leading outside the body. The other end of the protonephridium has cilia or a flagellum that draws body fluid in to form the filtrate. In most kinds of invertebrates, including earthworms, the organ is called the metanephridium (or sometimes simply nephridium). In the metanephridium the tubule is open at both ends, and the pressure of the body fluid forces the filtrate into the tubule.

Insects, crustaceans, spiders, and other arthropods have different types of excretory/osmoregulatory organs, but they too operate by filtration and active transport. In insects and spiders the main organs are Malpighian (pronounced mal-PIG-ee-yan) tubules, which are attached to the gut. For these terrestrial animals the problem is to conserve water, rather than eliminate it. Water, containing metabolic wastes and excess ions, is filtered into the Malpighian tubules and then joins the feces in the hind part of the gut. As the feces passes through the rectum, the water is pumped back into the body fluid.

In vertebrates the excretory/osmoregulatory organ is the kidney. The vertebrate kidney contains thousands or even millions of tubules, called nephrons , each one of which uses the same principles of filtration and active transport. One difference between kidneys and most other excretory/osmoregulatory organs is that kidneys filter blood rather than some other body fluid. Nephrons form a filtrate consisting of water, nutrients, ions, and other components of blood except cells and very large molecules. As the filtrate passes along the nephron tubule, nutrients, water, and other needed molecules are transported back into the bloodstream. Ions, metabolic wastes, and other excess molecules go into the resulting fluid. Finally, more water is removed from the fluid, transforming it into urine.

Exceptions

There are two exceptions to the generalization that excretion and osmoregulation are performed by the same systems. The first exception occurs in animals that spend their entire lives in the fairly constant environment of the ocean. For them, osmoregulation is not needed because the balance of water and ions in the oceans is osmotically suitable for cells. (This is not surprising, considering that life most likely evolved in the oceans.) A starfish, for example, does not have or normally need an osmoregulatory system. If a starfish is placed in water with a higher concentration of salts than in the ocean, however, water will be drawn out of it by osmosis, and it will shrink. Conversely, if the starfish is placed in fresh water, it will swell and burst as water is drawn into the more concentrated body fluids.

The second exception to the rule that osmoregulation and excretion are performed by the same system occurs in some sponges and other relatively simple animals that live in fresh water. These animals osmoregulate by collecting excess water in each cell within a chamber called the contractile vacuole. When full, the contractile vacuole contracts and expels the water through the plasma membrane of the cell. The contractile vacuole is also called the water expulsion vesicle.

see also Insect; Kidney; Osmoregulation; Porifera

C. Leon Harris

Bibliography

Hickman, Cleveland P., Jr., Larry S. Roberts, and Allan Larson. "Homeostasis." In Integrated Principles of Zoology, 11th ed. Boston: McGraw-Hill Higher Education, 2001.

Saladin, Kenneth S. "The Urinary System" and "Water, Electrolyte, and Acid-Base Balance." In Anatomy and Physiology. Boston: McGraw-Hill Higher Education, 2001.