The excretory system removes cellular wastes and helps maintain the salt and water balances in an organism. In providing these functions, excretion contributes to the body's homeostasis, the maintenance of constancy of the internal environment. When cells break down proteins, they produce nitrogenous wastes, such as urea. The excretory system serves to remove these nitrogenous waste products, as well as excess salts and water, from the body.
When cells break down carbohydrates during cellular respiration, they produce water and carbon dioxide as a waste product. The respiratory system gets rid of carbon dioxide every time we exhale. The digestive system removes feces, the solid undigested wastes of digestion, by a process called elimination or defecation. Organisms living in fresh and saltwater have to adjust to the salt concentration of their aqueous environment, and terrestrial animals face the danger of drying out. It is the job of the excretory system to balance salt and water levels in addition to removing wastes.
Different organisms have evolved a number of organs for waste removal. Protozoans, such as paramecium, have specialized excretory structures; flatworms, which lack a circulatory system, have a simple excretory organ; earthworms, grasshoppers, and humans, have evolved an excretory system that works with the circulatory system.
Nitrogenous waste products have their origin in the breakdown of proteins by cells. Cells catabolize amino acids to obtain energy. The first step of this process is deamination. During deamination, enzymes remove the amino group as ammonia (NH3). Ammonia is toxic, even at low concentrations, and requires large amounts of water to flush it out of the body. Many animals, including humans, create a less poisonous substance, urea, by combining ammonia with carbon dioxide. An animal can retain urea for some time before excreting it, but it requires water to remove it from the body as urine. Birds, insects, land snails, and most reptiles convert ammonia into an insoluble substance, uric acid. This way, water is not required water to remove urea from the body. This method of ammonia excretion is particularly advantageous for these animals, since they all lay eggs. If the embryo excreted ammonia inside the egg, it would rapidly poison its environment. Even urea would be dangerous. However, uric acid in solid form is a safe way to store nitrogenous wastes in the egg.
Excretion by organisms living in water
Some one-celled and simple multicellular aquatic organisms have no excretory organs; nitrogenous wastes simply diffuse across the cell membrane into the aqueous environment. In others, paramecium for example, a specialized organelle, the contractile vacuole, aids in excretion by expelling excess water and some nitrogenous waste. In freshwater, the inside of cells has a higher concentration of salt than the surrounding water, and water constantly enters the cell by osmosis. Radiating canals in paramecium collect excess water from the cell and deposit it in the contractile vacuole, which squeezes it out through a pore on the surface. This process requires energy supplied by ATP produced in the cell's mitochondria.
Saltwater-dwelling animals must survive in water that is has a higher salt concentration than in cells and body fluids. These animals run the risk of losing too much body water by osmosis or taking in too much salt by diffusion. Several adaptations protect them. The skin and scales of marine fish are relatively impermeable to saltwater. In addition, the salts in the water they continually drink are excreted by special cells in their gills. In fact, marine fish excrete most of their nitrogenous waste as ammonia through the gills and only a little as urea, which conserves water. Sharks and other cartilaginous fish, on the other hand, store large amounts of urea in their blood. As a result, the concentration of their blood is slightly greater than the surrounding seawater, and water does not enter by osmosis. Special cells in the rectal gland of these fish excrete whatever excess salt does enter the system.
Excretion by land animals
As in aquatic animals, the excretory system in land animals removes nitrogenous waste and helps establish a balance between salt and water in the body. Terrestrial animals, however, also run the risk of drying out by evaporation from the body surface and the lungs. The elimination of feces and the excretion of urine also bring about water loss. Drinking, foods containing large amounts of water, and producing water during cellular respiration help overcome the loss. Animals that produce uric acid need less water than those excreting urine. Flame cells in flatworms, the nephridia in segmented worms, Malpighian tubules in insects, and kidneys in vertebrates are all examples of excretory systems.
Planarians and other flatworms have an excretory system that consists of two or more longitudinal branching tubules that run the length of the body. The tubules open to the outside of the animal through holes or pores in the surface. The tubules end in flame cells, bulb-shaped cells that contain cilia. The cilia create currents that move water and wastes through the canals and out the pores. Flatworms lack a circulatory system, so their flame cells excretory system picks up wastes directly from the body tissues.
The cells of segmented worms, such as earthworms, produce urea that is excreted through long tubules called nephridia, that work in conjunction with the earthworm's circulatory system. Almost every segment of the earthworm's body contains a pair of nephridia. A nephridium consists of a ciliated funnel, a coiled tubule, an enlarged bladder, and a pore. The ciliated funnel collects wastes from the tissue fluid. The wastes travel from the funnel through the coiled tubule. Additional wastes from blood in the earthworm's capillaries enter the coiled tubule through its walls. Some of the water in the tubule is reabsorbed into the earthworm's blood. A bladder at the end of the nephridium stores the collected wastes. Finally the bladder expels the nitrogenous wastes through the pore to the outside.
Malpighian tubules are excretory organs that operate in association with the open circulatory system of grasshoppers and other insects. They consist of outpocketings of the digestive system where the midgut attaches to the hindgut. Blood in the open sinuses of the grasshoppers' body surrounds the Malpighian tubules. The ends of the tubules absorb fluid from the blood. As the fluid moves through the tubules, uric acid is precipitated. A lot of the water and other salts are reabsorbed into the grasshopper's blood. The remaining fluid plus uric acid passes out of the Malpighian tubule and enters the gut. Water is reabsorbed from the digestive tract. Finally, the uric acid is eliminated from the rectum as a dry mass.
The vertebrate excretory system works with circulatory system to remove wastes and water from blood, and convert them to urine. The urine is stored in a urinary bladder before it is expelled from the body. Kidneys are the main organs of excretion in vertebrates. Within the kidneys, working units called nephrons take in the liquid portion of the blood, filter out impurities, and return necessary substances to the blood stream. The remaining waste-containing portion is converted to urine and expelled from the body.
Excretion in humans
The main excretory system in humans is the urinary system. The skin also acts as an organ of excretion by removing water and small amounts of urea and salts (as sweat). The urinary system includes a pair of bean-shaped kidneys located in the back of the abdominal cavity. Each day, the kidneys filter about 162 quarts (180 L) of blood, enough to fill a bathtub. They remove urea, toxins, medications, and excess ions and form urine. The kidneys also balance water and salts as well as acids and bases. At the same time, they return needed substances to the blood. Of the total liquid processed, about 1.3 quarts (1.5 L) leaves the body as urine.
The size of an adult kidney is approximately 4 inches (10 cm) long and 2 inches (5 cm) wide. Urine leaves the kidneys in tubes at the hilus, a notch that occurs at the center of the concave edge. Blood vessels, lymph vessels, and nerves enter and leave the kidneys at the hilus. If we cut into a kidney, we see that the hilus leads into a space known as the renal sinus. We also observe two distinct kidney layers. There is the renal cortex, an outer reddish layer, and the renal medulla, a reddish brown layer. Within the kidneys, nephrons clear the blood of wastes, create urine, and deliver urine to a tube called a ureter, which carries the urine to the bladder. The urinary bladder is a hollow muscular structure that is collapsed when empty and pear-shaped and distended when full. The urinary bladder then empties urine into the urethra, a duct leading to outside the body. A sphincter muscle controls the flow of urine between the urinary bladder and the urethra.
Each kidney contains over one million nephrons, each of which consists of a tuft of capillaries surrounded by a capsule on top of a curving tube. The tuft of capillaries is called a glomerulus. Its capsule is cup-shaped and is known as Bowman's capsule. The glomerulus and Bowman's capsule form the top of a tube, the renal tubule. Blood vessels surround the renal tubule, and urine forms in it. The renal tubules of many nephrons join in collecting tubules, which in turn merge into larger tubes and empty their urine into the ureters in the renal sinus. The ureters exit the kidney at the hilus.
The job of clearing the blood of wastes in the nephrons occurs in three stages: filtration, reabsorption, and tubular secretion.
1. The first stage in clearing the blood is filtration, the passage of a liquid through a filter to remove impurities. Filtration occurs in the glomeruli. Blood pressure forces plasma, the liquid portion of the blood, through the capillary walls in the glomerulus. The plasma contains water, glucose, amino acids, and urea. Blood cells and proteins are too large to pass through the wall, so they stay in the blood. The fluid, now called filtrate, collects in the capsule and enters the renal tubule.
2. During reabsorption, which occurs in the renal tubules, needed substances in the filtrate travel back into the bloodstream. Then glucose and other nutrients, water, and essential ions materials pass out of the renal tubules and enter the surrounding capillaries. Normally 100 percent of glucose is reabsorbed. (Glucose detected in the urine is a sign of diabetes mellitus—too much sugar in the blood due to a lack of insulin.) Reabsorption involves both diffusion and active transport, which uses energy in the form of ATP. The waste-containing fluid that remains after reabsorption is urine.
3. Tubular secretion is the passage of certain substances out of the capillaries directly into the renal tubules. Tubular secretion is another way of getting waste materials into the urine. For example, drugs such as penicillin and phenobarbital are secreted into the renal tubules from the capillaries. Urea and uric acid that may have been reabsorbed are secreted. Excess potassium ions are also secreted into the urine. Tubular secretions also maintain the pH of the blood.
The volume of the urine varies according to need. Antidiuretic hormone (ADH), released by the posterior pituitary gland, controls the volume of urine. The amount of ADH in the bloodstream varies inversely with the volume of urine produced. If we perspire a lot or fail to drink enough water, special nerve cells in the hypothalamus, called osmoreceptors, detect the low water concentration in the blood. They then signal neurosecretory cells in the hypothalamus to produce ADH, which is transmitted to the posterior pituitary gland and released into the blood, where it travels to the renal tubules. With ADH present, the kidney tubules reabsorb more water from the urine and return it to the blood, and the volume of urine is reduced. If we take in too much water, on the other hand, the osmoreceptors detect the overhydration and inhibit the production of ADH. Reabsorption of water is reduced, and the volume of urine is increased. Alcohol inhibits ADH production and therefore increases the output of urine.
The liver also plays an important role in excretion. This organ removes ammonia and converts it into less toxic urea. The liver also chemically changes and filters out certain drugs such as penicillin and erythromycin. These substances are then picked up by the blood and transported to the kidneys, where they are put into the excretory system.
The urinary system must function properly to ensure good health. During a physical examination, the physician frequently performs a urinalysis. Urine testing can reveal diseases such as diabetes mellitus, urinary tract infections, kidney stones, and renal disease. Urography, taking x rays of the urinary system, also helps diagnose urinary problems. In this procedure, an opaque dye is introduced into the urinary structures so that they show up in the x rays. Ultrasound scanning is another diagnostic tool. It uses high frequency sound waves to produce an image of the kidneys. Biopsies, samples of kidney tissue obtained in a hollow needle, are also useful in diagnosing kidney disease.
Disorders of the urinary tract 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. They can obstruct the urinary passages and cause severe pain and bleeding. If they do not pass out of the body naturally, the physician may use shockwave treatment focused on the stone to disintegrate it. Physicians also use surgery to remove kidney stones.
Renal failure is a condition in which the kidneys lose the ability to function. Nitrogenous wastes build up in the blood, the pH drops, and urine production slows down. If left unchecked, this condition can result in death. In chronic renal failure, the urinary system declines, causing permanent loss of kidney function.
Hemodialysis and kidney transplant are two methods of helping chronic renal failure. In hemodialysis, an artificial kidney device cleans the blood of wastes and adjusts the composition of ions. During the procedure, blood is taken out of the radial artery in the patient's arm. It then passes through dialysis tubing, which is selectively permeable. 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 returns to the body. Kidney transplants also help chronic kidney failure. In this procedure, a surgeon replaces a diseased kidney with a closely matched donor kidney. In 2016, 100,791 people in the United States were waiting for donor kidneys, but only about 17,000 received them. Current research aims to develop new drugs to help kidney failure and better dialysis membranes for the artificial kidney.