Excretory system (comparative anatomy)

All cells, from the single-celled animals to the highly diversified cells of higher mammals, must maintain a constant internal environment regarding the kinds and amounts of specific ions, the pH of the protoplasm, the osmotic pressure, the water content of the cells, and the excretion of wastes. The kidney in vertebrates and the varied types of excretory organs found in lower animals perform this function. Although the excretory function is very important, an animal will die more quickly from a disturbance of the composition of the internal environment of the cell than it will from an accumulation of waste. Fortunately, the kidney performs functions that keep these from happening.

Maintaining Fluid Homeostasis

All cells live in a watery environment, which is maintained constantly through the processes of homeostasis. Maintaining homeostasis presents different problems for different organisms. For a primate, living in the air, water loss is a constant problem, and water must be conserved. A freshwater fish, on the other hand, takes in large quantities of water that has a lower concentration of salts than its body fluids, and it must excrete the excess. An ocean fish, living in water with a much greater concentration of salts than its body fluids, must obtain water from its environment and still avoid increasing the concentration of salts in the body fluids.

A kidney can perform all these functions, although in each of these cases, it acts in a different way. The freshwater fish must excrete large quantities of very diluted fluid to maintain salt conservation, whereas the saltwater fish must excrete a high concentration of salt in a very concentrated solution. The primate must be able to regulate the output of fluid as a function of water intake. Evolution has adapted the kidney of each animal to its environment.

Additionally, animals survive by metabolizing foodstuffs to provide the energy for movement. One of the major metabolic processes is the breakdown of protein to produce energy for the synthesis of other proteins and the rebuilding of body structures. In the breakdown of protein, nitrogen is freed from the organic molecule and must be excreted. One of the resultant nitrogen products is ammonia, which is toxic. The ammonia is converted into a less toxic material, urea (or uric acid, in some animals), before it is excreted by the kidney. Also, the metabolism of the body usually results in the production of acids, particularly if fat is metabolized. The body functions well only within a narrow range of acidity, so unless the excess acid is removed, serious problems quickly arise. The kidney serves the function of maintaining the pH at a constant value.

The Excretory Organs

The organs of excretion have taken many forms. Simple single-celled organisms such as amoebas can form contractile vacuoles, or walled-off spaces within the cell, in which water can be stored and waste products deposited. These vacuoles are periodically transported through the cytoplasm and excreted through the external cell membrane. The size and number of vacuoles are determined by water intake and the organism’s need to eliminate water, as well as by the accumulation of waste materials.

As animals developed more cell types and the number of cells increased dramatically, the need to provide a constant internal environment around the cells arose. The excretory organs became, of necessity, more complex in nature. In addition to excreting waste products, they developed abilities to retain some ions and excrete others, to retain or excrete water, and to retain or excrete acids or bases to maintain a constant environment.

Many organisms have no obvious means of regulating water and salt balance, and they apparently accomplish this feat through the skin or the gut; others have rudimentary organs of excretion. Many lower animals have nephridia, primitive versions of the kidney that excrete water and waste and regulate ion concentration. These are simple tubes into which body fluids pass; the fluids are excreted after chemical alteration. In animals such as worms that have segments, a pair of nephridia may be located in each segment. Some of them open into the body cavity, while others are closed. In some animals, these are well differentiated and are called flame cells. These tubular structures serve to regulate the internal environment of the body. For example, if the sodium concentration in the coelom, or internal cavity, is high, the nephridia excrete sodium, but if it is low, they reabsorb it. In the insect, the organ of excretion is the Malpighian tubule, which can regulate ion and water exchange. The accumulated fluid is flushed into the gut, where absorption of ions and water takes place.

The Kidney

In vertebrates, the kidney is the organ responsible for eliminating water and waste products. The kidney is a bean-shaped organ that receives a large blood supply from the heart. It has several auxiliary structures: the ureter, which collects fluid or urine from all the tubules; the bladder, which acts as a storage organ; and the urethra, which opens from the bladder to the outside of the body. The kidney consists of more than a million nephrons arranged symmetrically, with the lower part of each nephron pointing toward the hilus (the pole of the kidney where the ureter arises).

The kidney maintains homeostasis of the body through four basic mechanisms: filtration, reabsorption, secretion, and concentration. In filtration, a liquid portion of the blood is transferred to the tubule. There the cells proceed to reabsorb necessary materials, to secrete additional materials into the tubular liquid from the blood, and to concentrate the fluid in the tubule.

The nephron is the fundamental structure of the kidney. The nephron is a long, slender tube with different parts that are capable of secreting or reabsorbing ions, water, and other substances either to remove materials from the blood or to return materials to the blood, depending upon the needs of the body. One process it performs is the elimination of ammonia products and other waste materials.

The nephron consists of two major parts: a glomerulus and a tubule. The process of urine formation begins with filtration. The blood enters the kidney and then the glomeruli under high pressure from the heart. The pressure forces fluid from the blood into the tubule. The amount is tremendous; in humans, every day, some 180 liters (about 40 gallons of fluid) pass from the blood into the nephron. All the blood in the body passes through the nephron about thirty times per day. During this same period, seven hundred liters of blood pass through the kidneys, so only a small portion is actually filtered or transferred to the tubule. Most remarkable of all, only about 1.5 liters of urine are produced each day. The rest of this large volume is reabsorbed by the tubule.

As the fluid passes into the nephron, it enters a section of tubule at the beginning that reabsorbs much of the material needed by the body. Such things as the glucose needed for energy, amino acids for protein building, vitamins, and ions needed to maintain the correct concentration of the blood are removed and transported by the cells of the nephron back into the blood. At the same time, about 85 percent of the water of the filtrate is also transported back into the blood.

The cells of the nephron can secrete materials from the blood to the tubular fluid. This process is exactly the opposite of the reabsorption of substances. One of the most readily secreted substances is sodium. The cells of the body are high in potassium and low in sodium; since sodium is a constituent of every diet, the removal of excess sodium is necessary. Sodium is picked up from the blood that circulates around the nephron and is secreted into the tubule. The process of secretion also extends to other materials. The potent antibiotic penicillin was ineffective when it was first used to treat systemic infections because it was rapidly secreted by the tubules. A high enough concentration could not be accumulated in the blood to destroy bacteria. It became useful only when a derivative that was not secreted could be found.

The Loop of Henle

The tubular fluid that has been adjusted in concentration, volume of fluid, and concentration of ions and other materials now passes into a hairpin-shaped portion of the tubule called the loop of Henle. The loop of Henle adjusts the volume of filtrate. A hormone is produced by the brain (in the hypothalamus) that is capable of altering the permeability of the cells of the loop of Henle to water. The substance, a protein hormone called antidiuretic hormone (ADH), causes the reabsorption of water from the loop. Cells of the hypothalamus respond to the concentration of particles in the blood (its osmotic pressure) and adjust the amount of water that is reabsorbed from the tubule by secreting more or less ADH as necessary to maintain a constant concentration in the blood. The range of adjustment is remarkable. The volume of urine produced can range from about 0.5 liter to more than 30 liters per day, depending upon the need. If water is administered or restricted, the water concentration of the body changes. This causes a change in the production of ADH, which in turn increases or decreases the excretion of water to return the level to normal.

Sweating also causes water loss and thus decreases urine flow. Intake of large amounts of fluid will dilute the body fluids and cause an increased urine output. There is a constant adjustment, because water is lost by breathing, through the skin, and through excretions, and the kidney must make the proper corrections. Losses have been reduced to a minimum in animals such as the kangaroo rat, which lives in the desert and must conserve water. All its water intake is from seeds and other foods containing some water, and excretion is almost zero. The desert rat can concentrate urine to a level about five times that of the human.

As the urine passes from the loop of Henle, it enters upon a final adjustment in the latter portion of the tubule. Volume, concentration of material, and the like are adjusted to maintain homeostasis. Other alterations of the fluid are also made in the passage down the tubule. If the body becomes acidic, the cells of the tubule exchange sodium ions, which are neutral, for acid ions (H⁺), thus causing the body to lose acid. Conversely, if the body becomes basic (lacks acid or hydrogen ions), the reverse is true.

Studying Kidney Function

The kidney can be studied on many different levels. The output of the kidney, the urine, and the input, the blood, can be analyzed to determine function of both kidneys. In some animals, such as the frog, the individual nephrons are visible under a microscope. Using the stop-flow technique, the behavior of an individual portion of the nephron can be studied. In this technique, a very small needle is introduced into one portion of the nephron, and a small drop of oil is injected. Another drop is injected in an adjacent section of the nephron, thus isolating a section between the two drops. The exchanges that occur in that portion of the nephron can then be measured by taking samples of the fluid at intervals.

The whole kidney can be studied by examination of the kidney in relation to a reference substance. For example, the chemical inulin goes through the nephron without any alteration, so its excretion can be compared with other substances: If more of another substance appears in the urine in proportion to the inulin, the substance must have been excreted from the blood into the urine and secreted. If less is present, some material must have been reabsorbed from the tubule into the blood. This is called the clearance technique, and it is widely used to predict kidney function in health and disease. For less sophisticated testing, substances—such as certain dyes—that can be taken orally and are excreted by the kidney can be used to measure the rate of excretion. Radiopaque substances, substances that will appear on X-rays, can be used to detect overt kidney malfunction.

Principal Terms

Antidiuretic Hormone (ADH): A hormone produced in the hypothalamus that controls reabsorption of water in the loop of Henle

Contractile Vacuole: The excretory organ of several one-celled organisms

Filtration: The process of diffusion of plasma from the blood to the glomerulus and nephron

Glomerulus (pl. glomeruli): A capsule fitting around capillary blood vessels that receives the filtrate from the blood and passes it into the tubule

Loop of Henle: A slender hairpin turn in the tubule where most adjustment of the water balance of the body occurs

Malpighian Tubule: The primitive excretory organ of insects

Nephridia: The primitive forms of kidneys found in worms and lower organisms

Nephron: The basic excretory unit of the kidney

Tubule: The long, slender part of the nephron that is the location of almost all kidney function

Urea: A substance formed from by-products of protein metabolism and excreted by the kidney

Bibliography

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Hainsworth, F. Reed. Animal Physiology. Addison-Wesley, 1981.

Smith, Homer. From Fish to Philosopher. Doubleday, 1961.

Starr, Cecie, and Ralph Taggart. Biology: The Unity and Diversity of Life. 15th ed. Cengage Learning, 2019.

Valtin, Heinz. Renal Function. 3rd ed. Little, Brown, 1995.