EXCRETION

the process by which animals rid themselves of waste products and of the nitrogenous by-products of metabolism. Through excretion organisms control osmotic pressurethe balance between inorganic ions and waterand maintain acid-base balance. The process thus promotes homeostasis, the constancy of the organism's internal environment. Every organism, from the smallest protist to the largest mammal, must rid itself of the potentially harmful by-products of its own vital activities. This process in living things is called elimination, which may be considered to encompass all of the various mechanisms and processes by which life forms dispose of or throw off waste products, toxic substances, and dead portions of the organism. The nature of the process and of the specialized structures developed for waste disposal vary greatly with the size and complexity of the organism. Four terms are commonly associated with waste-disposal processes and are often used interchangeably, though not always correctly: excretion, secretion, egestion, and elimination. Excretion is a general term referring to the separation and throwing off of waste materials or toxic substances from the cells and tissues of a plant or animal. The separation, elaboration, and elimination of certain products arising from cellular functions in multicellular organisms is called secretion. Though these substances may be a waste product of the cell producing them, they are frequently useful to other cells of the organism. Examples of secretions are the digestive enzymes produced by intestinal and pancreatic tissue cells of vertebrate animals, the hormones synthesized by specialized glandular cells of plants and animals, and sweat secreted by glandular cells in the skins of some mammals. Secretion implies that the chemical compounds being secreted were synthesized by specialized cells and that they are of functional value to the organism. The disposal of common waste products should not, therefore, be considered to be of a secretory nature. Egestion is the act of excreting unusable or undigested material from a cell, as in the case of single-celled organisms, or from the digestive tract of multicellular animals. As defined above, elimination broadly defines the mechanisms of waste disposal by living systems at all levels of complexity. The term may be used interchangeably with excretion. the process by which animals rid themselves of the undigested waste products of food and the nitrogenous by-products of metabolism, regulate their water content, maintain acidbase balance, and control osmotic pressurethe balance between inorganic ions and water. The excretory system promotes homeostasis, the constancy of an organism's internal environment. The mechanisms that evolved to carry out excretory functions differ greatly in various organisms and environments. An animal living in a desert must be able to conserve water in its body, while a freshwater fish needs the capacity to rid itself of large amounts of water. The primary excretory product is ammonia, derived from the digestion of proteins. Since ammonia is highly toxic, it must be eliminated quickly and efficiently. This poses no problem in small aquatic animals; ammonia is very soluble and passes into the surrounding water by diffusion. In terrestrial animals (and large aquatic ones) ammonia must be converted to less toxic substancesurea in mammals and the insoluble uric acid in insects, birds, and reptilesbefore being excreted. In protozoans an organelle called a contractile vacuole maintains osmotic pressure. Nitrogenous wastes are lost through diffusion. The vacuole appears as an internal sac which fills with clear liquid, discharges its contents outside the cell, then fills again. The nephridia constitute the excretory organs of invertebrates like annelids (segmented worms). Typically the nephridia are paired tubules, one pair occurring in each segment of the worm's body. One end of the nephridium opens into the body cavity and the other to the outside environment. Since these organisms function as aquatic animals, they may lose nitrogenous wastes through their integument or via the nephridia. Fluid enters the nephridium from the body cavity and is probably propelled by cilia. As the waste fluid passes it becomes more dilute through reabsorption of body salts. It passes to the exterior through the nephridiopore. The excretory organ of mollusks is the renal gland, a wide tube opening at one end into the sac surrounding the heart and at the other end to the exterior environment. The urine is formed through filtration of the blood, and its composition altered by reabsorption and secretion. The paired coxal glands of aquatic arthropods pass from the coelomic sac where blood filtration occurs to external openings at the base of limbs, notably the antennae. Insects have evolved a very different type of excretory system. Malpighian tubules (varying in number from 2 to 100 depending upon species) open at one end into the blood space (body cavity) and at the other into the rectum, part of the alimentary canal. The primary urine is formed not by filtration but secretion of ions and water from the blood. In the rectum the composition of the urine is changed radically; soluble urate is converted to insoluble uric acid, and water and the soluble products of digestion are reabsorbed. Birds, like reptiles and insects, excrete uric acid into a continuation of the alimentary canal. Some marine birds have, in addition, salt glands (modified tear glands) which remove excess salt from their bodies and discharge the concentrated solution through the nostrils. Amphibians store large quantities of dilute urine in a large bladder which acts as a water reserve when the animal is on land. Freshwater fish must overcome the problem posed by water entering the body through osmosis and salts leaching out. To compensate, they produce large volumes of dilute urine and take in salts from the water through specialized cells in their gills. Nitrogenous wastes, for the most part, are diffused as ammonia through the skin. Marine fish, on the other hand, lose water through the skin and take in salt by osmosis. This is because their blood is only half as saline as seawater. They are unable to produce a highly concentrated urine, so they maintain homeostasis by swallowing great amounts of seawater and eliminating salt through specialized gill cells. The human excretory or urinary system is typical of that of all mammals. It consists of two kidneys where urine is produced by filtration, secretion, and reabsorption; the ureters, tubes that transport the urine; the bladder where the urine is stored; and the urethra through which the urine is voided. In humans, kidneys are paired, bean-shaped organs about five inches long, located in the small of the back. The concave portion lies nearest the backbone and is deeply cleft by the hilus through which arteries, veins, nerves, and lymphatics enter the kidney sinus. A cross section of the kidneys shows them to be made up of a darker outer cortex and inner medulla composed of rough cones with apexes projecting into the sinus. The functional units of the kidney are called nephrons, of which there are about a 1,000,000 in each kidney. They are made up of the renal corpuscle, or Bowman's capsule (a double-walled cup), the proximal tubule, Henle's loop, and the distal tubule. Urine formation begins in Bowman's capsule, which encloses a dense cluster of microscopic blood vessels, the glomerulus. Under the driving force of blood pressure, plasma filters from the blood. Proteins and about 80 percent of the water are held back in this process. The filtrate passes through the inner wall of the capsule and moves into the tubule. These tubules are an inch or two long. The proximal tubule passes from Bowman's capsule in the renal cortex into the medulla and makes a U-turn (Henle's loop). The distal convoluted tubule then reenters the cortex and joins with several other distal tubules to form a collecting tubule, which carries the urine to the renal pelvis and the ureters. The ureters move the urine to the bladder in peristaltic waves. The filtrate entering the tubule is different in composition from urine. In the proximal tubule almost all the water, salts, and glucose are reabsorbed by the network of blood vessels surrounding it. Concentration is effected in the constricted loop of Henle. The distal tubule regulates water, electrolyte, and hydrogen ion content of the filtrate. Uric acid is actively secreted into the filtrate here. The two ureters (1012 inches long) enter the hollow muscular bladder where urine collects until it is voided through the urethra in urination (or micturition). The urethra differs in males and females. In human males it is about eight inches long and is also the channel for semen in ejaculation. In women it is only one or two inches long and carries only urine. Malfunction of the excretory system can lead to dehydration or edema, and the dangerous buildup of waste and toxic substances. Acute renal failure is one of the primary diseases of the kidney. It is characterized by sudden failure of renal function so that little or no urine is produced, and water and waste products accumulate in the body. It may be caused by hemorrhage or shock leading to greatly decreased blood supply to the kidneys and resulting in renal necrosis (tissue death). A second cause is the accumulation of toxins in the kidney. Inflammatory diseases of the kidney (pyelonephritis, glomerulonephritis), high blood pressure, and obstruction of the lower urinary tract can lead to chronic renal failure. In this disease there is progressive degeneration of the nephrons resulting in uremic poisoning from the accumulated wastes. Both acute and chronic renal failure may be treated by dialysis, an artificial filtration of the blood through semi-permeable membranes to remove urea and other wastes, or by transplantation of another human kidney from a donor. The excretory tract is subject to benign and malignant tumours, infections and inflammations, and obstruction by calculi. The last are stones composed of inorganic substances (largely calcium, phosphate, or oxylate), or organic matter like uric acid. Additional reading Elimination General works containing coverage of the processes of excretion include: Paul B. Weisz and Richard N. Keogh, The Science of Biology, 5th ed. (1982), a comprehensive general text emphasizing molecular biology, ecology, and morphology; Ralph Buchsbaum, Animals Without Backbones, 3rd ed. (1987), an elementary, illustrated account of invertebrate animals; Alfred Sherwood Romer and Thomas S. Parsons, The Vertebrate Body, 6th ed. (1986), a general history of the vertebrate body emphasizing comparative aspects of structure and function; and C. Ladd Prosser (ed.), Comparative Animal Physiology, 3rd ed. (1973), a college-level text on the comparative aspects of functional systems in animals. See also Alberte Pullman, V. Vasilescu, and L. Packer (eds.), Water and Ions in Biological Systems (1985); and Albert Br and Gnther Ritzel (eds.), Xylitol and Oxalate: Metabolic Studies in Animals and Man (1985). Excretion and excretory systems Homer W. Smith, From Fish to Philosopher (1953, reissued 1961), is an introduction to vertebrate evolution in terms of kidney evolution. Among sources of information on the comparative structure and function of excretory systems are the following: Ernst Florey, An Introduction to General and Comparative Animal Physiology (1966); Edwin S. Goodrich, Studies on the Structure and Development of Vertebrates (1930, reprinted 1986), an older work of unchallenged authority on morphological and evolutionary aspects of all vertebrate systems, including the urogenital; Malcolm S. Gordon et al., Animal Physiology: Principles and Adaptations, 4th ed. (1982), on the regulation of water and salts in animals; and William S. Hoar, General and Comparative Physiology, 3rd ed. (1983), an established authoritative work. Appropriate sections of the multivolume series Handbook of Physiology, edited by John Field, provide surveys of the excretory function but presume a greater background knowledge than do the works of Hoar and Florey, listed above. See also A. Wessing (ed.), Excretion (1975), a collection of symposium papers with bibliographies. General features of excretory structures and functions The physiological process by which an organism disposes of its nitrogenous by-products is called excretion. The mechanisms for that process constitute the excretory systems, particularly such organs of vertebrate animals as elaborate and complicated as the kidney and its associated urinary ducts. The meaning of excretion is most easily understood in the context of vertebrate physiology. The animal swallows food (ingestion). In the stomach and intestine some of the food is broken down into soluble products (digestion) that are absorbed into the body (assimilation). In the body these soluble products undergo further chemical change (metabolism); some are used by the body for growth, but most provide energy for the various activities of the body. Metabolism involves the uptake of oxygen and the elimination of carbon dioxide in the lungs (respiration). Besides carbon dioxide, compounds of nitrogen arise from metabolism and are eliminated, chiefly by the kidney, in the urine (excretion). Food not digested is eliminated through the anus (defecation). These processes are characteristic of animals in general, but not of plants. A green plant takes in carbon dioxide from the atmosphere and nitrogen (as nitrate) from the soil. It uses the energy of sunlight to build these nutrients into the materials required for growth and in the process gives out oxygen (see photosynthesis). In a broad sense animals live on plants, and the by-products of animals are the raw materials on which plants grow. These mutually supporting activities of plants and animals are kept precisely in balance by the activities of bacteria. Bacteria convert the urine and feces of animals (and also the dead bodies of both plants and animals) to carbon dioxide and nitrate. In the living world as a whole, carbon and nitrogen are in continuous circulation, driven by the energy of sunlight (see biosphere). Over most of the earth, for most of time, no by-products accumulate. Occasionally the cycles get out of balance, as they must have done during the prehistoric period when coal was being formed in the earth as a consequence of the failure of bacteria to decompose all the remains of plants. Products of excretion Although every type of organism takes in some materials and eliminates others, excretion in the strict sense is a process found only in animals. For the purposes of this article excretion will be taken to mean the elimination of nitrogenous by-products and the regulation of the composition of the body fluids. The primary excretory product arising naturally in the animal body is ammonia, derived almost entirely from the proteins of the ingested food. In the process of digestion proteins are broken down into their constituent amino acids. Some of the amino-acid pool is then used by the animal to build up its own proteins, but a great deal is used as a source of energy to drive other vital processes. The first step in the mobilization of amino acids for energy production is deamination, the splitting off of ammonia from the amino-acid molecule. The remainder is oxidized to carbon dioxide and water, with the concomitant production of the energy-rich molecules of adenosine triphosphate (ATP; see metabolism). Since excessive levels of ammonia are highly toxic to most animals, they must be effectively eliminated. This is no problem in small aquatic animals because ammonia rapidly diffuses, is highly soluble in water, and escapes easily into the external medium before its concentration in the body fluids can reach a dangerous level. But in terrestrial animals, and in some of the larger aquatic animals, ammonia is converted into some less harmful compounds (detoxication). In mammals, including humans, it is detoxified to urea, which may be considered as being formed by the condensation of one molecule of carbon dioxide with two molecules of ammonia (though the biochemistry of the process is more complex than that). Urea is highly soluble in water but cannot be excreted in a highly concentrated solution because of the osmotic pressure (see below) it would exert. Because the conservation of water is important for most terrestrial animals, it is not surprising that many of them have evolved more economical methods for disposing of nitrogenous by-products. Birds, reptiles, and terrestrial insects excrete nitrogen in the form of uric acid, which is highly insoluble in water and can be removed from the body as a thick suspension or even as a dry powder.