Maintaining Fluid Homeostasis

All cells live in a watery environment, which is maintained constant through the processes of ho-meostasis. 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. Afreshwa-ter 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 is able to 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 dilute fluid in order to maintain salt conservation, whereas the saltwater fish must ex-

All cells, from the single-celled animals to the crete a high concentration of salt in a very concen-highly diversified cells of higher mammals, trated solution. The primate must be able to regu-must maintain a constant internal environment late the output of fluid as a function of water

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 Malphigian 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 intake. Evolution has adapted the kidney of each animal to its environment.

In addition, 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. In addition, 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 are able to 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 wastes 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; if it is low, they reabsorb it. In the insect, the organ of excretion is the Malphigian tubule, which is able to regulate ion and water exchange. The accumulated fluid is flushed into the gut, where absorption of ions and water takes place.

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