Tissue Fluid and Water Balance
► Most adaptations for maintaining salt and water balance and for excreting nitrogen wastes employ the same basic mechanisms: filtration of tissue fluid and active secretion and resorption of specific molecules.
► The problems of salt and water balance and nitrogen excretion that animals face depend on their environments, but in all animal excretory systems, there is no active transport of water.
► Marine animals can be osmoconformers or osmoregulators. Freshwater animals must be osmoregulators and must continually excrete water and conserve salts. Most animals are ionic regulators to some degree. Review Figure 51.1
► On land, water conservation is essential, and diet determines whether salts must be conserved or excreted. Marine birds excrete excess salt through nasal salt glands. Review Figure 51.2
► Aquatic animals can eliminate nitrogenous wastes such as ammonia by diffusion across their gill membranes. Terrestrial animals must detoxify ammonia by converting it to urea or uric acid before excretion. Review Figure 51.3
► Depending on the form in which they excrete their nitrogenous wastes, animals are classified as ammonotelic, ureotelic, or uricotelic.
► The protonephridia of flatworms consist of flame cells and excretory tubules. Tissue fluid is filtered into the tubules, which process the filtrate to produce a dilute urine. Review Figure 51.4
► In annelid worms, blood pressure causes filtration of the blood across capillary walls. The filtrate enters the coelomic cavity, where it is taken up by metanephridia. As the filtrate passes through the metanephridia to the outside, its composition is changed by active transport mechanisms. Review Figure 51.5. See Web/CD Activity 51.1
► The Malpighian tubules of insects receive ions and nitrogenous wastes by active transport across the tubule cells. Water follows by osmosis. Ions and water are resorbed from the rectum, so the insect excretes semisolid wastes. Review Figure 51.6
► The nephron, the functional unit of the vertebrate kidney, consists of a glomerulus, in which blood is filtered across the walls of a knot of capillaries, a renal tubule, which processes the filtrate into urine to be excreted, and a system of peritubular capillaries, which surround the tubule. Review Figure 51.7. See Web/CD Activity 51.2
► The adaptations of marine and terrestrial animals for conserving water are diverse. Marine bony fishes have few glomeruli and produce little urine. Cartilaginous fishes retain urea so that the osmolarity of their body fluids remains close to that of seawater. Amphibians remain close to water or have waxy skin coverings. Reptiles have scaly skin, lay shelled eggs, and excrete nitrogenous wastes as uric acid.
► Birds share the adaptations of reptiles; in addition, they can produce urine more concentrated than their tissue fluid. Only birds and mammals can produce such hypertonic urine.
The Mammalian Excretory System
► The concentrating ability of the mammalian kidney depends on its anatomy. Review Figure 51.9a,b
► The glomeruli and the proximal and distal convoluted tubules are located in the cortex of the kidney. Certain molecules are actively resorbed from the glomerular filtrate by the tubule cells, and other molecules are actively secreted. Straight sections of renal tubules called loops of Henle and collecting ducts are arranged in parallel in the medulla of the kidney. Review Figure 51.9c. See Web/CD Activity 51.3
► Salts and water are resorbed in the proximal convoluted tubule without the renal filtrate becoming more concentrated, although its composition changes.
► The loops of Henle create a concentration gradient in the tissue fluid of the renal medulla by a countercurrent multiplier mechanism. Urine flowing down the collecting ducts to the ureter is concentrated by the osmotic resorption of water caused by the concentration gradient in the surrounding tissue fluid. Review Figure 51.10
► Hydrogen ions secreted by the renal tubules are buffered in the urine by bicarbonate and other chemical buffering systems.
Review Figure 51.12
See Web/CD Tutorial 51.1 Regulation of Kidney Functions
► Kidney function in mammals is controlled by autoregulatory mechanisms that maintain a constant high glomerular filtration rate even if blood pressure varies.
► An important autoregulatory mechanism is the release of renin by the kidney when blood pressure falls. Renin activates angiotensin, which causes the constriction of peripheral blood vessels, causes the release of aldosterone (which enhances water resorption), and stimulates thirst.
► Changes in blood pressure and osmolarity influence the release of antidiuretic hormone, which controls the permeability of the collecting duct to water and therefore the amount of water that is resorbed from the urine. ADH stimulates the expression of proteins called aquaporins that serve as water channels in the membranes of collecting duct cells. Review Figure 51.13
► When the volume of blood returning to the heart increases and stretches the atrial walls, they release atrial natriuretic peptide (ANP), which causes increased excretion of salt and water.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.