Chapter Summary

• Excretory systems maintain the osmolarity and volume of the extracellular fluids and eliminate the waste products of nitrogen metabolism through the processes of filtration, reabsorption, and secretion. Urine is the output of excretory systems.

• There is no active transport of water, so water must be moved across membranes by a difference in either osmolarity or pressure.

• Water enters and leaves cells by osmosis. To achieve cellular water balance, animals must maintain the osmolarity of their extracellular fluids within an acceptable range.

• Marine animals can be osmoconformers or osmoregulators. Freshwater animals must be osmoregulators and must continually excrete water and conserve salts. Terrestrial animals are osmoregulators, but the nature of their regulation depends on environment and lifestyle. Review Figure 51.1

• Apart from regulating osmolarity of cells and extracellular fluids, animals must also regulate their ionic composition by conserving some ions and secreting others. Salt glands are adaptations for secretion of NaCl. Review Figure 51.2

• Aquatic animals that breathe water can eliminate nitrogenous wastes such as ammonia by diffusion across their gill membranes. Terrestrial animals and some aquatic 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. Extracellular fluid is filtered into the tubules, which process the filtrate to produce 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, which alter the composition of the filtrate by active transport mechanisms. Review Figure 51.5, 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 reabsorbed from the rectum, so the insect excretes semisolid wastes. Review Figure 51.6

• Marine bony fishes produce little urine. Cartilaginous fishes retain urea and TMAO, so the osmolarity of their body fluids remains close to that of seawater.

• Reptiles, including birds, have skin with low water permeability and excrete nitrogenous wastes as uric acid in a semisolid form.

• Mammals produce urine that is more concentrated than their extracellular fluids.

• The nephron, the functional unit of the vertebrate kidney, consists of a glomerulus, in which blood is filtered, a renal tubule, which use processes of active secretion and reabsorption to convert the glomerular filtrate into urine to be excreted, and a system of peritubular capillaries, which surround the tubule and support its functions of secretion and reabsorption. Review Figures 51.7, Activity 51.2

• The concentrating ability of the mammalian kidney is a function of its anatomy, which enables countercurrent exchange.

• The glomeruli and the proximal and distal convoluted tubules are located in the cortex of the kidney. Certain molecules are actively reabsorbed 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 51.9, Activity 51.3

• Salts, water, and valuable molecules such as glucose and amino acids are reabsorbed 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 interstitial fluid of the renal medulla by a countercurrent multiplier mechanism. Urine flowing down the collecting ducts to the ureter is concentrated by the osmotic reabsorption of water caused by the concentration gradient in the surrounding interstitial fluid. Review Focus: Key Figure 51.10, Animation 51.1

• Hydrogen ions secreted by the renal tubules are buffered in the urine by bicarbonate and other chemical buffering systems. Review Figure 51.11

• Kidney function in mammals is controlled by autoregulatory mechanisms that maintain a constant high glomerular filtration rate (GFR) even if blood pressure varies.

• The RAAS is an important autoregulatory mechanism. Renin is released by the kidney when blood pressure falls. Renin activates angiotensin, which causes the constriction of efferent glomerular arterioles and peripheral blood vessels, causes the release of aldosterone (which enhances water reabsorption), and stimulates thirst. Review Figure 51.13, Activity 51.4

• Changes in blood pressure and osmolarity influence the release of antidiuretic hormone (ADH), which controls the permeability of the collecting duct to water and therefore the amount of water that is reabsorbed from the urine. ADH stimulates the expression of and controls the intracellular location of aquaporins, which serve as water channels in the membranes of collecting duct cells. Review Figures 51.14, 51.15

• When the volume of blood returning to the heart increases and stretches the atrial walls, atrial natriuretic peptide (ANP) is released, which causes increased excretion of salt and water.