1. Salt and Water Balance and Nitrogen Excretion 40

2. Chapter 40 Salt and Water Balance and Nitrogen Excretion Key Concepts 40.1 Excretory Systems Maintain Homeostasis of the Extracellular Fluid 40.2 Excretory Systems Eliminate Nitrogenous Wastes 40.3 Excretory Systems Produce Urine by Filtration, Reabsorption, and Secretion

3. Chapter 40 Salt and Water Balance and Nitrogen Excretion Key Concepts 40.4 The Mammalian Kidney Produces Concentrated Urine 40.5 The Kidney Is Regulated to Maintain Blood Pressure, Blood Volume, and Blood Composition

4. Chapter 40 Opening Question How do excretory systems of animals maintain homeostasis of the interstitial fluid in the face of extreme challenges?

5. Concept 40.1 Excretory Systems Maintain Homeostasis of the Extracellular Fluid Excretory systems control volume, concentration, and composition of the extracellular fluid and excrete wastes. Four excretory functions: Regulate fluid volume in the body Regulate solute concentrations, or osmolarity, of extracellular fluid Maintain individual solutes Eliminate nitrogenous wastes Urine is the liquid waste product.

6. Concept 40.1 Excretory Systems Maintain Homeostasis of the Extracellular Fluid The osmolarity of a solution is the number of osmoles of active solutes per liter of solvent. The osmolarity of the extracellular fluid must be maintained for cellular water balance. If the osmolarity of the extracellular fluid is different than the cytoplasm, water will move into or out of the cells via osmosis, and cells may be damaged.

7. Concept 40.1 Excretory Systems Maintain Homeostasis of the Extracellular Fluid Animals in different environments face different osmolarity problems. On land, salt and water must be conserved. Terrestrial animals are osmoregulators and actively regulate the osmolarity of their extracellular fluid. Freshwater animals have to conserve salts but excrete excess water, so are also osmoregulators.

8. Concept 40.1 Excretory Systems Maintain Homeostasis of the Extracellular Fluid Marine animals are exposed to the high osmolarity of the ocean: Osmoconformers equilibrate their osmolarity with seawater. Artemia (brine shrimp) can survive in varied environmental osmolarities: In high osmolarity, Cl – is actively transported out through the gills, Na + ions follow. In low osmolarity, the transport of Cl – is reversed.

9. Figure 40.1 Osmoconformity Has Limits

10. Concept 40.1 Excretory Systems Maintain Homeostasis of the Extracellular Fluid Most animals are ionic regulators, conserving some ions and excreting others to maintain ionic composition of extracellular fluid. Ionic conformers allow their ionic composition, as well as their osmolarity, to match the environment. Commonly regulated ions are Na +, Cl –, K +, Ca 2+, H +, and HCO 3 – (bicarbonate).

11. Concept 40.1 Excretory Systems Maintain Homeostasis of the Extracellular Fluid H + concentration, or pH, is closely regulated—important to protein structure and function. A buffer is a substance that can absorb or release hydrogen ions. The major buffer in blood is bicarbonate (HCO 3 – ), which is formed from CO 2. Lungs eliminate CO 2 from blood, and kidneys reabsorb HCO 3 – and excrete H + to maintain pH.

12. Concept 40.2 Excretory Systems Eliminate Nitrogenous Wastes Animals must eliminate metabolic waste products: Carbohydrates and fat end up as water and CO 2 and are easily excreted Proteins and nucleic acids contain nitrogen, so metabolism produces nitrogenous waste Ammonia (NH 3 ) is the most common nitrogenous waste.

13. Concept 40.2 Excretory Systems Eliminate Nitrogenous Wastes Ammonia is soluble in water—aquatic animals who secrete NH 3 through gills are ammonotelic. Animals must convert NH 3 to urea or uric acid. Ureotelic animals mostly excrete urea. It is water-soluble but results in large water loss. Uricotelic animals mostly excrete uric acid. It is insoluble in water and precipitates out of the urine with little water loss.

14. Figure 40.2 Waste Products of Metabolism

15. Concept 40.2 Excretory Systems Eliminate Nitrogenous Wastes Most species secrete more than one nitrogenous waste. Humans are ureotelic but also excrete: Uric acid—from metabolism of nucleic acids and caffeine Ammonia—regulates pH of extracellular fluid by buffering urine

16. Concept 40.3 Excretory Systems Produce Urine by Filtration, Reabsorption, and Secretion In most systems urine is produced by filtering extracellular fluid. The resulting fluid is a filtrate—similar to blood plasma. Filtrate flows through tubules and is modified by reabsorption or secretion of solutes. The modified filtrate is excreted as urine.

17. Concept 40.3 Excretory Systems Produce Urine by Filtration, Reabsorption, and Secretion Annelids have segmented bodies, with a coelom in each segment. In earthworms, blood pumped under pressure causes blood to filter across capillary walls. Water, small molecules, and some waste products enter the coelom.

18. Concept 40.3 Excretory Systems Produce Urine by Filtration, Reabsorption, and Secretion Each earthworm segment contains a pair of metanephridia. A metanephridium begins as a nephrostome, or opening, which leads into a tubule. The tubule ends in a nephridiopore. Fluid enters through the nephrostomes, and tubule cells reabsorb or secrete molecules into it.

19. Figure 40.3 Metanephridia in Earthworms

20. Concept 40.3 Excretory Systems Produce Urine by Filtration, Reabsorption, and Secretion The insect excretory system consists of Malpighian tubules—blind-ended tubules that open into the gut. Tubule cells actively transport uric acid, K +, and Na + into the tubules. Water follows the solutes and moves contents toward the gut. Water and ions are recovered in the hindgut, and uric acid and other waste are excreted.

21. Figure 40.4 Malpighian Tubules in Insects (Part 1)

22. Figure 40.4 Malpighian Tubules in Insects (Part 2)

23. Concept 40.3 Excretory Systems Produce Urine by Filtration, Reabsorption, and Secretion Vertebrates are well-adapted to excrete excess water. Kidney—the main excretory organ Nephron—the main functional unit of the kidney, consisting of a renal tubule and the surrounding blood vessels Nephrons filter large volumes of blood and achieve bulk reabsorption.

24. Figure 40.5 The Vertebrate Nephron

25. Concept 40.3 Excretory Systems Produce Urine by Filtration, Reabsorption, and Secretion A nephron begins with Bowman’s capsule, which encloses the glomerulus. Blood enters through the afferent arteriole and leaves through the efferent arteriole. The glomerulus is highly permeable to water, ions, and small molecules, but impermeable to cells and large molecules.

26. Figure 40.6 A Tour of the Nephron (Part 1)

27. Figure 40.6 A Tour of the Nephron (Part 2)

28. Concept 40.3 Excretory Systems Produce Urine by Filtration, Reabsorption, and Secretion Filtration occurs when blood pressure drives water and solutes through fenestrations in glomerular capillaries. Filtration slits in Bowman’s capsule are formed by podocytes—specialized cells with projections that wrap around capillaries. Glomerular filtration rate is the rate of the filtered fluid entering the capsule.

29. Concept 40.3 Excretory Systems Produce Urine by Filtration, Reabsorption, and Secretion The filtrate entering the capsule is similar to blood plasma—its composition is adjusted as it passes along the renal tubule Peritubular capillaries transport substances to and from the renal tubules.

30. Concept 40.3 Excretory Systems Produce Urine by Filtration, Reabsorption, and Secretion Marine bony fishes must conserve water in their high osmolarity environment. They minimize water loss by producing very little urine. Some ions are not absorbed in their gut— NaCl is excreted through the gills.

31. Concept 40.3 Excretory Systems Produce Urine by Filtration, Reabsorption, and Secretion Cartilaginous fishes convert nitrogenous wastes to compounds and retain large amounts in the extracellular fluid. The fluid is similar in osmolarity to seawater so water is not lost by osmosis to the environment.

32. Concept 40.3 Excretory Systems Produce Urine by Filtration, Reabsorption, and Secretion Amphibians in dry environments reduce permeability of their skin to water. Estivation is a state of low metabolic activity and low water turnover. Some frogs fill a large bladder with dilute urine before estivation and gradually reabsorb it into the blood.

33. Concept 40.3 Excretory Systems Produce Urine by Filtration, Reabsorption, and Secretion Reptiles are amniotes and have three major adaptations that allow them to exist outside of water: Amniotic reproduction, shelled eggs Scaled epidermis that retards water loss Excretion of nitrogenous wastes as uric acid, with little water loss

34. Concept 40.3 Excretory Systems Produce Urine by Filtration, Reabsorption, and Secretion Mammals also have adapted to conserve water: Skin covering to reduce water loss Amniotic reproduction Evolution of a kidney to produce concentrated urine

35. Concept 40.4 The Mammalian Kidney Produces Concentrated Urine Mammals have kidneys that filter blood and produce urine. In each nephron, there is a specialized feature: the loop of Henle. Ion transport in this region creates an area of high osmolarity, so that water is able to be reabsorbed from the urine. This concentrates the urine and reduces water loss.

36. Concept 40.4 The Mammalian Kidney Produces Concentrated Urine Ureter—a duct from the kidney that leads to the urinary bladder Urethra—a tube for urine excretion leading from the urinary bladder, where urine is stored, to the outside of the body The ureter, renal artery, and renal vein enter the kidney on the concave side.

37. Figure 40.7 The Human Excretory System (Part 1)

38. Figure 40.7 The Human Excretory System (Part 2)

39. Concept 40.4 The Mammalian Kidney Produces Concentrated Urine Kidneys have an outer cortex that covers the inner medulla. The glomeruli and Bowman’s capsules are in the cortex. Proximal convoluted tubules—the initial, twisted segments of the renal tubules, located in the cortex

40. Figure 40.7 The Human Excretory System (Part 3)

41. Concept 40.4 The Mammalian Kidney Produces Concentrated Urine The renal tubule descends into the medulla and forms the loop of Henle, which is important for urine concentration. After forming the loop, the tubule returns to the cortex. The loop of Henle leads to the distal convoluted tubule. The distal convoluted tubules join the collecting duct in the cortex. Collecting ducts empty into the pelvis, which drains into the ureter.

42. Concept 40.4 The Mammalian Kidney Produces Concentrated Urine The vasa recta is a network of peritubular capillaries parallel to the loops of Henle and the collecting duct. Blood plasma that does not enter Bowman’s capsule goes via the efferent artery to the peritubular capillaries. These play an important role in secretion and reabsorption and in maintaining the high-osmolarity region.

43. Concept 40.4 The Mammalian Kidney Produces Concentrated Urine The proximal convoluted tubule (PCT) is responsible for the reabsorption of water and solutes—osmolarity does not change. PCT cells actively transport Na +, glucose, and amino acids. Water follows the transport of solutes. Next steps in urine processing: Reabsorb salts, leaving urea Set up conditions for hypertonic urine production

44. Concept 40.4 The Mammalian Kidney Produces Concentrated Urine Concentration of urine is due to a countercurrent multiplier mechanism in the loops of Henle. Tubule fluid flows in opposite directions in the two limbs of a loop of Henle. The loops increase osmolarity of extracellular fluid in a graduated way.

45. Figure 40.8 Concentrating the Urine

46. Concept 40.4 The Mammalian Kidney Produces Concentrated Urine Loop of Henle segments: Thick ascending limb—actively transports Na + (Cl – follows) and raises its concentration in the interstitial fluid Thin descending limb—loses water to the neighboring interstitial fluid with high Na + and Cl – concentration

47. Concept 40.4 The Mammalian Kidney Produces Concentrated Urine Thin ascending limb—receives concentrated fluid from descending limb and allows diffusion of Na + and Cl – into the interstitial fluid Fluid reaching the distal collecting duct is less concentrated—solutes in the medulla create a concentration gradient.

48. Concept 40.4 The Mammalian Kidney Produces Concentrated Urine The concentration gradient is preserved by the vasa recta. Blood flowing down the descending limb loses water and gains solutes. Concentrated blood flowing up the ascending limb gains water and loses solutes—water is thus returned to the bloodstream.

49. Concept 40.4 The Mammalian Kidney Produces Concentrated Urine Water reabsorption and fine-tuning of ionic composition begins in the distal convoluted tubule. Fluid that leaves the tubule and flows into the collecting duct as urine has different solute composition than blood plasma.

50. Concept 40.4 The Mammalian Kidney Produces Concentrated Urine In collecting duct the major solute in tubular fluid is urea. Fluid flows down collecting duct and loses water to interstitial fluid because of concentration gradient established by loops of Henle. Some urea also diffuses and adds to osmotic force—recycling this urea contributes to urine concentration.

51. Concept 40.4 The Mammalian Kidney Produces Concentrated Urine Renal failure results in: Salt and water retention (high blood pressure) Urea retention (uremic poisoning) Decreasing pH (acidosis) Dialysis treatment passes blood through membrane channels bathed in a plasma- like solution to remove wastes.

52. Concept 40.5 The Kidney Is Regulated to Maintain Blood Pressure, Blood Volume, and Blood Composition A constant glomerular filtration rate (GFR) requires blood supplied to the kidneys under adequate pressure. Autoregulatory mechanisms ensure blood supply and blood pressure. Hormones released by other organs also help regulate the kidneys.

53. Concept 40.5 The Kidney Is Regulated to Maintain Blood Pressure, Blood Volume, and Blood Composition If GFR begins to fall, the first autoregulatory response is dilation of afferent renal arterioles—increases glomerular blood pressure. Kidney releases renin if GFR still falls, this activates angiotensin.

54. Concept 40.5 The Kidney Is Regulated to Maintain Blood Pressure, Blood Volume, and Blood Composition Angiotensin: Constricts efferent renal arterioles Constricts peripheral blood vessels to raise blood pressure in the body Stimulates release of aldosterone to increase Na + uptake Stimulates thirst to increase water ingestion to raise blood volume and pressure

55. Figure 40.9 Renin-Angiotensin-Aldosterone System Helps Regulate GFR

56. Concept 40.5 The Kidney Is Regulated to Maintain Blood Pressure, Blood Volume, and Blood Composition The hypothalamus can stimulate release of antidiuretic hormone (ADH, also called vasopressin). ADH increases the permeability of membranes to water. Osmoreceptors that detect a rise in blood osmolarity will stimulate ADH release.

57. Concept 40.5 The Kidney Is Regulated to Maintain Blood Pressure, Blood Volume, and Blood Composition ADH causes aquaporins, or water channels, to be inserted in the membranes of cells in the collecting duct. More water is reabsorbed and urine is more concentrated. Alcohol inhibits release of ADH—excessive alcohol intake can cause substantial dehydration.

58. Concept 40.5 The Kidney Is Regulated to Maintain Blood Pressure, Blood Volume, and Blood Composition Atrial muscle fibers release atrial natriuretic peptide (ANP) when blood volume in the atria increases ANP decreases the reabsorption of Na + in the kidney. Increased loss of Na + and water decreases blood volume and pressure.

59. Figure 40.10 ADH Induces Insertion of Aquaporins into Plasma Membranes (Part 1)

60. Figure 40.10 ADH Induces Insertion of Aquaporins into Plasma Membranes (Part 2)

61. Answer to Opening Question Excretory systems include active transport of Na +, often with Cl –. Ion transport creates osmotic concentration gradients that move water across membranes. Depending on an animal’s environment, it will direct the absorption or secretion of solutes. Sea birds ingest salt water and use a salt- secreting organ with special transporters that take up NaCl and secrete it.

62. Figure 40.11 Salt Excretion in a Marine Bird