1. Lecture Outlines by Gregory Ahearn, University of North Florida Copyright © 2011 Pearson Education Inc. Chapter 35 The Urinary System

2. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Chapter 35 At a Glance  35.1 What Are the Basic Functions of Urinary Systems?  35.2 What Are Some Examples of Invertebrate Excretory Systems?  35.3 What Are the Functions of the Human Urinary System?  35.4 What Are the Structures of the Human Urinary System?  35.5 How Is Urine Formed and Concentrated?  35.6 How Do Vertebrate Kidneys Help Maintain Homeostasis?

3. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.1 What Are the Basic Functions of Urinary Systems?  Urinary systems help maintain homeostasis— the relatively constant internal environment required to preserve health and life –Urinary systems regulate the composition of the blood and extracellular fluid, the watery substance that bathes all cells –They also help control the concentration, or osmolarity, of dissolved substances in cells and in their extracellular environment

4. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.1 What Are the Basic Functions of Urinary Systems?  Urinary systems help maintain homeostasis— the relatively constant internal environment required to preserve health and life (continued) –A second function of urinary systems is excretion, or the removal of unwanted substances from the body –Urinary systems produce urine, which contains the waste products of cellular metabolism

5. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.1 What Are the Basic Functions of Urinary Systems?  Urinary system functions are performed through three basic processes –Blood or extracellular fluid is filtered, removing water and small dissolved substances –Nutrients are selectively reabsorbed back into the filtered fluid –Excess water, excess nutrients, and dissolved wastes are excreted from the body in urine

6. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.2 What Are Some Examples of Invertebrate Excretory Systems?  Protonephridia filter extracellular fluid in flatworms  The earliest excretory system probably served to maintain water balance, which is the primary function of the simple excretory system of flatworms –This early excretory system consists of protonephridia, which are tubules that branch throughout extracellular fluid that surrounds the flatworm’s tissues –This excretory system serves to collect excess water from the extracellular fluid using ciliated “flame cells,” and forces the fluid out of the animal through excretory pores –The large body surface of flatworms also serves as an excretory structure through which most cellular waste diffuse out

7. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e tubule excretory pore extracellular fluid flame cell cilia nucleus (a) Flatworms use protonephridia excretory pore eye spot Flatworms Use Protonephridia Fig. 35-1a

8. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.2 What Are Some Examples of Invertebrate Excretory Systems?  Malphigian tubules filter the hemolymph of insects –Insects have an open circulatory system where hemolymph (a fluid that serves as both blood and extracellular fluid) fills the hemocoel (the body cavity) and bathes the internal tissues and organs directly –Insect excretory systems consist of Malpighian tubules, small tubes that extend outward from the intestine and end blindly within the hemolymph

9. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.2 What Are Some Examples of Invertebrate Excretory Systems?  Malphigian tubules filter the hemolymph of insects (continued) –Wastes and nutrients move from the hemolymph into the tubules by diffusion and active transport, and water follows by osmosis –Urine is conducted into the intestine, where important solutes are secreted into the hemolymph by active transport –Insects produce very concentrated urine, which is excreted along with feces

10. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e abdomen Malpighian tubules intestine rectum hemocoel (filled with hemolymph) cellular and digestive wastes (b) Insects use Malpighian tubules Insects Use Malphigian Tubules Fig. 35-1b

11. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.2 What Are Some Examples of Invertebrate Excretory Systems?  Nephridia filter extracellular fluid in earthworms –In earthworms, mollusks, and several other invertebrates, excretion is performed by tubular structures called nephridia –In the earthworm, the body cavity (the coelom) is filled with extracellular fluid into which wastes and nutrients from the blood diffuse

12. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.2 What Are Some Examples of Invertebrate Excretory Systems?  Nephridia filter extracellular fluid in earthworms (continued) –Each nephridium begins with a funnel-like opening, the nephrostome, ringed with cilia that direct extracellular fluid into a narrow, twisted tubule surrounded by capillaries –As the fluid traverses the tubule, salts and nutrients are reabsorbed back into the capillary blood, leaving the wastes and water behind –The resulting urine is then excreted through an opening in the body called a nephridiopore –Each segment in an earthworm’s body contains a pair of nephridia

13. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e coelom (filled with extracellular fluid) nephrostome nephridium nephridiopore (c) Earthworms use nephridia capillary bed Earthworms Use Nephridia Fig. 35-1c

14. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.3 What Are the Functions of Vertebrate Urinary Systems?  Kidneys are the organs of the vertebrate urinary system where the blood is filtered and urine is produced –Because vertebrates live in such a wide variety of habitats, vertebrate kidneys face radically different challenges in maintaining constant conditions within their bodies

15. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.3 What Are the Functions of Vertebrate Urinary Systems?  The kidneys of humans and other mammals perform many homeostatic functions –The mammalian urinary system consists of the kidneys, ureters, bladder, and urethra –These organs filter the blood, collecting and then excreting the dissolved waste products in urine –During filtration, water and dissolved molecules are forced out of the blood –The kidneys then return to the blood nearly all of the water and nutrients required by the body –The urine retains wastes, which are expelled from the body

16. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.3 What Are the Functions of Vertebrate Urinary Systems?  The mammalian urinary system helps maintain homeostasis in several ways, by: –Regulating blood levels of ions such as sodium, potassium, chloride, and calcium –Maintaining proper pH of the blood by regulating hydrogen and bicarbonate ion concentrations –Regulating the water content of the blood

17. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.3 What Are the Functions of Vertebrate Urinary Systems?  The mammalian urinary system helps maintain homeostasis in several ways, by: (continued) –Retaining important nutrients such as glucose and amino acids in the blood –Eliminating cellular waste products such as urea –Secreting substances that help regulate blood pressure and blood oxygen levels

18. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.3 What Are the Functions of Vertebrate Urinary Systems?  Urea is a waste product of protein digestion –An important function of most urinary systems is to eliminate nitrogenous (nitrogen-containing) wastes that are formed when cells break down amino acids –Nitrogenous wastes from cells enter the blood as ammonia (NH 3 ), which is toxic –The livers of humans and other mammals convert ammonia into urea, which is less toxic –Urea is filtered from the blood by the kidneys and then excreted in the urine

19. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Urea Formation and Excretion Fig. 35-2 ammonia NH 3 amino acid Proteins in food are digested Amino acids are carried in the blood to body cells The cells convert the amino groups (-NH 2 ) to ammonia, which is carried in the blood to the liver 1 2 3 urea The liver converts ammonia to urea, which is less toxic In kidney nephrons, urea is filtered into the urine Urea is carried in the blood to the kidneys 4 6 5

20. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.4 What Are the Structures of the Human Urinary System?  The urinary system consists of the kidneys, ureters, bladder, and urethra –Kidneys are paired organs located on either side of the spinal column, just above the waist –Blood enters each kidney through the renal artery, and after the blood has been filtered, it exits through the renal vein –Urine leaves each kidney through a narrow, muscular tube called the ureter

21. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.4 What Are the Structures of the Human Urinary System?  The urinary system consists of the kidneys, ureters, bladder, and urethra (continued) –Rhythmic contractions of the ureter transports urine to the bladder, a hollow, muscular chamber that collects and stores blood –The bladder wall is lined with smooth muscle and is capable of considerable expansion, accommodating up to a pint of urine

22. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.4 What Are the Structures of the Human Urinary System?  The urinary system consists of the kidneys, ureters, bladder, and urethra (continued) –Urine is contained within the bladder by two sphincter muscles –The internal sphincter, located where the bladder joins the urethra, opens automatically during the reflexive contractions of the smooth muscle –The external sphincter, located slightly below the internal sphincter, is under voluntary control, allowing the brain to suppress urination unless the bladder becomes overly full –When open, the sphincters allow urine to flow into the urethra, a single narrow tube that conducts urine outside the body

23. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Author Animation: Overview of the Kidney

24. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e left renal artery left kidney left renal vein aorta left ureter urinary bladder urethra (in penis) vena cava The Human Urinary System Fig. 35-3

25. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.4 What Are the Structures of the Human Urinary System?  The structure of the kidney supports its function of producing urine –Each kidney contains a solid outer layer consisting of the renal cortex, which overlies an inner layer called the renal medulla –The renal medulla surrounds a branched, funnel- like chamber called the renal pelvis, which collects urine and funnels it into the ureter

26. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Cross-Section of a Kidney Fig. 35-4 renal artery renal vein ureter renal pelvis (cut away to show the path of urine) renal cortex enlargement of a single nephron and collecting duct renal medulla renal cortex renal pelvis nephron urine to the bladder renal medulla collecting duct

27. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.4 What Are the Structures of the Human Urinary System?  The renal cortex of each kidney is made up of more than 1 million microscopic filters called nephrons –Each nephron has two major parts –The glomerulus, which is a dense knot of capillaries where fluid is filtered out of the blood through the porous capillary walls –A long, twisted tubule, where urine formation occurs

28. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.4 What Are the Structures of the Human Urinary System?  The tubule has four major sections –The tubule begins with Bowman’s capsule, a cup-like chamber that surrounds the glomerulus and receives fluid filtered out of the blood from the glomerular capillaries –The remaining sections of the tubule return water and nutrients to the blood while retaining wastes

29. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.4 What Are the Structures of the Human Urinary System?  The tubule has four major sections (continued) –From Bowman’s capsule, the fluid is conducted to the proximal tubule –The loop of Henle carries the filtered fluid from the cortex deep into the medulla and back to the cortex –The distal tubule is in the cortex, and collects the filtrate from the loop of Henle and passes it on to the collecting ducts –The collecting duct is not part of the nephron, but collects fluid from many nephrons and deposits it in the renal pelvis

30. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Author Animation: Parts of the Nephron

31. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e collecting duct distal tubule proximal tubule glomerulus Bowman’s capsule arterioles venule branch of the renal vein branch of the renal artery loop of Henle capillaries An Individual Nephron and Its Blood Supply Fig. 35-5

32. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.4 What Are the Structures of the Human Urinary System?  The kidney’s blood supply allows it to fine-tune blood composition –To support their role in maintaining homeostasis, the kidneys have an enormous blood supply, receiving more than one quart of blood every minute –Blood flows to each kidney from the renal artery, which branches into arterioles that each supply single nephrons with blood for filtration –The arterioles branch into capillaries that form the glomerulus of each nephron –The capillaries empty into an outgoing arteriole that branches into more capillaries that surround the tubule –The capillaries carry blood into a venule that takes the blood to the renal vein and then the inferior vena cava

33. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.5 How Is Urine Formed and Concentrated?  Urine is produced in the nephrons of the kidneys in three stages –Filtration, during which water and most small dissolved molecules are filtered out of the blood –Tubular reabsorption, the process by which water and necessary nutrients are restored to the blood –Tubular secretion, during which wastes and excess ions that still remain in the blood are secreted into the urine

34. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Urine Formation and Concentration Fig. 35-6 1 Tubular secretion: Additional wastes are actively transported into the proximal and distal tubules from the blood 3 4 Filtration: Water, nutrients, and wastes are filtered from the glomerular capillaries into the Bowman’s capsule of the nephron Tubular reabsorption: In the proximal tubule, most water and nutrients are reabsorbed into the blood 2 Concentration: The loop of Henle produces a salt concentration gradient in the extracellular fluid; in the collecting duct, urine may become more concentrated than the blood as water leaves by osmosis blood leaving the glomerulus loop of Henle blood entering the glomerulus Bowman’s capsule collecting duct distal tubule proximal tubule

35. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.5 How Is Urine Formed and Concentrated?  As urine is formed, essentially all small organic nutrients, including amino acids and glucose, are filtered out of and then returned to the blood –Large quantities of water and many ions are also filtered out, but their return rate is continuously adjusted to meet the body’s changing needs –These ions include sodium (Na + ), chloride (Cl – ), potassium (K + ), calcium (Ca ++ ), hydrogen (H + ), and bicarbonate

36. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.5 How Is Urine Formed and Concentrated?  Urine is formed in the glomerulus and tubule of each nephron –Filtration, the first step in urine formation, occurs when water carrying small dissolved molecules and ions is forced through the walls of the capillaries that form the glomerulus –Blood cells and large proteins are too large to leave the capillaries, and so remain behind in the blood –The fluid filtered out of the glomerular capillaries, called filtrate, is collected in Bowman’s capsule and then continues through the tubule

37. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.5 How Is Urine Formed and Concentrated?  Urine is formed in the glomerulus and tubule of each nephron (continued) –Tubular reabsorption occurs primarily in the proximal tubule, although water and other nutrients are also reabsorbed in other tubule areas –It returns nearly all organic nutrients (such as glucose, amino acids, and needed vitamins) and most of the ions (Na +, Cl –, K +, Ca ++, H + and HCO 3 – ) to the blood –It restores most of the water that was filtered from the blood, as water follows the nutrients and ions by osmosis through aquaporins, proteins that form water pores

38. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.5 How Is Urine Formed and Concentrated?  Urine is formed in the glomerulus and tubule of each nephron (continued) –During tubular secretion, remaining wastes and excess ions move from the blood into the proximal and distal tubules –These wastes include excess K + and H +, small quantities of ammonia, many drugs, food additives, pesticides, and toxic substances, such as nicotine –Tubular secretion, which occurs primarily by active transport, takes place in both the proximal and distal tubules –When the filtrate leaves the distal tubule, it has become urine

39. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.5 How is Urine Formed and Concentrated?  The loop of Henle creates an extracellular concentration gradient in the renal medulla –The function of the loop of Henle is twofold –Some water and salt is reabsorbed from the filtrate as it passes through the loop –Most importantly, it creates a high salt and urea concentration in the extracellular fluid within the medulla

40. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.5 How Is Urine Formed and Concentrated?  The loop of Henle creates an extracellular concentration gradient in the renal medulla (continued) –To understand why a high salt concentration is important, we must start with an important role of the human kidney: water regulation –The kidneys help to maintain appropriate water content in body tissues by producing dilute, watery urine when fluid intake is high, and concentrated urine when fluid intake is low –Water can be conserved by allowing it to move out of the collecting duct by osmosis down its concentration gradient –The more concentrated the extracellular fluid, the more water that can leave the urine as it moves through the collecting duct

41. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.5 How Is Urine Formed and Concentrated?  The loop of Henle creates an extracellular concentration gradient in the renal medulla (continued) –The loop of Henle produces and maintains a high salt concentration gradient in the extracellular fluid in the medulla by transporting salt out of the filtrate –The salt and urea gradient causes an osmotic gradient between the filtrate and the surrounding extracellular fluid –The most concentrated fluid surrounds the bottom of the loop –The collecting duct passes through this gradient as it conducts urine from the distal tubule in the renal cortex into the renal pelvis

42. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.5 How Is Urine Formed and Concentrated?  The loop of Henle creates an extracellular concentration gradient in the renal medulla (continued) –As the filtrate descends into the loop of Henle and collecting duct, the following occur –It is exposed to the osmotic gradient surrounding the nephron –Water leaves the filtrate by osmosis and enters the surrounding capillaries –Filtrate becomes urine when it enters the collecting duct, and can be more than four times as concentrated as the blood

43. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Details of Urine Formation Fig. E35-2 FILTRATION TUBULAR REABSORPTION & TUBULAR SECRETION URINE CONCENTRATION renal cortex renal medulla osmosis diffusion active transport Bowman’s capsule loop of Henle proximal tubule distal tubule 1 2 3 4 5 6 7 8 H2O*H2O* H2OH2O H2O*H2O* H2O*H2O* H2OH2O H2OH2O H2OH2O H2OH2O NaCI urea NaCI H + NH 3 some drugs Na + nutrients HCO 3 – Ca 2+ Cl – K + collecting duct H + K + some drugs (extracellular fluid) NaCl Ca 2+

44. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.6 How Do Vertebrate Kidneys Help Maintain Homeostasis?  The kidneys regulate the osmolarity of the blood –One important function of the kidney is to regulate the water content of the blood –Human kidneys filter out about half a cup of fluid from the blood each minute, fine-tuning the composition of the blood and helping to maintain homeostasis –If the kidneys did not reabsorb this water, the rate of filtration would require that we drink nearly 50 gallons of water a day

45. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.6 How Do Vertebrate Kidneys Help Maintain Homeostasis?  The kidneys regulate the osmolarity of the blood (continued) –Consequently, the urinary system needs to restore nearly all of the water that is initially filtered out of the glomeruli –The amount of reabsorption is precisely regulated, and the ability of kidneys to reabsorb water is under the influence of antidiuretic hormone (ADH) –ADH is secreted by the posterior pituitary gland and is carried in the bloodstream –It stimulates cells of the distal tubule and collecting ducts to insert more aquaporin proteins into their membranes –The abundance of aquaporin membranes determines the permeability of the membranes to water

46. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.6 How Do Vertebrate Kidneys Help Maintain Homeostasis?  The kidneys regulate the osmolarity of the blood (continued) –Under normal conditions, some ADH is always present in the blood –Within the hypothalamus, receptors monitor blood osmolarity, which increases when water is lost

47. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Author Animation: Urine Formation

48. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.6 How Do Vertebrate Kidneys Help Maintain Homeostasis?  The kidneys regulate the osmolarity of the blood (continued) –For example, when water is lost during dehydration: –If blood osmolarity exceeds an optimal level, the hypothalamus stimulates the pituitary gland to release ADH into the bloodstream –In response to ADH, cells of the distal tubule and collecting duct insert more aquaporins into their membranes, increasing their permeability to water –The more concentrated extracellular fluid draws water out by osmosis, restoring water to the blood through nearby capillaries

49. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Dehydration Stimulates ADH Release and Water Retention Fig. 35-7 Receptors in the hypothalamus detect the increased blood osmolarity and signal the pituitary gland ADH increases the permeability of the distal tubule and the collecting duct, allowing more water to be reabsorbed into the blood Water is retained in the body and concentrated urine is produced The pituitary gland releases ADH into the bloodstream Heat causes water loss and dehydration through sweating 2 3 5 4 1

50. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.6 How Do Vertebrate Kidneys Help Maintain Homeostasis?  Kidneys release substances that help regulate blood pressure and oxygen levels –When blood pressure falls, the kidneys release the enzyme renin into the bloodstream –Renin catalyzes the formation of the hormone angiotensin in the blood

51. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.6 How Do Vertebrate Kidneys Help Maintain Homeostasis?  Angiotensin helps combat low blood pressure in three major ways –It stimulates the proximal tubules of the nephrons to reabsorb more Na + into the blood, causing water to follow by osmosis –It stimulates ADH release, causing more water to be reclaimed from the distal tubule and collecting duct –It causes arterioles throughout the body to constrict, which directly increases blood pressure

52. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.6 How Do Vertebrate Kidneys Help Maintain Homeostasis?  Kidneys release substances that help regulate blood pressure and oxygen levels (continued) –When blood oxygen levels are low, the kidneys release the hormone erythropoietin –This hormone stimulates the bone marrow to make more red blood cells –The higher number of red blood cells increases the oxygen carrying capacity of the blood

53. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.6 How Do Vertebrate Kidneys Help Maintain Homeostasis?  Vertebrate kidneys are adapted to diverse environments –Mammals have structurally different nephrons, depending upon the availability of water in their natural habitat –Mammals adapted to dry climates generally have long loops of Henle –Longer loops of Henle allow for allow a higher concentration of salt to be produced in the extracellular fluid of the medulla, allowing more water to be reclaimed from the collecting duct – An example of a mammal with very long-looped nephrons is the desert kangaroo rat

54. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e A Well-Adapted Desert Dweller Fig. 35-8

55. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.6 How Do Vertebrate Kidneys Help Maintain Homeostasis?  Vertebrate kidneys are adapted to diverse environments –In contrast, mammals adapted to habitats with an abundance of fresh water typically have short loops of Henle –For example, beavers, which live along streams, can only concentrate their urine to about twice their blood osmolarity –Humans have a mixture of long- and short- looped nephrons, and can concentrate urine to about four times the osmolarity of blood

56. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.6 How Do Vertebrate Kidneys Help Maintain Homeostasis?  Freshwater and saltwater environments pose special challenges for water regulation –Animals have evolved homeostatic mechanisms, including kidney adaptations, to maintain water and salt within their bodies, a process called osmoregulation –For example, freshwater fish live in a hypotonic environment –Water continuously leaks into their bodies by osmosis –Salts diffuse out –Freshwater fish acquire salt from their food and through their gills but never drink –Their kidneys retain salt and excrete large quantities of extremely dilute urine

57. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e (a) Freshwater fish The kidneys conserve salt and excrete large amounts of dilute urine Salt is pumped in by active transport Salt and some water enters in food water salt Water moves in by osmosis; salt diffuses out fresh water Osmoregulation in Fish Fig. 35-9a

58. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.6 How Do Vertebrate Kidneys Help Maintain Homeostasis?  Freshwater and saltwater environments pose special challenges for water regulation (continued) –Saltwater fish live in a hypertonic environment; seawater has a solute concentration of two to three times that of their body fluids –Water is constantly leaving their tissues by osmosis, and salt is constantly diffusing in and being taken in with food –To compensate for these effects, saltwater fish drink to restore their lost water, and excess salt they take in is excreted by active transport through their gills

59. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e (b) Saltwater fish Salt and water enter in food and by drinking seawater Some salt is excreted in small quantities of urine Water moves out by osmosis; salt diffuses in Salt is pumped out by active transport water salt salt water Osmoregulation in Fish Fig. 35-9b

60. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 35.6 How Do Vertebrate Kidneys Help Maintain Homeostasis?  Freshwater and saltwater environments pose special challenges for water regulation (continued) –Fish nephrons completely lack loops of Henle, and so fish cannot produce concentrated urine –To conserve water, the kidneys of most saltwater fish excrete very small quantities of urine containing salts not eliminated by their gills