Chapter 44

1. Chapter 44 Osmoregulation & Excretion

2. Main Ideas 1.) Since the cells & tissues of all animals must be maintained in a fluid environment, the relative concentrations of water & solutes in that environment is extremely important. 2.) Animals in diverse habitats show diverse adaptations for maintaining water/solute concentrations:

3. Main Ideas a.) Animals in marine environments must get rid of excess salt. b.) Animals in freshwater environments must reduce their uptake of water & conserve solutes. c.) Animals in deserts must conserve water.

4. Main Ideas 3.) In addition to balancing the water & solute concentrations of the body’s fluids, animals must get rid of wastes. a.) One HUGE concern: the breakdown of proteins & nucleic acids form ammonia (a nitrogenous waste) that is quite toxic & must be excreted from the body.

5. Main Ideas 4.) The homeostatic processes that control the water/solute & waste problem are: a.) Osmoregulation: regulation of solute concentrations & gain/loss of water. b.) Excretion: how animals get rid of N-containing waste products.

6. Osmosis Review 1.) Animal cells must balance water uptake with loss – since they lack cell walls, with too much water they burst, with too little they shrivel. 2.) Water moves across the plasma membrane by osmosis.

7. Osmosis Review a.) Osmosis occurs whenever 2 solutions separated by a membrane differ in either… i.) Osmotic pressure ii.) Osmolarity: this is the total solute concentration expressed as moles of solute per liter.

8. Osmosis Review 3.) Some situations… a.) If two solutions have the same solute concentration (osmolarity) there is no net movement of water between them (isoosmotic/isotonic). b.) When two solutions differ in solute concentration, the one with the higher concentration (higher osmolarity) is said to be hyperosmotic (hypertonic) to the other. Therefore, the one with the lower concentration is said to be hypoosmotic (hypotonic). i.) Water flows from an hypoosmotic solution to a hyperosmotic solution. (From where there is a higher water concentration to where there is a lower water concentration).

11. Osmoregulation Differences in Animals 1.) Some animals (which live in marine environments) are termed osmoconformers because they do not actively adjust their internal osmolarity & it is the same as their external environment. 2.) Most animals are osmoregulators. a.) For marine animals that do this, they must conserve water & excrete excess salt. b.) Freshwater animals must excrete excess water & conserve solutes. c.) Animals on land must conserve water by eating/drinking moist foods & using water produced by cellular respiration.

12. Osmoregulation Differences in Animals 3.) Adaptations of land animals for conserving water include: a.) Exoskeletons of insects b.) Layers of skin or scales (made with keratin in reptiles) c.) Nocturnal lifestyle d.) Internal gas exchange organs

13. Similarities in Osmoregulation in Animals The main goal of osmoregulation in all animals is to maintain the water/solute concentration in their cells. Most animals do this indirectly by maintaining the composition of the fluid surrounding their cells – in our case it would be our interstitial fluid. We actually control this by maintaining concentrations of water/solutes in our blood.

14. Similarities in Osmoregulation in Animals 1.) Most animals have specialized cells (or layers of these cells) called transport epithelium that regulate the movement of solutes. a.) Transport epithelia move specific solutes in controlled amounts in specific directions. b.) Transport epithelia are typically arranged in tubular networks with HUGE surface areas to maximize movement of water & solutes. c.) Our transport epithelia are in our kidneys – they have the dual function of controlling water & solute concentrations & excreting metabolic wastes (such as those containing N).

15. Concept Check 1.) The movement of salt from the surrounding water to the blood of a freshwater fish requires the expenditure of energy in the form of ATP. Why?

16. Nitrogenous Wastes 1.) Most metabolic wastes are dissolved in water when they are removed from the body. a.) Therefore, the amount & type of waste that needs to be excreted can have a big impact on an animal’s water balance.

17. Nitrogenous Wastes 2.) Forms of nitrogenous wastes (which result from the breakdown of proteins & nucleic acids): a.) Ammonia: very soluble in water but very toxic (can only be tolerated at low concentrations). Animals that excrete this from of N waste need a lot of water. i.) Therefore, ammonia excretion is most common in aquatic animals.

18. Nitrogenous Wastes b.) Mammals convert ammonia to urea in liver cells & then excrete urea. i.) Advantage: Has a low toxicity so does not require a lot of water to transport, store & excrete. ii.) Disadvantage: Body must expend energy to produce urea from ammonia.

19. Nitrogenous Wastes c.) Birds & reptiles convert ammonia to uric acid. i.) Advantage: low toxicity & insoluble in water which means it can be excreted as semi-solid paste with little water loss. Can also be stored in eggs with shells since it won’t dissolve in fluid around embryo & poison it. Therefore, uric acid is an evolutionary adaptation to a terrestrial environment. ii.) Disadvantage: requires even more energy to produce than urea.

21. Concept Check 1.) Dragonfly larvae which are aquatic, excrete ammonia, whereas adult dragonflies, which are terrestrial excrete uric acid. Explain.

22. Overview of Excretory Process 1.) Filtrations: Body fluid (blood, hemolymph) is collected & filtered through the membranes of transport epithelia. a.) Water, small solutes such as sugars, salts, amino acids & N wastes are forced from blood & into excretory organ. This fluid is now called the filtrate. 2.) Selective reabsorption: use of active transport to reabsorb needed molecules like glucose, some salts & amino acids.

23. Overview of Excretory Process 3.) Nonessential solutes & toxins are kept in filtrate & may be added to by secretion – actively pumping in more excess solutes or toxins. 4.) Excretion: the filtrate leaves the excretory organ & then the body.

24. Variation in Excretory Systems 1.) Flatworms have protonephridia: dead-end tubules that use cilia to draw water & solutes into tubule which empties into external environment. 2.) Annelids (earthworms) have metanephridia: internal openings surrounded by capillaries. Blood is filtered & waste leaves through pores that empty to external environment.

26. Variation in Excretory Systems 3.) Insects have Malpighian tubules: tubes that open into digestive tract & dead end at tips immersed in hemolymph. Tubules filter hemolymph & empty wastes into digestive tract.

27. Variation in Excretory Systems 4.) Vertebrates have kidneys to function in osmoregulation & excretion. Like other excretory organs, our kidneys are built of tubules that function in filtration.

30. Structure of the Kidney 1.) We have 2 kidneys – each about 10cm long. a.) They are supplied with blood by a renal artery & drained by a renal vein. b.) Urine exits each kidney through a ureter – these both empty into the urinary bladder. c.) Urine exits the bladder through a urtethra.

31. Structure of the Kidney 2.) The kidney has 2 regions: a.) An outer renal cortex b.) Inner renal medulla c.) Both regions are packed with microscopic tubules called nephrons – these are the functional units of the kidneys! i.) Each kidney contains ~1,000,000 nephrons with a total tubule length of 80 km!

32. Structure of the Kidney 3.) Structure of the nephron: a.) A single, long tubule & ball of capillaries. b.) The capillaries are called the glomerulus they are surrounded by an end of the tubule called Bowman’s capsule.

33. Overview of Nephron Function 1.) Blood pressure forces fluid from the glomerulus into Bowman’s capsule. a.) The filtrate contains water, salts, glucose, amino acids, vitamins, N wastes (urea), etc. 2.) The filtrate then passes through 3 regions of the nephron which are also surrounded by capillaries: a.) The proximal tubule b.) The loop of Henle c.) The distal tubule which empties into a collecting duct. d.) Filtrate from the collecting duct will empty into the ureter & then bladder.

34. Overview of Nephron Function 3.) Humans have 2 types of nephrons – some have shorter loops of Henle & some have very long ones (these are called juxtamedullary nephrons). a.) It is these nephrons with LONG loops of Henle that allow us to produce urine that is hyperosmotic to our body fluids. b.) This is an important adaptation for water conservation.

35. Overview of Nephron Function 4.) Each day the kidneys process about 180 L of initial filtrate (2 or 3 times your body weight). a.) Of this, almost all the sugar, vitamins, nutrients & water are reabsorbed leaving about 1.5 L of urine to be excreted.

36. Filtrate to Urine: A Closer Look 1.) From Bowman’s capsule, filtrate passes to the proximal tubule. a.) Reabsorption of salt, water, nutrients, bicarbonate out of tubule, into interstitial fluid & then back into capillaries. b.) Cells surrounding tubule secrete H+ into tubule to help control pH.

37. Filtrate to Urine: A Closer Look 2.) Filtrate enters the descending limb of the loop of Henle: a.) Continued reabsorption of water (meaning water moves out of tubule), not permeable to salt 3.) Filtrated enters ascending limb of loop of Henle: a.) Permeable to salt, not water

38. Filtrate to Urine: A Closer Look 4.) Filtrate enters distal tubule: a.) Regulates concentrations of K+ and NaCl by varying amounts of K+ secreted into filtrate and NaCl reabsorbed. 5.) Filtrate enters the collecting duct which is permeable to water so water continues to leave the filtrate and be reabsorbed.

39. Filtrate to Urine: A Closer Look a.) The filtrate becomes more and more concentrated and is hyperosmotic to body fluids. b.) This is a key adaptation to living a terrestrial lifestyle – the ability to reabsorb water & excrete very concentrated urine.

40. Note: The diffusion of salt out of ascending limb of loop of Henle helps maintain a gradient so that water flows out of descending limb. There is a similar relationship b/w the collecting duct & the ascending limb.

41. Filtrate to Urine: A Closer Look 6.) Much of the transport of salt & other molecules is done actively in the kidneys which means it requires energy. It takes a lot of energy to produce urine!

42. Regulation of Kidney Function The kidney can adjust the volume & osmolarity of urine depending on our water/salt concentrations & the rate of our urea production. How is all this regulated? 1.) Antidiuretic hormone (ADH): produced by hypothalamus & stored/released from the pituitary gland.

43. Regulation of Kidney Function a.) When osmolarity of blood rises (meaning too much solute, not enough water), ADH is released & causes the distal tubule & collecting duct to be more permeable to water which increases water reabsorption. b.) As osmolarity lowers, this triggers ADH to stop being released & reduces water permeability in the nephron again. (Negative feedback loop!) c.) Alcohol can disturb water balance & inhibit the release of ADH causing excessive water loss through urination leading to dehydration.

44. Regulation of Kidney Function 2.) A second homeostatic mechanism responds to a decrease in blood volume/blood pressure. This system of hormones is called the RAAS system. a.) If blood volume/pressure drops, hormones released cause arterioles to constrict and water and salt to be reabsorbed in the kidneys to increase blood volume.

45. Adaptations of Kidneys 1.) Depending on environment & needs, the vertebrate kidney has many adaptations. Some examples… a.) Desert animals typically have LONG loops of Henle to allow urine to become VERY concentrated & to reabsorb as much water as possible. b.) Aquatic mammals have shorter loops of Henle. c.) Freshwater fish have MANY nephrons. They must produce a lot of urine to excrete water continuously or their body fluids would become too dilute.