1. Excretion and Osmoregulation

2. Outline Introduction Comparative physiology of osmotic regulation
Mammalian kidney
Evolution of the vertebrate kidney

3. All organisms have an excretory system of some type in order to
Manage solutes in body fluids
Manage water content of the body
Remove metabolic end-products
Remove foreign substances

4. Two basic modes of excretion
Ultrafiltration- pressure filtrate of blood, withholds protein and large solutes but water and small solutes pass
Active transport- against a concentration gradient. If directed away from the organism, secretion. If directed toward the organism, reabsorption.

5. Three functions of excretory systems
Ultrafiltration
Secretion
Reabsorption

6. Osmotic Environments Aquatic environments (71% of Earth’s surface)
Sea waters- 3.5% salts
Mediterranean – 4% salts
Saturated with salts-
Great salt lake (NaCl)- no fish, some shrimp
Dead Sea (MgCl2)

7. Osmoconformers and osmoregulators
Osmoconformer- change osmotic concentration of body fluids to match environment
Osmoregulator- regulate composition of body fluids regardless of environment
Seawater-1000 mOsM- animals risk losing water to the environment
Fresh water-animals risk taking on too much water and losing salt

8. Two major categories of resistance to changes in osmotic environment
Stenohaline –limited tolerance to changes in osmotic composition of environment-most fish are stenohaline
Euryhaline- can tolerate wide fluctuations in environmental osmotic concentrations

10. Marine vertebrates Elasmobranchs, hagfish, crab-eating frogs
Osmolarity of body fluids equals that of sea-water, i.e mOsM
Lampreys, teleost fishes
Osmolarity of body fluids is one-third that of seawater, approx. 300 mOsM.

11. Marine Elasmobranchs
Maintain salt at one-third that of seawater, but add organic urea to body fluids-100 times greater than mammals
Total body fluid osmolarity is close to seawater
Produce trimethylamine oxide (TMAO)- neutralizes toxic effects of urea
Shark proteins including enzymes are dependent upon urea
Excess Na+ excreted by ‘rectal gland’

12. TMAO
urea

13. Teleosts
All teleosts maintain body salt concentration at 1/3 that of seawater
Marine teleosts must drink water and get rid of excess ions from water
Ion excretion (Na, Cl) is performed by gills
Mg, SO4, and other divalent ions secreted by kidneys

14. Saltwater

15. Freshwater

16. Amphibians Risk losing ions to freshwater
Actively reabsorb Na and Cl across skin

17. Terrestrial vertebrates
Face dehydration from any body surface
Birds and reptiles produce uric acid to conserve water
Develop skin which prevents water loss

18. Dipodomys microps, the Kangaroo rat

20. Kangaroo rats Abundant in deserts Do not drink water
If given a diet of barley or oats, body weight remains the same for months
Water (g) formed per gram of substrate:
Starch = 0.56
Fat=1.07
Protein=0.45

22. Nasal turbinates Increase water reabsorption from exhaled air
Nasal turbinates reduce temperature of exhaled air
More water vapor condenses on nasal epithelium during exhalation

24. Phylogeny of osmoregulatory and excretory organs

25. Contractile vacuole in Paramecium
Found only in freshwater organisms
Not a true excretory organ because it does not produce an ultrafiltrate
Excess water is channeled to vacuole, vacuole swells
Water expelled through pore

26. Paramecium- contractile vacuole

27. Protonephridia Blind ended excretory organ
Solenocyte (1 flagella), flame cell (many flagella)
Found in flatworms, rotifers
May or may not produce an ultrafiltrate
Since there is no circulatory system in these animals, flame cells must be located throughout the body

28. http://www. cartage. org

29. Metanephridia Only in eucoelomate animals Filters fluid from coelom
Produces an ultrafiltrate

30. Earthworm

31. Malpighian tubules Found in insects
KCl and NaCl transported from coelomic fluid into Malpighian tubules
Initial urine formed in tubules
Final urine formed in rectum
Produces a concentrated urine but not an ultrafiltrate

34. How it works
K+ actively transported into M. tubule, conc. 30 times body fluid
Passive Cl diffusion into M. tubule
Water follows ions
In hind gut, water is reabsorbed, uric acid precipitates

37. Direction of blood flow
Afferent arteriole
Glomerulus
Efferent arteriole
Vasa recta
Vein

38. Function of nephron
Glomerular filtration (urea, glucose, Na, K, Cl)-no proteins
Same proportion of ions, solutes and water in glomerulus as in blood plasma
No blood cells filtered
Filtration based upon molecular size

39. Function of nephron Tubular reabsorption 99.9% of water Most salts
Most reabsorption by the proximal tubules

40. Function of the nephron
Tubular synthesis
Some amino acids are deaminated
Tubular secretion
Regulates blood levels of K, H+ and HCO3

41. Brush border in proximal convoluted tubule

42. Glomerular filtration
125 mL/min, about 200 L/day
Affected by
Net hydrostatic pressure difference between capillary and Bowman’s capsule-favors filtration
Colloid osmotic pressure of blood, opposes filtration
Hydraulic permeability-sieve like properties of the filtration barrier
Capillary endothelium
Basement membrane
Bowman’s capsule

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52. Calculating GFR Patient injected with inulin
After a while, plasma sample drawn and inulin concentration measured in plasma
Concentration of inulin measured in urine
Urine volume determined
Since all inulin removed, GFR equals inulin clearance

53. Glomerular Filtration rate
GFR = VU/P
V = urine volume, mL/min
U = urine inulin, g/mL
P = plasma inulin, g/mL
GFR, mL/min
Inulin is freely filtered, not secreted, not reabsorbed

54. Tubular reabsorption
200 L (50 gallons) filtered, 1 L urine produced per day, 99% of water reabsorbed
99% of sodium reabsorbed
Plasma glucose clearance = 0
Transport maximum for glucose = 365 mg/min

58. Proximal tubule Reabsorbs 67% of Na in lumen All glucose reabsorbed
Water follows passively
66-75% of filtrate reabsorbed before loop of Henle
At the end of the proximal tubule, fluid is isosmotic even though Na and water reabsorbed

59. Concentrating mechanism
As fluid moves from proximal tubule to descending loop of Henle it is isosmotic with ECF

60. Descending loop of Henle
Not permebale to NaCl, urea
Permeable to water

61. Concentrating mechanism
In descending loop, water moves out of tubule because descending loop is permeable to water

63. Ascending limb of the loop of Henle
No transport of NaCl in thin limb
Permeable to NaCl, passive diffusion
Impermeable to water, urea

64. Concentrating mechanism
As fluid moves up ascending limb,
NaCl moves out from fluid passively in thin segment
NaCl is transported out in thick segment
No movement of water here as impermeable to water

66. Medullary thick limb Active transport of NaCl from lumen to ECF
Impermeable to water
Because of NaCl transport, urine is slightly hypo-osmotic here

67. Distal tubule Transport of K+, H+, NH3+ into lumen
Transports Na, Cl and HCO3- out of lumen
Permeable to water, water follows NaCl

69. Collecting duct
Permeable to water. Water leaves urine for higher osmotic gradient in extracellular fluid.
Permeable to urea at the distal end
Permeability to water is under hormonal control be ADH

71. Concentrating mechanism
Only birds and mammals are able to produce a concentrated urine
Only birds and mammals have a loop of Henle
Desert mammals have longer loops of Henle

72. Active transport of NaCl out of ascending thick limb and distal tubule
Water follows osmotic gradient and leaves distal tubules and descending limb
Urea becomes more concentrated- the only part of collecting duct that is permeable to urea is medullary portion
Because of higher osmolarity at bottom of loop, water tends to leave descending limb of loop, therefore higher osmotic conc of tubular fluid at bottom of loop

73. Because of high osmotic conc of tubular fluid, NaCl follows passively out of ascending limb

74. End result-high osmotic concentration of urea in inner medulla interstitium is due to urea leaving collecting duct.