The kidney- a fluid processing organ

1. The kidney- a fluid processing organ • The major function of the animal kidney is to regulate the composition of blood plasma by removing water, salts, and other solutes from the plasma in a controlled fashion • Effects of the kidney on blood composition can be studying by comparing the urine composition (U) to plasma composition (P) or the U/P ratios

2. The kidney- a fluid processing organ • The effects of kidney function on osmotic regulation depend on the osmotic U/P ratio • If U/P = 1, urine is isosmotic to plasma, no effect on water or solute excretion, plasma osmotic pressure unaltered • If U/P < 1, urine is hyposmotic to plasma, urine contains more water relative to solutes than plasma, plasma osmotic pressure is raised • If U/P > 1, urine is hyperosmotic to plasma, urine contains less water relative to solutes than plasma, plasma osmotic pressure is lowered

3. The kidney- a fluid processing organ • The effects of of kidney function on volume regulation depends on the amount of urine produced • Kidneys can play a role in volume regulation without a direct role in osmotic regulation Freshwater crabs of tropical regions -experience both volume and osmotic challenges -kidneys deal with volume challenge by excreting an equivalent amount of water that is gained by osmosis but are unable to produce a hypoosmotic urine -other tissues are involved in maintaining osmotic balance

4. The kidney- a fluid processing organ • The effects of kidney function on ionic regulation depend on ionic U/P ratios • Kidneys can play a role in ionic regulation without playing a direct role in osmotic regulation Marine teleost fish • hyposmotic to SW (lose water osmotically and gain ions by diffusion) • Produce a urine that is isosmotic to plasma (U/P=1), therefore urine production plays no direct role in osmotic regulation • However, urine ionic composition differs greatly from plasma , U/P ratios for Mg2+, SO42-, and Ca2+ >>>1 (lowers internal ionic composition)

5. III. Osmoregulation in the terrestrial environment • Functions of the mammalian kidney • Maintain water balance • Regulate concentration of ions in the ECF • Maintains long term arterial pressure • Maintains acid-base balance • Maintain proper ECF/ICF osmolarity • Excrete end products of metabolism • Excrete foreign compounds • Secrete erythropoietin and renin • Converts vitamin D into its active form

6. III. Osmoregulation in the terrestrial environment • Urinary system • Kidneys  Urine formation  Renal pelvis  Ureter  Urinary bladder  Urethra

7. III. Osmoregulation in the terrestrial environment • Structure of the mammalian kidney • Cortex (outer layer) • In contact with the renal capsule • Possesses many capillaries • Medulla (deeper region) • Composed of renal pyramids separated by renal columns • Renal pyramids project into minor calyces • Minor calyces unite to form major calyx • Major calyces form renal pelvis

8. Mammalian kidney (Eckert, Fig. 14-17)

9. III. Osmoregulation in the terrestrial environment • The nephron • Functional unit of the kidney • Two major components of the nephron: • Vascular component (glomerulus) • A tuft or ball of capillaries • Filters fluid from blood as it passes through • Tubular component • Filtered fluid from from the glomerulus (ultrafiltrate) passes to the tubular component and is converted to urine

10. III. Osmoregulation in the terrestrial environment • Renal circulation (2 capillary beds) • Glomerular capillaries • High pressure (50-60 mm Hg) • Allows for rapid filtration • Peritubular capillaries • Low pressure (10 mm Hg) • Allows for reabsorption • Some vessels form the vasa recta

11. III. Osmoregulation in the terrestrial environment • Blood flow through the kidney Afferent arterioles  Glomerular capillaries (ultrafiltration)  Efferentarterioles  Peritubular capillaries (wrapped around nephrons)  Renal tubules Renal venules  Renal vein

12. III. Osmoregulation in the terrestrial environment • Parts of a nephron • Bowman’s capsule • Invagination around the glomerulus which collects filtered fluid from the glomerulus • Juxtaglomerular apparatus • Specialized tubular and vascular cells lying next to the glomerulus • Produces renin

13. III. Osmoregulation in the terrestrial environment • Proximal tubule • Within the cortex • Reabsorption of selected solutes • Loop of Henle • U-shaped loop that dips into the medulla • Two sections: descending limb (cortexmedulla) and an ascending limb (medullacortex) • Establishes an osmotic gradient in medulla • Allows kidney to produce urine of varying concentration

14. III. Osmoregulation in the terrestrial environment • Distal tubule • Lies within the cortex • Empties into the collecting duct • Highly regulated reabsorption of Na+ and water • Secretion of H+ and K+ • Collecting duct • Drains fluid from the nephrons • Enters medulla and empties into the renal pelvis • Similar functions to the distal tubule

15. The nephron (Sherwood, Fig. 14-3)

16. III. Osmoregulation in the terrestrial environment • Types of nephrons • Cortical nephrons • Glomeruli in the outer cortex • Descending limb of the loop of Henle enters partially into the medulla • No vasa recta • Juxtamedullary nephrons • Glomeruli lie in the inner cortex • Descending limb enters entire length of medulla • Abundant in desert species • Vasa recta present

17. Cortical and juxtamedullary nephrons (Eckert, Fig 14-18)

18. III. Osmoregulation in the terrestrial environment • Processes contributing to urine formation • Glomerular filtration • Reabsorption from renal tubules into the peritubular capillaries • Secretion of substances from peritubular capillaries into the renal tubules Rate of urinary = Filtration – Reabsorprtion+Secretion excretion rate rate rate

19. Processes contributing to urine formation (Silverthorn, Fig. 18-3)

20. III. Osmoregulation in the terrestrial environment • Glomerular filtration rate: the amount of fluid that filters into the Bowman’s capsule per unit time • In humans, about 180 l/day • Kidneys excrete about 1 l/day, therefore most of the filtrate is returned to the vascular system (>99% reabsorbed) • GFR is about 20% of renal blood flow

21. III. Osmoregulation in the terrestrial environment • Glomerular capillary membrane • Three major layers: • Endothelium • Basement membrane • Podocytes (epithelial cells)

22. III. Osmoregulation in the terrestrial environment • Podocytes • Surround outer surface of the capillary membrane; cell body with several ‘arms’ or pedicels (foot processes) • Narrow slits between pedicels allow for the passage of molecules based on MW and charge • Glomerular capillaries are fenestrated, allowing for a high filtration rate • Most substances except large proteins are filtered

23. Structure of the glomerulus (Silverthorn, Fig. 18-4)

24. Structure of the podocytes (Silverthorn, Fig. 18-4)

25. III. Osmoregulation in the terrestrial environment • Forces involved in glomerular filtration • PG: glomerular hydrostatic pressure; promotes filtration (60 mm Hg) • PB: hydrostatic pressure in Bowman’s capsule; opposes filtration (18 mm Hg) • G: colloidal osmotic pressure of the glomerular capillary; opposes filtration (32 mm Hg) • B: colloid osmotic pressure of the Bowman’s capsule; promotes filtration (0 mm Hg)

26. III. Osmoregulation in the terrestrial environment • GFR depends largely on two factors • Net filtration pressure • Filtration coefficient • Surface area of glomerular capillaries • Permeability of glomerular capillary-Bowman’s capsule interface

27. III. Osmoregulation in the terrestrial environment • Regulation of GFR • Prevents extreme changes in renal excretion from occurring in response to small arterial pressure changes • Regulation is generally achieved by adjusting resistance to flow in the afferent arteriole • Afferent arteriole has large diameter and short length (low resistance) • Efferent arteriole and vasa recta have smaller diameter and are longer (offer higher resistance)

28. Creation of high filtration pressure at the renal glomerulus (Eckert, Fig. 14-20)

29. Control of GFR by modulating arteriolar resistance (Silverthorn Fig. 18-8)

30. Effect of vasoconstriction of the afferent arteriole on GFR (Silverthorn, Fig. 18-8)

31. Effect of vasoconstriction of the efferent arteriole on GFR (Silverthorn, Fig. 18-8)

32. III. Osmoregulation in the terrestrial environment • Mechanisms controlling GFR • Intrinsic (autoregulation) • Myogenic response of the arteriolar smooth muscle • Hormonal control • Involves the juxtaglomerular apparatus (JGA) • JGA- a specialized renal structure where regions of the nephron and afferent arteriole are in contact with each other • Macula densa and juxtaglomerular cells (granular cells)

33. Juxtaglomerular apparatus (Eckert, Fig. 14-24)

34. III. Osmoregulation in the terrestrial environment • Nervous control • Afferent arteriole innervated by sympathetic nervous system • Sympathetic activation causes constriction of glomerular cells and causes podocytes to contract • Nervous mechanism overrides autoregulatory mechanisms if there is a sharp decrease in BP

35. Nervous control of podocyte contraction (Sherwood, Fig. 14-15)

36. III. Osmoregulation in the terrestrial environment • Tubuloglomerular feedback • Changes in fluid flow sensed by macula densa • Paracrine factors can either cause vasoconstriction or vasodilation • Endothelin (vasoconstrictor); bradykinin and nitric oxide (vasodilators)

37. Tubuloglomerular Feedback (Fig. 18-10, Silverthorn)