Kidney Transport

1. Kidney Transport Reabsorption of filtered water and solutes from the tubular lumen across the tubular epithelial cells, through the renal interstitium, and back into the blood. Solutes are transported through the cells (transcellular route) by passive diffusion or active transport, or between the cells (paracellular route) by diffusion. Water is transported through the cells and between the tubular cells by osmosis. Transport of water and solutes from the interstitial fluid into the peritubular capillaries occurs by ultrafiltration (bulk flow).

2. Reabsorption 1 Na+ is reabsorbed by active transport. Filtrate is similar to interstitial fluid. Na+ 1 Tubular epithelium Extracellular fluid Tubule lumen Principles governing the tubular reabsorption of solutes and water Figure 19-11, step 1

3. Reabsorption 1 Na+ is reabsorbed by active transport. Filtrate is similar to interstitial fluid. 2 Electrochemical gradient drives anion reabsorption. Na+ 1 2 Anions Tubular epithelium Extracellular fluid Tubule lumen Figure 19-11, steps 1–2

4. Reabsorption 1 Na+ is reabsorbed by active transport. Filtrate is similar to interstitial fluid. 2 Electrochemical gradient drives anion reabsorption. Na+ 1 2 Anions 3 Water moves by osmosis, following solute reabsorption. 3 H2O Tubular epithelium Extracellular fluid Tubule lumen Figure 19-11, steps 1–3

5. Reabsorption 1 Na+ is reabsorbed by active transport. Filtrate is similar to interstitial fluid. 2 Electrochemical gradient drives anion reabsorption. Na+ 1 2 Anions 3 Water moves by osmosis, following solute reabsorption. 3 H2O Concentrations of other solutes increase as fluid volume in lumen decreases. Permeable solutes are reabsorbed by diffusion. 4 4 K+, Ca2+, urea Tubular epithelium Extracellular fluid Tubule lumen Figure 19-11, steps 1–4

6. Reabsorption • Transepithelial transport • Substances cross both apical (lumen) and basolateral membrane (interstitial space before capillary) • Paracellular pathway • Substances pass through the junction between two adjacent cells

7. Reabsorption Sodium reabsorption in the proximal tubule Figure 19-12

8. Basic mechanism for primary active transport of sodium through the tubular epithelial cell. The sodium-potassium pump transports sodium from the interior of the cell across the basolateral membrane, creating a low intracellular sodium concentration and a negative intracellular electrical potential. The low intracellular sodium concentration and the negative electrical potential cause sodium ions to diffuse from the tubular lumen into the cell through the brush border.

9. Reabsorption Sodium-linked glucose reabsorption in the proximal tubule Figure 19-13

10. Mechanisms of secondary active transport The upper cell shows the co-transport of glucose and amino acids along with sodium ions through the apical side of the tubular epithelial cells, followed by facilitated diffusion through the basolateral membranes. The lower cell shows the counter-transport of hydrogen ions from the interior of the cell across the apical membrane and into the tubular lumen; movement of sodium ions into the cell, down an electrochemical gradient established by the sodium-potassium pump on the basolateral membrane, provides the energy for transport of the hydrogen ions from inside the cell into the tubular lumen.

11. Reabsorption • Urea • Passive reabsorption • Plasma proteins • Transcytosis

12. Reabsorption Saturation of mediated transport Figure 19-14

13. Reabsorption Glucose handling by the nephron Figure 19-15a

14. Reabsorption Figure 19-15b

15. Reabsorption Figure 19-15c

16. Reabsorption Figure 19-15d

17. Secretion • Transfer of molecules from extracellular fluid into lumen of the nephron • Active process • Secretion of K+ and H+ is important in homeostatic regulation • Enables the nephron to enhance excretion of a substance • Competition decreases penicillin secretion

18. Cellular ultrastructure and primary transport characteristics of the proximal tubule The proximal tubules reabsorb about 65 per cent of the filtered sodium, chloride, bicarbonate, and potassium and essentially all the filtered glucose and amino acids. The proximal tubules also secrete organic acids, bases, and hydrogen ions into the tubular lumen.

19. Transport characteristics of the proximal tubule

20. Transport in loop of Henle

21. Cellular ultrastructure and transport characteristics of the early distal tubule and the late distal tubule and collecting tubule.

22. Cellular ultrastructure and transport characteristics of the medullary collecting duct.

23. Excretion • Excretion = filtration – reabsorption + secretion • Clearance • Rate at which a solute disappears from the body by excretion or by metabolism • Non-invasive way to measure GFR • Inulin or creatinine used to measure GFR

24. Inulin Clearance Efferent arteriole Filtration (100 mL/min) Glomerulus Peritubular capillaries Afferent arteriole 1 Nephron Inulin molecules KEY = 100 mL of plasma or filtrate 1 Inulin concentration is 4/100 mL Figure 19-16, step 1

25. Inulin Clearance Efferent arteriole Filtration (100 mL/min) Glomerulus Peritubular capillaries 2 Afferent arteriole 1 Nephron Inulin molecules KEY = 100 mL of plasma or filtrate 1 Inulin concentration is 4/100 mL 2 GFR = 100 mL /min Figure 19-16, steps 1–2

26. Inulin Clearance Efferent arteriole Filtration (100 mL/min) Glomerulus Peritubular capillaries 2 Afferent arteriole 1 Nephron Inulin molecules KEY = 100 mL of plasma or filtrate 3 100 mL, 0% inulin reabsorbed 1 Inulin concentration is 4/100 mL 2 GFR = 100 mL /min 3 100 mL plasma is reabsorbed. No inulin is reabsorbed. Figure 19-16, steps 1–3

27. Inulin Clearance Efferent arteriole Filtration (100 mL/min) Glomerulus Peritubular capillaries 2 Afferent arteriole 1 Nephron Inulin molecules KEY = 100 mL of plasma or filtrate 3 100 mL, 0% inulin reabsorbed 1 Inulin concentration is 4/100 mL 2 GFR = 100 mL /min 3 100 mL plasma is reabsorbed. No inulin is reabsorbed. 4 Inulin clearance = 100 mL/min 4 100% of inulin is excreted so inulin clearance = 100 mL/min 100% inulin excreted Figure 19-16, steps 1–4

28. GFR • Filtered load of X = [X]plasma GFR • Filtered load of inulin = excretion rate of inulin • GFR = excretion rate of inulin/[inulin]plasma = inulin clearance • GFR = inulin clearance

29. Excretion

30. Excretion The relationship between clearance and excretion Figure 19-17a

31. Excretion Figure 19-17b

32. Excretion Figure 19-17c

33. Micturition The storage of urine and the micturition reflex Figure 19-18a

34. Micturition 1 Stretch receptors fire. (b) Micturition Stretch receptors Sensory neuron 1 Internal sphincter External sphincter Figure 19-18b, step 1

35. Micturition 1 2 Stretch receptors fire. Parasympathetic neurons fire. Motor neurons stop firing. (b) Micturition Higher CNS input may facilitate or inhibit reflex. Stretch receptors Sensory neuron 1 Parasympathetic neuron 2 + – Motor neuron Tonic discharge inhibited Internal sphincter 2 External sphincter Figure 19-18b, steps 1–2

36. Micturition 1 2 3 Stretch receptors fire. Parasympathetic neurons fire. Motor neurons stop firing. Smooth muscle contracts. Internal sphincter passively pulled open. External sphincter relaxes. (b) Micturition Higher CNS input may facilitate or inhibit reflex. Stretch receptors Sensory neuron 1 Parasympathetic neuron 2 3 + – Motor neuron Tonic discharge inhibited Internal sphincter 2 3 External sphincter Figure 19-18b, steps 1–3