1. KIDNEY FUNCTIONS URINE FORMATION

2. Learning Outcomes:
Describe the function of each section of the nephron
Describe the process of urine formation
Describe how the kidneys maintain blood pH

3. Urine Composition Water Nitrogenous wastes (urea etc.) Salts
May contain H+ and other ions
May contain drugs, vitamins and other substances

4. Urine Formation 3 major steps:
Glomerular filtration - in the glomerular capsule
Tubular reabsorption - in the proximal tubule and loop of Henle
Tubular secretion - distal tubule

5. Different parts of the nephron have different permeability characteristics
Substances are filtered out of the blood by the nephron and excreted in urine
Some substances are reabsorbed back into the blood from the nephron

7. Pressure Filtration
Blood enters the glomerulus from the afferent arteriole
Blood pressure forces small molecules out of the capillaries into the capsule

8. Substances that leave the blood:
Water, salts, nutrients (glucose, amino acids), nitrogenous wastes (urea)
Filtrate resembles plasma

10. Selective Reabsorption
At the proximal tubule and loop of Henle
Some substances must be reabsorbed back into the blood
Examples:
Water, nutrients (glucose, amino acids), salts

12. Na+ ions, glucose, amino acids - move back into the peritubular capillaries by active transport (why active transport?)
Cl- ions follow passively
Water is reabsorbed by osmosis

13. Absorption of Water at the Loop of Henle
Function of the loop of Henle is to remove as much water as possible from the filtrate and return it to the blood

14. Descending limb is permeable to water but not salt
Ascending limb is permeable to salt but not water
Sodium is actively pumped out of the ascending limb

15. Medulla is hypertonic to the filtrate (contains high [Na+])
Water leaves the tubule by osmosis and enters the medulla, then the capillaries

16. Fig. 16.7

17. Fig. 16.6

18. Tubular Secretion At the distal tubule
Excess H+ ions, drugs, and other substances move from the blood into the nephron by active transport
Ions secreted or kept in the blood as needed, to maintain correct pH

19. Reabsorption of Water
More water is reabsorbed into the blood from the collecting duct
Collecting duct is in the hypertonic medulla so water is drawn out by osmosis

20. Fig. 16.6

21. Urine is now ready to be excreted from the body
Collects in the renal pelvis before going to the bladder

23. Path of Blood Renal artery & arterioles Afferent arteriole Glomerulus
Efferent arteriole
Peritubular capillary network
Venules to renal vein

24. Questions
How is the composition of blood in the renal artery different from that in the renal vein?
What is the importance of the hypertonic condition of the renal medulla?
Why is glucose transported actively instead of passively at the proximal tubule?

25. Fig. 16.6

26. Fig. 16.6