physiology notes:

Notes: Renal Physiology

1. Afferent arteriole: delivers blood to the nephron unit
2. Glomerulus: a ball of capillaries fed by afferent arterioles; site of filtration; located in the renal cortex area of kidney
3. Efferent arteriole: takes blood away from the glomerulus.
4. Glomerular capsule: (Bowman’s capsule); surrounds the glomerulus like a cup; it is continuous with the nephron tubule.
5. Distinguish between:
   1. Proximal convoluted tubule: first part of the nephron tubule network; found in the renal cortex; has many twists in its structure
   2. Nephron loop (Loop of Henle); made of a descending limb which projects into the renal medulla and an ascending limb which leaves the renal medulla to project back to the renal cortex
   3. Descending limb (see above)
   4. Ascending limb (see above)
   5. Distal convoluted tubule: twisted final portion of the nephron tubule; feeds into collecting duct
   6. Peritubular capillaries: capillary system fed by the efferent arteriole that surrounds the nephron tubule
   7. Collecting duct: fed by several nephron tubules; concentrates urine and empties into pelvis of kidney

6. Glomerular filtration: capillaries of glomerulus have fenestrae (holes) allowing passage of dissolved solutes but not proteins to leave glomerulus; filtrate flows into the capsule and is transferred to the nephron tubule.

7. Regulation of filtration: controlling the blood flow to the glomerulus; constriction of the afferent arterioles will decrease blood flow to glomerulus; filtration↓
8. Site of most reabsorption of solutes: proximal convoluted tubule
9. Reabsorption of
   1. Sodium: diffuses into tubule cells because tubule cells have a low Na+ concentration; this is maintained by the Na+/K+ pump that moves Na+ out of the tubule cells and into the peritubular capillaries
   2. Glucose and amino acids: moved against a concentration gradient by co-transport with Na+; each time a Na+ diffused in a glucose or amino acid is pulled in with it .

9. 3. water: drawn into the tubule cells by osmosis as the Na+ diffuses into the tubule.

10. Nephron loop: creates an increasing concentration of Na+ as the loop travels deeper into the renal medulla.

11. Descending vs. ascending limbs (permeability):

-descending limb is impermeable to Na+; no Na+ will enter or leave the tubule here; water will be drawn out.

-ascending limb Na+ and Cl- are actively pumped form the lumen of the tubule and into the tissue of the renal medulla; the Na+ and Cl- cannot enter the descending limb or peritubular capillaries so they are trapped in the renal medulla; ascending limb is impermeable to water

12. Renal medulla: has a Na+ concentration that increases deeper into medulla

13.Distal convoluted tubule: Na+ and K+ can be secreted or absorbed to fine tune the Na+ concentration of the blood

Collecting duct: picks up the fluid from several nephron loops; plunges through renal medulla; impermeable to Na+ but will allow water to exit; as filtrate moves down the collecting duct, more water is drawn out generating a concentrated urine; urine then moves to renal pelvis.

14. Regulation of absorption:

a. Antidiuretic hormone: affects tubule cells of collecting duct making them more permeable to water; when ADH↑ collecting ducts lose less water; urine is more concentrated

b. Aldosterone: activates Na+/K+ pump in distal convoluted tubule and the cortical part of collecting duct; Na+ is reabsorbed, K+ excreted; water will follow the Na+ into the capillaries due to osmosis

c. Atrial natriuretic factor: stimulates excretion of Na+ into distal convoluted tubule; water follows Na+ into DCT due to osmosis

15. Regulation of acid and base levels: H+ ions actively secreted into filtrate across the wall of the PCT.

Bicarbonate: reabsorbed by PCT; it is a base
In the filtrate, the high concentration of H+ causes it to bind to HCO3- forming Carbonic acid;

16. Carbonic anhydrase: enzyme breaking carbonic acid down into water and CO2;

CO2 diffuses back into the blood; this combines with H2O → carbonic acid;

Carbonic acid→ H+ and HCO3-

H+ is then pumped back into filtrate of PTC.

17. Control of urine pH: excretion of H+ and reabsorbing HCO3- gives urine a pH between 7 and 5.

18. High acidic urine: it may be necessary to remove large amounts of H+ and form highly acidic urine, as in acidosis; nephron cannot produce urine with a pH below 4.5; to get rid of more H+, the H+ are buffered by ammonia or HPO4- and removed attached to these ions.