Lecture 29

Question 1

what are the pressures affecting the GFR

Answer 1

• Ph = hydrostatic pressure (blood pressure)
• colloid osmotic pressure gradient due to proteins in plasma but not in bowman’s capsule
• Pfluid= fluid pressure created by fluid in Bowman’s capsule

Question 2

explain why GFR is nearly constant over a wide range of blood pressures

Answer 2

• changes very little despite fluctuations in MAP betwen 80-180 mmHG
• GFR is maintained by regulating renal blood flow

Question 3

explain how GFR is primarily regulated

Answer 3

• by varying flow through arterioles
• resistance changes in renal arterioles alter renal blood flow and GFR
• vasoconstriction of the afferent arteriole increases resistance and decreases renal blood flow, capillary blood pressure (Ph) and GFR
  => increase afferent arteriole resistance = decreased flow = decreased Ph and GFR
• increased resistance of efferent arteriole decreases renal blood flow but increases Ph and GFR
  => increase efferent arteriole resistance = blood pooling in glomerulus = increase Ph and GFR

Question 4

what is the myogenic response of autoregulation of GFR

Answer 4

• myogenic response to changes in blood pressure
  -> intrinsic ability of vascular smooth muscle
  -> stretch due to increased pressure causes vasoconstriction of afferent arterioles
• stretch activated cation channels on vascular smooth muscle
• depolarization leads to smooth muscle contraction

-> vasoconstriction of afferent arterioles leads to decreased blood flow into glomerulus and decrease Ph

Question 5

what is the tubuloglomerular (TGM) mechanism of autoregulation of GFR

Answer 5

• TGM = paracrine signaling
  -> occurs at juxtaglomerular apparatus
• region where ascending limp of loop of henle passes between afferent and efferent arterioles
• macula densa = specialized cells of the ascending tubule (the macula densa)
  -> senses increased flow due to high GFR
  -> release various paracrines
  -> paracrine act on afferent arteriole
  -> vasoconstriction

Question 6

explain the juxtaglomerular apparatus

Answer 6

• nephron loops back on itself so that the ascending limb of the loop of henle passes between the afferent and efferent arterioles
• the macula densa cells sense distal tubule flow and release paracrines that affect afferent arteriole diameter
• afferent arterioles also have granular cells = secrete renin (enzyme involved in salt and water balance)

Question 7

what are the steps of TGM feedback

Answer 7

1. GFR increases
2. Flow through tubule increases
3. Flow past macula densa increases
4. paracrine from macula densa to afferent arteriole
5. afferent arteriole constricts
   -> resistance in afferent arteriole increases
   -> hydrostatic pressure in glomerulus decreases
   ->GFR decreases
   = closed loop, negative feedback

Question 8

what is reabsorption

Answer 8

= movement from tubule lumen into blood
- most reabsorption occurs in the proximal tubule of the nephron

-> transepithelial transport
- substances cross both apical and basolateral membranes of transporting epithelium

-> paracellular pathway
- substances pass through tight junction of adjacent cells (smaller substances, H2O, K+)

Question 9

explain the steps of reabsorption for Na+

Answer 9

1. Na+ is reabsorbed by active transport
2. electrochemical gradient drives the anion reabsorption
3. water moves by osmosis, following solute reabsorption, concentration of other solutes increase as fluid volume in lumen decreases
4. permeable solutes are reabsorbed by diffusion through membrane transporters or by the paracellular pathway

Question 10

on which membrane of the epithelial cell would you need an active transporter for Na as it is reabsorbed

Answer 10

• basolateral side
• Na+ high in filtrate
• crosses to tubular epithelium (Na+ low) by Na+ channel
• goes against gradient (active transport) to extracellular fluid (where Na+ high)

Question 11

explain how Na+ is pumped via active transport

Answer 11

1. Na+ enters cell through various membrane proteins (ENaC), moving down its electrochemical gradient
2. Na+ is pumped out the basolateral side of the cells by the Na+ K+ ATPase
   - K+ leak channel for K+ to go back to extracellular fluid

Question 12

explain sodium linked secondary (indirect) active transport (glucose reabsorption)

Answer 12

1. Na+ moving down its electrochemical gradient uses the SGLT protein to pull glucose into the cell against its concentration gradient
2. glucose diffuses out the basolateral side of the cell using GLUT 4 protein
3. Na+ is pumped out by the Na+ K+ ATPase

Question 13

explain reabsorption in peritubular capillaries

Answer 13

• due to lower hydrostatic pressure
  -> net filtration pressure = Ph - colloid osmotic pressure = (-)
• negative value = flow into the peritubular capillaries

Question 14

what is secretion

Answer 14

• important in homeostatic regulation
  -> K+ and H+
• important in removing organic compounds from the body
  -> metabolites produced by the body (creatinine, urea, NH4+, urobilinogen)
  -> foreign substances (drugs)
• increasing secretion enhances nephron excretion

Question 15

explain excretion

Answer 15

• at the end of the collecting duct filtrate can not longer be modified
• output = urine
  -> a variable concentration of organic wastes, ions and water
• shouldn’t contain any glucose, amino acids, proteins, or other useful metabolites
  -> these substances have all been reabsorbed

Question 16

what is the formula for amount of solute excreted

Answer 16

amount filtered - amount reabsorbed + amount secreted = amount of solute excreted