Lecture 2: Physiology of Kidney - Filtration, Reabsorption, and Secretion

Question 1

what is the normal GFR?

Answer 1

125ml/min

Question 2

what arteriole supplies the glomerulus?

Answer 2

afferent arteriole

Question 3

what arteriole drains the glomerulus?

Answer 3

efferent arteriole

Question 4

after the glomerulus, where does the filtrate drain to?

Answer 4

proximal convoluted tubule

Question 5

what is a nephron composed of?

Answer 5

glomerular capillaries: afferent and efferent
+ Bowman’s capsule
+ proximal convoluted tubule
+ loop of Henle
+ distal convoluted tubule
+ collecting duct

Question 6

what substances are reabsorbed at the proximal convoluted tubule?

Answer 6

Na+ (65%)
H2O (65%)
K+ (60%)
Cl- (50-60%)
Mg2+
HCO3- (90%)
Ca2+ (60%)
glucose (should be 100%, in diabetes excess is excreted)
amino acids (100%)
urea (50%)
phosphate (85%, unless PTH inhibits process)
small proteins: insulin, albumin, haemoglobin
lipids diffuse passively

Question 7

why is it significant that 65% of sodium from the filtrate is reabsorbed at the proximal convoluted tubule into the circulation?

Answer 7

As 65% of the sodium is being reabsorbed, water will follow (approx. 65%).
Glucose and amino acids are also reabsorbed using sodium cotransport mechanisms > dependent on presence of sodium.

Question 8

define tubular reabsorption

Answer 8

The process by which substances from the kidney tubules (the filtrate) move from the kidney tubules into the bloodstream.

Question 9

define tubular secretion

Answer 9

The process of moving substances from the bloodstream into the kidney tubule filtrate.
Requires the presence of ATP.

Question 10

which substances are secreted into the proximal convoluted tubule?

Answer 10

drugs
H+
NH4+
creatinine

Question 11

what is the osmolality of the filtrate leaving the proximal convoluted tubule into the loop of henle?

Answer 11

300mOsm

Question 12

what substances are reabsorbed from the ascending limb of the loop of henle?

Answer 12

Na+ (25%)
K+ (30%)
2Cl- (30%)
(through sodium potassium chloride cotransporter)

Mg2+
Ca2+
(happens because K+ is secreted into ascending limb to balance charge)

Question 13

why does water leave (reabsorbed) the descending limb of the loop of henle?

15% of the water

Answer 13

because the ascending limb had released lots of salt ions into the medullary interstitium, water is drawn out of the descending limb through aquaporin 1 transporters

counter current multiplier mechanism

Question 14

what is the osmolality of the filtrate at the distal convoluted tubule?

Answer 14

100-200mOsm
20% water
10% Na+

Question 15

what substances are reabsorbed at the early distal convoluted tubule?

Answer 15

Na+
Cl-
Ca2+ (dependent on PTH)

Question 16

wh yis the vasa recta called the countercurrent exchanger?

Answer 16

• as you move down, the medullary gradient increases. Water leaves the vasa recta into he medullary interstitial space and salt will move into the vasa recta.
• as you move up, salt moves back out into medulla, but the vasa recta keeps some (325mOsm) and water moves back in.
• maintains interstitial gradient

Question 17

the late distal convoluted tubule is vere dependent on which hormone?

Answer 17

aldosterone

Question 18

how does aldosterone act on the late distal convoluted tubule?

Answer 18

• aldosterone secretion is stimulated by low blood sodium and high potassium and also angiotensin II.
• stimulates gene transcription to increase expression of sodium potassium pump transporters (requires ATP) so sodium can be reabsorbed into bloodstream and potassium secreted into distal convoluted tubule.
• in the presence of ADH can express aquaporin 2 channels to cause reabsorption of water to increase blood volume and pressure.

Question 19

how does ADH act on the collecting duct?

Answer 19

binds to V2 receptors on collecting duct tubule cells which causes dowsntream signalling events resulting in aquaporin 2 channels moving tubule membrane causing water to flow out into the bloodstream > increases blood volume and pressure and decreases plasma osmolality

Question 20

what substance is reabsorbed at the end of the collecting duct and contributes to concentrated urine, how does it do this?

Answer 20

urea
- as it flows out of collecting duct and into medullary interstitium, some is recycled back into tubules, however some remains in medullary interstitium, which draws more water out of the descending limb of the loop of henle.

Question 21

which substances are secreted into the collecting duct to be excreted in the urine?

Answer 21

• creatinine
• drugs
• NH3 + H+ > NH4-
• urea
• H+ if metabolic acidosis
• HCO3- if metabolic alkalosis

Question 22

which hormone determines how much water is excreted into the urine?

Answer 22

ADH

Question 23

which hormone determines how much sodium is secreted into urine?

Answer 23

aldosterone

Question 24

which factors can affect glomerular filtration?

Answer 24

• Starling forces
• glomerular capillary and endothelium permeability
• surface area of capillaries
• number of glomeruli
• blood supply into the glomerular capillaries

Question 25

what is the GFR in a healthy adult?

Answer 25

approx 120ml/min

Question 25

describe the glomerular filtration membrane

Answer 25

Lies between the glomerulus and Bowman’s capsule and is made up of the following:
- glomerular capillary endothelium - has fenestrations (small pores) and filters by size
- glomerular basement membrane - filters by negative charge
- podocytes (epithelial cells of Bowman’s capsule) - filters by shape