Lecture 2: Physiology of Kidney - Filtration, Reabsorption, and Secretion Flashcards
what is the normal GFR?
125ml/min
what arteriole supplies the glomerulus?
afferent arteriole
what arteriole drains the glomerulus?
efferent arteriole
after the glomerulus, where does the filtrate drain to?
proximal convoluted tubule
what is a nephron composed of?
glomerular capillaries: afferent and efferent
+ Bowman’s capsule
+ proximal convoluted tubule
+ loop of Henle
+ distal convoluted tubule
+ collecting duct
what substances are reabsorbed at the proximal convoluted tubule?
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
why is it significant that 65% of sodium from the filtrate is reabsorbed at the proximal convoluted tubule into the circulation?
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.
define tubular reabsorption
The process by which substances from the kidney tubules (the filtrate) move from the kidney tubules into the bloodstream.
define tubular secretion
The process of moving substances from the bloodstream into the kidney tubule filtrate.
Requires the presence of ATP.
which substances are secreted into the proximal convoluted tubule?
drugs
H+
NH4+
creatinine
what is the osmolality of the filtrate leaving the proximal convoluted tubule into the loop of henle?
300mOsm
what substances are reabsorbed from the ascending limb of the loop of henle?
Na+ (25%)
K+ (30%)
2Cl- (30%)
(through sodium potassium chloride cotransporter)
Mg2+
Ca2+
(happens because K+ is secreted into ascending limb to balance charge)
why does water leave (reabsorbed) the descending limb of the loop of henle?
15% of the water
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
what is the osmolality of the filtrate at the distal convoluted tubule?
100-200mOsm
20% water
10% Na+
what substances are reabsorbed at the early distal convoluted tubule?
Na+
Cl-
Ca2+ (dependent on PTH)
wh yis the vasa recta called the countercurrent exchanger?
- 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
the late distal convoluted tubule is vere dependent on which hormone?
aldosterone
how does aldosterone act on the late distal convoluted tubule?
- 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.
how does ADH act on the collecting duct?
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
what substance is reabsorbed at the end of the collecting duct and contributes to concentrated urine, how does it do this?
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.
which substances are secreted into the collecting duct to be excreted in the urine?
- creatinine
- drugs
- NH3 + H+ > NH4-
- urea
- H+ if metabolic acidosis
- HCO3- if metabolic alkalosis
which hormone determines how much water is excreted into the urine?
ADH
which hormone determines how much sodium is secreted into urine?
aldosterone
which factors can affect glomerular filtration?
- Starling forces
- glomerular capillary and endothelium permeability
- surface area of capillaries
- number of glomeruli
- blood supply into the glomerular capillaries
what is the GFR in a healthy adult?
approx 120ml/min
describe the glomerular filtration membrane
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
