Where does the majority of reabsorption occur?
Proximal tubule
What is a normal healthy GFR?
125 ml/min
Why is reabsorption specific?
Specific transport proteins
What is the osmolarity of the fluid at the start of the proximal tubules compared to that at the end?
The same
no change in osmolarity
Which process mainly occurs in the proximal tubule?
Reabsorption
Does secretion occur in the proximal tubule?
Some secretion
By which two means can substances be reabsorbed into peritubular capillaries?
Transcellular (through cell membranes)
Paracellular (through tight junctions)
What is primary active transport?
Energy used to transport a substance against its concentration gradient using a carrier
What is secondary active transport?
Active transport of a substance by carriers COUPLED to the concentration gradient of another ion
Secondary active transport usually occurs using the concentration gradient of which ion?
Sodium
What is facilitated diffusion?
Passive transport WITH A CARRIER
Where is 2/3rds of filtered sodium reabsorbed?
Proximal tubules
Where are NaKATPases found on the tubular cells of the proximal tubules?
Basolateral
empties into capillaries
What is the function of NaKATPases on the basolateral membrane of the tubular cells?
What does this achieve?
Active transport of 3 Na OUT and 2 K IN
Maintains concentration gradient of sodium
So it can diffuse in (be reabsorbed) at the apical membrane
Where do sodium ions go once they have been secreted OUT of tubular cells by NaKATPase?
Peritubular capillaries
leaving conc gradient from filtrate to tubular cells
The transport of sodium from the filtrate to the capillaries sets up which types of gradient?
CONCENTRATION GRADIENT FOR SODIUM (so more is reabsorbed from proximal tubules)
OSMOTIC GRADIENT FOR WATER (following sodium)
ELECTROCHEMICAL GRADIENT FOR CHLORINE (so it’s reabsorbed by the PARACELLULAR route)
By which routes are sodium and chlorine reabsorbed?
Sodium - transcellular
Chlorine - paracellular
Why doesn’t the osmolarity of the proximal tubules decrease as you go along?
Water and salt reabsorbed equally
By which means does glucose leave proximal tubules?
Co-transport with sodium (SGLT2)
How does glucose leave the tubular cells by the basolateral membrane?
FACILITATED DIFFUSION down its concentration gradient
GLUT2?
How does water follow sodium and glucose into the peritubular capillaries?
Paracellular reabsorption
Which two substances are reabsorbed via the paracellular route?
Water
Chloride ions
What proportion of glucose is reabsorbed in the proximal tubules?
100% normally
What is the equation for rate of filtration?
Rate of filtration = [substance]plasma x GFR
Does the rate of reabsorption of glucose increase in proportion with its rate of filtration?
Yes, up to the point where all the carriers are saturated
Which transporter is saturated when the rate of filtration of glucose is too high?
SGLT2
co-transporter
Which rate starts to increase if you pass the maximum rate of reabsorption of glucose?
Rate of excretion
glycosuria
What is the TM of glucose?
Transport maximum of glucose
RATE OF FILTRATION AFTER WHICH YOU START TO EXCRETE GLUCOSE
2 mmol/min
What is the
a) rate of filtration
b) plasma concentration
of glucose after which you start to get glycosuria?
a) 2 mmol/min, transport maximum
b) Around 12 mmol/L
After you reach the transport maximum for glucose, glucose is excreted by the kidneys, producing which sign?
Glycosuria
What value is
a) inulin/creatinine
b) PAH
used to determine?
a) GFR (because their clearance = GFR)
b) Renal blood flow (because its clearance = RBF)
100% of which substances are reabsorbed in the proximal tubules?
Glucose
Amino acids
What is the osmolarity of the proximal tubules all the way along it?
Why?
300 mosmol/L
Same volumes of water and NaCl reabsorbed, so osmolarity doesn’t change
What are the two parts of the Loop of Henle?
Descending limb
Ascending limb
What is the function of the Loop of Henle?
Generates solute concentration gradient between the renal cortex and medulla
The Loop of Henle allows for the production of (hypertonic / hypotonic) urine which is (dilute / concentrated) in solute.
hypertonic
high solute concentration (concentrated)
What process occurs in the Loop of Henle to produce concentrated urine?
Countercurrent multiplication
Which blood vessel, found in juxtamedullary nephrons, help concentrate the urine along with the Loop of Henle?
Vasa recta
What is reabsorbed in the descending limb of the Loop of Henle?
WATER
NOT NaCl
What is reabsorbed in the ascending limb of the Loop of Henle?
NaCl
NOT WATER
The descending limb reabsorbs ___.
The ascending limb reabsorbs ___.
descending limb = WATER
ascending limb = NaCl
What transporter is found at the basolateral membrane of every cell in the body?
NaKATPase
Which transporter, found on the luminal membrane of the cells of the ascending limb, transports ions across the membrane?
Na-K-2Cl co-transporter
Does water follow salt into the tubular cells in the ascending limb?
No
Can’t, tight junctions won’t let it through
What ions are absorbed into the interstitial fluid, then the capillaries at the ascending limb?
Na
Cl
Which drugs block the triple transporter in the ascending limb?
Loop diuretics
e.g furosemide
As a result of the action of the triple transporter and NaCl reabsorption in the ascending limb, what happens to the osmolarity of the ascending limb?
Osmolarity decreases
Interstitial fluid osmolarity (increases / decreases) as a result of NaCl reabsorption by the ascending limb.
increases
What happens when osmolarity of a compartment increases?
Osmosis of water from another compartment
As a result of osmosis into the interstitial space from the DESCENDING LIMB, what happens to the descending limb’s osmolarity?
Increases
runs round into ascending limb, NaCl reabsorbed, ascending limb’s osmolarity decreases, interstitium’s osmolarity increases so water reabsorbed from descending limb, descending limb’s osmolarity increases…
accumulates and accumulates, concentrating tubular fluid many many times
What process occurs in the Loop of Henle?
Countercurrent multiplication
Distal tubule’s fluid has a much higher osmolarity compared to proximal tubule
Establishes concentration gradient between cortex and medulla
What three substances contribute to the concentration gradient between the renal cortex and medulla?
Water
NaCl by countercurrent multiplication in Loop of Henle
Urea recycling
Which part of the kidney has the higher osmolarity?
Renal medulla
where countercurrent multiplication occurs
Which hormone requires there to be a concentration gradient between the renal cortex and medulla?
ADH
What is the purpose of ADH?
Controls water concentration by changing permeability of collecting ducts to water
Altering urine output and concentration
Allowing kidneys to produce hypertonic, concentrated urine if dehydrated or hypotonic, dilute urine if overloaded
Keeping in mind the osmolarities of the renal cortex and medulla, what happens to the osmolarity of the vasa recta as it goes
down into the medulla
back up into the cortex?
down - osmolarity INCREASES (water diffuses out down conc gradient, solute diffuses in down conc gradient)
up - the opposite, as osmolarity of interstitium DECREASES going back up to cortex
How is the vasa recta adapted so it doesn’t take up solute and urea and wash it out of the adrenal medulla?
Hairpin loops
Endothelium is freely permeable to NaCl and H20
Low volume blood flow
What two structures maintain the concentration gradient between the renal cortex and medulla?
Loop of Henle (via countercurrent multiplication and urea cycle)
Vasa recta