What is Resorbed Where and Why

Question 1

Describe the primary filtrate

Answer 1

As it leaves the renal corpuscle, the primary filtrate is iso-osmotic to the
plasma and has approximately the same composition as far as small
molecules are concerned

Question 2

What happens in the proximal tubule?

Answer 2

-Sodium is pulled through by the basal pump
-Glucose, amino acids etc are pulled through by the sodium gradient.
-Phosphate etc are pulled through by the sodium gradient.
-Potassium is dumped into
tubule lumen, again due to
the basal pump
-HCO3- is recovered, with a
bit of H+ cycling, again powered by the sodium gradient.

Question 3

Where is water in the Proximal tubule?

Answer 3

All of this solute movement
tries to lower the osmolarity
of the tubule, so water flows
passively from the tubule to
counteract this (through aquaporins)

Question 4

Where is chloride in the proximal tubule?

Answer 4

Chloride also leaves passively to stop its concentration
rising in the tubule
-Charge (sodium)
concentration gradient (water)

Question 5

How is the proximal tubule adapted to its function?

Answer 5

-Microvilli
-Pack a lot of length into a small space
=Large surface area

Question 6

What does the proximal tubule achieve overall?

Answer 6

-65% recovery of sodium, chloride, phosphate, calcium, amino acids…
-Slightly higher percentage of glucose
-Some recovery of water (65%)
=Concentration of urine still iso-osmotic
=No control of acid/base

Question 7

Why do we need to concentrate the urine and recover more ions?

Answer 7

• Renal filtrate flow 1.2L/min = 1700L/day
• Proximal tubule recovery 65% so loss would be 35% of 1700L = 595L
• Human drinks around 2L/day and eats 3g salt
• Without concentration, would need to drink 595L/day and eat 2kg salt

Question 8

How does the kidneys concentrate urine?

Answer 8

1. Do not have water pump
2. Na+/K+ ATPase
3. SLCs and ion channels that can parasitize the Na+ gradient to move ions and small molecules about
4. Osmosis- water follows ions
   Therefore need to provide more destination more concentrated than urine to recover water= area in tissues more concentrated in ions

Question 9

How is the rest of the tubule adapted to make a concentrated area of ions?

Answer 9

-Tight junctions so water cannot passively diffuse across- to make a super-concentrated area of ions
-No aquaporins so water cannot travel across
=hypertonic basal side area of ions as ion transport still occurs

Question 10

Describe the Loop of Henle

Answer 10

-Thin walled loop structure
=descending limb down towards middle if kidney (medulla)
=ascending limb, thickening at end

Question 11

Compare the descending and ascending limbs of the Loop of Henle

Answer 11

• Descending thin limb= permeable to water, impermeable to ions and urea (doesn’t pump ions)
• Ascending thin limb= impermeable to water, permeable to ions (active transport) and urea
• Thick ascending limb= active recovery of ions (driven by Na pump)- makes the area very salty for the descending limb

Question 12

Describe water movement in the descending limb and why it occurs

Answer 12

-Lots of aquaporins in descending limb so water drawn out of descending limb
=urine more concentrated
-Locally very hypertonic as many ions in concentrated urine recovered in ascending limb
*positive feedback system

Question 13

What typical osmolarity values are associated with the Loop of Henle?

Answer 13

• 0.29 osmole/kg in renal corpuscle
• 1.4 in descending limb as tubular contents gets much more concentrated
• 0.1 in thick ascending limb as gets more diluted because so much has been recovered

Question 14

What does the mechanism of the Loop of Henle recover?

Answer 14

-10% filtered water
-25% Na+ and Cl-
=75% water and 90% NaCl recovered so far from urine

Question 15

How does the kidneys stop the high osmolarity of the Henle’s loop area being washed away?

Answer 15

1. Loops in same area and all renal corpuscles elsewhere (0.29 renal corpuscles so cortex, 1.4 medulla)
2. Organised blood system (main transport system that could cause problems)

Question 16

How does the kidney stop out hypertonic region being swept away by blood flow in the tissues?

Answer 16

-The blood vessels emerging from the glomerulus go on to
form a secondary capillary system – the vasa recta
-Blood comes in up the concentration gradient and goes out the exact opposite way
=afferent arteriole, leaves glomerulus as afferent arteriole- goes down past the ascending limb and wraps around the loop of Henle of the same nephron

Question 17

Describe counter current exchange

Answer 17

• As blood comes down, enters hypertonic region so water drawn out, ions diffuse into it passively
• Blood more concentrated as it reaches bottom
• Up around descending limb= moving away from hypertonic region so ions given back and picks up water
• Oxygen short-circuits to ascending capillary so medulla is oxygen poor

Question 18

Describe the urine concentration in the distal tubule

Answer 18

• Active ion pumping in distal tubule to recover more salt

- Very dilute urine (losing salt not raising in volume)

Question 19

Describe the anatomy of the distal tubule

Answer 19

• For developmental reasons considered end of nephron
• Leads directly onto branched urine collecting duct system
• Collecting duct leads from cortex back through hypertonic zone and medulla of kidney to renal pelvis (basin-like urine collecting system in middle of kidney)

Question 20

Why is water reabsorbed from the collecting duct?

Answer 20

As the collecting duct takes urine back through the hypertonic zone= area of osmotic gradient which will pull water out into the medulla

• At top of collecting duct, water from dilute urine can equilibrate with plasma
• More salt recovery occurs (2-5%)
• Up to 24% of the filtered water recovered here (up to 99% recovered overall)

Question 21

Why is the water reabsorption regulated?

Answer 21

-Sometimes dont want to recover all the water you can
-Not all extra fluid as perspiration so volume of urine increases
-Active, evaporation of perspiration= short of water so recover more
=Homeostasis

Question 22

How is water reabsorption regulated in the collecting duct?

Answer 22

• Presence of aquaporins in cell membrane

- Either on plasma membrane (to mediate transport) or taken into vesicles inside cell to store them

Question 23

What is water regulation mediated by?

Answer 23

-AVP or vasopressin hormone

Question 24

Describe the transport of urea

Answer 24

• In collecting duct cells
• Passive transporters for urea
• Very concentrated in urine so diffuses down concentration gradient out of urine to aid hypertonic area of medulla

Question 25

How does the function of the loop of Henle depend on its anatomical arrangement?

Answer 25

-Separation between normal and hypertonic zones
-Route to pass again through hypertonic route
=Normal zone on outside of organ and hypertonic zone near place that urine collects
=Group several units around central urine collecting place

Question 26

Describe the anatomy of the kidney

Answer 26

-Cortex (renal corpuscles, PCT and DCT)
-Medulla (LoH, collecting duct)
-Renal pyramids drain into renal papilla into renal pelvis
Calyces= big branches of pelvis
-Kidneys surrounded by tough capsule for protection

Question 27

How is there individual variation in artery supply to the kidney?

Answer 27

-Sometimes renal artery will break up into arterioles before it has reached the kidney

Question 28

Why are the kidneys sensitive to ischaemia?

Answer 28

-The long runs of parallel arteries/ arterioles and veins/venules mean
that there is counter current exchange of oxygen, so that much gets shunted from artery to vein before the blood enters capillaries
-Low renal oxygen -> erythropoietin release -> more red cells made in bone marrow

Question 29

What things can be altered in the filtration of the kidneys/ urine production?

Answer 29

1. Blood flow to glomerulus
2. Na+ recovery
3. Urea recovery by collecting duct
4. Water permeability of collecting duct
5. Acid-base balance in collective duct

Question 30

How can blood flow to and in the kidney be controlled?

Answer 30

1. Systemic blood pressure
2. Constriction of afferent arterioles (lowers glomerulus pressure)
3. Constriction of efferent arterioles (raises glomerulus pressure- also in diabetic changes in vessels)

Question 31

What are the limits of the kidney’s ability to regulate blood flow in the glomerulus?

Answer 31

• 80 to 180 mmol pressure= safe window which management can hold pressures steady
• If too high= glomerular pressure to high
• Too low= filtration cant be remained at same rate

Question 32

What are the mechanisms that control pressure?

Answer 32

1. Direct pressure sensing in the afferent arteriole (myogenic mechanism)
2. Monitoring the performance of the nephron (tubuloglomerular feedback)

Question 33

Describe the myogenic mechanism

Answer 33

Stretch activated cation channels depolarize membrane and cause
smooth muscle to contract: fast and protective against acute surges

Question 34

Describe the tubuloglomerular feedback mechanism

Answer 34

Central heating takes measurement of room in order to feedback performance of system to boiler
-Measure by assessing salt concentration in distal tubule, feed information back to control constriction of arterioles

Question 35

What are the problems with tubuloglomerular feedback?

Answer 35

We need this control to be nephron-by nephron, not global, so we can’t have a
signal diffusing long distances.

Question 36

What is the solution for the tubuloglomerular problem?

Answer 36

Arrange a nephron so that the end of the distal tubule makes ‘kissing contact’ with the arterioles entering the glomerulus

• A special zone of the distal tubule (macula densa) forms where the contact is made with specialised cells
• Together= juxtaglomerular apparatus

Question 37

How does the macula densa work?

Answer 37

-Elevated glomerular blood pressure
=Filtrate flows faster
=Less time for solute recovery in PCT and LoH
=More NaCl remains in the distal tubule
=Macula densa cells pump out more NaCl than they usually can as more available
=Juxtaglomerular cells release adenosine
=Afferent arteriole constricts in response to adenosine