Unit 3 Lecture 4: Glucose Reabsorption/Osmotic Gradient in The Kidney

Question 1

Explain the steps of glucose reabsorption in from the tubule into the peritbular capillary?

Answer 1

1. Na+ enters the cell down an electrochemical gradient using SGLT-2 cotransporter (glucose is going against concentration gradient)
2. Na/K+ ATPase pumps move Na+ out of the cell onto the basolateral membrane
3. Glucose passively diffuses out of epithelial cell into interstitial fluid via GLUT 2 transporters (down concentration gradient)
4. GLUT 1 Transporters passively move glucose down it’s concentration gradient

Glucose is low in the tubules and moves against gradient to a high region then eventually to low region again (blood)

Question 2

Why use transporters when we could use osmosis?

Answer 2

Transporters move the substances much more quicker than just simple osmosis

Question 3

If we have a high amount of glucose entering the filtrate, what does that mean for the plasma glucose? What is their relationship?

Answer 3

We’ll have high amounts of plasma concentration of glucose
* How much glucose in the plasma dictates how much is being filtered (relationship direct)
* More plasma glucose ⇒ More glucose filtered by glomerulus

Question 4

What is Transport Maximum (Tm)? What determines whether Tm is reached?

Answer 4

Transport Maximum is the limit of SGLT-2 transporters to take glucose in
1. # of transporters
2. Rate of Action (how fast it is moved into the blood)

• Only a certain number of transporters are available to move a certain amount of glucose over
• Rate of action can be affected by hormonal changes

Question 5

What is Renal Threshold? What is the number that determines whether it has been exceeded

Answer 5

Plasma concentration of glucose in which the renal tubules become fully saturated to the point where we start to see it in the urine
300mosm/L is the limit and anything higher means glucose will appear in the urine
Lower than 300 mosm/L is normal and no glucose can really appear in the urine

Question 6

Explain how this graph works

Answer 6

• Green line refers to amount of glucose in urine (excreted) which means renal threshold has been crossed
• Dotted black line indicates reabsorption so below 300mosm/L of renal threshold means the glucose is being reabsorbed and not appearing in urine
• Red line shows that before renal threshold and Tm are reached whatever glucose is appearing is being reabsorbed back into the blood

Question 7

Unique features about the Nephron, specifically the descending limb & ascending limb of the loop of Henle?

Answer 7

Descending: Permeable to H20 but not to Na+ reabsorption
Ascending: Not permeable to H20 but Na+ reabsorption occurs (NaCl,K+ co transporter)

Question 8

What is osmolarity?

Answer 8

How many solutes there are in a liquid (typically per L of liquid)

Question 9

What is unique about the cortex region of the nephron in terms of osmolarity?

Answer 9

It is isosmotic; solutes between the interstitial fluid and loop of Henle at this region are entirely the same (no movement)
* 300mosm/L

Question 10

Interstitial fluid in the medullary region is hyperosmotic. Why is that?

Answer 10

Hyperosmotic being that there is more concentration of solutes in a given volume
* Loop of Henle generates hyperosmotic pressure in medullary region which draws water towards the interstitial space while more solute is pumped into the same interstitial space

Question 11

What is concurrent flow & countercurrent flow?

Answer 11

• The movement of two or more substances in the same direction
• The movement of substances in opposite directions but parallel to one another creating a concentration gradient

Question 12

What is important about the hairpin of the loop of Henle?

Answer 12

• The hairpin is the ascending and descending limbs; they both contribute to counter-current flow.
• With counter current the movement of substances occurs parallel to one another but in opposite directions
• Counter-current flow creates a concentration gradient in the surrounding interstitium, facilitating the reabsorption of water and solutes

Question 13

Explain the generation of the osmotic gradient in the Loop of Henle

Answer 13

1. In the ascending loop of the tubule, NaCl,K+ pumps move salt into the interstitial space (↑interstitial fluid osmolarity & ↓osmolarity of the tubular fluid)
2. Interstitial osmolarity increases
3. In the descending limb, H20 diffuses from the tubule into the interstitial space due to an osmotic gradient caused by salt in the interstitium. It equilibrates with osmolarity of interstitium
4. Cycle repeats

Purpose: Concentrate the urine but also preserve as much water as the body can as it is essential. Which is why urine becomes darker when we are dehydrated

Question 14

How does urine become more and more concentrated?

Answer 14

Through the continous absorption of salts and water into the interstitial fluid it becomes more concentrated

Question 15

In the Loop of Henle what specific region is there more water being reabsorbed?

Answer 15

Proximal region of the descending limb; More water moves out at the top and as we go down less water is left to move out which is why urine is so much more concentrated

Question 16

Why is the urine more concentrated in the hairpin of the loop rather than the top?

Answer 16

There are sodium pumps throughout the entirety of the ascending limb so more sodium is pumped at the bottom bringing in water and concentrating urine more compared to the top where not much sodium is left to go through the pumps and there is a lot more water at the proximal end of the descending limb so it is dilute

REMEMBER: Cortex is essentially isosmotic compared to the medullary region where it is seen as hyperosmotic

Question 17

Why would the reabsorption of Na and H20 not actually happen in the intersitium like we always assumed? Where is it being reabsorbed into?

Answer 17

• If there was Na+ and H20 being reabsorbed into the interstitium like we assumed it would cause a build up of fluid in there
• Vasa Recta is what takes the Na+ and H20 back to the blood; ONLY Juxtamedullary Nephrons

Question 18

Why would you want a long loop of Henle?

Answer 18

A longer loop of Henle indicates more reabsorption of water is required so that we can preserve whatever water that is in the filtrate rather than excreting it

Kangeroo mouse lives in desert with barely any water so it needs long loop of Henle to preserve more water; unlike beaver who lives in dams with water

Question 19

What does the Vasa Recta do in the osmotic gradient created in the loop of Henle

Answer 19

• Prevent hyperosmolarity and the diluting of interstitial fluid
• Capillary; it is permeable to water and solutes

Question 20

Unique feature of Vasa Recta compared to loop of Henle?

Answer 20

The flow of fluids and solutes moves in the opposite direction (counter-current) - absorbs water and solutes in the most effective way
* Absorbs sodium along the ascending limb and H20 along the descending limb
* Main function: REMOVE H2O

Note: Does not generate osmotic gradient, rather it maintains the osmotic gradient generated by the loop of Henle