Lecture 64 - Tubular Function & Electrolyte Balance

Question 1

Which tubular activity is bigger?

Answer 1

Resorption

NB 200L → 2 L

Question 2

In which structure is resorption the greatest?
How much reabsorption is done here?

Why?

Answer 2

In the PCT
70% of reabsorption

Don’t want to leave it until the last moment until we recover that water

Question 3

What is the general role of the distal nephron?

Compare this to PCT

Answer 3

Distal nephron: fine tuning

PCT: bulk processing

Question 4

What is the most important solute to be reabsorbed?

Why?

Answer 4

Sodium

Reabsorption of sodium drives many other processes. Without reabsorbing Na, we can't reabsorb other things
• water
• Cl
• glucose
• K+
• H+

Consumes 80% of kidney’s oxygen supply

Question 5

Describe Na reabsorption, and the downstream effects of this

Answer 5

1. Na diffuses into proximal tubule
2. Na pumped out into ECF by Na/K ATPase
   - sets up electrochemical gradient -
3. Anions follow down gradient (Cl)
4. Water moves from hypotonic lumen into hypertonic ECF through aquaporins
5. K+, Ca2+ and urea are concentrated in the nascent urine

Question 6

What does the Na/K ATPase do?

Thus, where do we want this pump to be?

Answer 6

Pumps sodium out of the cell

It needs to be on the basolateral side of the tubule cell

Question 7

What are some compounds that are actively reabsorbed?

Answer 7

• Amino acids
• Glucose
• Sodium
• Lactate

Question 8

What is the difference between primary and secondary active transport?

Answer 8

Primary: Na/K ATPase on basolateral side

Secondary: movement of Na into tubule cells due to the gradient set up by the pump

Question 9

What is the process whereby Na moves from the lumen of the tubule into the tubular cells?

Answer 9

Secondary active transport

The primary active transport sets up the gradient so this secondary transport can occur

Question 10

Describe glucose reabsorption

Answer 10

1. SGLT; Na diffusing into cell down a concentration gradient, glucose pulled in against its concentration gradient
2. Glucose diffuses out of the cell with GLUT transporter across basolateral side

Question 11

Why is there glucose in the urine in diabetes mellitus?

Answer 11

There is a maximum capacity of the nephron to reabsorb glucose

In diabetes, plasma glucose concentration is very high, and the capacity is over-whelmed.

Not all glucose is reabsorbed

Ie reabsorption is saturable

Question 12

Describe saturation of reabsorption

Answer 12

There are only so many transporters

After a certain point, all the transporters are used up, and no more reabsorption can occur

Question 13

What is Tm?

Answer 13

Transport maximum

The transport rate at saturation

Question 14

What colour is the blood in the kidney veins?

Answer 14

Red

Not all of the oxygen delivered to the kidney is used

Question 15

What things are happening during fine tuning of the urine concentration

Answer 15

H+ and K+ secretion

Fine tuning pH of the blood

Question 16

Where is Na normally found in the body?

Answer 16

ECF:
• interstitial fluid
• plasma

Question 17

What is the average salt intake through the diet?

Answer 17

150 mmol

Question 18

What is the salt concentration in ECF?

Answer 18

150 mmol

Question 19

What happens if we have a normal salt intake, and no extra salt is excreted?

Answer 19

Increase in ECF osmolarity

Shrinkage of cells

Question 20

How long does the body take to respond to an increased sodium intake?

Answer 20

around 3 days

- not overnight -

Question 21

What is a positive balance?

Answer 21

More salt taken in than excreted

Increase in weight

Question 22

How long does it take for the body to respond to a decreased sodium intake?
What happens then?

Answer 22

A couple of days

Negative balance - decreasing weight.

Question 23

Compare ECF in interstitial fluid and plasma

Answer 23

75% in interstitial fluid

25% in plasma

Question 24

What infusion would we give people who are haemarrhoging?

Answer 24

Albumin

Increase oncotic pressure

Question 25

Which detects Na balance?

Answer 25

Stretch receptors in hypothalamus

Question 26

What causes the release of renin?

Answer 26

1. Drop in BP
2. Detect by renal arterial pressure receptors
3. Release of renin from granular cells in juxtaglomerular apparatus

Question 27

Where is Na/K ATPase normally found?

Answer 27

On the basolateral side of the tubule cells

Question 28

What is meant by bulk processing?

Answer 28

Non-selective reabsorption that occurs in the PCT

Question 29

What is the main driver of reabsorption in the PCT?

Answer 29

Na/K ATPase on the basolateral side

Question 30

Describe secondary active transport in PCT

Answer 30

Carriers on the apical side

Stuff moves down gradient set up by Na/K ATPase

Question 31

How is pH balanced maintained?

Answer 31

Secretion of H+

H+/Na+ antiporter on apical membrane of PCT

Question 32

What is happening in the descending LOH?

Answer 32

Permeable to water, and impermeable to salts

1. Water moves down concentration gradient into medulla
2. Filtrate becomes very concentrated

Question 33

What is happening in the ascending LOH?

Answer 33

Permeable to salts, impermeable to water

1. Na/K ATPase pumping on basolateral side
2. Apical side: NKCC diffusing ions into tubule cells

Question 34

When water moves out of the descending LOH, why isn’t the medulla diluted?

Answer 34

Water moves into the vasa recta

Vasa recta always reabsorbs water due to counter current flow.

Question 35

Describe the function of NKCC

Answer 35

One Na, one K, and two Cl moved across the apical membrane, from the lumen of the tubule into tubule cell.

By secondary active transport
(gradient set up by Na/K ATPase)