Lecture 64 - Tubular Function & Electrolyte Balance Flashcards
Which tubular activity is bigger?
Resorption
NB 200L → 2 L
In which structure is resorption the greatest?
How much reabsorption is done here?
Why?
In the PCT
70% of reabsorption
Don’t want to leave it until the last moment until we recover that water
What is the general role of the distal nephron?
Compare this to PCT
Distal nephron: fine tuning
PCT: bulk processing
What is the most important solute to be reabsorbed?
Why?
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
Describe Na reabsorption, and the downstream effects of this
- Na diffuses into proximal tubule
- Na pumped out into ECF by Na/K ATPase
- sets up electrochemical gradient - - Anions follow down gradient (Cl)
- Water moves from hypotonic lumen into hypertonic ECF through aquaporins
- K+, Ca2+ and urea are concentrated in the nascent urine
What does the Na/K ATPase do?
Thus, where do we want this pump to be?
Pumps sodium out of the cell
It needs to be on the basolateral side of the tubule cell
What are some compounds that are actively reabsorbed?
- Amino acids
- Glucose
- Sodium
- Lactate
What is the difference between primary and secondary active transport?
Primary: Na/K ATPase on basolateral side
Secondary: movement of Na into tubule cells due to the gradient set up by the pump
What is the process whereby Na moves from the lumen of the tubule into the tubular cells?
Secondary active transport
The primary active transport sets up the gradient so this secondary transport can occur
Describe glucose reabsorption
- SGLT; Na diffusing into cell down a concentration gradient, glucose pulled in against its concentration gradient
- Glucose diffuses out of the cell with GLUT transporter across basolateral side
Why is there glucose in the urine in diabetes mellitus?
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
Describe saturation of reabsorption
There are only so many transporters
After a certain point, all the transporters are used up, and no more reabsorption can occur
What is Tm?
Transport maximum
The transport rate at saturation
What colour is the blood in the kidney veins?
Red
Not all of the oxygen delivered to the kidney is used
What things are happening during fine tuning of the urine concentration
H+ and K+ secretion
Fine tuning pH of the blood
Where is Na normally found in the body?
ECF:
• interstitial fluid
• plasma
What is the average salt intake through the diet?
150 mmol
What is the salt concentration in ECF?
150 mmol
What happens if we have a normal salt intake, and no extra salt is excreted?
Increase in ECF osmolarity
Shrinkage of cells
How long does the body take to respond to an increased sodium intake?
around 3 days
- not overnight -
What is a positive balance?
More salt taken in than excreted
Increase in weight
How long does it take for the body to respond to a decreased sodium intake?
What happens then?
A couple of days
Negative balance - decreasing weight.
Compare ECF in interstitial fluid and plasma
75% in interstitial fluid
25% in plasma
What infusion would we give people who are haemarrhoging?
Albumin
Increase oncotic pressure
Which detects Na balance?
Stretch receptors in hypothalamus
What causes the release of renin?
- Drop in BP
- Detect by renal arterial pressure receptors
- Release of renin from granular cells in juxtaglomerular apparatus
Where is Na/K ATPase normally found?
On the basolateral side of the tubule cells
What is meant by bulk processing?
Non-selective reabsorption that occurs in the PCT
What is the main driver of reabsorption in the PCT?
Na/K ATPase on the basolateral side
Describe secondary active transport in PCT
Carriers on the apical side
Stuff moves down gradient set up by Na/K ATPase
How is pH balanced maintained?
Secretion of H+
H+/Na+ antiporter on apical membrane of PCT
What is happening in the descending LOH?
Permeable to water, and impermeable to salts
- Water moves down concentration gradient into medulla
- Filtrate becomes very concentrated
What is happening in the ascending LOH?
Permeable to salts, impermeable to water
- Na/K ATPase pumping on basolateral side
- Apical side: NKCC diffusing ions into tubule cells
When water moves out of the descending LOH, why isn’t the medulla diluted?
Water moves into the vasa recta
Vasa recta always reabsorbs water due to counter current flow.
Describe the function of NKCC
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)
