4.5. Regulation of Extra-Cellular Volume - Osmotic Diuresis

Question 1

In what terms is Osmotic Diuresis an important an important situation to consider?

Answer 1

Hypovolaemia - leading to hypotension

Question 2

What does osmotic diuresis show?

Answer 2

1. How renal function can be disrupted

2. The effects of uncontrolled diabetes producing a hyperglycaemic coma

Question 3

What happens when Blood Glucose levels are not kept controlled?

Answer 3

The high plasma glucose level exceeds the maximum re-absorptive capacity (Tm) in the proximal tubule

Question 4

What does the glucose, which remains in the tubule, do?

Answer 4

It exerts an osmotic effect to retain H2O in the tubule

Question 5

What happens to the sodium concentration (Na) in the tubule lumen, due to the increase in the osmotic effect of Glucose?

Answer 5

It is decreased, due to the Na being present in a larger volume (due to the retention of water in the tubule)

Question 6

What happens as a result of the decreased concentration of Na in the tubule lumen, due to the increase in the osmotic effect of Glucose?

Answer 6

1. As the Na gains access to the proximal tubule cells via passive diffusion down a concentration gradient, Na reabsorption will decrease
2. This decreases the ability to reabsorb glucose, as they share a transporter

Question 7

What happens to the ability of H2O to move into the interstitium from the descending limb of the loop of henle, as a result of the increased Na and Glucose concentration?

Answer 7

This is decreased, due to the osmotic effect the exert

Question 8

What happens to the fluid which is delivered to the ascending limb, due to the decrease in H2O leaving the descending limb?

Answer 8

It is less concentrated

Question 9

What happens to the less concentrated fluid which is delivered to the ascending limb?

Answer 9

As the NaCl pumps are gradient limited, the medullary interstitial gradient is much less.
Therefore there is a reduction in the volume of NaCl and H2O reabsorbed until the interstitial gradient is abolished

Question 10

What does the loss of the interstitial gradient, due to the effects of retained tubule Na and Glucose, result in at the distal tubule?

Answer 10

A large volume of NaCl and H2O being delivered

Question 11

What does a large volume of NaCl and H2O being delivered at the distal tubule inhibit?

Answer 11

Renin release - due to the Macula Densa detecting the high rate of NaCl delivery

Question 12

What does the inhibition of Renin release cause?

Answer 12

A decreased reabsorption at the distal tubule

Question 13

What is ultimately caused by the retention of Glucose in the Urine?

Answer 13

The loss of a large volume of isotonic urine, which can cause an decrease in perfusing volume

Question 14

What does a decrease in perfusing volume cause?

Answer 14

ADH release due to baroreceptor stimulation, but this cannot be effective due to the interstitial gradient being run down

Question 15

As the interstitial gradient has been run down (due to the effects of the retained Na and Glucose in the tubule lumen), and there is no effective way to reabsorb NaCl and H2O, what can occur?

Answer 15

NaCl and H2O depletion, leading to thirst, and severe hypotension which can result in a coma

Question 16

What can cause osmotic diuresis?

Answer 16

Any solute left in the tubule lumen

Question 17

What is the specific problem of Diabetes Mellitus, in relation to osmotic diuresis?

Answer 17

The problem is not self-limiting as the liver will continue to produce glucose

Question 18

What can loop diuretics cause?

Answer 18

Potassium ion (K-) wasting as the active transport mechanism for NaCl in the ascending limb of the loop of henle involves this ion as well (NaK2Cl-cotransporter protein)