Session 6: Group Work

Question 1

Which two physiological mechanisms exist to help maintain plasma osmolality?

Answer 1

Thirst

ADH leading to urination

Question 2

Which of the two physiological mechanisms that control plasma osmolality is its first line of defence.

Answer 2

ADH

Question 3

Where is the hormone that regulates water reabsorption made?

Answer 3

(ADH)

Made in hypothalamus specifically the supraoptic nucleus and the paraventricular nucleus.

Question 4

Where after synthesis is ADH stored?

Answer 4

In the posterior pituitary gland

Question 5

What is the most important physiological effect of this hormone?

Answer 5

Water reabsorption by upregulation of aquaporin on the apical membrane in the collecting duct.

Also causes vasoconstriction

Question 6

Describe the action of this hormone in the kidney.

Answer 6

Increased expression aquaporin channels on the apical membrane in the collecting duct. This increases water permeability and more water can be reabsorbed.

Question 7

How would you describe the osmolality of the glomerular filtrate (tubular fluid) as it reaches the top of the ascending limb of the loop of Henle compared to plasma?

Answer 7

Hypo-osmotic

Question 8

How would you describe the interstitial osmolalility changes that occur from the cortex to the papillary region of the kidney?

Answer 8

Increases in concentration as you go further down into the medulla

Question 9

How does this corticomedullary concentration gradient allow the kidney to produce concentrated urine?

Answer 9

Allows an increasing concentration of urine as the urine descends the collecting duct.

Since the collecting duct is descending as well and the interstitium in the collecting duct is very concentrated the urine will be hypo-osmotic compared to it. This allows water reabsorption in the collecting duct.

Question 10

What role does urea play in the concentrating ability of the kidney.

Answer 10

Works as an osmotic agent to concentrate urine further.

It is then recycled in response to ADH.

Question 11

Explain why water leaving the descending limb of the loop of Henle and the collecting duct does not dilute the concentration of the interstitial fluid thus destroying the concentration gradient. (Counter current mechanism)

Answer 11

Because the water does not stay in the interstitium. The water moves into the concentrated vasa recta as it ascends the descending limb. This allwows the concentration gradient to be maintained.

Question 12

What effect has the elevated serum glucose had on her plasma osmolality?

Answer 12

Her plasma osmolality has increased leaving her plasma hyperosmotic.

Question 13

In the nephron, what is the site of glucose reabsorption?

Answer 13

PCT

Question 14

What normally happens to the total volume of filtrate by the end of this region?

Answer 14

Decrease total volume as there is a lot of reabsorption of ions, glucose and amino acids. There are also aquaporin channels so water can move down the concentration gradient.

65% water is reabsorbed.

Question 15

What has happend to the osmolality of the filtrate at the end of this region?

Answer 15

It is isosmotic

Question 16

Can you explain why the patient would complain of polyuria?

Answer 16

Not all glucose is being reabsorbed because it has reached its reabsorption limit.

This means that there will be glucose in the urine. The glucose is an osmotic agent and will lead to water moving into the tubule or rather staying in the tubule.

This leads to increase urination.

Question 17

What effect on the patient’s plasma volume will have occurred following the change to the volume of urine produced?

Answer 17

Decrease in plasma volume and decrease in BP

Question 18

Can you explain why your patient has polydipsia?

Answer 18

Hypothalamic osmoreceptors detect hyperosmotic plasma. This stimulates the hypothalamus to produce ADH or PP to secrete ADH.

Baroreceptors will detect low plasma volume in the carotid sinus and the aortic arch. This leads to the same response as with the osmoreceptors.

Also baroreceptors in the kidneys will be stimulated to start the RAAS system.

If the ADH can’t keep up with the water loss, thirst will be induced by the hypothalamus instead.

Question 19

Think about his electrolytes and osmolality. What conclusion do you draw about this patents water balance and why?

Answer 19

He does not get enough water. Water is low and he is dehydrated.

Question 20

Using your knowledge about ADH, can you think of an explanation for his clinical presentation and blood and urine results?

Answer 20

ADH is not working.

Probably something wrong with hypothalamus or PP due to trauma

Question 21

Describe the changs in plasma osmolality and urine osmolality in this patient.

Answer 21

Plasma has decreased

Urine has increased

Question 22

What is your interpretation of her urine sodium result and what can you infer from this?

Answer 22

That her kidneys are working and doing what they are told.

Question 23

In this patient what is the most likely underlying cause?

Answer 23

SIADH