Regulation of Osmolarity

Question 1

What hormone is responsible for water balance?

Answer 1

Vasopressin/ADH

Question 2

Where is ADH synthesised?

Answer 2

In the supraoptic and paraventricular nuclei of the hypothalamus

Question 3

How is ADH secretion controlled?

Answer 3

When the plasma osmolarity increases, the rate of discharge of ADH-secreting neurones in the supraoptic and paraventricular nuclei is increased so more ADH is released from the posterior pituitary

Question 4

What receptors mediate changes in neuronal discharge in ADH secretion?

Answer 4

Osmoreceptors in the anterior hypothalamus

Question 5

How is an increase in osmolarity detected?

Answer 5

Increased osmolarity means there are more particles in the ECF, so water moves out of the cells and shrinks them. This activates sensitive ion channels and results in increased neural discharge and ADH secretion

Question 6

How is a decrease in osmolarity detected?

Answer 6

Decreased osmolarity means there are less particles in the ECF. so water move into the cells and causes the cells to swell. This causes a decrease in neural discharge and ADH secretion

Question 7

What is the normal plasma osmolality?

Answer 7

280-290mOsm/Kg H2O

Question 8

What is the result on ADH secretion of an increase in osmolarity without an increase in tonicity?

Answer 8

Solutes that can penetrate membranes move together with water and don’t produce any tonicity so no change in ADH secretion

Question 9

What is the site of water regulation?

Answer 9

The collecting duct

Question 10

How is reabsorption of water and solutes varied?

Answer 10

ADH controls the permeability of the collecting duct

Question 11

How does ADH alter the permeability of the collecting duct?

Answer 11

Vasopressin binds to membrane receptors, which activates cAMP second messenger system. This results in the cell inserting AQP2 water pores into the apical membrane and water is absored by osmosis into the blood

Question 12

What is ureas role in the production of concentrated urine?

Answer 12

In the prescence of ADH, H2O moves out of the collecting ducts and concentrates the urea in the ducts. CD membranes are relatively permeable to urea, particularly towards medullary tips. As urea moves towards the tips there is an increasing tendency for it to move out down its concentration gradient and the permeability of the late medullary CD to urea is enhanced by ADH

Question 13

Why is urea retained during maximum anti-diuresis?

Answer 13

Urea us retained in order to save water and reinforce medullary gradient in regions of thing ascending limb of LoH

Question 14

How does ECF volume affect ADH secretion?

Answer 14

There is an inverse relationship between rate of ADH secretion and rate of discharge of stretch receptor afferents in the low and high pressure areas of the circulation.

Question 15

Where are the low and high pressure receptors found in the circulation?

Answer 15

Low pressure receptors- found in both atria and great veins

High pressure receptors- carotid and aortic arch baroreceptors

Question 16

How are the low and high pressure receptors involved in ADH secretion?

Answer 16

Moderate decreases in ECF volume affect the atrial receptors, decreasing their discharge and increasing ADH release. If the ECF volume changes enough to affect blood pressure then carotid and aortic receptors will also contribute to changes in ADH secretion

Question 17

What stimuli increases ADH release?

Answer 17

Pain
Emotion
Stress
Exercise
Nicotine
Morphine

Question 18

What stimuli decrease ADH release?

Answer 18

Alcohol