Osmoregulation L10

Question 1

Where is osmolality detected?

Answer 1

anterioventral third ventricle (AV3V) in the brain. The blood-brain barrier is incomplete at this point.

Question 2

AV3V neurones send signals to the hypothalamus to stimulate production of ADH. Which areas of the hypothalamus are responsible for ADH production?

Answer 2

The cell bodies of neurones present in the Supraoptic and Paraventricular nuclei.

Question 3

Once ADH is manufactured in the hypothalamus, where does it go?

Answer 3

vesicles transport ADH down infudibulum to posterior pituitary gland where ADH is secreted into circulation.

Question 4

What stimulus causes an increase in ADH release?

Answer 4

increase in osmolality

Question 5

Which peptide is ADH released into circulation with?

Answer 5

neurophysin

Question 6

What is the half-life of ADH?

Answer 6

10mins *unstable in circulation

Question 7

Describe the structure of AQP1 in the apical membrane.

Answer 7

• multisubunit oligomer
• 4 identical subunits (tetramer) - four ‘tubes’
• water passes through all four subunits
• a large glycan is attached to one subunit

Question 8

Describe the feedback mechanism which causes thirst.

Answer 8

• low water intake > increase in plasma osmolality
• detected by AV3V
• AV3V neurones project to median preoptic area of hypothalamus
• increases thirst

Question 9

Why do we stop feeling thirsty once we drink water?

Answer 9

• water quickly absorbed from gut
• decrease in plasma osmolality
• supression of ADH release
• supression of thirst

*decreasing osmolality by a small amount (so that it returns to normal) will completely shut off production of ADH

Question 10

Why does drinking sea water cause you to become dehydrated?

Answer 10

• sea water has an osmolality of 2000mOsm/Kg
• so if you drink 1kg (1L) of sea water, you are ingesting 2000mOsm of NaCl.
• with maximal ADH release, our urine can only reach an osmolality of 1400mOsm.
• Therefore to remove the 2000mOsm of salt from the body, you need to urinate 1.4L of water
• (2000/1400 = roughly 1.4)
• so you drink 1L of sea water and urinate out 1.4L of water > by drinking sea water you are losing water from the body > dehydration

Question 11

Why is important to make sure formula milk is dilute enough before feeding it to babies?

Answer 11

• babies can only produce urine with a maximum osmolality of 500mOsm.
• if osmolality of milk is higher than this, baby will become dehydrated for same reasons as drinking sea water.

Question 12

If a person ingests 2800mOsm of salt in one day, how much urine would they need to produce that day in order to remove all the salt secreted into the filtrate?

Answer 12

minimum of 2L of urine

Question 13

can carbohydrates, fats and proteins we eat affect osmolality?

Answer 13

carbs and proteins can as are absorbed into the circulation in water-soluble forms

fats cannot, water-insoluble

Question 14

Why do salts such as Na+ and Cl- affect plasma osmolality more than proteins and carbohydrates?

Answer 14

• carbohydrates are converted into simple sugars which are transferred into cells
• proteins>AA which are absorbed into cells
• Na+ and Cl- stay in extracellular space > contribute to osmolality

Question 15

How can diabetes mellitus affect plasma osmolality?

Answer 15

• high glucose concentration in plasma (over 33mM - osmolality of 320mOsm)
• hyperosmolality increases thirst
• can lead to cellular dehydration and hyponatraemia in blood due to water movement into plasma from cells
• can cause mental problems, seizures, increased blood viscosity and increased clotting risk.