51.1 Regulation of Body Fluid osmolarity and Volume

Question 1

Summarise the concepts of how plasma osmolarity and volume are regulated.

Answer 1

• Osmolarity -> Alterations in water balance
• Volume -> Alterations in Na⁺ balance.

Question 2

Draw and explain a diagram to show how water retention and sodium retention are linked.

Answer 2

• When there is increased intake of sodium, there is increased concentration and osmolarity of body fluid
• ADH increases water retention, so concentration falls but volume increases
• ANP increases salt excretion (while angiotensin-aldosterone system increases sodium retention, so it is downregulated)
• Increased salt excretion leads to lower osmolarity, so ADH is down-regulated and water is not retained
• This means that the volume also falls back to normal

In this way, the two mechanisms allow maintenance of osmolarity and volume.

Question 3

How does aldosterone raise blood volume?

Answer 3

-Increases ENaC transcription in collecting duct
-Increases Na/K ATPase (basolateral) and Na/Cl (apical) in the DCT
Increases sodium reabsorption

Question 4

How does ANP affect sodium handling?

Answer 4

Inhibits Na/H on apical PCT cells = less reabsorption of soidum

Question 5

What are the effects of angiotensin in regulating blood volume? (5)

Answer 5

-Triggers ADH secretion to increase water reabsorption
-Vasoconstriction of vessels to raise pressure
-Phosphorylation of Na/H exchanger by PKC in PCT = increasing Na and water retention; increased K+ excretion
-Release of aldosterone by zona glomerulosa
- Increased sympathetic activity

Question 6

What are the three axes which control volume?

Answer 6

ADH
Renin-angiotensin-aldosterone
ANP (and BNP)

Question 7

What are the two circumventricular organs which detect changes in plasma osmolarity? Why are they able to do this?

Answer 7

Organum Vasculosum Laminae Terminalis (OVLT)
Subfornical Organ (SFO)
Have highly permeable capillaries (no BBB) so can easily detect changes in blood

Question 8

What are the two types of receptors that ADH acts on?

Answer 8

V1 and V2

Question 9

What are V1 receptors coupled to?

Answer 9

Gq coupled

Question 10

What are V2 receptors coupled to?

Answer 10

Gs proteins

Question 11

What causes renin secretion by the granular cells in the JGA?

Answer 11

-Reduced sodium delivery to the macula densa cells (signal to JGA via prostaglandins if Na is high and adenosine is low)
-Increased sympathetic activity resulting in beta 1 stimulation
-Decreased glomerular filtration rate (lowered volume in the macula densa)
-reduced perfusion pressure

Question 12

What does the hypothalamus make in response to increased plasma osmolarity?

Answer 12

ADH which travels to the neurohypophysis for secretion

Question 13

What hormone is secreted during high blood pressure?

Answer 13

ANP by atrial myocytes

Question 14

What is Conn’s syndrome?

Answer 14

primary hyperaldosterism caused by the excessive secretion of aldosterone

Question 15

What is the consequence of V2 activation? What occurs as a result?

Answer 15

Activation of V2 results in increased transcription and insertion of AQP2 channels into the collecting duct of kidneys, resulting in increased water reabsorption to try and reduce blood osmolarity

Question 16

What stimulates ADH release by the posterior pituitary gland?

Answer 16

Increased blood osmolarity
Haemorrhage
Angiotensin II

Question 17

What type of hormone is aldosterone?

Answer 17

Steroid hormone

Question 18

What type of receptor does angiotensin II work on?

Answer 18

AT1 which are Gq coupled

Question 19

Where are plasma osmolarity changes detected?

Answer 19

Anterior hypothalamus
Circumventricular organs (OVLT and SFO)

Question 20

Where are V1 receptors found?

Answer 20

Vasa recta = vasocontriction reduces washout

Question 21

Why is it important to regulate extracellular fluid volume?

Answer 21

Needed to maintain blood pressure and generate adequate tissue perfusion

Question 22

Why is it important to regulate osmolarity?

Answer 22

Hypotonic or hypertonic solutions can result in changes in cellular volume

Question 23

Why is it that sodium controls EC volume?

Answer 23

Sodium is the main osmotic constituent (alongside Cl- and HCO3-) so where Na moves, water follows