4.2. Regulation of Extra-Cellular Volume - Renal Response to Hypovolaemia

Question 1

What can cause a loss of ECF Volume?

Answer 1

1. Vomiting
2. Diarrhoea
3. Excessive Sweating

Question 2

What happens after there is an initial loss of ECF Volume?

Answer 2

1. The loss of water from the ECF causes a decreased perfusing volume - a reduced venous pressure
2. There is therefore a reduced venous return, so there is a decrease in atrial pressure and the End-Diastolic Volume is also reduced
3. Due to the decreased cardiac output there is:
   a) Reduced Systemic Volume
   b) Reduction in blood pressure

Question 3

What does a decrease in blood pressure cause?

Answer 3

A decrease in the carotid sinus baroreceptor inhibition of sympathetic discharge

Question 4

What does the decrease in atrial pressure, and carotid sinus baroreceptor inhibiton of sympathetic discharge, cause?

Answer 4

An increase in the secretion of ADH (Vasopressin)

Question 5

What can an increase in ADH cause?

Answer 5

Hypo-osmolarity

Question 6

What does the sypathetic discharge (from the carotid sinus baroreceptor) cause?

Answer 6

Vasoconstriction

Question 7

What does vasoconstriction cause?

Answer 7

An increase int he Total Peripheral Resistance which increases blood pressure back towards normal

Question 8

How does the vasoconstriction, in association with the sympathetic discharge of the carotid sinus baroreceptors, effect the kidney?

Answer 8

1. It increases Renal Arterial Constriction

2. It increases Renin production

Question 9

Why does renal arterial constriction occur?

Answer 9

Due to an increase in renal nerve vaso-constriction activity

Question 10

What does an increase in Renin production lead to?

Answer 10

An increase in Angiotensin II

Question 11

What does an increase in Angiotensin II cause?

Answer 11

1. An increase in proximal tubule Na+ reabsorption

2. An increase in Aldosterone production

Question 12

Why does an increase in proximal tubule Na reabsorption occur due to Angiotensin II?

Answer 12

Due to the:

1. Decrease in Peritubular Capillary Hydrostatic Pressure
2. Increase in oncotic pressure

Question 13

What does an increase in Aldosterone production cause?

Answer 13

An increase in the distal tubule reabsorption of Na

Question 14

What happens in the Peritubular Capillaries in Hypovolaemia?

Answer 14

There is an automatic readjustment of Starling’s forces, to increase the amount of NaCl and H2O which is reabsorbed

Question 15

What changes occur in the peritubular capillaries (in comparison with normovolaemia)?

Answer 15

1. Hydrostatic pressure is less than normal because of the greater efferent arteriole constriction, caused by Angiotensin II
2. Oncotic pressure is greater than normal because of the lost NaCl and H2O (due to the vomiting, sweating etc.) so the concentration of plasma protein is increased

Question 16

How much of the filtrate does the body reabsorb in:

1. Hypovolaemia?
2. Normovolaemia?
3. Hypervolaemia?

Answer 16

1. 75%
2. 70%
3. 65%

Question 17

What maintains the Glomerular Filtration Rate, and the Vasoconstriction of the Afferent / Efferent Arterioles?

Answer 17

Autoregulation

Question 18

What is the result of autoregulation?

Answer 18

There is little effect on the GFR until volume depletion is severe enough to cause a considerable decrease in the mean arterial blood pressure