During a marathon, David has been sweating profusely and has not drunk enough fl

Question

During a marathon, David has been sweating profusely and has not drunk enough fluids. As a result, when he reaches the finish line, David is proud but exhausted and dehydrated. When the stewards ask for a urine sample after the race, David only manages to produce a very small volume of dark coloured urine.

What has happened to David in terms of the following parameters and why has this happened: [2.5 marks]
- Plasma osmolarity
- Plasma volume
- Mean arterial pressure
- Renin concentrations
- Angiotensin II levels

Explain the mechanism(s) by which the changes in Renin and Angiotensin II levels have affected his urine production [2.5 marks]

Explanation / Answer

Plasma osmolarity: Since, the subject D in the given case did not take plenty of fluids and went on for a strenous exercise (running marathon in this case), he got dehydrated, the plasma osmolarity increases.

Explanation: As the water content is reduced in the plasma due to excessive sweating and continuous metabolism without water replacement (as the subject did not take enough fluids), the colloid osmotic pressure is increased as the volume of fluids is less when compared to macromolecules like albumin, proteins and electrolytes. This results in increased plasma osmolarity

Plasma volume: The plasma volume increases

Explanation: During dehydration, there is loss of fluid which leads to reduced blood flow to the kidney that stimulates the kidney to release renin which in turns activates angiotensin I followed by angiotensin II that stimulates the release of aldosterone. This results in Sodium and water reabsorption which causees increase in plasma volume and decrease in urine production (Decreased urine production is mentioned in the question and fits the description)

Mean arterial Pressure (MAP): Decreases

Explanation: MAP = Cardiac output x Total Peripheral Resistance

In dehydration there is decreased blood volume, decreased venous return, decreased Cardiac output and blood pressure which ultimately results in reduction of mean arterial pressure

Renin Concentrations: Increases

Explanation: Renin angiotensisn aldosterone system is activated as a compensatory mechanism to Dehydration to correct the fluid volume deficiency. This causes increase in renin as the blood osmolarity is high, blood pressure is low and fluid volume is low ultimately increasing renin concentrations

Angiotensinn II levels: Increases (Explanation is providied above under renin )

Mechanism by which the changes in Renin and Angiotensin II levels have affected his urine production:

During dehydration, there is loss of fluid which leads to reduced blood flow to the kidney that stimulates the kidney to release renin which in turns activates angiotensin I followed by angiotensin II that stimulates the release of aldosterone. This results in Sodium and water reabsorption which causees increase in plasma volume and decrease in urine production

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