AAMC 10 bio

[chiddler]

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What changes in blood volume and pressure would be expected as a result of aldosterone deficiency?

Answer: Decreased volume and decreased pressure.

I got this answer right BUT the answer tells me two reasons for the answer. I used the first reason. This is the second and I would like help understanding it, please.

"Because H2O passively follows Na+ during reabsorption in the kidney, less Na+ would result in less H2O reabsorption into the blood stream. This would decrease blood volume."

Thanks. If this isn't clear, I can add more of the answer so please just mention.

Specific question: Why does H2O follow sodium during reabsorption? Is this a general rule of thumb or only in response to aldosterone?

 

[MedPR]

What changes in blood volume and pressure would be expected as a result of aldosterone deficiency?

Answer: Decreased volume and decreased pressure.

I got this answer right BUT the answer tells me two reasons for the answer. I used the first reason. This is the second and I would like help understanding it, please.

"Because H2O passively follows Na+ during reabsorption in the kidney, less Na+ would result in less H2O reabsorption into the blood stream. This would decrease blood volume."

Thanks. If this isn't clear, I can add more of the answer so please just mention.

I figured that was the first reason.. That's the only reason I can think of since it is basically the definition of the effect of Aldosterone.

I'm not sure what part you are having a hard time with, so I'll try to address everything as best I can.

Aldosterone increases sodium reabsorption in the DCT and collecting duct. As sodium is reabsorbed, the oncotic pressure within the peritubular capillary network (not the vasa recta, since vasa recta is only in medulla) goes up and increased oncotic pressure = increased water reabsorption since water flows from high to low concentration. SO, since we are talking about aldosterone deficiency, less sodium will be reabsorbed in the DCT, so less water will be reabsorbed, and more water will leave as urine. Less water in the blood = lower blood volume (since blood is primarily made of water) = lower blood pressure (since blood pressure is hydrostatic pressure and hydrostatic pressure is based on volume within a tube).

 

[chiddler]

The other reason is just the edema-like effects of reduced blood osmolarity.

I didn't know aldosterone increased water reabsorption like that. And ADH must work in the collecting tube, right? Not the peritubular capillaries like aldosterone?

 

[MedPR]

The other reason is just the edema-like effects of reduced blood osmolarity.

I didn't know aldosterone increased water reabsorption like that. And ADH must work in the collecting tube, right? Not the peritubular capillaries like aldosterone?

Aldosterone works mainly in the distal convoluted tubule. It increases sodium reabsorption in the DCT and the reabsorbed sodium moves into the peritubular capillary network. ADH works mainly in the collecting duct.

Aldosterone works that way because water follows sodium for the most part.

 

[chiddler]

yup i understand thanks very much.

 

[Morsetlis]

When you think of water-follows-sodium, think aldosterone.

When you think of changing sodium concentration, think of ADH.

Water usually follows sodium.

An entire explanation of renal physiology may be too much material for the MCAT. Just think, water follows sodium.