Low plasma volume

[duke2121]

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[Majik]

Bit of confusion here, would like it be cleared up. So I understand that when plasma osmolarity increases so does plasma concentration of ADH. However, when there is low plasma volume, why does ADH increase to increase water reabsorption.

When plasma osmolarity increases, there's more "stuff" in the plasma. As such, the plasma is very concentrated. To resolve this, we need to dilute our plasma. Our body has multiple checks to fix this issue. One way to resolve this is by releasing ADH (from the Posterior Pituitary) into our blood stream. This hormone acts in the distal convoluted tubule (of the nephron) to reabsorb water from the kidney (as a result, urine becomes very concentrated). This reabsorbed water enters our blood stream to dilute the concentrated plasma. Angiotensin II also assists ADH by reabsorbing salt from the nephron to make our blood even more saltier. Even though this seems to make the situation worse, the purpose of this is to (indirectly) assist ADH in reabsorbing more water into the blood stream. Both of these effects decrease plasma osmolarity and increase BP. Low plasma volume is essentially the same situation as just discussed (Low plasma volume = High plasma osmolarity).

Here's a few things to remember:

Low Osmolarity ---> Angiotensin II acts to increase plasma osmolarity.
High Osmolarity ---> We need more water to dilute the saltyness of the blood. ADH and Angiotensin II help do this.

Low Plasma Volume ---> We need more water to raise our Blood Pressure. ADH and Angiotensin II help do this.
High Plasma Volume ---> Angiotensin II acts to increase plasma osmolarity.

Realize that High Plasma Osmolarity/Low Plasma Volume ...and... Low Plasma Osmolarity/High Plasma Volume are essentially the same conditions worded differently.

 

[duke2121]

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