ADH secreting tumor essay question

ADH increases water permeability in the kidneys (by inserting aquaporins into the collecting duct of nephrons). This makes it so that you reabsorb more water from your urine back into your body. You will pee less water and the pee will be more concentrated in solutes.

If you reabsorb more water without reabsorbing more salt, your plasma osmolarity will decrease (more water + less salt = less osmolarity). In this sense, ADH lowers your plasma osmolarity when it is too high.

There is a distinction between plasma osmolarity and plasma volume that should be made. When you are dehydrated, it usually means you don't have enough water and therefore your plasma volume decreases. This says nothing about plasma osmolarity. It is possible to be dehydrated with low plasma osmolarity.

With that being said, ADH is released when there is low plasma volume and inhibited when there is high plasma volume (the effect of plasma volume on ADH is not as great as plasma osmolarity). When you are dehydrated, you will release more ADH, which will reabsorb water, making you have more blood volume and lower osmolarity.

Note that when you are dehydrated and have low plasma osmolarity, ADH will be halted because of the osmolarity effect, so you will remain dehydrated. This is the reason why people make electrolyte water.

Maintaining plasma osmolarity is important for cell excitability and maintaining volume. The solutes have gradients and charges (like K+ and Ca2+) that affect cell excitation. For instance, hypocalcemia is when you have low Ca2+ levels which causes you to have higher excitability.

Another effect of plasma osmolarity is on plasma volume. Ever had a doctor tell your dad not to eat salty food when he has high blood pressure? This is because elevated plasma osmolarity will create an osmotic gradient to fill your plasma with more water. This will make your blood pressure go up.

Summary:

ADH lowers plasma osmolarity. Dehydration and water loading affects plasma volume, not osmolarity, so it is hard to say what ADH will do. Plasma osmolarity measures electrolyte concentrations which affect cell excitability (like your nervous system function). Plasma osmolarity also affects plasma volume which has direct effects on blood pressure.