How does osmolarity affect blood pressure?
A loss of water volume in the body increases blood osmolarity and decreases blood pressure. When your body senses either an increase in osmolarity, a decrease in blood pressure, or both, it reacts with different homeostatic mechanisms to try to increase water volume back to normal levels, restore blood pressure, and ensure adequate circulation.
Blood pressure is a measure of how strongly blood pushes against the walls of arteries, which (among other things) is related to how much blood volume you have, which is related to how much water volume you have.
If the blood pressure gets too low because of inadequate volume - i.e. you bled a lot or you're dehydrated - there's not enough blood to adequately flow throughout the body and vital nutrients won't get delivered where it needs to go (and waste won't be taken away).
Since adequate blood circulation and maintaining blood pressure is so important to living, your body has multiple ways of detecting if there’s something wrong and reacting quickly to compensate:
1) by monitoring blood pressure and blood flow:
The Juxtaglomerular Apparatus (“JGA”) in your kidney has baroreceptors that monitor blood pressure and flow. If decreased, the JGA produces & releases a hormone called Renin which activates the Renin-Angiotensin-Aldosterone System (RAAS) which ultimately results in vasoconstriction (blood vessels getting smaller in diameter in order to increase blood pressure) and increased sodium (Na+) and water resorption in the kidneys to restore water volume.
2) by monitoring blood osmolarity:
Osmolarity is a measure of how many osmotically active solute particles there are in an unit volume of water. In the case of blood, we're talking primarily about sodium, Na+.
Higher blood osmolarity = more Na+ in 1 unit volume of water.
Lower blood osmolarity = less Na+ in 1 unit volume of water.
Or if the amount of Na+ in blood is constant:
Higher osmolarity = same Na+ in less volume of water.
Lower osmolarity = same Na+ in more volume of water.
If the blood osmolarity is higher than normal, your body’s osmoreceptors (monitors of change in osmotic pressure) think that there’s been a loss of volume.
Your body reacts to replace this water loss by releasing a hormone ( Vasopressin aka Anti-Diuretic Hormone (ADH) ) via the hypothalamus to activate a chain of events that leads to increased water resorption by your kidneys. Your brain also induces feelings of thirst so that you will want to drink more water.
Figure (a) is a great summary of the Hypothalamus-Vasopressin/ADH-Kidneys loop while Figure (b) shows the Renin-Angiotensin-Aldosterone System (RAAS):
Detected drop in blood pressure and/or increase in blood osmolarity
= Body assumes there’s been a loss of water volume
--> Body compensates by activating multiple pathways to replace water volume and boost blood pressure.