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Dehydration and Blood Pressure

Dehydration and Blood Pressure

Why is water important for blood pressure?

FUN FACT: water makes up 60% of our body weight! About two-thirds of that water weight comes from the water inside healthy cells. The remaining third of water either sits between your cells, or else travels around your body as part of blood. While its the oxygen, nutrients, antibodies, and other essential molecules that make blood as essential as it is, blood would be nothing without water. Water makes up the volume of blood, and allows for the circulation of essential nutrients to organs and tissues [1].

The volume of the blood in your body dictates the pressure generated by your heart to move the blood through your veins and arteries. If the volume of blood is off—or, if there is an incorrect volume of water in your body—your heart will have difficulty keeping its normal, healthy rhythms.

How are dehydration and blood pressure related?

heart anatomy figure

This can be tricky to understand, so let’s break this down: Imagine your heart is a pump that works to circulate your blood around your body. When you’re healthy and properly hydrated, your heart pumps with a certain amount of force to move blood around your body. If you’re dehydrated, the volume of blood decreases, so the pump has to work harder (apply more force) to get the same amount of blood to organs and tissues as it is used to. If you're over-hydrated, and your body has a larger volume of blood than normal, your heart will begin to pump with less force. So, if you’re dehydrated (low blood volume), your heart rate increase. If you’re over-hydrated (higher blood volume), your heart rate decreases.

For the first litre of fluid you lose as you become dehydrated, blood vessels can constrict and become narrower to counter the change in volume and maintain blood pressure [2]. If more than 1L is lost, your blood pressure starts to fall directly in proportion to the degree of your dehydration [3]. You may feel your heart beating fast to try to stop this, but even though your heart is beating faster, less blood is pushed out per heartbeat, and your blood pressure decreases [4].

What role do electrolytes play in dehydration and blood pressure?

We’ve discussed the role of water when it’s part of the blood, but what about the water that’s between and in your cells? How does water even get in and out of cells?

waterfall river

Electrolytes help the movement of water in and out of your cells, which require very specific concentrations of electrolytes inside and outside to function normally. The difference in electrolyte concentrations inside and outside of a cell creates what is known as a concentration gradient. Just like gravity forces things from a high point to a low point (like a river down a waterfall), salts and sugars in your body move from areas of high concentration to areas of low concentration. For cells, the outer cell membrane acts as the boundary between areas of high and low concentration, or, put another way, between the inside and outside of the cell. Sometimes, instead of the salt or sugar passing through the cell membrane, it’s easier for water to move in or out of the cell: out of the cell to dilute a higher concentration of electrolytes outside of the cell, or into the cell to increase the concentration of electrolytes outside the cell.

Normally, Potassium is kept at a much higher concentration in the cells than outside, whereas Sodium is kept at much higher concentration outside than inside. Water moves with and in response to these concentrations. This means that electrolytes, by affecting the volume of water in and in between your cells, can have an effect on blood pressure.

How does dehydration have different effects on blood pressure?

salt shaker

When you’re dehydrated, you can lose water, salt, or a mixture of the two. It’s most common for humans to lose both salt and water at the same time. If you’ve lost too much fluid, water found surrounding your cells can move into the bloodstream to help maintain blood pressure and keep your organs and tissues well supplied. But, when your salt level is also reduced, water can’t move as effectively into the bloodstream to counter low blood pressure. Dehydration with salt depletion is known to have a bigger effect on blood pressure than water loss on its own [5].

Why are changes in blood pressure a problem?

A fall in blood pressure decreases your cardiac output, the amount of blood pumped around the body by the heart per minute [6]. A decreased blood pressure also makes it harder for the body to cool down, as decreased blood flow to the skin can decrease sweat rates [7]. In extreme cases, a decreased sweat rate can increase your body temperature to the point that it causes heat stroke. If allowed to drop far enough, decrease in blood pressure can even lead to shock, and sometimes organ damage.

How can I tell if my blood pressure has fallen?

When you stand up, gravity pulls your blood towards your feet. Normally, the blood vessels in your lower body will constrict to stop all the blood from pooling in your legs. But with dehydration, there usually isn’t enough fluid to maintain pressure throughout your body when you stand; you’ll likely feel faint as blood rushes from your head. Your skin will also feel cold as less blood is pumped to the surface, in an attempt to maintain the central circulation above all else. You may also notice an increase in heart rate, as we’ve discussed [2].

To avoid these side effects, you should sip fluids to rehydrate regularly. Mixing a pack of Hydrant into a glass of water makes it easy to make sure your electrolyte balance is in check too!

 


Editor: Elizabeth Trelstad, www.hellobeaker.com
 

References: 

[1] Jequier, E & Constant, F, Water as an essential nutrient: the physiological basis of hydration, European Journal of Clinical Medicine, 64:2, 115-123, 2010
[2] Kumar, P & Clark, M, Clinical Medicine, Elsevier Saunders, 2005
[3] Gisolfi, C, Fluid balance for optimal performance, Nutrition reviews, 54:4, S159-68, 1996
[4] Gozalez-Alonso, J et al, Dehydration markedly impairs cardiovascular function in hyperthermic endurance athletes during exercise, Journal of applied physiology, 82:4, 1229-1236, 1997
[5] Elkinton, J et al, Hemodynamic changes in salt depletion and dehydration, Obstetrical and Gynecological Survey, 1:5, 620, 1946
[6] Ritz, P et al, Effects of changes in water compartments on physiology and metabolism, European Journal of Clinical Nutrition, Suppl 2, S2-5, 2003
[7] Fortney, S et al, Effect of blood volume on sweating rate and body fluids in exercising humans, Journal of applied physiology¸ 51:6, 1594-1600, 1981