Your Blood Is Your Air Conditioner

Everyone thinks sweat is what cools you down.

And technically, it is. But sweat is actually the last step in a much bigger cooling system.

Before sweat can do its job, your body has to get heat from your core to your skin. And the primary vehicle responsible for that job is your blood.¹

Your Blood Is Moving Heat

When your body heats up, blood vessels near the skin dilate. This sends more warm blood from your core toward the surface of your body, where that heat can be released into the environment.¹

Then comes the sweat.

As sweat reaches the surface of your skin and evaporates, it removes heat from the body. But without sufficient blood flow bringing heat to the skin, there is less heat available for sweat to remove.²

Think of your circulatory system like an air conditioner:

Your core is the room.

Your blood is the coolant.

Your skin is the radiator.

Your sweat is the final cooling mechanism.

And your body can move an incredible amount of blood through this system. At rest, skin blood flow may be only around 300 mL per minute. During severe heat stress, it can increase to approximately 7 to 8 liters per minute.³ That means your skin can receive a massive amount of your total cardiac output simply to help get heat out of your body. In some conditions, skin blood flow can account for roughly 50 to 60% of cardiac output.⁴

Your heart is essentially working overtime to keep your personal air conditioner running.

This Is Where Hydration Matters

There is a problem, though.

Sweating means losing water. As you lose fluid, your plasma volume decreases, which means there is less circulating blood volume available to maintain both blood pressure and the enormous blood flow required for cooling.⁵

Your body responds by protecting what matters most.

As dehydration progresses, the cardiovascular system has to balance competing demands. Blood flow to the skin and sweating can be reduced in an effort to maintain cardiovascular stability.⁵ This means less blood is available to transport heat from your core to your skin.

And now your air conditioner is losing capacity.

Research has found that hypohydration can reduce both sweating and skin blood flow at a given core temperature, increasing the amount of heat stored in the body.⁶

This is why hydration in the heat is about more than simply replacing the water you sweat out.

You are maintaining the circulation that allows your body to cool itself.

The Takeaway

Sweat gets all the credit for keeping you cool, but your cooling system starts much deeper.

Heat builds up → blood carries it to the skin → sweat reaches the surface → evaporation removes the heat.

Interrupt the blood-flow step, and the entire system becomes less effective.

So the next time you are training in the heat, remember:

Your blood is your air conditioner.

And keeping that air conditioner supplied with enough fluid is one of the most important things you can do to support your body's ability to regulate temperature.

Sources

¹ Charkoudian N. Skin Blood Flow in Adult Human Thermoregulation: How It Works, When It Does Not, and Why. Mayo Clinic Proceedings. 

² Ashworth KJ, et al. Sweat evaporation in humans: A molecular and thermodynamic perspective. Experimental Physiology. 

³ Taylor NAS, et al. Occupational Exposure to Heat and Hot Environments. National Institute for Occupational Safety and Health. 

⁴ Charkoudian N. Skin Blood Flow in Adult Human Thermoregulation. 

⁵ Kenefick RW, Cheuvront SN. Physiological adjustments to hypohydration: Impact on thermoregulation. Autonomic Neuroscience. 

⁶ Sawka MN, Montain SJ, Latzka WA. Hydration effects on thermoregulation and performance in the heat. Comparative Biochemistry and Physiology. 