Sweat Is Not Only for Cooling You Down

When a fever comes, sweat comes. It comes to cool you down.

Half of that is right. But sweat has one more piece of work besides lowering heat. The reason I ask about sweat so often in the consulting room lies on that other side.

Send more blood and the fluid increases

A place that is injured or inflamed receives more blood. It is because the cells that take charge of immunity and the components of plasma have to be carried there. (The Swelling Has Gone Down, but It Still Feels Stiff)

In that process the walls of the vessels grow loose, and plasma leaks out into the tissue. The fluid between the tissues increases.

Up to here it is work the body does with intent. The trouble is what follows.

The body is not a sack but a compartment

The place where the fluid has increased is closed in on all sides. Fascia and connective tissue surround it, so it cannot swell freely however much fluid rises.

When fluid increases in a narrow compartment, the pressure rises. And that pressure does not act inward alone. It presses on the capillaries and the lymphatic vessels from outside instead.

This is the centre of the story. Past a certain line, the force with which the tissue presses grows greater than the force pushing blood in. Then the blood flow is blocked instead.

The work of sending more blood turns over into the work of keeping blood out. What meant to help becomes the hindrance.

So a way out is needed

There has to be a way for the increased fluid to leave, or a turning-over like this occurs. The lymphatic vessels are one such way, and there is one more. The sweat glands.

Here one thing locks into place. The material the sweat glands use to make sweat is that very fluid between the tissues. Sweat is not water newly made at the skin — it is that pooled fluid drawn up and put out.

So when fluid leaves as sweat, the pressure between the tissues comes down. The capillaries and lymphatic vessels that had been pressed do their work again, and only then does what was sent come back out.

The pores are a valve that lets pressure out. Cooling the heat is one among the several things that valve does.

So I look at sweat in the consulting room

I ask whether sweat comes, whether it comes of itself, whether it does not come at all. What I am looking at is whether the valve is open.

Why it matters is this. When the help being given works by increasing blood flow, the way out has to be open first. Pour more fluid into a closed compartment and only the pressure goes up. With the way out open, the same work becomes a washing through.

Situations that look alike part ways depending on whether sweat comes or not. For me this is a fairly large fork in the road.

The medicinal materials divide the work

Cinnamon twig is the centre of this part. There is a study in which the compound that gives cinnamon its scent was applied to people's forearms, and blood flow in the skin at that spot increased. The response was distinct between ten and twenty minutes after application. A channel in the peripheral sensory nerves appears to be one of the paths involved. Cinnamon twig takes charge in this way of pushing blood out to the periphery.

Ginger's pungent compounds attach to the same channel that the pungent compound of chilli attaches to — TRPV1. They adjust the signals by which vessels open and tighten. This side, though, is so far confirmed in cells and animals.

Jujube supplies the material that forms the groundwork of connective tissue, and settles the nerve side, which bears on how strong pain is.

Licorice holds the threshold of the nerves so that pain signalling does not grow too large.

All four touch different places. The widening side, the signal-evening side, the groundwork-filling side, the threshold-holding side. The force they push with differs and their directions differ, yet the result goes in one direction. It is like several people pushing different parts of a large ball to roll it one way.

That is why any one of them, taken on its own, looks out of place.

Where this story continues

How the pressure between the tissues comes to the hand I have written down in What You Can Tell by Laying a Hand on the Belly.

How water crosses the cell membrane in the first place, and how the body opens and closes that door, I have written in Water Does Not Move on Its Own.

When to see a doctor first

If sweating suddenly becomes excessive, if it soaks you at night, if it comes on one side only, or if weight is dropping along with it — before a story like this, examination and testing come first.

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Sources

• A review setting out that the sweat glands use the fluid between the tissues as the material for sweat — Temperature, 2019
• A study measuring that applying the cinnamon compound to people's forearms increases blood flow in the skin at that spot — Clinical Pharmacology & Therapeutics, 2023
• A study showing how ginger's pungent compounds open the same channel as chilli's (cell stage) — British Journal of Pharmacology, 2019

Written by Dr. Heo Ji-young (Ph.D. in Korean Medicine Pathology, Kyung Hee University · former Research Professor of Herbology, Kyung Hee University)