Inside and Outside the Vessel — The Scale That Divides Water

I often see people who have been ill a long time, or who have eaten poorly, become swollen. It is commonly taken to be from "drinking too much water" or "eating too salty." But this kind of swelling is often the opposite. It is not that water has overflowed, but that the force to hold water in its place has weakened — that is why it swells.

To tell this story, we first have to see that the water in the body always sits between two forces.

The Force That Sends Water Out, and the Force That Pulls It Back

The water inside a vessel does not sit still. Two forces pull against each other in opposite directions.

One is the pushing force. The pressure of the water inside the vessel pushes against the thin wall, sending water bit by bit toward the tissue. This pushing pressure is called hydrostatic pressure.

The other is the holding force. Inside the vessel float proteins that do not dissolve in water and remain behind. These proteins pull water and draw it back into the vessel. This force by which protein pulls water is called osmotic pressure, and most of it is produced by a single protein called albumin.

So at the vessel wall there is always a tug-of-war. The pushing hydrostatic pressure sends water out; the pulling osmotic pressure draws water in. Depending on which side is stronger, water goes out or comes back.

It Goes Out and Comes Back, and the Lymph Gathers What Remains

In a healthy body, this tug-of-war changes direction along the way.

At the front of the vessel (the arterial end), the pushing force is strong, so a little water goes out to the tissue. Toward the back (the venous end), the pushing force weakens, and this time the holding force wins, so water comes back in. As much as went out generally comes back, and the small amount that remains is quietly gathered up by the lymph. This balance of coming and going is called Starling forces, after the old physiologist who described it.

The body does not swell not because water never goes out, but because the water that went out properly comes back, and what remains is gathered up.

Swelling Means This Scale Has Tipped

So swelling is less a matter of water having increased than of this scale having tipped to one side. There are a few ways that happens.

Sometimes the pushing force grows too strong. When you stand a long time and blood pools in the legs, or when the heart is weak and blood backs up and collects in the veins, the pushing hydrostatic pressure rises and water keeps going out.

Sometimes the holding force weakens. When you eat poorly and protein runs short, when the liver is weak and makes less albumin, or when the kidney leaks and loses albumin, the force to pull water back drops. The "person who eats poorly and swells" I mentioned earlier is exactly this case. It swells not because water has overflowed, but because there are not enough hands to hold it.

And there are cases where the wall itself loosens and leaks, and cases where the gathering lymph is blocked. These two are covered separately in The Body Has Vessels That Are Meant to Leak and Sweat Is Not Only for Cooling. Even the same swelling calls for looking at a different place, depending on which force has tipped.

That Is Why, With Swelling, I Look at "Which Force"

When people see swelling, they often try to remove the water first. I put one step before that. Is it swelling from a strengthened pushing force, swelling from a weakened holding force, or swelling from a leaking wall or a blocked lymph? I first look at which side of the scale has tipped.

A test that measures the amount of water tells you how much water is in the body, but it does not always tell you why that water changed places — which side of the scale grew heavier. That is why "I'm swollen but the tests say nothing much" happens. Swelling is often a matter of the balance of forces more than the amount of water.

But This Kind of Swelling Needs a Doctor First

Let me make one thing clear. If only one leg suddenly swells and hurts, if you swell along with shortness of breath, or if your face and eyelids swell severely every morning — this may be a signal beyond the scale. A test to check the heart, kidney, and vessels comes first.

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We commonly read swelling as "too much water." But the water in the body always sits between a pushing force and a holding force, and swelling is often the mark of that scale tipping to one side.

A body that swells after eating poorly is not a signal that you drank too much water, but the body's report that it lacks the force to hold water in place.

Related Reading

• The Body Has Vessels That Are Meant to Leak — swelling that leaks because the wall opens
• Sweat Is Not Only for Cooling — the valve that drains off excess water

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References

• Plasma proteins (especially albumin) create the osmotic pressure that pulls water into the vessel — albumin accounts for about 80% of it — StatPearls, Physiology: Colloid Osmotic Pressure, 2023
• Swelling arises when the water going out exceeds the water coming back — the many ways the balance of hydrostatic and osmotic pressure breaks down — Pathophysiology of Edema Formation (NCBI Bookshelf)

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