Capillaries do all their work in bed – they’re not lazy, but they’re leaky!

Capillaries leaking in bed

Our tiny capillaries are the most important blood vessels in the body, responsible for supplying virtually all the oxygen and nutrients that our organs depend on.  So how do they behave, under the immense burden of this great responsibility?  They never even climb out of bed!

A capillary bed is the name for a group of capillaries that all receive blood from the same source.  The source vessel is called a metarteriole, and from there, a branching network of capillaries originates.  The large number of branches provides a huge surface area across which diffusion can rapidly deliver the materials demanded by the surrounding tissues.

But there’s something else going on in most capillary beds – they’re leaky.  But don’t worry, it’s perfectly normal!  There are holes in the capillary walls, allowing the fluid component of blood (plasma) to leak out, and this is how the body’s interstitial fluid is produced.  The bulk flow of fluid across the capillary wall, back and forth between the blood and surrounding tissues, permits more rapid exchange than diffusion alone can provide.

Capillary leakiness varies greatly from one organ to the next, depending on the need for exchange.  The liver is the “water purification plant” of the body, and requires tremendous amounts of exchange to do its job – it therefore has among the leakiest capillaries.  At the other extreme, the brain is like a “cleanroom” where contaminants are excluded to prevent “misfires” of the delicate neural machinery – so its capillaries are almost watertight (the blood-brain barrier).

Given that most capillaries are leaky, what happens to all that water?  Does the bedroom flood completely, until a plumber is called to the scene?  Well, not quite.  The answer to this problem is the lymphatic system – it’s the storm drain of the body, which collects all the excess interstitial fluid of the body, and carries it right back to the blood…to be leaked out, all over again, by our hard-working capillaries.

Your crowded capillaries

Red blood cells in capillary

Your blood is crowded with red blood cells, or erythrocytes — they occupy around 45% of blood volume in most people (that number is the hematocrit).  This becomes more evident when you look at the capillaries, the tiniest blood vessels.  These vessels are literally what keeps you alive from one moment to the next — providing virtually all the oxygen and nutrients, and removal of wastes, that allows your organs to survive — brain, muscles, skin, bones, and so on… even the heart itself.

Red blood cells travel in single file down your capillaries.  This is no accident, because exchange here depends on the rate of diffusion, the passive drift of molecules from regions of high to low concentration.  Diffusion is a cheap, but slow, way for a cell to get its “groceries”, so the only way this can work is to minimize the diffusion distance those molecules have to travel.

Capillaries accomplish this in two ways.  The narrow vessel means that an oxygen molecule, for example, is never more than half a cell’s diameter away from the wall of the vessel.  But also, the wall itself is extremely thin (endothelium — a type of simple squamous epithelium), further minimizing the diffusion distance.

On top of this, your capillaries branch many times to permeate the tissues.  It’s been estimated that if all your capillaries were laid end-to-end, they would total 50,000 miles — enough to wrap twice around the Earth.  Packed into your tissues, this provides an enormous contact surface, across which exchange can occur.

Not only does this branching increase contact, it also decreases the velocity of blood flow — in the same way that a wide hose ejects water more slowly than one with a thumb placed partway over the end.  The speed of blood flow in the aorta (the largest artery) is about 1 ft./sec. but goes down to around 1/1000 that speed in the capillaries — about 3/10 of a millimeter (equivalent to around 30 cells) each second.

So, picture a long, but crowded buffet line that moves briskly along — each cell has a chance to exchange everything it needs to (albeit without much elbow room) before it shoots out the other end of the capillary, to head back toward the lungs.