diffusion – Leif Saul | Life Science Cartoons

The many faces of areolar connective tissue

Areolar connective tissue, like connective tissues in general, holds us together. Like all connective tissues, it contains a lot of nonliving material — the extracellular matrix. In this case the matrix is loose and unspecialized, with a large amount of interstitial fluid, making it an ideal “filler” between many structures in the body. In particular, it is found on the back side of almost every epithelium in the body, including the lining of blood vessels. As a result, every molecule that crosses between the blood and surrounding tissues, has to diffuse across areolar connective tissue — the “middleman” of exchange.

Epithelia line not just the blood vessels, but every other surface and cavity of the body. This means they can function not only as an exchange surface, but also a barrier to microorganisms. Here again, areolar connective tissue plays a vital role — as the “second line of defense”, harboring immune cells that attack any invaders that breach our defenses.

I’ve already paid homage to some of our other connective tissues. The dense connective tissues are distinguished by large amounts of collagen, making them strong, though flexible. Bone tissue contains a rigid mineral component making it an ideal structural support. Areolar connective tissue, by comparison, is weak and shapeless. But this unpretentious mass of matrix and cells is arguably even more important for our survival.

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.