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.