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.

Don’t have a heart attack!

heart with obstructed coronary artery

This week’s image may seem melodramatic, but I’m sure if you were a heart undergoing a heart attack, you’d feel something like this.

Heart attacks teach us that the heart is like any other organ – it consists of cells that require oxygen and nutrients to survive.  The other thing we learn is that, despite the large amount of blood in the heart, almost none of its contents are directly available to the heart’s cells.  That’s because diffusion is only effective across short distances, so the thickness of the heart’s walls prevents all but the inner surface cells (endothelium) from acquiring their vital resources in this way.

The solution is for the heart to have its own network of blood vessels, the coronary arteries, penetrating deep into the heart walls to supply its own tissues with blood.  The suffering heart depicted here is actually anatomically correct (minus the face and heads), allowing us to see all the major blood vessels that deliver blood to and from the body.

Oxygenated blood, from the lungs, reaches the heart through the four pulmonary veins, entering horizontally from the sides.  It enters the left side of the heart, which ejects this oxygen-rich blood (shown in red) into the aorta, the largest artery in the body.  The aorta emerges diagonally from the left side and arches over the top of the heart, branching many times to provide oxygen to the entire body.

From the aorta, the right coronary artery curves around the heart’s right side (on the left side of the page), and the left coronary artery curves toward the heart’s left side, each providing many branches of their own.  If one of these becomes blocked by a blood clot or fatty deposits in the artery walls, then a heart attack results – meaning the heart tissue dies due to the obstructed blood supply, and the resulting lack of oxygen to that region.  This is different from cardiac arrest, in which the heart stops beating – a problem with the electrical conducting system of the heart.  (However, the two problems can be related, if a heart attack damages part of the conducting system.)

Let’s look at the rest of the blood flow, in a healthy heart.  From a coronary artery, tiny branches (capillaries) supply oxygen to the heart’s tissues by diffusion, and drain into cardiac veins (not shown) which drain into the right side of the heart.  This is also where oxygen-depleted blood enters from the rest of the body, through the superior and inferior vena cava (vertical vessels on the left side of the page).  From there, it is pumped through the pulmonary trunk (large vessel crossing in front of the aorta) back to the lungs.