Red river

Blood is actually connective tissue, the only liquid type in the human body.

How much blood we have depends on how big we are — blood accounts for about 8 percent of body weight, on average about five quarts (roughly five liters). More than half of blood is plasma, a yellowish fluid that itself is mostly water.Blood splatter

Plasma carries all the things the cells need when it begins its journey out from the heart. The bulk of its cargo is the 25 trillion red blood cells filled with oxygen, but it also carries infection-fighting white blood cells and platelets that will clot the blood when needed, as well as vitamins, electrolytes, hormones and other materials.

Animals that use a protein called hemoglobin to store oxygen have bright red blood when it's fully oxygenated, because hemoglobin contains iron. (Spider blood, for example, contains copper-rich hemocyanin, and is blue when oxygenated.)

Red blood cells are manufactured in bone marrow, and circulate in the blood for about four months. They look like fat plates, flat but curved, a shape that allows them to squeeze into capillaries. They have no nucleus, devoting as much space as they can to hemoglobin.

One entire circulation of the blood through the body of the average resting adult, given that five quarts of blood, takes about one minute. Without the life-giving oxygen it carries, the most vulnerable cells, including those in the brain, would begin to die within about five minutes, and organs would start shutting down just a few minutes later.

Image from Wikimedia Commons

The heart of the matter

The circulatory system is all about distributing oxygen around the body. The mighty heart — which never rests as long as we live — the 60,000 miles worth of blood vessels, and blood itself, all come down to this: Every cell in our bodies needs a fresh supply of oxygen every few minutes, or it will die. And so will we.

Diagram of the human heart

The human heart

The heart is at the center of the circulatory system, a hollow organ composed of muscle and connective tissue. In humans, the heart has four chambers — two atria or "entrances," and two ventricles or "bellies" — and weighs less than a pound.

The heart beats optimally about 70 times a minute throughout our lives, beginning within three weeks after conception, for a total of about 3 billion pulses in a lifetime of  80 years.

The heart pumps blood to the lungs, where it picks up its cargo of oxygen, and then on to the rest of the body.  Valves in the heart and the blood vessels ensure that blood travels in one direction only, away from the heart in the arteries, and toward the heart in the veins.

Red blood cells travel single-file through the capillaries, the fine vessels that connect the arteries and the veins, to deliver oxygen and other nutrients to the cells. Here the blood takes on waste, especially carbon dioxide, which it will deposit in the lungs for expulsion into the air.

A septum in the middle of heart keeps waste-filled blood returning from its journey through the body separate from oxygenated blood fresh the lungs.

Here in the heart, over and over, the journey begins again.

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