“A simple experiment confirms the composite nature of blood. Pour a quantity of red blood cells into any clear glass and simply wait. Horizontal bands of color will appear as various cells settle by weight, until the final multi-layered result resembles an exotic cocktail. The deepest reds; comprising clumps of red cells, sink to the bottom; plasma, a thin yellow fluid, fills the top part of the flask; platelets and white cells congregate in a pale gray band in between.

What the telescope does to nearby galaxies, the microscope does to a drop of blood: it unveils the staggering reality. A speck of blood the size of this letter ‘o’ contains 5,000,000 red cells, 300,000 platelets and 7,000 white cells. The fluid is actually an ocean stocked with living matter. Red cells alone, if removed from a single person and laid side by side, would carpet an area of 3,500 square yards.

Red cells and white cells make appearances in other portions of this book and are examined in detail elsewhere. But the body’s survival depends just as surely on the cells with a delicate flower-like shape, the platelets. Their function remained hidden until recently. Now scientists recognize that platelets, which circulate only six to twelve days in blood, play a crucial role in the life-saving process of clotting; they serve as mobile first-aid boxes by detecting leaks, plugging them, and tidying up the debris.

When a blood vessel is cut, the fluid that sustains life beings to leak away. In response, tiny platelets melt, like snowflakes, spinning out a gossamer web of fibrinogen. Red blood cells collect in this web, like autos crashing into each other when the road is blocked. Soon the tenuous wall of red cells thickens enough to stanch the flow of blood.

Platelets have a very small margin of error. Any clot that extends beyond the vessel wall and threatens to obstruct the vessel itself will stop the flow of blood through the vessel and perhaps lead to a stroke or coronary thrombosis and possibly death. On the other hand, people whose blood has no ability to clot live short lives: even a tooth extraction may prove fatal. The body cannily gauges when a clot is large enough to stop the loss of blood but not so large as the impede the flow within the vessel itself.*

*India has a very feared species of snake, the ‘eleven-step adder,’ so named because its toxic bite is said to allow the victim time for just eleven more steps. Like all vipers, it kills with a clotting toxin. If its fang penetrates a major vein, say, in the leg, all the blood in the channel between the heart and leg clots at once. If the toxin merely reaches a minor vessel, an amazing thing happens. The poison draws platelets to the tissue like a magnet. Elsewhere in the body platelets simply vanish so that the blood cannot clot anywhere. Then, the smallest scratch will kill the victim or he may bleed internally in brain or intestine. Bleeding cannot be stopped. Thus a viper’s toxin call kill in two opposite ways: a devastating clot or any equally devastating inability to clot. The Haffkeine Institute in Bombay milks these adders and uses infinitesimal amounts of the dried toxin to treat excess bleeding in patients.”

From In His Image, by Dr. Paul Brand and Philip Yancey