EXPLAINING ANGINA: HEALTHY BLOOD
The heart should beat efficiently, be in peak condition, and have smooth, wide blood vessels that supply it with enough oxygen and glucose to deal with any demand put upon it, without clotting or sticking. For this to happen, the blood itself must be in peak condition.
Blood is certainly thicker than water, and it contains within it a blend of substances and cells that makes it much less than free-flowing.
First, there is the serum, the watery fluid in which all the constituents of the blood float. It carries minerals, salts, and glucose—all the soluble substances needed for the continuing health of the tissues and organs, and the waste products that need disposing of through the kidneys.
Then there is the plasma, which is the name given to the serum plus a mixture of proteins and fats, mainly derived from food. Cholesterol and related fatty-protein compounds, the lipoproteins, are part of the plasma. Very fatty plasma, high in cholesterol, is more viscous than normal, it is sticky to the touch, like the stickiness left after frying food in a pan.
Whole blood is plasma plus the cells—the solid constituents. They include the white cells (leukocytes), which act to resist infection and inflammation; platelets, fragments of cells that initiate clotting in arteries; and the red blood cells (erythrocytes) that transport oxygen around the body.
A fluid engineer would be horrified to be asked to design a pump for a closed system of wide and narrow tubes filled with such a mixture of solid, fatty, and watery substances. The difficulties in calculating the various pressures and flow rates are huge. For example, in each blood constituent—serum, plasma, and cells—there are substances that can make it much more viscous. As a fluid becomes stickier, the pressure needed to push it through a small tube becomes much higher, and that means a much greater effort from the heart. The demand of the myocardium for oxygen rises steeply.
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