Dark Banquet - Bill Schutt [41]
To help carry out myriad circulatory functions, the average human has between 1.2 and 1.5 gallons (4.5 to 5.7 liters) of blood in their body at any given time. Blood plasma constitutes 55 percent of that volume, while cells (red and white blood cells) and platelets (classified together with cells as “formed elements”) make up the remaining 45 percent. Water makes up about 92 percent of blood plasma, with dissolved stuff making up the remaining 8 percent. The majority of these solutes are proteins produced in the liver.
To be considered a tissue, blood needs to be composed of several cell types—and it is. Blood cells (or corpuscles) come in two flavors: erythrocytes (red blood cells) and leukocytes (white blood cells). Erythrocytes (from erythrós, the Greek for “red”) are by far the most numerous, making up over 99 percent of blood cells. Function-wise, they’re very much like the old Kentucky Fried Chicken (as opposed to the new KFC). That is “they do one thing and they do it right.” The one thing they do is carry oxygen, and they do it right because each erythrocyte is literally stuffed full of an iron-containing pigment molecule called hemoglobin. Hemoglobin acts like an oxygen magnet, picking it up where it’s plentiful (like in the lungs right after you take a breath) and dumping it off in places where it’s in short supply (like tissues, whose cells require a constant supply of O2> and nutrients). Hemoglobin is so effective at carrying O2> (compared to say, water) that without it a person would need to have seventy-five gallons of fluid circulating in their body to carry the required O2>. And while this would certainly be an exciting development for the fabric industry (bathing-suit sizes would reflect the number of yards of material used to make them), it probably wouldn’t be much fun for anyone else.
There is, however, a downside to hemoglobin’s efficiency and this is related to the fact that it’s even more strongly attracted to carbon monoxide than it is to oxygen. This means that hemoglobin binds to the potentially deadly gas even when there’s oxygen present—a property that makes carbon monoxide extremely toxic even in small amounts. With the body’s hemoglobin tied up with carbon monoxide, the tissues in the brain quickly become starved for oxygen. This leads to a loss of consciousness followed soon after by severe brain damage and death.*59
All right, back to blood cells.
Erythrocytes are tiny—only about one three-thousandths of an inch in diameter—and each is shaped like an old rubber stickball being squeezed from opposite sides (nonstickball players often refer to this shape as a biconcave disk). Red blood cells are so focused on their single-minded, oxygen-carrying quest that when mature they lack nuclei or any of the organelles many of us once committed to the rote memorization region of our brains. Erythrocytes are formed in red bone marrow and enter into circulation at a rate of two million per second (which means that they’re destroyed and recycled at the same rate by the spleen and liver). There are so many erythrocytes that if they were laid side by side in a single layer they would cover about thirty-five hundred square yards. This provides an incredible amount of surface area for oxygen to cross into tissues.
White blood cells, on the other hand, are a different and altogether more diverse lot than erythrocytes. For starters, they have a nucleus and they do not contain hemoglobin (so they don’t carry oxygen). Leukocytes have been divided in two major groups (granulocytes and agranulocytes) based on whether or not their cytoplasm (sort of like a matrix inside each cell) looks grainy when stained for viewing under a microscope.*60
Functionally, some white blood cells (neutrophils and macrophages) are like blood cell versions of amoebas. Highly energetic, macrophages can usually be found scarfing up microscopic material through a process known as phagocytosis. Rather than looking for food, wandering