Coursing ceaselessly through arteries and veins, blood is vital for the life of every tissue in the body. This complex fluid contains the foods and fuel that provide energy and the materials for repairing damaged cells and building new ones. Blood also helps demolish and remove worn-out cells, other wastes, and harmful foreign bodies. It also takes heat from the body core to the extremities. And blood brings to wounds the materials that minimize blood loss and promote healing.
A man weighing about 155 pounds (70 kilograms) contains about 1.3 gallons (5 liters) of blood; a child half his weight has only half that quantity. Most blood is manufactured in bone marrow, although some components come from the lymphatic system. The four main ingredients of blood are plasma, red cells, white cells, and platelets.
This pale yellow fluid accounts for 55 to 65 per cent of blood by volume, and it is in this that the blood cells are suspended. Plasma is 90 per cent water and 10 per cent dissolved substances, chiefly salts, proteins, and some lipids. Most salts are ionized and can diffuse out from capillaries into the surrounding tissues, but the proteins are too large to escape. The resulting difference of concentration inside and outside capillaries creates osmotic pressure, which helps maintain a healthy balance between the fluid in the capillaries and in the tissues.
Important plasma proteins are albumin, globulin, and fibrinogen. Albumin (a substance also found in egg white) helps maintain blood volume and pressure. Globulin contains various antibodies each kind combining chemically with a specific kind of foreign body such as a bacterium or virus which help to neutralize disease-inducing germs. Fibrinogen plays a crucial part in blood clotting. Plasma deprived of clotting factors forms the watery liquid known as serum, which often oozes from a minor injury to the skin.
Red blood cells
These account for more than 99 per cent of the total volume of all blood cells. A mature red blood cell is a disk with concave sides, and is only 7 microns in diameter. Erythrocytes, as red cells are also known, bring oxygen from the lungs to tissues. They can do this because red cells contain the oxygen-attracting compound hemoglobin, which picks up oxygen molecules from the lungs. These oxygen molecules transform hemoglobin into oxyhemoglobin, a compound that colors the erythrocyte bright red. When erythrocytes surrender oxygen to tissues in exchange for carbon dioxide, blood becomes more purple in color, which explains the general color difference between bright red arterial blood and the duller-colored blood in veins.
Typically, a normal human body contains about 25 billion red blood cells; each microliter of blood contains from four to six million red blood cells. Bone marrow produces over 100 million every minute to make up for the millions destroyed. Each erythrocyte loses its nucleus before it leaves the marrow, then survives about 120 days before it is broken down in the spleen, liver, or blood vessels.
White blood cells
White blood cells, or leukocytes, are larger than red blood cells but far less plentiful a mere 4,000 to 10,000 per microliter compared to 5 million red blood cells. White blood cells protect and scavenge, many of them moving actively to sites of danger. There are three main types of leukocyte: granulocytes, monocytes, and lymphocytes. These account, respectively, for 70 per cent, 10 per cent, and 20 per cent of all white blood cells. The first two come from bone marrow, the third comes from lymph glands.
Granulocytes cells that have a granular appearance when stained and viewed under the miscroscope include cells that swarm upon infected tissue and devour bacteria by a process called phagocytosis. Monocytes produce macrophages that settle primarily in the spleen and liver, where they engulf old red cells and foreign bodies. Lymphocytes are the core of the body’s immune system: some kinds “remembering” specific types of foreign body and recognizing fresh invasions by them, others producing antibodies that coat these foreign bodies and make them easy prey for granulocytes and macrophages.
These smallest blood particles, of which there are from f 50,000 to 400,000 per microliter of blood, play a crucial part in clotting. They collect where an injured blood vessel is leaking blood, then stick together, thereby partly plugging the hole in the vessel wall. Platelets also react with clotting factors to convert the soluble fibrinogen in blood plasma into a mesh of fibrin threads, creating a net that traps other cells, to plug the gap completely.
Lymph and the lymphatic system
While blood circulates through arteries and veins, the blood-based fluid lymph flows through the lymphatic system.
Lymph is colorless, like plasma, but with little protein and no blood cells. Leaking from capillaries, lymph bathes the body’s cells and drains into the lymphatic system, taking with it waste matter, dead cells, and bacteria. Tissues pressing on the smaller thin-walled tubes (the larger ones have one-way valves) drive lymph into two major ducts that drain ultimately into broad veins at the sides of the neck. On the way, lymph flows through lymph nodes that filter out wastes and produce bacteria-destroying lymphocytes and other defensive cells. During illness, node activity increases, producing enlarged lymph glands that can be felt in armpit, neck, and groin.
Besides the lymph nodes, the tonsils, adenoids, intestines, spleen, and the thymus gland all generate protective lymphocytes.