Blood

Blood is a complex liquid that circulates throughout an animal's body and keeps the body's cells alive. Blood transports oxygen and nutrients, carries away waste, and helps fight germs that invade the body. Each different function of the blood is carried out by a different type of blood cell.

The importance of blood to life was recognized by primitive humans, but they had no understanding of why it was so. They did not know that the cells in an animal's body are very specialized and that these cells cannot get their own food or dispose of their waste. Primitive humans also did not know that it is the blood that carries oxygen and nutrients to the cells, while also carrying away waste products, and that blood plays a major role in fighting disease by defending the body from invasions of microorganisms and parasites. These ancient humans believed that blood had mystical properties, and in many ways they were right since blood has the ability to sustain life.

SCIENCE LEARNS TO UNDERSTAND BLOOD

The first real scientific contribution that concerned blood was made by the English physician, William Harvey (1578–1657). Harvey made the major discovery that blood circulates constantly throughout the body in a one-way direction. Harvey said the heart was a pump which pushed the blood through a one-way, closed-loop circulation, and he was able to demonstrate this by tying off arteries and veins. He proved that blood did not wash back and forth like the ocean tides, but instead kept constantly moving in the same direction. Once this mechanical aspect of blood circulation was understood, science moved toward an understanding of the composition and actual function of blood.

After the seventeenth-century invention of the microscope, the Dutch naturalist Jan Swammerdam (1637–1680) discovered the red blood cell in frog's blood. In 1673 and 1674, his countryman, Anton van Leeuwenhoek (1632–1723), was the first to see and describe red blood cells in human blood. In the next century, white blood cells (known as leukocytes) were first seen, and finally platelets (tiny fragments that cause clotting) were first observed in 1842.

PARTS OF BLOOD

By the start of the twentieth century, the composition of blood was known. It was found to be composed of red blood cells, white blood cells, platelets, and plasma. Besides knowing what blood was made of, scientists also were beginning to understand each parts real function.

Red Blood Cells. Red blood cells are called erythrocytes, from the Greek erythros, meaning red. These cells collect oxygen from the lungs and carry it to all the cells in the body. Under a microscope they look like flattened disks, and they do not contain a nucleus (the control center for most of a cell's activities). Erythrocytes get their distinctive red color from a pigment called hemoglobin. Hemoglobin is an iron-containing protein that does the actual oxygen-carrying for the blood. People who have a diet low in iron may not have enough hemoglobin, and they would tend to tire easily since their blood does not carry enough oxygen. Hemoglobin also carries carbon dioxide away from the cells to be disposed by the lungs. Red blood cells are made in the bone marrow (a soft, fatty tissues that fills most of the bone cavities and is the source of red blood cells and many white blood cells) and live for about four months.

White Blood Cells. White blood cells are called leukocytes and form part of the body's defense against invasion. They are larger than red blood cells, and as part of the body's immune system, their job is to fight infection. They do this by engulfing and killing the invading organism.

Platelets. Blood also contains tiny fragments called platelets or thrombocytes. These small fragments go into action when the body is injured. They help cause blood to clot or form a plug to stop bleeding. They also help repair damaged blood vessels. Clotting or coagulation of the blood prevents foreign organisms from getting in and minimizes blood loss.

Plasms. All of these specialized cells and cell fragments (red cells, white cells, and platelets) are suspended in a liquid called plasma. Plasma is the liquid portion of the blood. It has often been compared to seawater in the amount of sodium chloride (common salt) it contains.

BLOOD TYPES AND DISORDERS

There are slight but important differences that people have in their blood, and this accounts for what are known as blood types or blood groups. There are four major human blood groups (A, B, AB, and O), and people can only receive blood from someone else of the same blood group. These groups are determined by certain proteins in the blood.

Blood disorders take the form of problems with red blood cells, the worst of which are hereditary diseases like sickle cell anemia. Leukemia is a type of white blood cell disorder in which these cells reproduce uncontrollably. They should only make up about one percent of the blood. Platelet disorders can result in bleeding problems or dangerous clots. Future breakthroughs in blood research may soon result in cures for these conditions, such as artificial blood or some acceptable blood substitute that will be able to carry oxygen to the cells.

[See alsoBlood Types; Circulatory System; Heart; Respiratory System ]