The cardiovascular system includes the heart and the blood vessels. The heart pumps blood, and the blood vessels channel and deliver it throughout the body. Arteries carry blood filled with nutrients away from the heart to all parts of the body. The blood is sometimes compared to a river, but the arteries are more like a river in reverse. Arteries are thick-walled tubes with a circular covering of yellow, elastic fibers, which contain a filling of muscle that absorbs the tremendous pressure wave of a heartbeat and slows the blood down. This pressure can be felt in the arm and wrist - it is the pulse. Eventually arteries divide into smaller arterioles and then into even smaller capillaries, the smallest of all blood vessels. One arteriole can serve a hundred capillaries. Here, in every tissue of every organ, blood's work is done when it gives up what the cells need and takes away the waste products that they don't need. Now the river comparison really does apply. Capillaries join together to form small veins, which flow into larger main veins, and these deliver deoxygenated blood back to the heart. Veins, unlike arteries, have thin, slack walls, because the blood has lost the pressure which forced it out of the heart, so the dark, reddish-blue blood which flows through the veins on its way to the lungs oozes along very slowly on its way to be reoxygenated. Back at the heart, the veins enter a special vessel, called the pulmonary arteries, into the wall at right side of the heart. It flows along the pulmonary arteries to the lungs to collect oxygen, then back to the heart's left side to begin its journey around the body again.
Normally, 7-8% of human body weight is from blood. In adults, this amounts to 4.5-6 quarts of blood. This essential fluid carries out the critical functions of transporting oxygen and nutrients to our cells and getting rid of carbon dioxide, ammonia, and other waste products. In addition, it plays a vital role in our immune system and in maintaining a relatively constant body temperature. Blood is a highly specialized tissue composed of more than 4,000 different kinds of components. Four of the most important ones are red cells, white cells, platelets, and plasma. All humans produce these blood components--there are no populational or regional differences.
| Human erythrocytes or "red cells"
(cell diameter about .0003 inches)|
Red cells, or erythrocytes, are relatively large microscopic cells without nuclei. In this latter trait, they are similar to the primitive prokaryotic cells of bacteria. Red cells normally make up 40-50% of the total blood volume. They transport oxygen from the lungs to all of the living tissues of the body and carry away carbon dioxide. The red cells are produced continuously in our bone marrow from stem cells at a rate of about 2-3 million cells per second. Hemoglobinis the gas transporting protein molecule that makes up 95% of a red cell. Each red cell has about 270,000,000 iron-rich hemoglobin molecules. People who are anemic generally have a deficiency in red cells, and subsequently feel fatigued due to a shortage of oxygen. The red color of blood is primarily due to oxygenated red cells. Human fetalhemoglobinmolecules differ from those produced by adults in the number of amino acid chains. Fetalhemoglobin has three chains, while adults produce only two. As a consequence, fetalhemoglobin molecules attract and transport relatively more oxygen to the cells of the body.
White cells, or leukocytes, exist in variable numbers and types but make up a very small part of blood's volume--normally only about 1% in healthy people. Leukocytes are not limited to blood. They occur elsewhere in the body as well, most notably in the spleen, liver, and lymph glands. Most are produced in our bone marrow from the same kind of stem cells that produce red blood cells. ...