Blood Cells
The cellular portion of blood consists of red blood cells, white blood cells, and platelets. Red blood cells transport oxygen, white blood cells perform a variety of protective functions, and platelets help in the clotting process. Platelets are actually fragments of cells derived from larger cells in bone marrow.
Red Blood Cells The most numerous cells in the blood are the red blood cells, or erythrocytes (eh-RITH-roh-syts). One milliliter of blood contains about 5 million red blood cells. Red blood cells transport oxygen. They get their color from hemoglobin. Hemoglobin is the iron-containing protein that binds to oxygen in the lungs and transports it to tissues throughout the body where the oxygen is released.
Red blood cells are shaped like disks that are thinner in the center than along the edges. These cells are produced from cells in red bone marrow. As these cells gradually become filled with hemoglobin, their nuclei and other organelles are forced out. Thus, mature red blood cells do not have nuclei. Red blood cells circulate for an average of 120 days before they are worn out from squeezing through narrow capillaries. Old red blood cells are destroyed in the liver and spleen.
White Blood Cells White blood cells, or leukocytes (LOO-koh-syts), do not contain hemoglobin. They are much less common than red cells, which outnumber them almost 1000 to 1. Both white and red blood cells are produced from the same population of blood-forming stem cells found in the bone marrow. Unlike red blood cells, however, white blood cells contain nuclei. They may live for days, months, or even years.
White blood cells are the “army” of the circulatory system—they guard against infection, fight parasites, and attack bacteria. There are many types of white blood cells, and they perform a wide variety of important functions. Some protect the body by acting as phagocyte, or “eating cells,” that engulf and digest bacteria and other disease-causing microorganisms. Some white blood cells react to foreign substances by releasing chemicals known as histamines. These chemicals increase blood flow into the affected area, producing redness and swelling that are often associated with allergies. A special class of white blood cells, known as lymphocytes, produce antibodies that are proteins that help destroy pathogens. Antibodies are essential to fighting infection and help to produce immunity to many diseases.
White blood cells are not confined to the circulatory system. Many white blood cells are able to slip out of capillary walls, travel through the lymphatic system, and attack invading organisms in the tissues of the body. In many ways, white blood cells are the first lines of defense when the body is invaded by disease-causing organisms, making them part of the immune system as well.
Like an army with units in reserve, the body is able to increase the number of white blood cells dramatically when a “battle” is underway. A sudden increase in the white blood cell count is one of the ways in which physicians can tell that the body is fighting a serious infection.
Platelets and Blood Clotting Blood is essential to life. An injury can cause the body to lose this essential fluid. Fortunately, blood has an internal mechanism to slow bleeding and begin healing. A minor cut or scrape may bleed for a few seconds or minutes, but then it stops. Clean it up with soap and water, cover it with a bandage, and it begins to heal. Have you ever wondered why the bleeding stops?
The answer is that blood has the ability to form a clot. The figure at right summarizes the process. Blood clotting is made possible by plasma proteins and cell fragments called platelets. There are certain large cells in bone marrow that can break into thousands of small pieces. Each fragment of cytoplasm is enclosed in a piece of cell membrane and released into the bloodstream as a platelet.
Blood Clotting
When platelets come into contact with the edges of a broken blood vessel, their surfaces become very sticky, and a cluster of platelets develops around the wound. These platelets then release proteins called clotting factors. The clotting factors start a series of chemical reactions that are quite complicated. In one reaction, a clotting factor called thromboplastin (thrahm-boh-PLAS-tin) converts prothrombin, which is found in blood plasma, into thrombin. Thrombin is an enzyme that helps convert the soluble plasma protein fibrinogen into a sticky mesh of fibrin filaments. These filaments stop the bleeding by producing a clot. The figure below shows the tangle of microscopic fibers in an actual blood clot.
Formation of a Blood Clot Strands of fibrin trap blood cells, forming a net that prevents blood from leaving a damaged blood vessel.
Blood Clotting Problems If the wound is small, within a few minutes the mesh of platelets and fibrin seals the wound, and bleeding stops. Most of the time, this clotting reaction works so well that we take it for granted. However, if one of the clotting factors is missing or defective, the clotting process does not work well. Hemophilia is a genetic disorder that results from a defective protein in the clotting pathway. People with hemophilia cannot produce blood clots that are firm enough to stop even minor bleeding. They must take great care to avoid injury. Fortunately, hemophilia can be treated by injecting extracts containing the missing clotting factor.
Although the first successful transfusions of human blood were carried out in the 1820s, many recipients had severe reactions to the transfused blood, and several died. Today we know why. We inherit one of four blood types—A, B, AB, or O—which are determined by antigens on our blood cells. Antigens are substances that trigger an immune response. People with blood type A have A antigens on their cells, those with type B have B antigens, those with AB blood have both A and B, and those with type O have neither A nor B antigens.
When blood types match, the transfusion is successful. However, transfusions are successful in some cases even when the blood types of the donor and the recipient do not match. Use the table to answer the questions that follow.
Drawing Conclusions Which blood type is sometimes referred to as the “universal donor”? Which is known as the “universal recipient”?
Drawing Conclusions In a transfusion involving the A and O blood types, does it make a difference which blood type belongs to the recipient and which to the donor?
Applying Concepts Write a brief explanation for the results in the chart using information about phenotypes and genotypes in blood group genes. (Hint: Review Section 14-1 if needed.)