Characteristics of Red Blood Cells, Red Blood Cell Counts, and Destructions of Red Blood Cells
Red Blood Cell Production and Its Control and Dietary Factors Affecting RBC Production
Types of White Blood Cells, White Blood Cell Counts, and Functions of White Blood Cells
Nutrients and Gases, Nonprotein Nitrogenous Substances, and Plasma Electrolytes
Blood Vessel Spasm, Platelet Plug Formation, and Blood Coagulation
Antigens and Antibodies, ABO Blood Group, and Rh Blood Group
Blood is a type of connective tissue. Cells found in blood include red blood cells and white blood cells. Blood also contains cell fragments called platelets. The fluid part of blood is called plasma.
An average-sized adult contains about 5 liters of blood. However blood volume varies from person to person depending on their size, amount of adipose tissue, and concentrations of certain ions in blood.
A percentage of blood cells in a sample of blood is called a hematocrit. A normal person would have a hematocrit around 45%. Most of the cells are red blood cells. Only about 1% of the cells are white blood cells. The rest of blood (55%) is plasma.
Click here to see a sample of blood and its components
Red blood cells are also called erythrocytes. They are biconcave-shaped cells that are small enough to pass through the smallest blood vessels called capillaries. Mature red blood cells do not contain nuclei. They lose their nuclei in order to make room for a pigment called hemoglobin. Hemoglobin is the pigment in red blood cells that carries oxygen. Hemoglobin carrying oxygen is called oxyhemoglobin and is bright red; when it is not carrying oxygen it is called deoxyhemoglobin and is dark red.
Click here to see red blood cells
A red blood cell count is the number of red blood cells in one cubic millimeter of blood. A normal red blood cell count will be somewhere between 4 million to 6.5 million red blood cells per cubic millimeter of blood. A cubic millimeter of blood is only 20 drops of blood!
Because the function of a red blood cell is to transport oxygen throughout the body, a low red blood cell count would reflect a decreased ability to carry oxygen.
When red blood cells age, macrophages in the liver and spleen destroy them. When a red blood cell is destroyed a pigment called biliverdin is released from the cell. The liver usually converts bkilverdin into an orange colored pigment called bilirubin. Bilirubin is used to make bile which is needed for the digestion of fats. However, sometimes bilirubin is not used to make bile; instead it persists in the blood stream. This causes a person's skin to appear yellowish - a condition known as jaundice.
Click here to see a summary of red blood cell destruction
During development red blood cells are made in the yolk sac, liver and spleen. However, once a baby is born, most red blood cells are produced by red bone marrow.
The average life span of a red blood cell is only about 120 days so red bone marrow is constantly making new cells.
The hormone erythropoietin is responsible for regulating the production of red blood cells. Erythropoietin is mostly produced by the kidneys and stimulates the red bone marrow to produce red blood cells. The kidneys know to release this hormone when oxygen concentrations in the blood get low.
The cell type that makes new blood cells is called a hemocytoblast.
Vitamin B12 and folic acid are two dietary factors that affect red blood cell production. These vitamins are necessary for DNA synthesis so any actively dividing tissue such as red bone marrow is affected when DNA cannot be produced. Iron is also necessary to make hemoglobin. Too few red blood cells or too little hemoglobin results in a condition known as anemia. Anemia is a decreased ability to transport oxygen.
White blood cells are also called leukocytes. Some leukocytes have granules in their cytoplasm and thus they are granulocytes. Granulocytes include neutrophils, eosinophils, and basophils. Some leukocytes do not have granules in their cytoplasm and thus they are called agranulocytes. Agranuloctyes include monocytes and lymphocytes.
Neutrophils account for about 55% of all white blood cells. They are phagocytic cells so are important for destroying bacteria, viruses, and toxins in the blood stream.
Eosinophils account for about 3% of all white blood cells. They are effective in getting rid of parasitic infections such as worm infections. They also help to control inflammation and allergic reactions.
Basophils account for less than 1% of all white blood cells. They release substances such as histamine and heparin that promote inflammation.
Monocytes account for about 8% of all white blood cells. They are phagocytic cells so are important for detroying bacteria, viruses, and toxins.
Lymphocytes account for about 33% of all white blood cells. They provide immunity for the body. Immunity is discussed in a later chapter.
Click here to see pictures of the various types of white blood cells
A white blood cell count is the number of white blood cells in 1 cubic millimeter of blood (about 20 drops of blood). A normal white blood cell count will be between 5 thousand and 10 thousand cells per cubic millimeter of blood. A white blood cell count above normal is termed leukocytosis. Leukocytosis will typically occur when a person has a bacterial infection. A white blood cell count below normal is termed leukopenia. Some viral infections cause leukopenia.
A differential white blood cell count lists percentages of the different types of leukocytes in a sample of blood. This is a useful test as the numbers of different white blood cells change in different diseases. For example, neutrophil numbers increase at the onset of a bacterial infection while monocyte numbers will not increase for about two weeks after a bacterial infection. Eosinophil numbers increase during worm infections. In AIDS, lymphocyte numbers fall.
Click here to see a table listing some causes of abnormal white blood cell counts
White blood cells have defensive functions. Some stay in the blood stream and fight infections there. Some however, leave the blood stream by squeezing through blood vessel walls to reach other tissues of the body. Squeezing through a blood vessel wall is called diapedesis.
Platelets are fragments of cells that are found in the blood stream. Platelets are also called thrombocytes and are important in the clotting of blood. Platelets come from cells called megakaryocytes that are in red bone marrow. An normal average platelet count would be between 130,000 to 360,000 platelets per cubic millimeter of blood.
Plasma is the liquid portion of blood. It is mostly water but also contains a mixture of organic and inorganic molecules.
There are three major types of proteins found in plasma:
1) albumins - smallest of the plasma proteins; made by the liver; important for pulling water into the blood stream to help maintain blood pressure
2) globulins - made by the liver and lymphatic organs; they transport lipids and some vitamins in plasma; some become antibodies
3) fibrinogen - important for blood coagulation
Nutrients in plasma include amino acids, glucose, nucleotides and lipids that have all been absorbed from the digestive tract.
Because lipids are not water soluble (plasma is mostly water), they must combine with molecules called lipoproteins to be transported. The different types of lipoproteins are chylomicrons, very low-density lipoproteins (VLDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL).
The gases dissolved in plasma include oxygen, carbon dioxide, and nitrogen.
Molecules that contain nitrogen but are not proteins make up a group called nonprotein nitrogenous substances. They include amino acids, urea and uric acid. Urea and uric acid are waste products of metabolism.
Many electrolytes (ions) are dissolved in plasma. They include sodium ions, potassium ions, calcium ions, magnesium ions, chloride ions, bicarbonate ions, phosphate ions, and sulfate ions. the importance of these ions are discussed in a later chapter.
Hemostasis refers to the stoppage of bleeding. This is important when blood vessels are damaged and bleeding begins. There are three processes that occur is hemostasis - 1) blood vessel spasm, 2) platelet plug formation and 3) blood coagulation.
When a blood vessels is broken, the smooth muscle in its wall will contract and cause the blood vessel to spasm. This spasm reduces the amount of blood lost through the vessel.
When a blood vessel is broken, blood is exposed to surrounding connective tissue. The connective tissue triggers platelets to stick to the broken area of the blood vessel. The platelets also stick to each other to form a platelet plug. The platelet plug stops the bleeding temporarily.
Click here to view platelet plug formation
A blood clot eventually replaces the platelet plug. The formation of a blood clot is called blood coagulation. In this process the plasma protein fibrinogen is converted to fibrin. Once fibrin forms it sticks to the damaged area of the blood vessel creating a meshwork that entraps blood cells and platelets. The resulting mass is called a blood clot which stops bleeding until the vessels has repaired itself.
When a blood vessels is injured, it is normal for a blood clot to form. However, sometimes blood clots form on the side of a blood vessel that has not been injured; this abnormal blood clot is called a thrombus. The danger of a thrombus is that a portion of it can break off and start moving through the blood stream. The moving portion of the thrombus is called an embolus. The danger of an embolus is that it can block small blood vessels in the heart, causing a heart attack. It can also block small blood vessels in other organs. If it blocks a blood vessel in the brain, a stroke results.
Click here to a summary of the events of blood clotting
Agglutination is the clumping of red blood cells following a blood transfusion. Agglutination is not desirable as it leads to severe anemia.
Agglutination occurs because proteins called antigens on the surface of red blood cells bind to antibodies in plasma. To prevent agglutination antigens should not be mixed with antibodies that will bind to them.
If a person has type A blood, then they have antigen A on the surface of their red blood cells. A person with type A blood also has antibody B in their plasma. Antibody B will only bind to antigen B.
If a person has type B blood, then they have antigen B on the surface of their red blood cells. A person with type B blood also has antibody A in their plasma.
If a person with type A blood is given type B blood then the antibody B in the person with type A blood will bind with the red blood cells of type B blood because those cells have antigen B on their surface. Remember that when antigens bind with antibodies, agglutination occurs and red blood cells are destroyed. This is why a person with type A blood should not be given type B blood and vice versa.
If a person has type O blood then they have neither antigen A nor antigen B on the surface of their red blood cells. However, they do have both antibody A and antibody B in their plasma. Type O blood is called the universal donor because it can be given to most people regardless of their blood type. Type O blood will not agglutinate when given to other people because it does not have the antigens to bind to antibodies.
If a person has type AB blood then they have both antigen A and antigen B on the surface of their red blood cells. They have neither antibody A nor antibody B in their plasma. People with type AB blood are universal recipients because most of them can receive all blood types. They can receive all blood types because they lack the antibodies in their plasma that bind to antibodies.
Click here for a summary of ABO blood types
If a person is Rh positive then they have red blood cells that contain the Rh antigen. If a person is Rh negative then their red blood cells do not contain the Rh antigen. If a person who is Rh negative is given Rh positive blood, then the Rh negative person will make antibodies that will bind to the Rh antigens and agglutination of red blood cells will occur.
If a female is Rh negative and she mates with an Rh positive male, there is a 50:50 chance her fetus will be Rh positive. The Rh positive fetus's blood mixes with the mothers blood and the mother will develop antibodies against the fetus's blood cells with the Rh antigen. The first Rh positive fetus usually does not suffer from these antibodies because it takes so long for the mother's body to generate the antibodies. However, when she conceives a second Rh positive fetus, the fetus's blood will be attacked by the antibodies. The fetus then develops a condition called erythroblastosis fetalis and the baby is born severely anemic.
Today erythroblastosis fetalis is prevented by giving an Rh negative woman the drug RhoGAM. RhoGAM prevents the Rh negative mother from making antibodies against the Rh antigen.
Click here for a summary of the importance of the Rh factor in pregnancy