INTRODUCTION
Blood, vital fluid found in humans and other animals that provides important nourishment to all body organs and tissues and carries away waste materials. Sometimes referred to as “the river of life,” blood is pumped from the heart through a network of blood vessels collectively known as the circulatory system.
An adult human has about 5 to 6 liters (1 to 2 gal) of blood, which is roughly 7 to 8 percent of total body weight. Infants and children have comparably lower volumes of blood, roughly proportionate to their smaller size. The volume of blood in an individual fluctuates. During dehydration, for example while running a marathon, blood volume decreases. Blood volume increases in circumstances such as pregnancy, when the mother’s blood needs to carry extra oxygen and nutrients to the baby.
ROLE OF BLOOD
Blood carries oxygen from the lungs to all the other tissues in the body and, in turn, carries waste products, predominantly carbon dioxide, back to the lungs where they are released into the air. When oxygen transport fails, a person dies within a few minutes. Food that has been processed by the digestive system into smaller components such as proteins, fats, and carbohydrates is also delivered to the tissues by the blood. These nutrients provide the materials and energy needed by individual cells for metabolism, or the performance of cellular function. Waste products produced during metabolism, such as urea and uric acid, are carried by the blood to the kidneys, where they are transferred from the blood into urine and eliminated from the body. In addition to oxygen and nutrients, blood also transports special chemicals, called hormones, that regulate certain body functions. The movement of these chemicals enables one organ to control the function of another even though the two organs may be located far apart. In this way, the blood acts not just as a means of transportation but also as a communications system.
The blood is more than a pipeline for nutrients and information; it is also responsible for the activities of the immune system, helping fend off infection and fight disease. In addition, blood carries the means for stopping itself from leaking out of the body after an injury. The blood does this by carrying special cells and proteins, known as the coagulation system, that start to form clots within a matter of seconds after injury.
Blood is vital to maintaining a stable body temperature; in humans, body temperature normally fluctuates within a degree of 37.0° C (98.6° F). Heat production and heat loss in various parts of the body are balanced out by heat transfer via the bloodstream. This is accomplished by varying the diameter of blood vessels in the skin. When a person becomes overheated, the vessels dilate and an increased volume of blood flows through the skin. Heat dissipates through the skin, effectively lowering the body temperature. The increased flow of blood in the skin makes the skin appear pink or flushed. When a person is cold, the skin may become pale as the vessels narrow, diverting blood from the skin and reducing heat loss.
COMPOSITION OF BLOOD
About 55 percent of the blood is composed of a liquid known as plasma. The rest of the blood is made of three major types of cells: red blood cells (also known as erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes).
A. Plasma
Plasma consists predominantly of water and salts. The kidneys carefully maintain the salt concentration in plasma because small changes in its concentration will cause cells in the body to function improperly. In extreme conditions this can result in seizures, coma, or even death. The pH of plasma, the common measurement of the plasma’s acidity, is also carefully controlled by the kidneys within the neutral range of 6.8 to 7.7. Plasma also contains other small molecules, including vitamins, minerals, nutrients, and waste products. The concentrations of all of these molecules must be carefully regulated.
Plasma is usually yellow in color due to proteins dissolved in it. However, after a person eats a fatty meal, that person’s plasma temporarily develops a milky color as the blood carries the ingested fats from the intestines to other organs of the body.
Plasma carries a large number of important proteins, including albumin, gamma globulin, and clotting factors. Albumin is the main protein in blood. It helps regulate the water content of tissues and blood. Gamma globulin is composed of tens of thousands of unique antibody molecules. Antibodies neutralize or help destroy infectious organisms. Each antibody is designed to target one specific invading organism. For example, chicken pox antibody will target chicken pox virus, but will leave an influenza virus unharmed. Clotting factors, such as fibrinogen, are involved in forming blood clots that seal leaks after an injury. Plasma that has had the clotting factors removed is called serum. Both serum and plasma are easy to store and have many medical uses.
B. Red Blood Cells
Red blood cells make up almost 45 percent of the blood volume. Their primary function is to carry oxygen from the lungs to every cell in the body. Red blood cells are composed predominantly of a protein and iron compound, called hemoglobin, that captures oxygen molecules as the blood moves through the lungs, giving blood its red color. As blood passes through body tissues, hemoglobin then releases the oxygen to cells throughout the body. Red blood cells are so packed with hemoglobin that they lack many components, including a nucleus, found in other cells.
The membrane, or outer layer, of the red blood cell is flexible, like a soap bubble, and is able to bend in many directions without breaking. This is important because the red blood cells must be able to pass through the tiniest blood vessels, the capillaries, to deliver oxygen wherever it is needed. The capillaries are so narrow that the red blood cells, normally shaped like a disk with a concave top and bottom, must bend and twist to maneuver single file through them.
Blood Type
There are several types of red blood cells and each person has red blood cells of just one type. Blood type is determined by the occurrence or absence of substances, known as recognition markers or antigens, on the surface of the red blood cell. Type A blood has just marker A on its red blood cells while type B has only marker B. If neither A nor B markers are present, the blood is type O. If both the A and B markers are present, the blood is type AB. Another marker, the Rh antigen (also known as the Rh factor), is present or absent regardless of the presence of A and B markers. If the Rh marker is present, the blood is said to be Rh positive, and if it is absent, the blood is Rh negative. The most common blood type is A positive—that is, blood that has an A marker and also an Rh marker. More than 20 additional red blood cell types have been discovered.
Blood typing is important for many medical reasons. If a person loses a lot of blood, that person may need a blood transfusion to replace some of the lost red blood cells. Since everyone makes antibodies against substances that are foreign, or not of their own body, transfused blood must be matched so as not to contain these substances. For example, a person who is blood type A positive will not make antibodies against the A or Rh markers, but will make antibodies against the B marker, which is not on that person’s own red blood cells. If blood containing the B marker (from types B positive, B negative, AB positive, or AB negative) is transfused into this person, then the transfused red blood cells will be rapidly destroyed by the patient’s anti-B antibodies. In this case, the transfusion will do the patient no good and may even result in serious harm. For a successful blood transfusion into an A positive blood type individual, blood that is type O negative, O positive, A negative, or A positive is needed because these blood types will not be attacked by the patient’s anti-B antibodies.
White Blood Cells
White blood cells only make up about 1 percent of blood, but their small number belies their immense importance. They play a vital role in the body’s immune system—the primary defense mechanism against invading bacteria, viruses, fungi, and parasites. They often accomplish this goal through direct attack, which usually involves identifying the invading organism as foreign, attaching to it, and then destroying it. This process is referred to as phagocytosis.
White blood cells also produce antibodies, which are released into the circulating blood to target and attach to foreign organisms. After attachment, the antibody may neutralize the organism, or it may elicit help from other immune system cells to destroy the foreign substance. There are several varieties of white blood cells, including neutrophils, monocytes, and lymphocytes, all of which interact with one another and with plasma proteins and other cell types to form the complex and highly effective immune system.
BLOOD DISEASES
Many diseases are caused by abnormalities in the blood. These diseases are categorized by which component of the blood is affected.
Red Blood Cell Diseases
White Blood Cell Diseases
Coagulation Diseases
Blood, vital fluid found in humans and other animals that provides important nourishment to all body organs and tissues and carries away waste materials. Sometimes referred to as “the river of life,” blood is pumped from the heart through a network of blood vessels collectively known as the circulatory system.
An adult human has about 5 to 6 liters (1 to 2 gal) of blood, which is roughly 7 to 8 percent of total body weight. Infants and children have comparably lower volumes of blood, roughly proportionate to their smaller size. The volume of blood in an individual fluctuates. During dehydration, for example while running a marathon, blood volume decreases. Blood volume increases in circumstances such as pregnancy, when the mother’s blood needs to carry extra oxygen and nutrients to the baby.
ROLE OF BLOOD
Blood carries oxygen from the lungs to all the other tissues in the body and, in turn, carries waste products, predominantly carbon dioxide, back to the lungs where they are released into the air. When oxygen transport fails, a person dies within a few minutes. Food that has been processed by the digestive system into smaller components such as proteins, fats, and carbohydrates is also delivered to the tissues by the blood. These nutrients provide the materials and energy needed by individual cells for metabolism, or the performance of cellular function. Waste products produced during metabolism, such as urea and uric acid, are carried by the blood to the kidneys, where they are transferred from the blood into urine and eliminated from the body. In addition to oxygen and nutrients, blood also transports special chemicals, called hormones, that regulate certain body functions. The movement of these chemicals enables one organ to control the function of another even though the two organs may be located far apart. In this way, the blood acts not just as a means of transportation but also as a communications system.
The blood is more than a pipeline for nutrients and information; it is also responsible for the activities of the immune system, helping fend off infection and fight disease. In addition, blood carries the means for stopping itself from leaking out of the body after an injury. The blood does this by carrying special cells and proteins, known as the coagulation system, that start to form clots within a matter of seconds after injury.
Blood is vital to maintaining a stable body temperature; in humans, body temperature normally fluctuates within a degree of 37.0° C (98.6° F). Heat production and heat loss in various parts of the body are balanced out by heat transfer via the bloodstream. This is accomplished by varying the diameter of blood vessels in the skin. When a person becomes overheated, the vessels dilate and an increased volume of blood flows through the skin. Heat dissipates through the skin, effectively lowering the body temperature. The increased flow of blood in the skin makes the skin appear pink or flushed. When a person is cold, the skin may become pale as the vessels narrow, diverting blood from the skin and reducing heat loss.
COMPOSITION OF BLOOD
About 55 percent of the blood is composed of a liquid known as plasma. The rest of the blood is made of three major types of cells: red blood cells (also known as erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes).
A. Plasma
Plasma consists predominantly of water and salts. The kidneys carefully maintain the salt concentration in plasma because small changes in its concentration will cause cells in the body to function improperly. In extreme conditions this can result in seizures, coma, or even death. The pH of plasma, the common measurement of the plasma’s acidity, is also carefully controlled by the kidneys within the neutral range of 6.8 to 7.7. Plasma also contains other small molecules, including vitamins, minerals, nutrients, and waste products. The concentrations of all of these molecules must be carefully regulated.
Plasma is usually yellow in color due to proteins dissolved in it. However, after a person eats a fatty meal, that person’s plasma temporarily develops a milky color as the blood carries the ingested fats from the intestines to other organs of the body.
Plasma carries a large number of important proteins, including albumin, gamma globulin, and clotting factors. Albumin is the main protein in blood. It helps regulate the water content of tissues and blood. Gamma globulin is composed of tens of thousands of unique antibody molecules. Antibodies neutralize or help destroy infectious organisms. Each antibody is designed to target one specific invading organism. For example, chicken pox antibody will target chicken pox virus, but will leave an influenza virus unharmed. Clotting factors, such as fibrinogen, are involved in forming blood clots that seal leaks after an injury. Plasma that has had the clotting factors removed is called serum. Both serum and plasma are easy to store and have many medical uses.
B. Red Blood Cells
Red blood cells make up almost 45 percent of the blood volume. Their primary function is to carry oxygen from the lungs to every cell in the body. Red blood cells are composed predominantly of a protein and iron compound, called hemoglobin, that captures oxygen molecules as the blood moves through the lungs, giving blood its red color. As blood passes through body tissues, hemoglobin then releases the oxygen to cells throughout the body. Red blood cells are so packed with hemoglobin that they lack many components, including a nucleus, found in other cells.
The membrane, or outer layer, of the red blood cell is flexible, like a soap bubble, and is able to bend in many directions without breaking. This is important because the red blood cells must be able to pass through the tiniest blood vessels, the capillaries, to deliver oxygen wherever it is needed. The capillaries are so narrow that the red blood cells, normally shaped like a disk with a concave top and bottom, must bend and twist to maneuver single file through them.
Blood Type
There are several types of red blood cells and each person has red blood cells of just one type. Blood type is determined by the occurrence or absence of substances, known as recognition markers or antigens, on the surface of the red blood cell. Type A blood has just marker A on its red blood cells while type B has only marker B. If neither A nor B markers are present, the blood is type O. If both the A and B markers are present, the blood is type AB. Another marker, the Rh antigen (also known as the Rh factor), is present or absent regardless of the presence of A and B markers. If the Rh marker is present, the blood is said to be Rh positive, and if it is absent, the blood is Rh negative. The most common blood type is A positive—that is, blood that has an A marker and also an Rh marker. More than 20 additional red blood cell types have been discovered.
Blood typing is important for many medical reasons. If a person loses a lot of blood, that person may need a blood transfusion to replace some of the lost red blood cells. Since everyone makes antibodies against substances that are foreign, or not of their own body, transfused blood must be matched so as not to contain these substances. For example, a person who is blood type A positive will not make antibodies against the A or Rh markers, but will make antibodies against the B marker, which is not on that person’s own red blood cells. If blood containing the B marker (from types B positive, B negative, AB positive, or AB negative) is transfused into this person, then the transfused red blood cells will be rapidly destroyed by the patient’s anti-B antibodies. In this case, the transfusion will do the patient no good and may even result in serious harm. For a successful blood transfusion into an A positive blood type individual, blood that is type O negative, O positive, A negative, or A positive is needed because these blood types will not be attacked by the patient’s anti-B antibodies.
White Blood Cells
White blood cells only make up about 1 percent of blood, but their small number belies their immense importance. They play a vital role in the body’s immune system—the primary defense mechanism against invading bacteria, viruses, fungi, and parasites. They often accomplish this goal through direct attack, which usually involves identifying the invading organism as foreign, attaching to it, and then destroying it. This process is referred to as phagocytosis.
White blood cells also produce antibodies, which are released into the circulating blood to target and attach to foreign organisms. After attachment, the antibody may neutralize the organism, or it may elicit help from other immune system cells to destroy the foreign substance. There are several varieties of white blood cells, including neutrophils, monocytes, and lymphocytes, all of which interact with one another and with plasma proteins and other cell types to form the complex and highly effective immune system.
BLOOD DISEASES
Many diseases are caused by abnormalities in the blood. These diseases are categorized by which component of the blood is affected.
Red Blood Cell Diseases
White Blood Cell Diseases
Coagulation Diseases
BLOOD BANKS
The Red Cross and a number of other organizations run programs, known as blood banks, to collect, store, and distribute blood and blood products for transfusions. When blood is donated, its blood type is determined so that only appropriately matched blood is given to patients needing a transfusion. Before using the blood, the blood bank also tests it for the presence of disease-causing organisms, such as hepatitis viruses and human immunodeficiency virus (HIV), the cause of acquired immunodeficiency syndrome (AIDS). This blood screening dramatically reduces, but does not fully eliminate, the risk to the recipient of acquiring a disease through a blood transfusion. Blood donation, which is extremely safe, generally involves giving about 400 to 500 ml (about 1 pt) of blood, which is only about 7 percent of a person’s total blood.
BLOOD IN NONHUMANS
One-celled organisms have no need for blood. They are able to absorb nutrients, expel wastes, and exchange gases with their environment directly. Simple multicelled marine animals, such as sponges, jellyfishes, and anemones, also do not have blood. They use the seawater that bathes their cells to perform the functions of blood. However, all more complex multicellular animals have some form of a circulatory system using blood. In some invertebrates, there are no cells analogous to red blood cells. Instead, hemoglobin, or the related copper compound heocyanin, circulates dissolved in the plasma.
The blood of complex multicellular animals tends to be similar to human blood, but there are also some significant differences, typically at the cellular level. For example, fish, amphibians, and reptiles possess red blood cells that have a nucleus, unlike the red blood cells of mammals. The immune system of invertebrates is more primitive than that of vertebrates, lacking the functionality associated with the white blood cell and antibody system found in mammals. Some arctic fish species produce proteins in their blood that act as a type of antifreeze, enabling them to survive in environments where the blood of other animals would freeze. Nonetheless, the essential transportation, communication, and protection functions that make blood essential to the continuation of life occur throughout much of the animal kingdom.
