Anatomy of the blood

A milliliter of blood has approximately 4.2 million to 6 million red blood cells (RBCs), 4500 to 11,000 white blood cells (WBCs), and 150,000 to 450,000 platelets. The average 150-pound (68-kg) person has approximately 5 liters of blood circulating throughout the body. The following sections discuss plasma, platelets, WBCs, and RBCs in more detail.

Blood composition

Blood is made up of two components:

• Liquid portion: Called plasma and makes up about 55% of the total blood volume.
• Formed elements: Includes RBCs, WBCs, and platelets. The formed elements are suspended (freely float) in the plasma and make up about 45% of the total blood volume.

The following sections describe the components of the blood.

Plasma

Plasma is the liquid portion of whole blood. Plasma contains 90% water. Water is the solvent for all the dissolved substances that travel in the plasma. Plasma also contains the following components:

• Plasma proteins: Made in the liver; plasma proteins include the following:
• Albumin: It is the most abundant protein in plasma, and it attracts water. It normally stays in the blood vessels; thus, it helps to maintain the blood volume.
• Globulins: Antibodies are a type of globulin; they protect against infections.
• Clotting factors: Include fibrinogen and prothrombin, which are important in the blood clotting or coagulation process.
• Complements: Enzymes (proteins) that are normally inactive in the blood. When activated, they create protein rings that destroy pathogens.
• Inorganic substances and electrolytes (sodium, calcium, potassium, chloride, magnesium, bicarbonate).
• Organic substances (amino acids, glucose, fats, cholesterol, hormones).
• Waste products (urea, uric acid, ammonia, creatinine).

Organic and inorganic substances are needed for metabolism and cellular function, repair, and reproduction. Cellular metabolism waste products are dissolved in the plasma and are transported to the liver, kidneys, and lungs for excretion.

Plasma has a very narrow pH range (7.35 to 7.45), which is necessary to make sure that red blood cells can transport the optimal level of oxygen in the blood. The range also allows needed chemical reactions in the body to continue to work. Most chemical reactions in the body are pH-dependent. Without a stable blood pH, homeostasis is unlikely to be maintained.

Thrombocytes

Thrombocytes, also called platelets, have an irregular round or oval shape. They are formed in the bone marrow from stem cells. They are just a small piece or fragment of a much larger cell called a megakaryocyte, which is in the bone marrow. Platelets aid in coagulation, the process of changing a liquid to a solid. When an injury to a blood vessel occurs, platelets rush to the area and agglutinate, or clump together. This stops or slows the bleeding, thus controlling the blood flow (hemostasis).

Leukocytes

Leukocytes, also called white blood cells (WBCs), protect the body from invading pathogens. There are two main types:

• Granulocytes: Have granules within the cytoplasm, and they also have a segmented or multilobed nucleus. There are three types of granulocytes: neutrophils, eosinophils, and basophils. Each has its own role in immunity.
• Agranulocytes: Lack granules in the cytoplasm, and they have one nucleus. Granulocytes originate in the bone marrow and mature after entering the lymphatic system. There are two types of agranulocytes: monocytes and lymphocytes.

Erythrocytes

Erythrocytes, or red blood cells, have the important function of transporting oxygen (O2) and carbon dioxide (CO2) throughout the body. Hemoglobin (Hgb), a molecule on the RBC, is capable of binding, transporting, and releasing oxygen and carbon dioxide. Hemoglobin is made up of protein and iron.

RBC production is stimulated by erythropoietin, a hormone produced in the kidneys. Hematopoiesis of RBCs takes place in the red bone marrow. Stem cells produce immature RBCs that contain a nucleus. As an RBC matures, it loses its nucleus and is released into the bloodstream. RBCs are biconcave (curved inward on both sides), which allows for more surface area for the hemoglobin, thus allowing more O2 and CO2 to be carried. The shape also allows more flexibility as the RBC moves through the tiny capillaries.

Red blood cells have a life span of approximately 120 days. When an RBC is no longer useful, it is broken down into iron and bilirubin in the spleen. The iron is stored mainly in the liver and is recycled into new RBCs. The bilirubin is further broken down into bile, which is excreted by the liver.

Blood types

Two major classification systems are used with blood types, the ABO system and the Rh system. In the ABO system, each person has an A, a B, an ABO, or an O blood type. In the Rh system, a person is either Rh positive or Rh negative. For example, a person may have O-negative blood. In the ABO system, the person has type O blood and is Rh negative. These two systems are explained in more detail in the following sections.

ABO System

In the ABO system, each blood type is unique based on the specific antigens present or absent on the surface of the RBCs and specific antibodies circulating in the plasma.** Antigens** are foreign substances to the body, and the immune system makes specific antibodies to fight off specific antigens. When a person receives a blood transfusion, the blood given must be compatible with the person’s blood. If an antibody is present in the recipient’s blood, the corresponding antigen cannot be in the donor’s blood. For example, type O blood contains both A and B antibodies; thus blood with A or B antigens cannot be given. The only type of blood that is compatible with type O is type O. Another example is a person with type AB blood, which contains no antibodies. These people can receive all four blood types because they have no antibodies to attack the donor blood. Type AB blood is considered the universal recipient because it does not contain any antibodies.** Type O** is considered the universal donor because it does not contain any antigens, and everyone can receive type O blood.

Rh System

In the Rh system, there is a collection of 47 antigens that act together as a group and are referred to as the Rh antigen or Rh factor. When the Rh antigen is present, the person is said to be Rh positive. About 85% of the population in the United States is Rh positive, and 15% is Rh negative.

Rh antibodies are different from ABO antibodies. Rh antibodies are not preformed (present before antigen exposure), as are ABO antibodies. The first time a person is exposed to the Rh antigen, the immune system produces antibodies. This process is called sensitization. Nothing is noticeable during the first exposure, although subsequent exposures to the Rh antigen will cause lysis (destruction) of RBCs.

Blood typing is the process of determining a person’s blood type using the ABO and Rh systems. If the person receives the wrong type of blood, a transfusion reaction can occur, which may be a serious, life-threatening situation. For instance, if a person with type A blood is transfused with type B blood, the anti-B antibodies in the recipient’s blood will attack the B antigens in the donor’s blood, causing agglutination (blood clumping) or lysis (blood cell rupture).

Erythroblastosis fetalis

The Rh factor is an important consideration during pregnancy for Rh-negative mothers due to erythroblastosis fetalis (also called hemolytic disease of the newborn [HDN]). In this disorder, an Rh-negative mother who is carrying an Rh-positive fetus will develop antibodies to the Rh-positive blood if fetal blood exposure occurs. If another pregnancy occurs with an Rh-positive fetus, the mother’s immune system produces antibodies that cross the placenta and destroy the fetal red blood cells.

To prevent this disorder, it is standard practice for the pregnant mother’s blood to be tested (typed) early in pregnancy if the blood type is unknown. Mothers with Rh-negative blood receive an injection of RhoGAM (Rho[D] Immune Globulin [Human]) around weeks 26 to 28 of pregnancy. After the baby is born, the cord blood is tested. If the baby has Rh-positive blood, the mother receives another injection of RhoGAM within 72 hours of delivery.

MICRhoGam (a smaller dose of RhoGAM) should be given to Rh-negative women 72 hours after a threatened or actual termination of pregnancy (spontaneous [miscarriage] or induced) through week 12 of the pregnancy. For threatened or actual exposure to Rh-positive blood (e.g., through an invasive procedure, trauma, ectopic pregnancy, or pregnancy termination), Rh-negative women should receive RhoGAM within 72 hours. RhoGAM and MICRhoGAM are Rho (D) human immune globulin, a blood product that prevents Rh immunization by binding to Rh-positive cells that sneak into the bloodstream, making them invisible to the mother’s immune system.