Anatomy and Physiology

The hemopoietic system consists of blood, lymphatic tissue, bone marrow, and the spleen. Circulating blood contains both plasma and blood cells, with the plasma constituting approximately 55% of the total blood volume. The plasma is about 90% water and 10% solutes such as proteins, glucose, amino acids, and lipids. Plasma proteins are classified as globulins, albumins, and clotting factors. With the exception of immunoglobulins (Igs), all of these proteins are synthesized by the liver. Immunoglobulins are synthesized by the lymphatic system, specifically by mature lymphocytes termed plasma cells. Immunoglobulins are responsible for fighting infectious organisms and include IgA, IgB, IgM, IgD, and IgE. Albumins regulate the passage of water and solutes through the capillaries. Clotting factors found within the plasma are primarily fibrinogen, the precursor of the fibrin clot.

Three basic types of blood cells—erythrocytes, leukocytes, and thrombocytes (Table 9-1)—make up the remaining 45% of the total blood volume.

Erythrocytes, or red blood cells (RBCs), are very small in relation to the other blood cells. They do not possess a nucleus and are shaped like biconcave disks. Erythrocytes are responsible for transporting oxygen and carbon dioxide to and from the various organs of the body. This is accomplished via the hemoglobin in erythrocytes, which allows oxygen or carbon dioxide molecules to attach to the cell for transport. Individuals with a hemoglobin level of less than 12 grams (g) per 100 milliliters (mL) of blood have anemia and are considered “anemic” because less than normal oxygen or carbon dioxide transportation occurs in them. The total percentage of RBCs in blood volume is determined by a laboratory test termed hematocrit.

Erythrocytes are formed by specialized cells called hemocytoblasts, which are located in the myeloid tissue found within red bone marrow. These cells live approximately 120 days and are phagocytosed by the reticuloendothelial system, which consists of specialized cells in the liver, spleen, and bone marrow. During phagocytosis, the iron within the hemoglobin is released, and bilirubin is formed. The released iron is used again in the development of new erythrocytes, and bilirubin is excreted in bile.

Erythrocytes may contain various antigens that determine blood type. This is especially critical for blood transfusions because blood type incompatibility could have fatal results. Erythrocytes may contain no antigens, either the A or the B antigen, or both the A and the B antigens. The resulting blood types are O (no antigen), A, B, and AB. If incompatible blood types are mixed (e.g., a type O patient receives type AB blood), erythrocytes from the donor clump together in the serum of the recipient. Because the type O recipient does not possess the A or B antigen, antibodies are formed to fight against the foreign RBCs. This is termed agglutination. Eventually, donor erythrocytes are destroyed, and the rejection possibly results in immediate shock. In some cases, the reaction may be delayed and may result in fever, pain, and ultimate renal failure as the kidneys try to excrete the byproducts from the destruction of the donor erythrocytes. Cross-matching of blood types to eliminate the risk of transfusion of incompatible blood is essential.

The person with type O blood is considered the universal donor because this type does not contain any antigens and can be given to anyone regardless of the recipient’s blood type. The person with type AB blood is considered the universal recipient because this type of blood possesses both antigens, which enables the person to receive any type of blood (Table 9-2). In addition, the Rh blood factor should be considered. The Rh factor is termed such because it was first discovered in the blood of the rhesus monkey. Approximately 85% of the human population has this factor; these people are classified as Rh-positive. Individuals not possessing the Rh factor are Rh-negative. The Rh factor becomes a problem if an Rh-positive father transmits the Rh factor to a fetus carried by an Rh-negative mother. The first pregnancy generally progresses normally, but in subsequent pregnancies, the mother’s anti-Rh antibodies, made during the first pregnancy, attack the Rh-positive fetal blood. This scenario can be avoided by Rh-immunization of the mother before pregnancy.

Leukocytes, or white blood cells (WBCs), may be classified as granular or nongranular. Granular leukocytes contain cytoplasmic granules and irregular nuclei. They are formed within the red bone marrow and include basophils, neutrophils, and eosinophils. The names of these cells correspond to the manner in which they respond to certain dyes for microscopic inspection. Nongranular leukocytes do not contain cytoplasmic granules, and they possess regular nuclei. They are mainly formed in the lymphatic tissue of the spleen and include lymphocytes and monocytes. Leukocytes play an important role in the body’s defense system. They are able to move out of capillaries into tissue to “attack” and phagocytose foreign substances. The life span of leukocytes varies, depending on the type of cell. Granular leukocytes live for only about 2 weeks, whereas lymphocytes may live for years. Normal blood contains between 5000 and 9000 leukocytes per milliliter. Changes in the number of leukocytes often indicate the presence of disease.

The third major type of blood cells is the thrombocytes, or platelets. These cells are necessary for blood to clot properly and respond within seconds to initiate the coagulation process. Thrombocytes are also formed in the myeloid tissue within the red bone marrow and have a life span of approximately 10 days. The normal platelet count is 140,000 to 340,000 per cubic millimeter (mm³). Platelet activation is increased by an inflammatory response. Vascular damage results in a physiologic response to maintain homeostasis, including vasoconstriction, the development of a platelet plug, the activation of blood coagulation (thrombin), and the formation of a blood clot. This process is critical in preventing hemorrhage and is dependent on the availability of the proper number of platelets (Fig. 9-1).

The lymphatic system is a subsystem of the circulatory system. Its major function is ensuring immunity through production of lymphocytes and antibodies, but it is also responsible for absorbing fat from the intestinal tract and for manufacturing blood under certain circumstances. The lymphatic system is composed of both lymphatic vessels and nodes. Lymphatic vessels contain a milky liquid substance termed lymph. Lymph nodes are small ovoid bodies attached in a chainlike pattern along the vessels. They filter out particles and foreign materials from blood. Major areas of lymph node chains include the neck, the mediastinum, and the axillary, retroperitoneal, pelvic, and inguinal regions. These lymph nodes often become enlarged when the body is invaded by an infectious agent and in cases of neoplastic disease.

Mature lymphocytes are the most important cells in the development of immunity. T lymphocytes are derived from lymphatic tissue of the thymus gland, and B lymphocytes are derived from bone marrow. These two types of lymphocytes work together with macrophages to ingest foreign substances and process specific foreign antigens. Through a complex process, an antibody that is capable of attacking the foreign antigen is formed. An antibody is an immunoglobulin produced by plasma cells and is categorized into one of five classifications: IgG, IgM, IgA, IgD, and IgE. The formation of antibodies is the systemic response that could have a negative effect on tissue grafts and organ transplants. The human body sees the transplant as foreign, so the lymphocytes and macrophages try to destroy the foreign antigens, which results in rejection of the graft or transplant.

Although the risk of whole-body radiation exposure is of little concern in diagnostic radiology, it is important for the radiographer to remember that exposure to x-rays or gamma-rays can have a harmful effect on blood marrow and lymphoid tissue. It takes a whole-body dose of approximately 0.5 to 0.75 Gray (Gy) (50 to 75 rad) to cause a detectable change in blood cells. The most radiosensitive blood cells are lymphocytes, followed by leukocytes and thrombocytes.

The spleen is the largest lymphoid organ and is located in the upper left quadrant of the abdomen. Its chief function is production of lymphocytes and plasma cells. It serves as a reservoir for blood and contains mononuclear phagocytes that cleanse the blood and lymphocytes to fight infectious bloodborne microorganisms. The splenic artery arises from the trifurcation of the celiac trunk off the abdominal aorta and supplies fresh blood to the spleen. This blood is cleansed by macrophages, which remove old or defective blood cells and microorganisms. The splenic vein empties into the portal vein. The spleen is occasionally ruptured in abdominal trauma and can be removed without detrimental effects.

Myeloid tissue,

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