Learning Objectives
1. Describe the principal functions of blood and its homeostatic mechanisms.
2. Identify the plasma proteins and their chief functions.
3. Describe the structure and function of red blood cells, white blood cells, and platelets.
4. Discuss the importance of chemotaxis and phagocytosis in fighting invading organisms and other foreign materials.
5. Briefly describe the mechanism of blood clotting.
6. Identify the four blood groups and define the term Rh factor. Explain the concept of the universal donor and the universal recipient.
7. Describe lymphatic circulation and the filtration role of lymph nodes.
8. Describe the circle of Willis and the blood-brain barrier, including the function of each.
9. Explain the process of hepatic-portal circulation.
10. Discuss at least three normal changes in the hematologic and lymphatic systems caused by aging.
|
IMPORTANT TERMINOLOGY |
||
|
agglutination |
hematopoiesis |
plasma |
|
albumin |
hemorrhage |
platelet |
|
anastomose |
hemostasis |
prothrombin |
|
coagulation |
leukocyte |
Rh factor |
|
embolus |
lymph |
spleen |
|
endocytosis |
lymph node |
thrombin |
|
erythrocyte |
lymphocyte |
thrombocyte |
|
fibrin |
monocyte |
thrombus |
|
fibrinogen |
phagocytosis |
tonsil |
|
globulin |
pinocytosis |
type and crossmatch |
|
Acronyms |
|
|
APC |
RBC |
|
BBB |
Rh + |
|
Hb |
Rh- |
|
Hgb |
WBC |
|
MABP |
|
The hematologic system consists of blood (i.e., plasma and formed elements) and bone marrow, the primary organ that manufactures blood cells. The lymphatic system consists of the lymphatic vessels and tissues. Other organs and structures, such as the spleen, liver, and kidneys, also perform specific related functions.
Structure and Function
The hematologic system has three general functions: transportation, regulation, and protection. These functions involve delivery of nutrients and oxygen to cells, removal of wastes, blood volume regulation, blood cell and antibody production, and blood coagulation. The lymphatic system transports dietary fats, drains interstitial fluid, and provides immunity to help protect the body from infection. It also recycles and returns excess proteins that may escape from blood vessels to the systemic circulation. Box 23-1 lists some of the functions of the hematologic and lymphatic systems.
Key Concept The hematologic and lymphatic systems have transportation and protective functions in the body Also, blood functions in regulatory processes, and lymph functions in the manufacture of formed elements, removal of foreign substances, and absorption and storage of substances in the body
BOX 23-1. Functions of the Hematologic and Lymphatic Systems
Blood
Transportation
♦ Transports oxygen to body cells and carbon dioxide away from body cells
♦ Exchanges oxygen for carbon dioxide at cellular level
♦ Transports water; nutrients, and other needed substances, such as salts (electrolytes) and vitamins, to body cells
♦ Aids in body heat transfer
♦ Transports waste products from cells to be removed from the circulation (e.g., kidney removes excess water; electrolytes, and urea; liver removes bile pigments and drugs; lungs remove carbon dioxide)
♦ Transports hormones from sites of origin to organs they affect
♦ Transports enzymes
Regulation
♦ Contributes to regulation of body temperature
♦ Assists in maintenance of acid-base balance
♦ Assists in maintenance of fluid-electrolyte balance
Protection
♦ Fights disease and infection (leukocytes)
♦ Promotes clotting of blood (platelets and specialized factors)
♦ Provides immunity due to antibodies and antitoxins (specialized cells)
Lymph
Transportation
♦ Carries fluid away from tissues
♦ Carries wastes away from tissues
Absorption
♦ Absorbs fats and transports fats to blood (lacteals)
♦ Stores blood (spleen)
♦ Destroys worn-out erythrocytes
Protection
♦ Filters waste products out of blood
♦ Filters foreign substances out of blood (including dead blood cells, bacteria, smoke by-products, cancer cells)
♦ Destroys bacteria
♦ Participates in antibody production to fight foreign invasion
Manufacture
♦ Manufactures lymphocytes and monocytes
♦ Manufactures erythrocytes (spleen in fetus)
BLOOD
Blood is a versatile vascular fluid that is heavier, thicker, and more viscous than water. Although it is a liquid, it has a unique quality that contributes to its ability to form solid clots. The primary objective of blood is to maintain a constant environment for all body tissues. It maintains this homeostasis via its viscosity (thickness), and by its ability to carry dissolved substances and to move to all body parts. Blood is responsible for transportation of oxygen, carbon dioxide, nutrients, heat, waste products, and hormones to and from the cells. It also helps regulate pH, body temperature, and cellular water content. It contributes to protection from blood loss and foreign body invasion.
Blood is considered a connective tissue because it is made primarily of cells that share many characteristics with other connective tissues in terms of origin and development (Cohen & Wood, 2005). It differs from other connective tissues, however, in that its cells are not fixed, but move freely to all body cells.
Hematopoiesis (hemopoiesis) refers to the production and maturation of blood cells. The red bone marrow manufactures blood cells, or “formed elements,” of blood. (In the embryo, RBCs are produced in the liver and spleen as well.) Other tissues, such as lymph nodes, spleen, and thymus, contribute to additional production and maturation of agranular white blood cells. Erythropoiesis refers to the formation of red blood cells (erythrocytes). A glycoprotein-type hormone, erythropoietin is secreted by the kidneys in the adult. This hormone stimulates the stem cells in bone marrow to produce red blood cells. Dietary elements such as iron, cobalt, copper, amino acids, and certain vitamins are also required for erythropoiesis.
Key Concept A form of erythropoietin,derived in DNA technology, may be used to treat the type of anemia caused by insufficient or ineffective RBCs. This is called recombinant human erythropoietin (RHE) or epoetin alfa.
Blood is composed of both plasma and formed elements. It is carried through a closed system of vessels pumped by the heart.The volume of circulating blood differs with individual body size; however, the average adult body contains approximately 4-6 L of blood. Table 23-1 lists some normal laboratory values for blood components.
Plasma
Blood plasma is the fluid portion of circulating blood. It constitutes 55% of blood volume. Plasma is 90% water. Its remaining 10% consists primarily of plasma proteins, but it also includes salts (electrolytes), nutrients, nitrogenous waste products, gases, hormones, and enzymes.
Salts contained in plasma are sodium (Na+), calcium (Ca+), potassium (K+), and magnesium (Mg++). Plasma also contains ions of other elements such as bicarbonates, sulfates, chlorides, and phosphates.Plasma absorbs these salts from food for use by body cells. The maintenance of these salts within plasma controls the chemical and acid-base balance of the blood and contributes to the entire body’s homeostasis.
Plasma Proteins
Four groups of plasma proteins are manufactured in the liver. Albumin, the largest group, accounts for 60%-80% of plasma proteins. Its important function is to provide thickness to the circulating blood volume, thus maintaining osmotic pressure. (Osmotic pressure draws water from surrounding tissue fluid into capillaries and, thus, maintains fluid volume and blood pressure.) Loss of albumin can result in dramatic fluid shifts, edema, hypotension, and even death.Fibrinogen and prothrombin are two other plasma proteins essential for blood clotting.
TABLE 23-1. Selected Approximate Normal Laboratory Values*
|
MALE |
FEMALE |
NEWBORN |
|
|
Hemoglobin (Hgb) |
14-18 g/dL |
12-16 g/dL |
16.5-19.5 g/dL Children vary by age. |
|
Cell Counts Erythrocytes (red blood cells [RBCs]) |
4.6-6.2 million/mm3 |
4.2-5.4 million/mm3 |
Children vary by age. |
|
Leukocytes (total) (white blood cells [WBCs]) Differential (Diff) in percentages (all adults) |
(All adults) 5,000-9,000 million/mm3 |
||
|
Band neutrophils |
3-5 |
||
|
Segmented neutrophils |
54-62 |
||
|
Lymphocytes |
25-33 |
||
|
Monocytes Eosinophils |
3-7 1-3 |
||
|
Basophils |
0-1 |
||
|
Platelets |
150,000-400,000/mm3 |
*Values vary slightly by laboratory
Globulin is the fourth type of plasma protein. Two types of globulin (alpha and beta) are formulated in the liver and act as carriers for molecules, such as fats. Gamma globulins (immunoglobulins [Ig]) are antibodies. (Antibodies are materials synthesized by the body in response to antigens [foreign invaders], thus providing immunity against infection and disease.
Key Concept Albumin, the largest group of plasma proteins, helps maintain blood pressure and circulating fluid volume. The three other circulatory plasma proteins are fibrinogen, prothrombin, and globulin.
Formed Elements
The remaining blood volume consists of formed elements. These elements are red blood cells (RBCs), white blood cells (WBCs), and platelets. Figure 23-1 illustrates the types of WBCs and RBCs.
Red Blood Cells
Red blood cells, also called erythrocytes (erythro = red; cyte = cell) or red corpuscles, are flattened, biconcave disks. (Biconcave means that both faces of the cell are thinner in the center than at the edges. In cross section, the RBC is shaped like a dumbbell.) When RBCs mature, they have no nucleus (and therefore, no DNA; so RNA cannot be synthesized). Thus, they cannot reproduce. They are also unable to synthesize protein. RBCs consist mainly of hemoglobin, in a surrounding medium, the stroma. Each cell is enclosed in a cell wall.
Erythrocytes are the most numerous of the blood cells. About 25 trillion RBCs are found in the body. Approximately 3,000 RBCs could be placed side by side within a 1-inch space. They are made from stem cells in red bone marrow, mostly in the large bones. The RBCs are fragile and wear out quickly.
Macrophages in the liver and spleen ingest old, used RBCs and salvage iron, which is transported to bone marrow to make new RBCs. The life of an individual RBC is about 120 days. (The body manufactures about 180 million RBCs every minute!)
Each RBC contains more than 250 million molecules of the compound hemoglobin (Hgb or Hb). Hemoglobin is composed of the iron-containing pigment heme and a protein, globin. (Iron is the pigment that makes RBCs appear red.) As blood passes through the lungs, the iron in hemoglobin attracts and binds to oxygen in a loose combination. (Hemoglobin allows blood to carry more than 60 times the amount of oxygen as would plasma alone.) When hemoglobin is saturated with oxygen (oxyhemoglobin), the blood is bright red. As blood circulates through the capillaries, the hemoglobin gives its oxygen to various cells of the body. Although RBCs are smaller than most other human cells, they are larger in diameter than the smallest capillaries. This is believed to force the transfer of the oxygen from RBCs into individual body tissues. Most of the carbon dioxide waste is carried away from cells as bicarbonate, which is dissolved in the plasma. Deoxygenated blood is much darker (almost maroon) in color. (Some RBCs may be stored in the spleen and dumped into the blood during exertion or stress. This would increase the oxygen-carrying capacity of the blood.) In addition to the transportation of oxygen, RBCs have several other functions, which are presented in Box 23-2.
FIGURE 23-1 · Normal types of blood cells. Erythrocytes are the red blood cells. Also shown are platelets (thrombocytes). All the other cells shown are types of white blood cells (leukocytes). Granulocytes (granular leukocytes) are basophils, neutrophils, and eosinophils. Agranulocytes (agranular leukocytes) are monocytes and lymphocytes.
BOX 23-2.
Functions of Red Blood Cells
In addition to the transportation of oxygen, red blood cells (RBCs) have the following functions:
♦ If RBCs experience stress or shear because blood vessels are constricted, they release adenosine triphosphate (ATP), which causes vessel walls to relax and dilate.
♦ RBCs also produce ATP (the energy-carrier) by fermentation.
♦ When hemoglobin is deoxygenated, RBCs release substances which assist in dilation of blood vessels and facilitate blood flow to oxygen-poor areas.
♦ RBCs store iron in the body; RBCs themselves are stored in the spleen.
♦ RBCs are involved in the immune response. If RBCs are lysed by pathogens, the hemoglobin releases free radicals that break down the cell walls of the pathogens.
♦ Myoglobin, a compound related to hemoglobin, stores oxygen in muscle cells.
♦ RBCs are important in acid-base balance and have an influence on specific gravity of blood because they contribute viscosity (thickness) to the blood.
Nursing Alert Hyperbaric oxygenation (high pressure oxygenation) involves administration of oxygen as the total body is subjected to greater than normal atmospheric pressure. This forces oxygen into the body and allows it to be carried, not only by hemoglobin, but also by other portions of the blood. It is used in situations such as carbon monoxide poisoning and gas gangrene (caused by the anaerobic organism Clostridium).
Key Concept
♦ Iron in the hemoglobin picks up oxygen in the lungs.This oxygen is exchanged for carbon dioxide at the cellular level, which is returned to the lungs in plasma, to complete the cycle. The RBCs do not use any of the oxygen they transport.
♦ The average female has fewer RBCs than males.
♦ A deficiency of RBCs is one form of anemia.
♦ During inflammation, RBCs may "stack up” due to the elevation of specific serum proteins. This is called a "rouleaux formation.”
♦ People living in high altitudes have more RBCs because each RBC carries less oxygen.
Nursing Alert Pulse oximetry directly measures arterial oxygen content. This is based on the color variations between oxygenated and deoxygenated blood (colorimic technique).
Specific blood tests include the RBC count (number of RBCs per volume of blood); hematocrit (percentage of blood volume occupied by RBCs), and the Hgb (hemoglobin) level. Glycosylated (glycated) Hgb is a gauge of blood sugar control in diabetes.
RBCs must be in an isotonic solution to survive (see Fig. 17-4).
