Blood Components, Formation, and Diagnostic Tests

Learning Objectives

• Describe the composition and functions of plasma.
• Understand the function and life span of blood cells.
• Trace the process of hematopoiesis from stem cell to mature blood cell.
• Describe the components of a complete blood count with differential.
• Describe the various diagnostic tests used in determining laboratory values for RBCs, WBCs, and thrombocytes.

Nursing Concepts

• Clotting
• Perfusion

Composition of Blood and Formation of Blood Cells

• Blood is a specialized connective tissue. It consists of:
  • Blood cells (erythrocytes, leukocytes, and thrombocytes)
  • Plasma: Extracellular fluid
• Blood volume:
  • Adult assigned male at birth: 5 to 6 L
  • Adult assigned female at birth: 4 to 5 L
• Blood circulates throughout the body within the circulatory system, acting as a transport vehicle.
• Blood components can be identified in the lab using a centrifuge, separating into distinct layers.
  • Bottom Layer: Erythrocytes (RBCs), 42-47% of blood volume.
  • Intermediate Layer: Leukocytes (WBCs), 1% of blood volume, buffy coat.
  • Above WBCs: Thrombocytes (not discernible to the naked eye).
  • Top Layer: Plasma, translucent yellowish fluid, 55% of blood volume.

Key Points on Blood Composition

• RBCs: Oxygen and carbon dioxide transport.
• WBCs: Immunity and inflammation.
• Thrombocytes: Blood clotting.
• Plasma Composition: By weight
  • 90-91% water
  • 6.5-8% proteins
  • 1-2% other small molecular substances
• Plasma Functions:
  • Transport vehicle for nutrients, chemical messengers, metabolites, and other materials.
  • Carries hormones to target organs
  • Transports blood urea nitrogen to kidneys
  • Transports RBCs for oxygen distribution
  • Participates in electrolyte and acid-base balance
  • Contains plasma proteins for osmotic regulation
  • Plasma absorbs and distributes heat.

Plasma Components (Table 21.1)

• Water: 90-91% of plasma volume.
• Proteins: 6.5-8% of plasma volume.
  • Albumin: 54% of plasma proteins.
  • Globulins: 38% of plasma proteins.
  • Fibrinogen: 7% of plasma proteins.
• Other Substances: 1-2% of plasma volume, including hormones, enzymes, carbohydrates, fats, amino acids, gases, electrolytes, and excretory products.

Plasma Proteins

• Most abundant solutes in plasma.
• Major types: Albumin, globulins, and fibrinogen.
• Produced by the liver (except bloodborne hormones and gamma globulins).
  • Albumin: Most abundant plasma protein (54%).
    • Maintains plasma osmotic pressure and blood volume.
    • Carrier for certain substances and acts as a blood buffer.
  • Globulins: 38% of plasma proteins.
    • Alpha globulins: Transport bilirubin and steroids
    • Beta globulins: Transport iron and copper.
    • Gamma globulins: Antibodies of the immune system.
  • Fibrinogen: 7% of plasma proteins.
    • Soluble protein that polymerizes to form insoluble fibrin during blood clotting.
  • Serum: Liquid remaining after fibrinogen and clotting factors are removed from plasma.
  • Remaining 1% of plasma proteins: Hormones, enzymes, complement, and carriers for lipids.

Blood Cells

• Formed elements: RBCs, WBCs, and thrombocytes.
• Originate in the bone marrow.
• Only WBCs are true cells; RBCs lack nuclei and organelles, and thrombocytes are cell fragments.
• Most blood cells do not divide, so division of bone marrow cells is needed to renew them.

Blood Cell Count (Table 21.2)

• Red blood cell count
  • Male: 4.2–5.4 × 10^6/μL (4.2–5.4 × 10^{12}/L)
  • Female: 3.6–5.0 × 10^6/μL (3.6–5.0 × 10^{12}/L)
• White blood cell count: 4.8–10.8 × 10^3/μL (4.8–10.8 × 10^9/L)
• Differential count
  • Granulocytes
    • Neutrophils
      • Segs: 50–73%
      • Bands: 0–4%
    • Eosinophils: 0–4%
    • Basophils: 0–2%
  • Lymphocytes: 20–44%
  • Monocytes: 3–8%
• Platelet count: 150–400 × 10^3
• RBC indices
  • MCV (mean corpuscular volume): 80–100 fL
  • MCH (mean cell hemoglobin): 27–34 pg/cell
  • MCHC (mean corpuscular hemoglobin concentration): 31–35 g/dL
• Values for infants and children vary significantly according to age.

Erythrocytes (RBCs)

• Most numerous of the formed elements.
• Small, biconcave disks with an average diameter of 6 to 8 μm.
  • Thickness at the edge: approximately 2.6 μm.
  • Thickness at the center: 0.8 μm.
• Large surface area and high elasticity allow them to deform to move through small capillaries.
• Primary function: Carry oxygen to the body’s tissues via hemoglobin.
• Contribute to carbon dioxide transport and regulate acid–base balance.
• Life span: approximately 120 days.
• 90% are phagocytosed in the bone marrow, spleen, and liver.
• 10% break down in blood vessels, releasing insignificant amounts of hemoglobin.
• Bone marrow continually releases new RBCs.

Leukocytes (WBCs)

• 10 to 12 μm in diameter, larger than RBCs.
• Constitute only 1% of the total blood volume.
• Originate in the bone marrow and circulate throughout the lymphoid tissues.
• Crucial to defense against disease:
  • Responsible for the immune response
  • Identify and destroy cancer cells
  • Participate in the inflammatory response and wound healing
• Classified into two groups based on the presence or absence of cytoplasmic granules:
  • Granulocytes: Contain specific granules
  • Agranulocytes: Lack specific granules

Granulocytes

• Spherical with distinctive multilobar nuclei.
• All are phagocytes, engulfing microbes and other substances.
• Granules contain chemicals and enzymes that break down engulfed microbes.
• Three types: Neutrophils, eosinophils, and basophils, based on staining properties of granules.
  • Eosinophils: Stain with eosin (acid dye).
  • Basophils: Stain with a basic dye.
  • Neutrophils: Mechanism of staining is unknown.

Neutrophils

• Constitute 50% to 73% of the total WBCs.
• Also called polymorphonuclear leukocytes (PMNs).
• Stained neutrophil is pale pink or tan.
• Maintain normal host defenses against bacteria, fungi, cell debris, and foreign substances.
• Increase in number during bacterial and systemic fungal infections.
• Life span of 3 to 5 days.
• Circulating neutrophils are in equilibrium with marginating cells along blood vessel walls.
• In response to chemotactic factors, marginating neutrophils exit the circulation at sites of inflammation.
• Bone marrow releases large numbers of neutrophils during acute systemic bacterial infections.
• Immature neutrophils, called band cells, are released as part of this response.

Eosinophils

• Similar in size to neutrophils, with bilobed nuclei.
• Cytoplasmic granules stain a red color.
• Reside primarily in the connective tissues.
• Constitute 0% to 4% of the total WBCs.
• Important host defense roles in allergic reactions, parasitic infections, and chronic immune responses (e.g., asthma).
• Increase in number in response to allergic responses, worm infestations, and asthma.
• Release enzymes like histaminase to inactivate histamine and other inflammatory substances.
• Attach to parasites and kill them through the release of hydrolytic enzymes and toxic proteins.

Basophils

• Similar in size to neutrophils, least numerous of WBCs (0% to 2% of total).
• Dark-blue granules are large and abundant, sometimes concealing the nucleus.
• Granules contain heparin (anticoagulant), histamine (vasodilator), and inflammatory mediators (leukotrienes).
• Similar to mast cells in function.
• Involved in allergic and hypersensitivity reactions.

Agranulocytes

• Also known as mononuclear leukocytes.
• Distinguished from granulocytes by finer granules and single-lobed nucleus.
• Two types: Lymphocytes and monocytes.

Lymphocytes

• Account for 20% to 44% of total blood WBCs.
• Main functional cells of the immune system.
• Move between blood and lymph tissue.
• Defend against microorganisms through the immune response.
• Three types: B lymphocytes, T lymphocytes, and natural killer (NK) cells.
  • B lymphocytes (B cells): Differentiate to form antibody-producing plasma cells; involved in humoral-mediated immunity.
  • T lymphocytes (T cells): Differentiate in the thymus; activate other immune cells (helper T cells) and are involved in cell-mediated immunity (cytotoxic T cells).
  • NK cells: Participate in innate or natural immunity and destroy foreign cells.
• Lymphocytes have unique surface markers that help define their function and diagnose disease.
• Breakdown of lymphocytes: 60% to 80% T cells, 10% to 20% B cells, and less than 20% NK cells.
• Major histocompatibility antigens (human leukocyte antigens, HLAs) are expressed on lymphocytes and are responsible for multiple aspects of the human immunologic response.

Monocytes and Macrophages

• Monocytes are the largest WBCs, constituting approximately 3% to 8% of the total leukocyte count.
• Distinguished by a large amount of cytoplasm and a dark-stained, kidney-shaped nucleus.
• Survive for months to years in the tissues; circulating monocyte lifespan is approximately 3 days.
• Produced in the bone marrow and transform into macrophages after leaving the vascular system.
• Specific activity of macrophages depends on their location:
  • Histiocytes in loose connective tissue
  • Microglial cells in the brain
  • Kupffer cells in the liver
• Monocytes and macrophages comprise the mononuclear phagocyte system (reticuloendothelial system).
• Primary role is host defense, engulfing larger quantities of foreign material than neutrophils.
• Convert into antigen-presenting cells (APCs), activating lymphocytes and presenting antigen to T cells.
• Play an important role in chronic inflammation.
• Granulomatous inflammation: Macrophages form a capsule around insoluble materials that cannot be digested:
  • Foreign body granulomas: Incited by inert foreign bodies (e.g., talc, surgical sutures).
  • Immune granulomas: Caused by insoluble particles inciting a cell-mediated immune response (e.g., tubercle in primary tuberculosis infections).

Thrombocytes

• Circulating cell fragments of megakaryocytes derived from the myeloid stem cell.
• Contribute to the formation of the platelet plug to help control bleeding.
• Cytoplasmic granules release mediators for the blood coagulation process.
• No nucleus, cannot replicate, and last approximately 7 to 10 days in circulation.
• Removed by the spleen and liver.

Formation of Blood Cells (Hematopoiesis)

• Production of blood cells.
• Begins in the yolk sac during the second week of embryonic development.
• Transitions to the liver and spleen around the second month of gestation.
• Taken over by the bone marrow at approximately 7 months of gestation.
• Bone marrow continues to serve as the primary site of blood cell production throughout life.
• In children, this happens primarily in the distal long bones.
• In adults, hematopoiesis is largely restricted to the bone marrow and lymphatic tissue.

Medullary and Extramedullary Hematopoiesis

• Medullary hematopoiesis: Blood cell production within the bone marrow.
  • Blood-forming population: Self-renewing stem cells, differentiated progenitor cells, and functional mature blood cells.
  • Red bone marrow: Hematopoietically active, produces RBCs.
  • Yellow bone marrow: Predominantly fat cells, inactive in terms of blood cell generation.
• During active skeletal growth, red marrow in long bones is gradually replaced by yellow marrow.
• In adults, red marrow is largely restricted to the flat bones of the pelvis, ribs, and sternum.
• During increased demand for red cell replacement, yellow bone marrow can convert to red marrow.
• Extramedullary hematopoiesis: Blood cell production outside the bone marrow.
  • Liver and spleen retain hematopoietic ability.
  • If the bone marrow cannot produce sufficient blood cells, the liver and spleen will resume hematopoiesis.

Blood Cell Precursors

• All blood cells are derived from pluripotent stem cells.
• Pluripotent cells can produce several types of cells.
• Blood cell precursors of erythrocyte, myelocyte, lymphocyte, and megakaryocyte are derived from these cells.
• Pluripotent stem cells have a lifelong potential for proliferation and self-renewal.
• Pluripotent stem cells develop into functionally mature cells through differentiation.
• Differentiation leads to committed, unipotential cells (colony-forming units, CFUs).
• Progenitor cells have limited capacity for self-renewal but can differentiate into lineage-specific precursor cells.
• Precursor cells have morphologic characteristics, undergo cell division and differentiation, and give rise to mature lymphocytes, myelocytes, megakaryocytes, or erythrocytes.

Hematopoiesis Key Points

• White blood cells are formed from hematopoietic stem cells that differentiate into committed progenitor cells, which in turn develop into the myelogenous and lymphocytic lineages needed for the formation of the different blood cell types.
• The growth and reproduction of the different stem cells are controlled by colony-stimulating factors (CSFs) and other cytokines and chemical mediators.

Regulation of Hematopoiesis

• Numbers and total mass of circulating blood cells remain relatively constant.
• Blood cells are produced in different numbers according to needs and regulatory factors.
• Regulation is partially controlled by hormone-like growth factors called cytokines.
• Cytokines: Short-lived mediators that stimulate proliferation, differentiation, and functional activation of blood cells.
• Many cytokines derived from lymphocytes or bone marrow stromal cells stimulate the growth and production of new blood cells.
• Colony-stimulating factors (CSFs): Promote the growth of hematopoietic cell colonies in the laboratory.
  • Erythropoietin (EPO): Stimulates RBC production.
  • Granulocyte-monocyte colony-stimulating factor (GM-CSF): Stimulates progenitors for granulocytes, monocytes, erythrocytes, and megakaryocytes.
  • Granulocyte colony–stimulating factor (G-CSF): Promotes the proliferation of neutrophils.
  • Macrophage colony–stimulating factor: Induces macrophage colonies.
  • Thrombopoietin (TPO): Stimulates the differentiation of thrombocytes.
• Other cytokines: Interleukins, interferons, and tumor necrosis factor support stem cell proliferation, lymphocyte development, and aid the multiple functions of the CSFs.
• Genes for most hematopoietic growth factors have been cloned, and their recombinant proteins have been generated for management of a wide range of clinical conditions (e.g., EPO, TPO, G-CSF, and GM-CSF).
• Used to treat bone marrow failure caused by chemotherapy or aplastic anemia, the anemia of kidney failure and cancer, hematopoietic neoplasms, infectious diseases, and congenital and myeloproliferative disorders.
• Growth factors increase peripheral stem cells for transplantation and accelerate cell proliferation after bone marrow engraftment.

Disorders of Hematopoietic Stem Cells

• Underproliferation: Failure to produce one or several cell types.
  • Aplastic anemia: Multipotent stem cells fail to grow and provide cells for differentiation.
  • Pancytopenia: Reduction in the number of all blood cell types.
    • Anemia: Low RBC levels
    • Thrombocytopenia: Low thrombocyte levels.
    • Granulocytopenia: Low granulocyte levels.
• Myeloproliferative diseases: Hypercellular bone marrow and elevated blood cell counts.
  • Polycythemia: Unregulated overproduction of the red cell mass.
  • Thrombocytosis: Bone marrow produces too many thrombocytes.
  • Leukemias: Abnormal proliferation of WBCs.
• Management of these and many other disorders uses hematopoietic stem cells as part of the treatment.
• Stem cell transplants correct bone marrow failure, immunodeficiencies, hematologic defects and malignancies, and inherited errors of metabolism.
• Sources of stem cells: Bone marrow and umbilical cord blood.
• Stem cell transplants may be derived from the recipient (autologous) or from a histocompatible donor (allogeneic).
• Autologous transplants are often used to replenish stem cells after high-dose chemotherapy or irradiation.
• Umbilical cord blood from HLA-matched donors is a transplant option for children and carries less risk of graft-versus-host disease.

In Summary

• Blood is composed of:
  • Plasma
  • Plasma proteins
  • Fixed elements or blood cells
  • Substances (hormones, enzymes, electrolytes, and byproducts of cellular waste)
• Blood cells consist of erythrocytes or RBCs, leukocytes or WBCs, and thrombocytes or platelets.
• Mature blood cells have a relatively short life span and must be continuously replaced.
• All blood cells arise from pluripotent stem cells located in the bone marrow.
• Pluripotent stem cells differentiate into unipotential CFUs, which are the progenitors for each of the blood cell types.
• Blood cell production is largely regulated by chemical messengers called cytokines and growth factors (CSFs).

Diagnostic Tests

• Provide information about:
  • Oxygen-carrying capacity of the blood (RBCs)
  • Presence of infection or tissue injury (WBCs)
  • Ability of blood to clot (thrombocytes)
• Blood specimens can be obtained through skin puncture (capillary blood), venipuncture, arterial puncture, or bone marrow aspiration.
• Each specimen has a specific procedure that correlates to what collection tube to use; whether the specimen should be gently agitated after collection; whether the specimen needs to be refrigerated, iced, or not; and other specific details that must be followed in order to generate accurate findings.

Blood Count

• Provides information regarding the number of blood cells and their structural and functional characteristics.
• Complete blood count (CBC): Determines the number of RBCs, WBCs, and thrombocytes per unit of blood.
• Measurements of hemoglobin, hematocrit, mean corpuscular volume (MCV), mean corpuscular hemoglobin concentration (MCHC), and mean cell hemoglobin (MCH) are usually included in the CBC.
• Abnormally low hematocrit and hemoglobin levels indicate the presence of anemia.
• Abnormally high hematocrit and hemoglobin levels indicate the presence of plasma volume depletion or polycythemia.
• The MCV, MCHC, and MCH (RBC indices) reflect the size, shape, or color of cells; used to determine the type of anemia.
• Low MCV indicates a small cell (microcytic).
• Low MCHC and MCH indicate a pale cell (hypochromic).
• These findings are commonly seen in iron deficiency anemia, thalassemia, and lead poisoning.
• White cell differential count: Determination of the relative proportions (percentages) of individual white cell types.
• Knowing which type of WBC is increased or decreased can help identify the underlying cause of a condition.
• Granulocytes are often increased during an acute bacterial infection.
• An elevation in band neutrophil cells indicates a large infection.

Erythrocyte Sedimentation Rate

• Laboratory test used to detect inflammation.
• RBCs aggregate and fall to the bottom of a tube, forming sediment.
• The ESR is the distance in millimeters that a red cell column travels in 1 hour.
• Normal values are 0 to 10 mm/hour for people assigned male at birth and 1 to 20 mm/hour for people assigned female at birth.
• The rate of fall of the aggregates is accelerated in the presence of fibrinogen and other plasma proteins that are often increased in inflammatory diseases.
• Increased ESR indicates inflammation is present.
• ESR is used for monitoring the fluctuations in the clinical course of a disease such as chronic fatigue syndrome, lupus erythematosus, or polymyalgia rheumatica (PMR).

Bone Marrow Aspiration and Biopsy

• Tests of bone marrow function are done on samples obtained using bone marrow aspiration or bone marrow biopsy.
• Bone marrow aspiration: Performed with a special needle inserted into the bone marrow cavity, through which a sample of marrow is withdrawn.
  • Sites used: Posterior iliac crest, anterior iliac crest, sternum, and spinous processes T10 through L4.
  • Sternum is not commonly used in children
• Technique is used primarily to determine the type of cells present and their relative numbers for diagnostic purposes.
• Bone marrow biopsy: Done with a special biopsy needle inserted into the posterior iliac crest.
  • Removes a sample of bone marrow tissue and allows study of the architecture of the tissue.
• Used to determine the marrow-to-fat ratio and the presence of fibrosis, plasma cells, granulomas, and cancer cells.
• The major hazard of these procedures is the slight risk of hemorrhage, which is increased in people with a reduced platelet count or any type of bleeding tendency.

In Summary Diagnostic Tests

• Diagnostic tests of the blood include the CBC, the ESR, and bone marrow aspiration and biopsy.
• The CBC is used to describe the number and characteristics of the RBCs, WBCs, and thrombocytes.
• The ESR is used to detect inflammation.
• Bone marrow aspiration is removal of the fluid portion of marrow from within the bone marrow cavity. Bone marrow aspirate analysis focuses on cellular morphology and determination of a differential cell count.
• A bone marrow biopsy removes a sample of solid bone marrow, which permits study of the marrow’s overall cellularity and detection of focal lesions and the extent of the marrow by pathologic processes.

Geriatric Considerations

• Active bone marrow is replaced with fat as people age, limiting the body’s response to needs for blood cells.
• In older patients, hematopoietic function is hindered due to alterations in stem cell production resulting in mutations that have been replicated over time.
• Older patients have higher levels of erythropoietin; however, aging is accompanied by a decreased responsiveness to erythropoietin stimulation.
• Changes in hematopoiesis that occur with aging decrease the adequacy of immune responses in older adults.

Pediatric Considerations

• Hemoglobin levels are relatively high at birth, typically reaching at least 14 g/dL or higher.
• Hemoglobin levels drop precipitously after birth due to increased oxygenation that results from breathing combined with decreased production of erythropoietin.
• At age 2 to 3 months, levels are commonly as low as 11 g/dL.
• Premature infants have lower hemoglobin levels than full-term infants, and the drop in hemoglobin after birth may be as low as 7 to 8 g/dL resulting in significant anemia.
• For children age 6 months to 6 years, typical hemoglobin levels are 10.5 to 11.5 for girls and 12.0 to 13.5 for boys.