Hematology: Blood Composition and Function

• Hematology is the branch of medicine focused on the study, cause, prognosis, treatment, and prevention of blood-related diseases.

Blood Composition and Function

• Blood consists of cells suspended in a fluid medium called plasma.
• Blood functions as a vehicle for transporting gases, nutrients, metabolic waste products, cells, and hormones.

Blood Cells

• There are three major functional classes/lineages of blood cells:
  • Red Blood Cells (Erythrocytes)
  • White Blood Cells (Leukocytes)
  • Platelets (Thrombocytes)
• All blood cells are formed in the bone marrow through a process called hematopoiesis.

Hematopoiesis

• Hematopoiesis is the process by which the formed elements of blood are produced, occurring in the red bone marrow.
• Hematopoiesis: Multipotential hematopoietic stem cells (Hemocytoblasts) gives rise to common myeloid progenitor cells and common lymphoid progenitor cells.
  • Common Myeloid Progenitor Cells give rise to Megakaryocytes (platelets), Erythrocytes, Mast cells, Myeloblasts
  • Common Lymphoid Progenitor Cells give rise to Natural killer cells, Small lymphocytes that become either B Lymphocytes or T Lymphocytes

Erythropoietin

• Erythropoietin regulates red cell production.
• It is produced and released by peritubular capillary lining cells within the kidney.
• Stimuli for erythropoietin production include the availability of O₂ for tissue metabolic needs and a drop in hemoglobin below 100-120 g/L.

Erythrocytes (RBCs)

• The principal function of erythrocytes is oxygen and CO2 transport.
• Hemoglobin is synthesized in the bone marrow.
• Before its release, the erythrocyte nucleus is extruded, resulting in a biconcave disc shape.
• A fully differentiated RBC consists of an outer plasma membrane enclosing hemoglobin and a limited number of enzymes for cell maintenance.
• Due to their high content of hemoglobin, cells are stained pink (basic protein).
• Pale-staining center is due to its biconcave disc shape.
• The average lifespan of an erythrocyte is 120 days.
• Old/senescent or damaged RBCs are removed from circulation by the spleen and liver.

Anemia

• Anemia is defined as having a hemoglobin level below 130 g/L in men and below 120 g/L in women.
• Anemia is often recognized by abnormal screening laboratory tests rather than its symptoms.
• Anemia corresponds to a decrease in red blood cells.

Anemia Symptoms: Gradual Onset

• Common in young persons
• May not have symptoms until the anemia is severe (Hgb < 70-80 g/L)

Anemia Symptoms: Moderate Anemia

• Fatigue
• Dizziness
• Loss of stamina
• Breathlessness
• Increased heart rate
• Insomnia
• Pallor

Anemia Symptoms: Severe Anemia

• Severe pallor
• Shortness of breath, especially on exertion
• Cold hands & feet
• Chest pains

Causes of Anemia

• Blood loss
• Increased destruction of blood cells (hemolysis & autoimmune diseases, severe infections)
• Decreased production (hypoproliferation) (deficiency in nutrients, inflammatory disorders, bone marrow disorders/failure, leukemia & other marrow infiltrating diseases)
• Red cell maturation disorders (deficiency in nutrients , thalassemia, myelodysplasia)

Acute Anemia

• Sudden anemia is usually due to: Acute Blood loss, Acute Hemolysis
• Can cause an increase in erythropoiesis.

Acute Blood Loss

• Sudden loss @ 10-15% of total blood volume
  • Decrease in blood pressure & blood flow to organs
• Sudden blood loss @ > 30% of total blood vol.
  • Patient prefers to remain supine, as BP drops on sitting up
• Sudden blood loss @ > 40%
  • Hypovolemic shock (confusion, delirium, sweating)
  • Decreased organ blood flow → emergency

Hemolysis

• Hemo = blood, lysis = breakdown
• Breakdown of blood cells due to various causes, usually autoimmune or genetic
• One of the most common forms of anemia.
• Examples/causes: Autoimmune disorders, Paroxysmal nocturnal hemoglobinuria, Hemoglobinopathies

Infectious Causes of Anemia

• Severe infections or Sepsis
  • Cytokine storm causes great consumption of blood cells including red blood cells
  • Overwhelming infection can cause bleeding problems and cause bleeding elsewhere.
• Tuberculosis
  • Consumption of blood cells due to cytokines released during inflammation.
  • Blood loss from bleeding inside lungs causing blood-streaked phlegm (hemoptysis).

Hypoproliferative Anemias

• Comprise at least 75% of all cases
• Majority are due to Mild to moderate Iron Deficiency, Long-term (chronic) Inflammation
• Usually does not affect RBC size (normocytic) & hemoglobin content (normochromic)

Iron Deficiency Anemia

• One of the most common types of anemia
• Due to insufficient iron inside the body
• Causes: Blood loss (e.g. menses), Lack of iron in diet, Poor absorption of iron (e.g. intestinal disorders), Pregnancy

Inflammatory Causes of Anemia

• Rheumatic Arthritis
  • Inflammatory effect on bone marrow
  • Anti-inflammatory drugs used to treat RA can cause gastric erosions and small bleeds in stomach
• Autoimmune Disorders (e.g. SLE)
  • Hemolysis or breakdown of normal blood cells by self-targeting antibodies or killer cells

Aplastic Anemia

• Loss of hematopoiesis in the bone marrow

Maturation Disorders

• Presence of anemia with poor bone marrow response (low reticulocyte count)
• Causes: Nuclear maturation defects (e.g. Vitamin B12 or folic acid deficiency), Severe Iron Deficiency, Cytoplasmic maturation defects (e.g. Thalassemia, sideroblastic anemia), Myelodysplasia

Polycythemia

• Defined as the increase in hemoglobin above normal.
• May be real, or only due to decrease in plasma volume (making the blood more concentrated)
• Erythrocytosis is an increase in red cell mass.
• Polycythemia is any increase in red cells.

Leukocytes

• Five types of leucocytes are normally present
• Divided into 2 main groups based on nuclear shape & granules: Granulocytes, Mononuclear Leukocytes
• Principal function: components of the body defense systems in the different tissues
• Leukocytes carry surface proteins for binding to receptors in blood vessels before migrating into the tissues

Naming Conventions:

Neutrophils

• Most common type of leukocyte (40-75%).
• Only leave the circulation in large numbers in response to disease.
• Most prominent feature: highly lobulated nucleus.
• In females, condensed X chromosome = Barr body.
• Elevated in infection especially in bacterial infections or blood disorders such as leukemia.

Neutrophilia

• Excess in neutrophils/segmenters
• Causes: Increased neutrophil production from bacterial infections or inflammation, increased marrow release., stress, excitement and vigorous exercise.
• Another cause is ingestion of Certain drugs (steroids, NSAIDs, epinephrine, Li).

Neutropenia

• Low or absent neutrophils
• Very dramatic consequences – increase in susceptibility to infections
• Causes: Decreased production because of Chemotherapy drugs or Certain infections (especially viral), Overwhelming infections and Autoimmune disorders

Lymphocytes

• Smallest of the white cells, but second most common of leukocytes (20-50%)
• Distinguishable feature: round dense nucleus & small/scanty basophilic cytoplasm
• Functions: Immunological defense (signalling, direct killing, etc)

Lymphocytosis

• Increase in number or proportion of lymphocytes
• Causes: Acute viral infections (dengue, hepatitis, etc), Lymphocytic leukemias, Lymphomas, Tuberculosis

Lymphocytopenia

• Lower than normal number of lymphocytes
• Causes: Autoimmune disorders, Certain cancers, Cancer treatment, Bone marrow disorders, Viral infections, Malnutrition

Monocytes

• Largest of the white cells, constituting 2-10%
• Distinguishable feature: indented nucleus → horseshoe/kidney shape
• Functions: Migrate into tissues & become macrophages

Monocytosis

• Increase in monocytes
• Causes: Tuberculosis, Subacute bacterial endocarditis, Some viral infections (influenza), Certain cancers & leukemias, Certain immune deficiency disorders

Monocytopenia

• Low monocyte count
• Causes: Acute infections, stress, Treatment with steroids, Aplastic anemia, Some forms of leukemia

Eosinophils

• Account for 1-6% of leukocytes in blood
• Diurnal variation: Increases in AM, Decreases in afternoon
• Distinguishable feature: bright red granules & bilobed nucleus

Eosinophilia

• Increased eosinophils (>500 per µL)
• Occurs most commonly in the setting of parasite infections
• Other causes: Allergies (hay fever, rhinitis), Allergic reaction to drugs (aspirin, some antibiotics), Asthma, Eczema

Basophils

• Distinguishing feature: large basophilic granules & bilobed nucleus
• Share structural & functional similarity with mast cells.

Basophilia

• Increased levels of basophils (>200 per µL)
• Causes: allergic reactions, chronic inflammation, leukemia, and certain tumors.

Summary of Leukocytes and Elevation

Platelets /Thermbocytes

• Small disc-shaped non-nucleated structures in the blood representing cell fragments derived from megakaryocytes in the bone marrow
• Normal levels is 150,000 to 400,000/mL
• Functions:
  • Stops bleeding (hemostasis)
  • Form plugs to occlude sites of vascular damage
  • Promote clot formation
  • Modulate coagulation & vascular repair

Thrombocytes (Platelets)

• Round or oval, biconvex discs that are usually clumped together
• Lack of platelets (thrombocytopenia) lead to increased risk of bleeding.
• High levels of platelets (thrombocytosis) leads to risk of inappropriate blood clotting (thrombosis)

Thrombocytosis

• Increase in platelet count of > 450 x 109/L
• Possible causes: Infection, Bone marrow disruption
• Effects: Clotting disorders & complications (e.g. stroke, heart attack), Redness/throbbing of hands & feet, enlargement of the spleen

Thrombocytopenia

• Decrease in platelet count of < 150 x 109/L
• Possible causes: Infections (especially virals), Bone marrow aplasia, Liver disease
• Effects: Bleeding disorders (e.g. nosebleeds, strokes), Presence of petechiae

Laboratory Exams:

• Complete Blood Count (CBC)
• Peripheral Blood Smear (PBS)
• Iron Studies
• Bone Marrow Examination

Complete Blood Count (CBC)

• Readily available blood test that focuses on Red Blood Cells (RBC), White Blood Cells (WBC) and Platelets, and their various parameters.
• Red Blood Cell Count, White Blood Cell Count, Platelet Count

Complete Blood Count (CBC).: Red Blood Cell Count

• Hemoglobin
• Hematocrit
• Reticulocyte count (if abnormal)

Complete Blood Count (CBC).: Red Blood Cell Indices

• Mean cell volume (MCV)
• Mean cell hemoglobin (MCH)
• Red cell distribution width (RDW)
• Mean Cell Hemoglobin Concentration (MCHC)

Complete Blood Count (CBC).: White Blood Cell Count

• Cell Differential
• Nuclear segementation or Neutrophils
• Lymphocytes
• Monocytes
• Eosinophils
• Basophils

Histological Methods to Study Blood

• Standard method of examination is to make a spread (smear) on a glass slide.

Distinctive Staining Characteristics

• Basophilia (deep blue) – characteristic of DNA in nuclei, RNA & ribosomes
• Azurophilia (purple) – typical of lysosomes & one of the granules in leukocytes
• Eosinophilia (pink) – feature of hemoglobin & granules of eosinophils (eosin is acidic)
• Neutrophilia (salmon pink) – cytoplasmic granules of neutrophil leukocytes

Peripheral Blood Smear

• Microscopic analysis of blood taken from a peripheral site such as the vein of the arm.
• Can reveal the number and appearance or morphology of the different blood cells as well as the presence of blood borne parasites or other pathological findings.

Erythrocyte (RBC)

• Cells are stained pink due to their high content of hemoglobin (basic protein).
• Pale-staining center is due to its biconcave disc shape.

RBC Morphology

• Some abnormal shapes are: Tear drop cell, Sickle cell, Acanthocyte, Spherocyte, Target cell and Schistocyte (Fragmented RBC)

Reticulocytes

• Immature red blood cells which have shed their nucleus but still retain residual nuclear material.
• Cannot normally be seen in routinely stained blood spreads and can only be seen through supravital staining techniques.

Iron Studies

• Focused analysis on the levels of free iron in the serum, storage form of iron & its binding capacity.
• Includes: Serum Iron level, Total Iron Binding Capacity (TIBC), Serum Ferritin

Bone Marrow Aspiration

• Focused analysis of the bone marrow contents
• Reveals existing hematopoietic islands or clusters and presence or absence of bone marrow disorders or failure.