Comprehensive Study Guide to Hematology, Blood Group Serology, and Transfusion Medicine

General Physiology and Composition of Blood

• Definition of blood: Blood is a specialized liquid connective tissue composed of three distinct groups of formed cellular elements—erythrocytes (glóbulos rojos or hematíes), leukocytes (glóbulos blancos), and platelets (plaquetas or trombocitos)—suspended within a liquid extracellular matrix termed blood plasma (plasma sanguíneo).

• Primary physiological role: Blood circulates throughout the cardiovascular system to deliver essential oxygen ($\text{O}_2$) and nutrients to every living cell in the human body, while simultaneously removing metabolic wastes.

• Hematopoiesis (hematopoyesis): The physiological process governing the formation, developmental differentiation, and maturation of all blood cells from an uncommitted pluripotent stem cell (célula madre) originating within the bone marrow (médula ósea). Under normal homeostatic conditions, a precise dynamic equilibrium coordinates cell production with peripheral destruction.

• Four primary functions of blood:

  • Gas and solute transport: Conveys respiratory gases ($\text{O}_2$ and carbon dioxide, $\text{CO}_2$), nutritive organic substances, and regulatory hormones throughout the body.
  • Internal physiological regulation: Maintains systemic homeostasis through the active regulation of internal body temperature, water balance, and systemic electrolyte concentrations.
  • Organismal defense: Provides immune defense through leukocytes and circulating immunoglobulins against invasive pathogens and pathogenic substances.
  • Hemostasis (hemostasia): The innate physiological capacity of the body to arrest and cease hemorrhages (detención de una hemorragia) spontaneously via vascular and clotting mechanisms.

• Quantitative volumetric proportions of blood:

  • In a standard adult reference human blood volume of $5\,000\,\text{mL}$ ($5\,\text{L}$), blood plasma accounts for approximately $3\,000\,\text{mL}$.
  • Blood plasma fraction: Accounts for approximately $55\%$ of total circulating blood volume.
  • Erythrocyte fraction: Accounts for approximately $41\%$ of total circulating blood volume.
  • Leukocyte and thrombocyte fraction (buffy coat layer): Accounts for approximately $4\%$ of total circulating blood volume.

Formed Cellular Elements: Erythrocytes, Leukocytes, and Platelets

• Erythrocytes (glóbulos rojos or hematíes):

  • Structure and morphology: Anucleated cells characterized by a biconcave disc shape (discos bicóncavos) featuring a depressed central zone. Highly flexible, permitting passage through microvascular capillary beds without membrane disruption.
  • Hemoglobin content and pigment: Packed with hemoglobin (hemoglobina), a specialized respiratory metalloprotein composed of a protein component (globin, globina) paired with an iron-containing chemical moiety (heme group, grupo hemo). Hemoglobin confers the red pigmentation to blood.
  • Respiratory gas exchange function: Hemoglobin binds oxygen at respiratory alveolar surfaces to transport it to peripheral tissue beds, and binds carbon dioxide ($\text{CO}_2$) produced by cellular metabolism to carry it back to pulmonary tissues for exhalation.
  • Reference cell counts:
  • Adult females: Approximately $4.5 \times 10^6\,\text{erythrocytes/mm}^3$ to $5.0 \times 10^6\,\text{erythrocytes/mm}^3$ ($4{,}5\text{ a }5\text{ millones de eritrocitos por milímetro cúbico}$).
  • Adult males: Approximately $5.0 \times 10^6\,\text{erythrocytes/mm}^3$ to $5.5 \times 10^6\,\text{erythrocytes/mm}^3$ ($5\text{ a }5{,}5\text{ millones de eritrocitos por milímetro cúbico}$).
  • Erythrocyte lifespan and turnover: Mean circulating lifespan is approximately $120\,\text{days}$. Senescent or damaged erythrocytes undergo clearance and enzymatic breakdown within the liver (hígado) and the spleen (bazo).
  • Erythropoiesis (eritropoyesis): The specialized biological process of forming and developing new erythrocytes to balance clearance. Occurs in distinct anatomical sites across ontogeny:
  • Fetal life (vida fetal): Yolk sac (saco vitelino), bone marrow (médula ósea), spleen (bazo), liver (hígado), and lymph nodes (ganglios linfáticos).
  • Childhood (niños): Active bone marrow across all skeleton bones, as well as the spleen (bazo).
  • Adulthood (adultos): Restricted to the red bone marrow of axial and flat bones, specifically the vertebrae (vértebras), sternum (esternón), ribs (costillas), and iliac bones (huesos ilíacos).

• Leukocytes (glóbulos blancos or leucocitos):

  • Reference cell count: Normal circulating concentrations range from $4\,800\,\text{leucocytes/mm}^3$ to $10\,000\,\text{leucocytes/mm}^3$ of whole blood.
  • Granulocytes (granulocitos): Morphologically identified by specific cytoplasmic granules:
  • Neutrophils (neutrófilos): Specialize in cellular defense via phagocytosis (fagocitosis), engulfing and ingesting foreign particulate matter, pathogens, and cellular debris.
  • Basophils (basófilos): Directly participate in systemic inflammatory cascades; synthesize and store heparin (heparina, an anticoagulant agent) and histamine (histamina, a vasoactive amine).
  • Eosinophils (eosinófilos): Attenuate and modulate the intensity of systemic allergic reactions; possess specialized phagocytic capabilities directed toward circulating antigen-antibody immune complexes (complejos antígeno-anticuerpo).
  • Agranulocytes (agranulocitos): Characterized by the absence of visible cytoplasmic granules:
  • Lymphocytes (linfocitos): Indispensable cellular mediators of adaptive immunity synthesized in the bone marrow. Differentiate into two lineages:
    • T lymphocytes (linfocitos T): Mediate cellular adaptive immunity (inmunidad celular).
    • B lymphocytes (linfocitos B): Mediate humoral adaptive immunity (inmunidad humoral) via the synthesis and secretion of target-specific antibodies (anticuerpos).
  • Monocytes (monocitos): Circulating mononuclear phagocytes that migrate into tissues to become macrophages (macrófagos), combating infectious agents via phagocytosis.

• Platelets (plaquetas or trombocitos):

  • Reference cell count: Normal physiological concentrations range from $150\,000\,\text{platelets/mm}^3$ to $400\,000\,\text{platelets/mm}^3$.
  • Origin: Formed inside the red bone marrow (médula ósea roja) through the cellular fragmentation of large, multinucleated precursor cells known as megakaryocytes (megacariocitos).
  • Morphological features: Cytoplasmic fragments that completely lack a cell nucleus; incapable of cell division or biological self-replication.
  • Lifespan and clearance: Remain functionally active in peripheral circulation for approximately $8\,\text{days}$ to $12\,\text{days}$. Destroyed and phagocytosed primarily by resident macrophages within the spleen (macrófagos del bazo).
  • Physiological role: Actively mediate blood coagulation cascades and drive the physical consolidation of the primary platelet plug (tapón plaquetario) at vascular lesions.

Blood Plasma: Composition and Physiological Functions

• Plasma composition by percentage:

  • Water: Constitutes $90\%$ of total plasma volume, serving as the universal biological solvent.
  • Plasma proteins: Constitute $7\%$ of plasma volume, representing the most functionally prominent organic fraction.
  • Dissolved solutes: Constitute the remaining $3\%$ of plasma volume.

• Dissolved plasma solutes:

  • Inorganic electrolytes: Sodium ($\text{Na}^+$), potassium ($\text{K}^+$), calcium ($\text{Ca}^{2+}$), chloride ($\text{Cl}^-$), among others.
  • Circulating nutrients: Glucose, amino acids, fatty acids, and lipids.
  • Endocrine signaling hormones: Circulating systemically to target organ receptors.
  • Blood gases: Dissolved oxygen ($\text{O}_2$), nitrogen ($\text{N}_2$), and carbon dioxide ($\text{CO}_2$).
  • Nitrogenous metabolic waste products: Urea, uric acid (ácido úrico), creatinine (creatinina), and bilirubin (bilirrubina).

• Major plasma proteins and physiological functions:

  • Albumin (albúmina): Responsible for maintaining systemic blood osmotic (colloid oncotic) pressure. Because albumin and other bulky plasma proteins cannot traverse intact capillary walls, the intravascular protein concentration remains higher than that of the extravascular interstitial space, creating a transcapillary concentration gradient that prevents fluid loss from the vascular tree.
  • Fibrinogen (fibrinógeno) and prothrombin (protrombina): Vital protein components that drive the blood coagulation cascade and facilitate platelet plug stabilization.
  • Immunoglobulins (inmunoglobulinas): Circulating protective antibody proteins synthesized by antigen-stimulated B lymphocytes to execute humoral immune responses.
  • Transport proteins: Specialized plasma globulins that bind and transport insoluble lipids, fat-soluble molecules, and systemic metallic ions throughout the vascular circuit.

Cardiovascular Circulation and Systemic Gas Exchange

• Left heart mechanics and arterial outflow: Oxygen-rich blood returning from the pulmonary beds enters the left atrium (aurícula izquierda) via the pulmonary veins (venas pulmonares). Blood then traverses the atrioventricular valve into the left ventricle (ventrículo izquierdo), which contracts forcefully to eject oxygenated blood into the systemic aorta (arteria aorta).

• Microvascular systemic exchange: From the aorta, blood flows through systemic branching arteries, terminal arterioles, and into capillary networks throughout every organ and tissue bed. Capillaries exchange oxygen and systemic nutrients for cellular metabolic wastes and carbon dioxide ($\text{CO}_2$).

• Venous return to the right heart: Venous capillaries collect into postcapillary venules and systemic veins, which funnel into the superior and inferior venae cavae (venas cavas). The venae cavae deliver deoxygenated, $\text{CO}_2$-rich blood into the right atrium (aurícula derecha).

• Systemic versus pulmonary circuit tracking:

  • Systemic circuit (circuito mayor): Blood is ejected from the left ventricle through the aorta to the systemic capillary beds, and returns to the heart via the venae cavae into the right atrium.
  • Pulmonary circuit (circuito menor): Deoxygenated blood exits the right ventricle into the pulmonary arterial trunk and pulmonary arteries, reaches alveolar lung capillaries where it releases $\text{CO}_2$ and binds $\text{O}_2$, and returns via the pulmonary veins into the left atrium.

Blood Group Classification: ABO and Rh Systems

• Fundamental concept of blood groups: Classification systems for human blood determined by specific carbohydrate or protein antigens displayed on the outer plasma membrane of erythrocytes. The presence or absence of these antigens governs immunological compatibility or incompatibility between donor and recipient.

• The ABO System (Sistema ABO):

  • Historical discovery: Discovered in 1901 by Austrian physician Karl Landsteiner.
  • Antigenic foundation: Based upon the presence or complete absence of two discrete surface antigens on the erythrocyte membrane: Antigen A (antígeno A) and Antigen B (antígeno B).
  • The four phenotypic ABO groups:
  • Group A (Grupo A): Erythrocytes express surface antigen A. The immune system naturally contains isoantibodies against antigen B, reacting immunologically against any transfused blood bearing antigen B.
  • Group B (Grupo B): Erythrocytes express surface antigen B. The immune system contains isoantibodies against antigen A, reacting immunologically against transfused blood bearing antigen A.
  • Group AB (Grupo AB): Erythrocytes express both antigen A and antigen B simultaneously. The immune system synthesizes neither anti-A nor anti-B antibodies, recognizing both antigens as self.
  • Group 0 (Grupo 0 / cero): Erythrocytes express neither antigen A nor antigen B on their outer membrane. The immune system synthesizes antibodies against both antigen A and antigen B, reacting against blood presenting A, B, or both antigens.

• The Rh System (Sistema Rh):

  • Historical discovery: Discovered in 1940 by Karl Landsteiner through immunological experimental work conducted on Rhesus monkeys (monos Rhesus).
  • Antigenic foundation: Governed by a collection of specific blood antigens, of which Antigen D (antígeno D) is the most clinically significant and immunogenic.
  • Phenotypic classifications:
  • Rh-positive ($\text{Rh}^+$): Individuals whose erythrocytes express surface antigen D.
  • Rh-negative ($\text{Rh}^-$): Individuals whose erythrocytes completely lack surface antigen D. These individuals do not naturally possess pre-formed anti-Rh antibodies, but will rapidly synthesize anti-Rh antibodies if parenterally exposed to $\text{Rh}^+$ erythrocytes, precipitating immune agglutination (aglutinación).

Blood Transfusion Principles and Immunological Compatibility

• Clinical definition of blood transfusion: A specialized medical-surgical procedure transferring blood from one individual (donor, donante) into another (recipient, receptor). Because blood constitutes a living fluid tissue, blood transfusion is clinically defined as a tissue transplant (trasplante de tejido).

• Transfusion indications: Routine procedure utilized to replace massive volume loss following major surgery, acute traumatic hemorrhage, or severe pathological blood consumption. While whole blood (sangre completa) can be transfused, clinical practice predominantly utilizes targeted component therapy, administering packed erythrocytes, isolated leukocytes, platelet concentrates, or fresh frozen plasma according to the specific clinical deficits of the patient.

• Complications and adverse risks:

  • Local procedural risks: Minor localized inflammatory reactions or bacterial site infections at the venipuncture puncture site (punto de punción).
  • Severe immunological incompatibility reactions: Hemolysis (hemólisis), defined as the acute rupture and destruction of donor erythrocytes with uncontrolled release of free hemoglobin into the recipient's plasma.
  • Systemic consequences: Acute renal failure (fallo renal), severe acquired anemia, and circulatory shock (shock circulatorio), a critical medical emergency where systemic tissue perfusion fails and cells are deprived of oxygen and essential nutrients, causing profound arterial hypotension, compensatory tachycardia, and multi-organ failure (fallo orgánico), which can be fatal.

• Rules of donor-recipient compatibility:

  • Homologous identity: Every blood group is universally compatible with, and can both donate to and receive from, its exact own identical blood group type.
  • Universal donor (donante universal): Blood group $0\,(-)$ serves as the universal donor for all recipients. Because $0\,(-)$ erythrocytes display neither antigen A, antigen B, nor antigen D, they cannot trigger antibody-mediated agglutination or acute hemolytic reactions in any recipient.
  • Universal recipient (receptor universal): Blood group $\text{AB}\,(+)$ serves as the universal recipient. Because $\text{AB}\,(+)$ individuals express antigens A, B, and D simultaneously, their immune system recognizes all donor erythrocyte phenotypes as self and carries no circulating isoantibodies against them.

• Comprehensive compatibility matrix between donor and recipient phenotypes:

  • Donor $\text{A}^+$: Can safely donate to recipients $\text{A}^+$ and $\text{AB}^+$. Incompatible with recipients $\text{B}^+$, $0^+$, $\text{A}^-$, $\text{B}^-$, $\text{AB}^-$, and $0^-$.
  • Donor $\text{B}^+$: Can safely donate to recipients $\text{B}^+$ and $\text{AB}^+$. Incompatible with recipients $\text{A}^+$, $0^+$, $\text{A}^-$, $\text{B}^-$, $\text{AB}^-$, and $0^-$.
  • Donor $\text{AB}^+$: Can safely donate exclusively to recipient $\text{AB}^+$. Incompatible with recipients $\text{A}^+$, $\text{B}^+$, $0^+$, $\text{A}^-$, $\text{B}^-$, $\text{AB}^-$, and $0^-$.
  • Donor $0^+$: Can safely donate to recipients $\text{A}^+$, $\text{B}^+$, $\text{AB}^+$, and $0^+$. Incompatible with recipients $\text{A}^-$, $\text{B}^-$, $\text{AB}^-$, and $0^-$.
  • Donor $\text{A}^-$: Can safely donate to recipients $\text{A}^+$, $\text{AB}^+$, $\text{A}^-$, and $\text{AB}^-$. Incompatible with recipients $\text{B}^+$, $0^+$, $\text{B}^-$, and $0^-$.
  • Donor $\text{B}^-$: Can safely donate to recipients $\text{B}^+$, $\text{AB}^+$, $\text{B}^-$, and $\text{AB}^-$. Incompatible with recipients $\text{A}^+$, $0^+$, $\text{A}^-$, and $0^-$.
  • Donor $\text{AB}^-$: Can safely donate to recipients $\text{AB}^+$ and $\text{AB}^-$. Incompatible with recipients $\text{A}^+$, $\text{B}^+$, $0^+$, $\text{A}^-$, $\text{B}^-$, and $0^-$.
  • Donor $0^-$: Can safely donate to all recipients ($\text{A}^+$, $\text{B}^+$, $\text{AB}^+$, $0^+$, $\text{A}^-$, $\text{B}^-$, $\text{AB}^-$, and $0^-$).

Maternal-Fetal Rh Incompatibility (Incompatibilidad Rh entre Madre y Feto)

• Etiology and pathophysiology: Occurs when an $\text{Rh}^-$ mother carries an $\text{Rh}^+$ fetus inherited from an $\text{Rh}^+$ father.

• Primary pregnancy exposure: During an initial pregnancy, adverse clinical sequelae are rare because maternal and fetal circulations remain separated by the placental barrier under normal physiological gestation.

• Sensitization at parturition: At the time of delivery (parto) or through transplacental fetomaternal hemorrhage, fetal $\text{Rh}^+$ erythrocytes enter the maternal bloodstream. The maternal immune system identifies the foreign antigen D on fetal cells and becomes sensitized, mounting a primary immune response that produces maternal anti-Rh antibodies.

• Subsequent pregnancies and pathogenesis: While maternal anti-Rh antibodies cause no harm to the $\text{Rh}^-$ mother herself, in subsequent pregnancies with another $\text{Rh}^+$ fetus, these maternal antibodies cross the placenta. Recognizing antigen D as a foreign substance, they bind and destroy fetal erythrocytes, precipitating hemolytic disease of the newborn (enfermedad Rh del recién nacido or enfermedad Rh).

• Clinical prophylaxis and therapeutic prevention: To prevent maternal isoimmunization, an $\text{Rh}^-$ mother at risk of carrying an $\text{Rh}^+$ child receives an exogenous administration of anti-Rh immunoglobulin (inmunoglobulina Rh). This prophylaxis functions as a vaccine that clears circulating fetal $\text{Rh}^+$ erythrocytes from maternal blood before her adaptive immune system can recognize them, completely inhibiting endogenous anti-Rh antibody synthesis.

Review Questions and Applied Case Scenarios

• Cardiovascular circulatory pathways:

  • Pulmonary circuit sequence: Deoxygenated blood exits the right ventricle via the pulmonary arteries, traverses the lung microvasculature, and returns via the pulmonary veins into the left atrium.
  • Systemic circuit sequence: Oxygenated blood exits the left ventricle via the systemic aorta, traverses tissue capillaries, and returns to the heart via the venae cavae into the right atrium.
  • Superior vena cava red blood cell pathway: A red blood cell entering the superior vena cava travels into the right atrium, passes through the tricuspid atrioventricular valve into the right ventricle, and is ejected through the pulmonary valve into the pulmonary arterial trunk and pulmonary arteries en route to the lung.
  • Pulmonary vein red blood cell pathway: A red blood cell exiting the right lung through a pulmonary vein enters the left atrium, passes through the mitral (bicuspid) atrioventricular valve into the left ventricle, and is pumped across the aortic valve into the ascending aorta and the aortic arch (cayado de la aorta).

• Compatibility case assessments:

  • Donor $\text{A}^+$ to Receptor $\text{B}^+$
  • Result: Incompatible (No).
  • Immunological rationale: Recipient $\text{B}^+$ possesses circulating anti-A antibodies that attack and hemolyze donor erythrocytes presenting antigen A.
  • Donor $\text{A}^-$ to Receptor $\text{AB}^-$
  • Result: Compatible (Sí).
  • Immunological rationale: Recipient $\text{AB}^-$ expresses antigen A (and antigen B) and therefore lacks anti-A antibodies; both individuals are Rh-negative, avoiding anti-Rh sensitization.
  • Donor $\text{B}^+$ to Receptor $0^-$
  • Result: Incompatible (No).
  • Immunological rationale: Recipient $0^-$ plasma contains anti-B antibodies as well as the capacity to form anti-D antibodies against both antigen B and the Rh factor.
  • Donor $\text{B}^-$ to Receptor $\text{AB}^-$
  • Result: Compatible (Sí).
  • Immunological rationale: Recipient $\text{AB}^-$ possesses both A and B surface antigens, synthesizes no anti-B antibodies, and shares identical Rh-negative status.
  • Donor $\text{AB}^+$ to Receptor $0^+$
  • Result: Incompatible (No).
  • Immunological rationale: Recipient $0^+$ plasma contains circulating anti-A and anti-B antibodies that immediately attack donor erythrocytes expressing antigens A and B.
  • Donor $\text{AB}^-$ to Receptor $\text{AB}^+$
  • Result: Compatible (Sí).
  • Immunological rationale: Recipient $\text{AB}^+$ possesses antigens A, B, and D, making this individual the universal recipient without circulating antibodies against any donor antigens.