Circulatory system

Overview and Physical Characteristics of Blood

• Functions of Blood:

  • Transport: Delivers hormones, nutrients, gases ($\text{O}_2$ and $\text{CO}_2$), and metabolic wastes from one point in the body to another.

  • Protection: Contains antibodies, white blood cells, and clotting factors to defend against infection and prevent blood loss.

  • Fluid Balance Regulation: Regulates water distribution between intravascular and interstitial compartments. Because blood is predominantly water, shifting blood volume shifts body water.

  • Temperature Control (Thermoregulation): Water possesses a high heat capacity, making it an effective heat sponge. During physical exertion, blood absorbs metabolic heat generated by skeletal muscles and diverts it to superficial cutaneous blood vessels, allowing heat to dissipate via sweat evaporation.

• Definition of Hematology:

  • The specialized branch of medicine and biology dedicated to the study of blood, blood-forming organs, and blood disorders.

• Whole Blood Composition:

  • Plasma: The liquid extracellular matrix, accounting for approximately $55\%$ of whole blood volume; composed of $92\%$ water alongside proteins and dissolved solutes.

  • Formed Elements: The cellular components, accounting for approximately $45\%$ of whole blood volume; composed primarily of red blood cells, white blood cells, and cell fragments.

• Centrifugation and Blood Transfusion:

  • When whole blood is centrifuged, heavy cellular components settle to the bottom, separating into packed red blood cells (hematocrit), a middle buffy coat (leukocytes and platelets), and plasma at the top.

  • In clinical blood transfusions, donors typically provide packed red blood cells (hematocrit) rather than whole blood.

  • Plasma containing donor antibodies is discarded to prevent transfusion reactions. Trace donor antibodies or minor coagulation may occur in packed cell transfusions, but they rarely trigger significant clinical complications.

• Physical Properties of Blood:

  • Temperature: Slightly higher than core body temperature ($\sim 38^\circ\text{C}$ or $100.4^\circ\text{F}$) due to continuous friction and thermal absorption.

  • Viscosity: Approximately $5\text{ times}$ thicker (more viscous) than water due to suspended formed elements and plasma solutes.

    • Clinical Implication: In uncontrolled diabetes mellitus, elevated blood glucose dramatically increases blood viscosity. Hyperviscous blood struggles to flow through microvasculature, leading to peripheral circulation impairment, diabetic retinopathy (vision issues), and erectile dysfunction.

  • Blood pH Parameters:

    • Physiological pH Range: $7.35$ to $7.45$

    • Average Physiological pH: $7.4$

    • Acidosis Effect: If blood pH drops below $7.35$, the quaternary structure of hemoglobin alters shape (denatures). This structural shift reduces hemoglobin's binding affinity for oxygen, decreasing systemic oxygen transport. Patients display tachypnea (rapid breathing) and tachycardia (elevated heart rate) to compensate for severe hypoxia.

  • Blood Volume:

    • Average Adult Volume: $5\,\text{L}$ (used as standard baseline in cardiovascular calculations).

    • Gender Differences: Males typically possess a higher blood volume ($5\,\text{L}$ to $6\,\text{L}$) compared to females ($4\,\text{L}$ to $5\,\text{L}$) due to body mass differences and hormonal influences.

  • Color Variations:

    • Oxygenated Blood: Bright red due to oxygen bound to hemoglobin in arterial circulation.

    • Deoxygenated Blood: Dark red/burgundy due to oxygen dissociation in venous circulation.

Hematocrit and Blood Plasma Dynamics

• Hematocrit Value:

  • Definition: The percentage of total whole blood volume occupied by red blood cells (erythrocytes).

  • Clinical Significance: Used to evaluate oxygen-carrying capacity and detect conditions such as anemia.

  • Anemia: A clinical condition defined by an abnormally low red blood cell count or reduced hemoglobin concentration, resulting in tissue hypoxia, reduced cellular ATP production, and systemic fatigue.

  • Gender Variations: Males generally exhibit higher hematocrit values than females.

    • Hormonal Mechanism: Androgens (such as testosterone) stimulate the renal secretion of Erythropoietin (EPO), driving erythrocyte production.

    • Menstrual Influence: Periodic blood loss during menstruation lowers baseline red blood cell counts in females.

  • Hormonal Regulation: Erythropoietin (EPO) directly increases hematocrit by targeting red bone marrow to accelerate erythropoiesis.

• Colloid Osmotic Pressure (Oncotic Pressure):

  • Mechanism: Synthesized primarily by the liver, plasma proteins (predominantly albumins) remain trapped within blood capillaries because they are too large to pass through vessel walls.

  • Osmotic Force: Trapped proteins and solutes create colloid osmotic pressure ($\text{OP}$), drawing water out of the interstitial connective tissues and holding it within the intravascular space.

  • Blood Volume Maintenance: High intravascular solute/protein concentration ("the party in the vessel") maintains proper blood volume and systemic blood pressure.

  • Pathology of Liver Disease: In liver cirrhosis, liver cancer, or severe protein malnutrition, protein synthesis drops. Reduced plasma albumin lowers colloid osmotic pressure, causing fluid to leak out of capillaries into interstitial spaces and body cavities.

    • Ascites: Pathological accumulation of serous fluid within the peritoneal cavity, presenting as a distended, fluid-filled abdomen seen in end-stage liver failure.

• Osmolarity, Dehydration, and Edema:

  • High Blood Osmolarity: Hypertonic blood (e.g., during dehydration) draws water from the interstitial fluid into blood vessels to maintain circulating blood volume at the expense of tissue hydration.

    • Clinical Marker: Skin turgor test ("pinch test"); dehydrated interstitial tissue lacks fluid, causing pinched skin to tent rather than snap back.

  • Low Blood Osmolarity: Hypotonic blood causes fluid to leave blood vessels and pool in interstitial spaces.

    • Edema: Abnormal accumulation of fluid in peripheral connective tissues, causing tissue swelling in extremities.

  • Hypertension Management: Patients with high blood pressure are advised to restrict dietary sodium ($\text{Na}^+$) intake. Elevated blood sodium increases plasma osmotic pressure, drawing extra water into the bloodstream, expanding blood volume, and exacerbating elevated blood pressure.

Hemopoiesis and Stem Cell Lineages

• Hemopoiesis (Hematopoiesis):

  • Definition: The continuous physiological process of producing all formed elements of blood (red blood cells, white blood cells, and platelets).

  • Production Rate: Approximately $3 \times 10^6\,\text{erythrocytes}$ ($3\text{ million}$ cells) are produced every second to replace dying cells.

  • Hemopoietic Tissue Location:

    • Adults: Exclusively within red bone marrow (myeloid tissue) located in axial skeleton, girdles, and proximal epiphyses of long bones. Yellow bone marrow consists primarily of adipose tissue and does not perform hemopoiesis.

    • Embryos/Fetuses: Takes place in multiple sites, including the yolk sac, liver, spleen, and lymphatic tissues before migrating to bone marrow.

• Stem Cell Differentiation Pathways:

  • Hemocytoblast: The universal, pluripotent stem cell located in red bone marrow capable of differentiating into any formed element.

  • Lymphoid Stem Cell Line:

    • Divergence pathway restricting differentiation exclusively to Lymphocytes (B cells, T cells, NK cells).

  • Myeloid Stem Cell Line:

    • Divergence pathway giving rise to all other formed elements:

      1. Erythrocytes (Red Blood Cells)

      2. Megakaryocytes (parent cell of Platelets)

      3. Neutrophils

      4. Eosinophils

      5. Basophils

      6. Monocytes

• Chemical Signal Control:

  • Erythropoietin (EPO): Directs hemocytoblasts down the myeloid line specifically toward erythrocyte production.

  • Colony-Stimulating Factors (CSFs): Cytokines and growth factors that direct hemocytoblasts to differentiate into specific leukocyte populations (e.g., granulocytes, monocytes).

Erythrocytes: Structure, Function, and Lifespan

• Key Erythrocyte Properties:

  • Location of Origin & Stem Cell: Produced in red bone marrow from pluripotent stem cells (hemocytoblasts).

  • Primary Function: Transport respiratory gases, specifically oxygen ($\text{O}_2$) and carbon dioxide ($\text{CO}_2$).

  • Primary Protein Composition: Packed almost entirely with Hemoglobin.

  • Three-Dimensional Shape: Biconcave disc structure (characterized by two central indentations/dimples).

  • Lifespan: Exactly $120\,\text{days}$ in circulation.

• Structural Adaptations for Gas Transport:

  • Increased Surface Area: The biconcave geometry provides a higher surface-area-to-volume ratio than a sphere, accelerating gas diffusion into and out of the cell.

  • Anucleate & Lack of Organelles: During maturation (taking approximately $1\,\text{week}$), normoblasts eject their nucleus, mitochondria, Golgi apparatus, and endoplasmic reticulum. Removing organelles leaves maximum internal space for hemoglobin storage.

  • Anaerobic Respiration: Lacking mitochondria, mature RBCs generate ATP exclusively via anaerobic glycolysis. They do not consume any of the oxygen they transport.

  • Rouleaux Formation & Flexibility: The biconcave shape allows red blood cells to bend, fold, and flex as they line up single-file in stacks called rouleaux to squeeze through narrow capillaries with diameters equal to or smaller than the cell itself.

  • Inability to Repair: Lacking a nucleus and ribosomes, erythrocytes cannot synthesize new proteins or repair cellular damage. Mechanical trauma from scraping against vessel walls limits their functional lifespan to $120\,\text{days}$, ending in cellular senescence without mitosis.

Hemoglobin Structure and Destruction Pathway

• Hemoglobin ($\text{Hb}$) Structural Composition:

  • Quaternary Protein Structure: Composed of four globular polypeptide subunit chains:

    • Two Alpha ($\alpha$) chains

    • Two Beta ($\beta$) chains

  • Globin Portion: The collective polypeptide chains made of amino acids. Carbon dioxide ($\text{CO}_2$) binds directly to amino acids on globin chains to form Carbaminohemoglobin.

  • Heme Group: A non-protein organic ring complex (Biliverdin) embedded at the center of each of the four polypeptide chains.

  • Iron Core ($\text{Fe}^{2+}$): Situated at the center of each Heme group. Each iron atom reversibly binds one molecule of oxygen ($\text{O}_2$).

  • Oxygen Carrying Capacity: Each single hemoglobin molecule contains $4\text{ iron atoms}$, allowing it to carry up to $4\,\text{O}_2\text{ molecules}$.

    • $100\%$ Saturation: $4\,\text{O}_2\text{ molecules}$ bound per hemoglobin molecule.

    • $75\%$ Saturation: $3\,\text{O}_2\text{ molecules}$ bound.

    • $50\%$ Saturation: $2\,\text{O}_2\text{ molecules}$ bound.

  • Chemical States:

    • Oxyhemoglobin: Hemoglobin bound to oxygen; exhibits a bright red color.

    • Deoxyhemoglobin: Hemoglobin devoid of bound oxygen; exhibits a dark red color.

• Erythrocyte & Hemoglobin Destruction Flowchart (5-Step Pathway):

  • Destruction Context: After $120\,\text{days}$, senescent or damaged erythrocytes are phagocytized by macrophages located within the liver, spleen, and red bone marrow at a rate of $3 \times 10^6\text{ cells per second}$.

  • Step 1: Hemoglobin released from phagocytized red blood cells is broken down into Heme and Globin components.

  • Step 2: The Globin protein is hydrolyzed into individual Amino Acids, which are released into the blood and recycled to synthesize new proteins or hemoglobin.

  • Step 3: The Heme complex is disassembled into Biliverdin (a green organic pigment) and an Iron ($\text{Fe}$) ion.

  • Step 4: Iron binds to the plasma transport protein Transferrin in the blood and is recycled, traveling to the red bone marrow for new hemoglobin synthesis. (Iron deficiency prevents hemoglobin synthesis, resulting in iron-deficiency anemia).

  • Step 5: Biliverdin is converted into Bilirubin (a yellow-orange pigment), which is transported to the liver and excreted in bile into the digestive tract.

    • Elimination: Intestinal bacteria convert bilirubin into pigments that give feces its characteristic brown color. Small amounts absorbed into blood are excreted by kidneys, giving urine its yellow color.

    • Jaundice: Biliary duct obstruction or excessive erythrocyte hemolysis leads to hyperbilirubinemia. Bilirubin accumulates in peripheral connective tissues, producing yellowing of the sclera and skin.

Erythropoietin (EPO) and Oxygen Homeostasis

• Feedback Mechanism:

  • Stimulus: Tissue hypoxia (reduced oxygen delivery detected by renal cells).

  • Endocrine Response: Juxtaglomerular cells in the kidneys secrete Erythropoietin (EPO) into the bloodstream.

  • Target Tissue: Progenitor cells in the red bone marrow.

  • Effector Action: Accelerates erythrocyte maturation and stimulates hemocytoblast division rates (erythropoiesis).

  • Homeostatic Restoration: Increased circulating RBC count elevates blood oxygen-carrying capacity, raising tissue oxygen levels and shutting off EPO secretion via negative feedback.

• Physiological Triggers for EPO Release:

  • High Altitude: Low atmospheric pressure decreases alveolar oxygen loading, inducing systemic hypoxia.

  • Aerobic Exercise: Increased metabolic consumption of oxygen by skeletal muscle creates tissue oxygen demand.

  • Lung Pathology: Conditions limiting respiratory surface area or airflow (e.g., emphysema, asthma, severe bronchoconstriction) impair arterial blood oxygenation.

Leukocytes: Granulocytes and Agranulocytes

• General Characteristics of Leukocytes (White Blood Cells):

  • Primary Function: Defense against biological pathogens, neutralisation of toxins, destruction of mutated/cancerous cells, and clearance of cellular debris. They play no role in respiratory gas transport.

  • Distribution: Less than $0.1\%$ of leukocytes circulate in blood at any given time; the vast majority reside within connective tissues and organs of the lymphatic system (lymph nodes, spleen, tonsils).

  • Origin: Derived from hemocytoblasts in red bone marrow via both myeloid and lymphoid stem cell lines.

• Granulocytes (Possess visible cytoplasmic granules under light microscopy):

  • Neutrophils:

    • Anatomy: Multi-lobed nucleus ($3$ to $5$ lobes; termed polymorphonuclear leukocytes or PMNs) with faint, neutral-staining granules.

    • Function: First responders to tissue infection; highly phagocytic toward bacteria.

    • Mechanism: Release oxidative chemical blasts via exocytosis of hydrogen peroxide ($\text{H}_2\text{O}_2$) to destroy pathogens. Neutrophils die during the attack.

    • Lifespan: Short operational lifespan ($\sim 24\,\text{hours}$).

    • Pus Formation: Mass accumulation of dead neutrophils, cellular debris, and fluid forms viscous pus at infection sites.

  • Eosinophils:

    • Anatomy: Bilobed nucleus with prominent, bright orange-red acidic granules.

    • Function: Attack large, multicellular organisms (parasitic worms/helminths) by exocytosing cytotoxic enzymes; modulate allergic inflammatory reactions.

    • Diagnostic Marker: Elevated circulating counts indicate parasitic infections or active allergic reactions.

  • Basophils:

    • Anatomy: Bilobed or obscured nucleus masked by dense, dark purple/black basic granules.

    • Function: Release pro-inflammatory mediators into blood.

    • Granule Content: Contains Histamine (promotes vasodilation and increased capillary permeability, triggering mucus production) and Heparin (an anticoagulant preventing premature blood clotting).

    • Tissue Identity: Termed Mast Cells once they migrate out of blood circulation into interstitial connective tissue.

• Agranulocytes (Lack prominent light-microscopic cytoplasmic granules):

  • Monocytes:

    • Anatomy: Largest circulating leukocyte; features a prominent kidney- or horseshoe-shaped nucleus surrounded by abundant pale cytoplasm.

    • Function: Leave blood circulation to transform into tissue Macrophages.

    • Action: Voracious phagocytes that engulf tissue debris, dead neutrophils, and pathogens. Act as antigen-presenting cells to recruit specific adaptive immune responses.

    • Lifespan: Can survive in connective tissues for months to several years.

  • Lymphocytes:

    • Anatomy: Slightly larger than erythrocytes; features a dense, spherical nucleus occupying almost the entire cell volume, framed by a thin rim of blue cytoplasm (cytoplasmic halo).

    • Function: Primary cells of specific (adaptive) immunity; identify and eliminate specific pathogen strains.

    • Distribution: Reside inside lymphatic tissues (lymph nodes, spleen) where pathogens are filtered and brought to them.

    • Lifespan: Can persist and circulate for decades as memory cells.

Leukocyte Recruitment Mechanisms

• Tissue Injury Response Sequence:

  • Chemical Release: Damaged tissue cells and localized mast cells exocytose inflammatory mediators (histamine, prostaglandins).

  • Endothelial Stickiness: Inflammatory chemicals cause local capillary endothelial cells to express adhesion molecules, making vessel walls sticky.

  • Margination & Diapedesis (Emigration): Circulating neutrophils slow down, roll along the sticky endothelium, compress their bodies, and squeeze through narrow intercellular gaps between endothelial cells to enter the connective tissue.

  • Chemotaxis: Migrated leukocytes move along a chemical concentration gradient toward the highest density of inflammatory signaling molecules (following the chemical scent directly to the site of pathogen invasion or tissue damage).

Platelets (Thrombocytes) and Hemostasis

• Structural Characteristics of Platelets:

  • Cellular Nature: Platelets are not intact cells; they are small, membrane-enclosed cytoplasmic fragments.

  • Origin: Derived from giant polyploid cells in red bone marrow called Megakaryocytes.

  • Mechanism of Formation: Megakaryocytes extend long cytoplasmic processes (proplatelets) through endothelial windows into bone marrow blood sinusoids. The force of flowing blood shears these extensions into small, circulating platelets.

  • Motility: Possess ameboid movement capability, enabling them to change shape and extend pseudopods upon activation.

• Primary Function in Hemostasis:

  • Blood Clotting (Coagulation): Platelets aggregate at sites of vascular endothelial disruption to form a temporary platelet plug, adhering to exposed underlying collagen fibers and secreting clotting factors to initiate blood clot formation.

Clinical Terminology and Blood Pathology

• Erythrocyte Conditions:

  • Anemia: Abnormally low red blood cell count, low hematocrit, or deficient functional hemoglobin, resulting in tissue hypoxia.

  • Polycythemia: Abnormally high concentration of circulating red blood cells, elevating blood viscosity and increasing cardiovascular workload.

• Leukocyte Conditions:

  • Leukopenia: Abnormally low total white blood cell count, compromising immune defense.

  • Leukocytosis: Abnormally high white blood cell count, typically reflecting an active physiological response to infection, severe inflammation, or hematologic malignancy (e.g., leukemia).

• Thrombocyte Conditions:

  • Thrombocytopenia: Deficient circulating platelet count, predisposing individuals to spontaneous bleeding, purpura, and impaired hemostasis.

  • Thrombocytosis: Excess circulating platelet count, elevating the risk of inappropriate intravascular thrombosis (blood clot formation).