Comprehensive Study Notes on Body Fluids and Circulation

Principles of Internal Transport and Body Fluids

• Necessity of Internal Transport:

  • Every living cell requires a continuous supply of essential substances, including nutrients and oxygen ($O_2$), to drive cellular respiration and metabolic processes.

  • Metabolic activity yields potentially toxic byproducts, such as carbon dioxide ($CO_2$) and nitrogenous wastes, which require prompt and continuous clearance to maintain homeostatic cellular environments.

• Transport Mechanisms in Lower Organisms:

  • Primitive aquatic invertebrates, including sponges (phylum Porifera) and cnidarians (phylum Coelentrata), do not possess specialized circulatory fluids or vascular systems.

  • These organisms utilize environmental water circulating through body pores, gastrovascular cavities, and canals to deliver nutrients, facilitate gas exchange, transfer gametes, and eliminate metabolic wastes.

• Transport Fluids in Complex Animals:

  • Higher organisms have evolved specialized internal circulating fluids to handle transport across substantial diffusion distances.

  • The primary circulatory fluid in vertebrates is blood.

  • The secondary circulatory fluid is lymph (also known as interstitial fluid or tissue fluid), which bridges the metabolic interface between circulating blood and individual body cells.

• Core Components of the Blood Vascular System:

  • Pumping Organ: A muscular motor mechanism, the heart, which provides the hydrostatic driving force.

  • Circulating Fluid: Blood, which transports respiratory gases, metabolites, and regulatory molecules.

  • Conduit Network: Blood vessels (arteries, arterioles, capillaries, venules, and veins) through which blood flows in a directed pathway.

Blood Composition: Fluid Matrix, Plasma, and Formed Elements

• Physical Nature of Blood:

  • Blood is classified as a specialized connective tissue possessing a fluid extracellular matrix called plasma.

  • Unlike classical connective tissues (such as bone, cartilage, or loose connective tissue), blood completely lacks structural fibers within its normal circulating fluid matrix, preventing luminal occlusion or vascular clogging.

• Constituent Volume Proportions:

  • Plasma: Constitutes approximately $55\%$ of total blood volume.

  • Formed Elements: Cellular and fragmentary components accounting for the remaining $45\%$ of blood volume.

• Plasma Constituents and Characteristics:

  • Physical Properties: A straw-colored, pale yellow, viscous fluid.

  • Water Content: Accounts for $90\%$ to $92\%$ of plasma volume, serving as a primary solvent.

  • Protein Fraction: Accounts for $6\%$ to $8\%$ of plasma by weight, comprising three major functional classes:

    • Fibrinogen: Synthesized in the inactive state; converts into insoluble fibrin fibers during coagulation to generate a hemostatic plug.

    • Globulins: Primarily immunoglobulins ($\text{IgA}$, $\text{IgG}$, etc.) synthesized by plasma cells; mediate humoral defense and immunity.

    • Albumins: Major contributors to colloidal osmotic pressure, regulating osmolarity and transcapillary fluid balance.

  • Electrolytes and Dissolved Ions: Minor dissolved components (accounting for $2\%$ to $4\%$ of plasma) including cations ($Na^+$, $Ca^{2+}$, $Mg^{2+}$) and anions ($Cl^-$, $HCO_3^-$).

  • Dynamic Solutes in Transit: Organic nutrients whose circulating levels fluctuate based on dietary ingestion and tissue consumption, including glucose, free amino acids, lipids, and excretory waste products (urea, uric acid).

• Definition of Serum:

  • Serum represents blood plasma from which all clotting factors (specifically fibrinogen) have been removed:         $\text{Serum} = \text{Plasma} - \text{Clotting Factors}$

Cellular Components: Erythrocytes, Leukocytes, and Platelets

• Erythrocytes (Red Blood Cells / RBCs):

  • Abundance: Most abundant formed element, numbering $5.0 \times 10^6$ to $5.5 \times 10^6\,\text{mm}^{-3}$ of blood ($50$ to $55\,\text{lakhs}$ per cubic millimeter).

  • Erythropoiesis: Produced in the red bone marrow of adults. Early embryonic stages synthesize erythrocytes in primitive structures such as the yolk sac and fetal liver.

  • Morphology and Cytology: Biconcave, circular, discoid cells lacking a nucleus (enucleated) and other major organelles at functional maturity to maximize intracellular volume for hemoglobin packaging.

  • Hemoglobin Content: An iron-containing conjugated metalloprotein present at concentrations of $12\,\text{g}$ to $16\,\text{g}$ per $100\,\text{ml}$ of whole blood.

  • Lifespan and Degradation: Circulate for approximately $120\,\text{days}$ ($\sim 4\,\text{months}$) before destruction within the spleen, known as the "graveyard of RBCs," where components like iron are salvaged.

  • Function: Transportation of respiratory gases ($O_2$ bound as oxyhemoglobin and $CO_2$ bound as carbaminohemoglobin).

• Platelets (Thrombocytes):

  • Origin and Structure: Non-nucleated cellular fragments derived via fragmentation from giant bone marrow precursor cells called megakaryocytes.

  • Abundance: Range from $150{,}000$ to $350{,}000\,\text{mm}^{-3}$ of blood ($1.5$ to $3.5\,\text{lakhs}$ per cubic millimeter).

  • Function: Release procoagulant factors that initiate hemostasis; critical deficiency leads to severe coagulopathies and persistent blood loss.

• Leukocytes (White Blood Cells / WBCs):

  • Characteristics: Nucleated, colorless cells lacking hemoglobin; count ranges from $6{,}000$ to $8{,}000\,\text{mm}^{-3}$.

  • Granulocytes (Presence of visible cytoplasmic granules):

    • Neutrophils: Constitute $60\%$ to $65\%$ of circulating leukocytes; exhibit neutral staining; phagocytose foreign micro-pathogens.

    • Eosinophils: Constitute $2\%$ to $3\%$; take up acidic dyes; resist parasitic infections and modulate allergic reactions.

    • Basophils: Least abundant at $0.5\%$ to $1\%$; take up basic dyes; release histamine, serotonin, and heparin to mediate inflammation, modulate vasodilation, and maintain patency (similar to tissue mast cells).

  • Agranulocytes (Absence of cytoplasmic granules):

    • Lymphocytes: Constitute $20\%$ to $25\%$; divided into B lymphocytes and T lymphocytes; execute humoral and cell-mediated adaptive immune responses.

    • Monocytes: Constitute $6\%$ to $8\%$; large motile phagocytes that differentiate into macrophages within peripheral tissues.

  • Relative Abundance Hierarchy:         $\text{Neutrophils} > \text{Lymphocytes} > \text{Monocytes} > \text{Eosinophils} > \text{Basophils}$         Mnemonic: "Never Let Monkeys Eat Bananas"

Blood Group Systems: ABO Antigens and Rh Incompatibility

• The ABO Blood Group System:

  • Governed by specific carbohydrate/glycoprotein surface antigens expressed on the outer erythrocyte plasma membrane: Antigen A and Antigen B.

  • Reciprocal pre-formed antibodies (isoagglutinins) exist naturally within the plasma against absent red blood cell antigens:

    • Blood Group A: Possesses surface Antigen A; produces Anti-B antibodies in plasma.

    • Blood Group B: Possesses surface Antigen B; produces Anti-A antibodies in plasma.

    • Blood Group AB: Possesses both Antigen A and Antigen B; produces neither Anti-A nor Anti-B antibodies in plasma (Universal Recipient).

    • Blood Group O: Devoid of both Antigen A and Antigen B; produces both Anti-A and Anti-B antibodies in plasma (Universal Donor).

  • Transfusion Compatibility: Antigen-antibody mismatch triggers complement-mediated intravascular agglutination and massive hemolysis.

• The Rh Blood Group System:

  • Based on the Rh antigen, first discovered on erythrocytes of the Rhesus monkey (Macaca mulatta).

  • Individuals expressing the Rh antigen are designated Rh-positive ($\text{Rh}^+$, accounting for approximately $80\%$ of the human population).

  • Individuals lacking the surface Rh antigen are designated Rh-negative ($\text{Rh}^-$, accounting for approximately $20\%$ of humans).

• Rh Incompatibility (Erythroblastosis Fetalis):

  • Clinical Setting: Occurs when an $\text{Rh}^-$ mother carries an $\text{Rh}^+$ fetus (the Rh gene being inherited from an $\text{Rh}^+$ father).

  • Sensitization Phase: During the first pregnancy, maternal and fetal circulations remain separated by the placental barrier. During parturition, micro-ruptures allow fetal $\text{Rh}^+$ erythrocytes into the maternal bloodstream, prompting maternal synthesis of anti-Rh antibodies.

  • Subsequent Pregnancies: If a subsequent fetus is $\text{Rh}^+$, maternal anti-Rh IgG antibodies cross the placenta into fetal circulation, binding to and lysing fetal erythrocytes.

  • Consequences: Massive fetal hemolysis resulting in profound hemolytic anemia, severe kernicterus/jaundice, hydrops fetalis, or intrauterine death.

  • Prophylaxis: Administration of exogenous anti-Rh antibodies (RhoGAM) to the $\text{Rh}^-$ mother immediately post-delivery of an $\text{Rh}^+$ infant, clearing fetal cells before active maternal sensitization occurs.

Blood Coagulation Cascade and Hemostasis

• General Hemostatic Mechanism:

  • Vascular disruption initiates rapid localized clot formation to prevent exsanguination.

  • The mature blood clot (coagulum) is composed of a three-dimensional mesh of insoluble fibrin polymer threads trapping formed elements and platelets.

• Cascade Pathway (The "Packet of Fiber" Pathway):

  • Tissue damage and platelet activation prompt the release of thromboplastin.

  • Step 1 (P - Thromboplastin): Extrinsic and intrinsic triggers assemble the enzyme complex thrombokinase (prothrombinase).

  • Step 2 (K - Thrombokinase): Thrombokinase converts the inactive plasma proenzyme prothrombin into active thrombin in the presence of calcium ions ($Ca^{2+}$).

  • Step 3 (T - Thrombin): Thrombin enzymatically cleaves soluble fibrinogen into insoluble fibrin monomers that polymerize across the wound margins.

  • Summary Scheme: Thromboplastin (P) activates Thrombokinase (K), which generates Thrombin (T), producing Fibrin fibers.

• Cofactor Requirements:

  • Calcium ($Ca^{2+}$): Designated officially as Clotting Factor IV; serves as an indispensable divalent ionic cofactor across multiple steps.

  • Vitamin K: Fat-soluble vitamin essential for hepatic post-translational carboxylation of clotting factors (Factors II, VII, IX, and X).

  • Thirteen numbered clotting factors have been historically described; twelve are functionally recognized (Factor VI was reclassified).

Lymphatic System and Tissue Fluid Dynamics

• Microvascular Fluid Dynamics:

  • Arterial blood enters dense terminal networks of microvascular capillaries.

  • Elevated hydrostatic pressure at the arterial end forces water, electrolytes, low-molecular-weight nutrients, and hormones across permeable capillary walls into extracellular voids, creating interstitial fluid (tissue fluid).

  • Colloidal plasma proteins and formed elements remain confined inside capillary walls.

• Transition from Interstitial Fluid to Lymph:

  • Cellular tissues extract delivered $O_2$ and nutrients while discharging metabolic wastes ($CO_2$, nitrogenous end-products) into the surrounding interstitial fluid.

  • While most fluid re-enters capillaries at the venous limb, unabsorbed tissue fluid drains into open-ended lymphatic capillaries.

  • Once enclosed within the specialized endothelial conduits of the lymphatic network, this fluid is designated as lymph.

• Lymphatic Architecture and Return:

  • Lymph vessels converge into larger lymphatic trunks, which empty back into central venous circulation via the subclavian veins near the heart.

  • Interspersed along the lymphatics are organized encapsulated lymphoid masses called lymph nodes (concentrated heavily in the axillary and inguinal regions), which filter particulate matter and house lymphocytes that initiate immune responses against drained pathogens.

Circulatory Pathways and Patterns Across Animal Phyla

• Open vs. Closed Circulatory Systems:

  • Open Circulatory Systems:

    • Observed in arthropods and non-cephalopod molluscs.

    • Blood pumped by the heart empties into large cavities or sinuses (hemocoel), directly bathing tissues without an intervening continuous capillary wall.

  • Closed Circulatory Systems:

    • Observed in annelids, cephalopods, and all vertebrates (chordates).

    • Blood flows through a continuous, closed circuit of muscular vessels (arteries, capillaries, veins), allowing precise physiological regulation of regional hydrostatic pressure and perfusion.

• Vertebrate Heart Chamber Configurations:

  • Two-Chambered Heart (Fishes / Pisces):

    • Composed of one atrium and one ventricle.

    • Operates via Single Circulation: Deoxygenated venous blood passes once through the heart, which pumps it to the branchial gill capillary bed for oxygenation; newly oxygenated blood travels directly to systemic tissues before returning deoxygenated to the atrium.

  • Three-Chambered Heart (Amphibians and Most Reptiles):

    • Composed of two atria and one shared ventricle.

    • Operates via Incomplete Double Circulation: The left atrium receives oxygenated blood from lungs/skin; the right atrium receives deoxygenated systemic blood. Both empty into a single ventricular chamber, leading to partial mixing of blood streams before ventricular ejection.

  • Four-Chambered Heart (Crocodilians, Birds / Aves, Mammals):

    • Composed of two discrete atria and two discrete ventricles.

    • Operates via Complete Double Circulation: The left and right sides of the heart are physically isolated, preventing mixing of oxygenated and deoxygenated blood.

Anatomy and Histology of the Human Cardiovascular System

• Gross Structural Anatomy of the Heart:

  • Embryological Derivation: Mesodermally derived organ.

  • Anatomical Position: Located within the thoracic mediastinum between the lungs, tilted toward the left side. This orientation creates the cardiac notch in the left lung, making it slightly smaller than the right lung.

  • Dimensions: Approximately equal in size to an individual's clenched fist.

  • Pericardium: Enclosed by a protective double-walled fibroserous sac (pericardium) containing lubricating pericardial fluid to reduce mechanical friction.

  • Internal Compartments: Four distinct chambers: thin-walled superior reception chambers (right and left atria) and thick-walled inferior muscular pumping chambers (right and left ventricles).

• Internal Septa:

  • Interatrial Septum: A thin, membranous muscular partition separating the right and left atria.

  • Interventricular Septum: A thick, muscular divider separating the right and left ventricles.

  • Atrioventricular (AV) Septa: Dense, fibrous sheets separating atria from their corresponding ventricles, perforated by valved orifices.

• Valvular Apparatus:

  • Tricuspid Valve: Formed of three muscular cusps; guards the right atrioventricular orifice.

  • Bicuspid Valve (Mitral Valve): Formed of two muscular cusps; guards the left atrioventricular orifice.

  • Chordae Tendineae: Fibrous collagenous cords originating from ventricular papillary muscles and inserting into the margins of the AV valves, preventing cuspal eversion into the atria during ventricular systole.

  • Semilunar Valves: Crescentic tri-flap valves located at the base of the pulmonary artery and the ascending aorta that prevent regurgitant backflow into the ventricles.

• Histology of Blood Vessels:

  • Both arteries and veins exhibit three concentric tunics:

    1. Tunica Intima: Innermost lining composed of a monolayer of simple squamous endothelium resting on a basal lamina.

    2. Tunica Media: Middle layer comprised of concentric smooth muscle cells and elastic lamellae. In arteries, this layer is thick and robust, yielding a narrow central lumen and higher systemic pressure; in veins, it is thin and flexible, producing a wide central lumen and lower operational pressure.

    3. Tunica Externa (Adventitia): Outermost coat consisting of fibrous connective tissue with collagen fibers.

• Vascular Circuits:

  • Systemic Circulation: High-pressure circuit: Left ventricle $\rightarrow$ Aorta $\rightarrow$ Systemic Arteries $\rightarrow$ Arterioles $\rightarrow$ Capillaries $\rightarrow$ Venules $\rightarrow$ Vena Cava $\rightarrow$ Right Atrium.

  • Pulmonary Circulation: Low-pressure circuit: Right ventricle $\rightarrow$ Pulmonary Artery $\rightarrow$ Alveolar Capillaries $\rightarrow$ Pulmonary Veins $\rightarrow$ Left Atrium.

  • Hepatic Portal System: A specialized vascular arrangement where venous outflow from the stomach and intestines drains via the hepatic portal vein directly to the liver sinusoids before entering systemic venous circulation.

  • Coronary Circulation: Dedicated arterial and venous circuit supplying the cardiac myocardium directly through the coronary arteries branching off the base of the ascending aorta.

Cardiac Nodal Tissue and Electrical Conduction

• Myogenic Cardiac Properties:

  • Human cardiac tissue is myogenic; it generates intrinsic, rhythmic electrical action potentials without requiring external neurogenic signals.

• Components of the Intrinsic Conduction System:

  • Sinoatrial Node (SA Node):

    • Specialized nodal tissue situated in the upper right wall of the right atrium near the entry of the superior vena cava.

    • Acts as the primary Pacemaker by generating action potentials at the highest frequency: $70$ to $75\,\text{impulses/min}$ (averaging $72\,\text{beats/min}$).

  • Atrioventricular Node (AV Node):

    • Situated in the lower left portion of the right atrium, adjacent to the atrioventricular septum.

    • Acts as a Pacesetter and relay station, introducing a slight functional delay that allows atrial emptying before ventricular excitation.

  • Bundle of His (Atrioventricular Bundle):

    • Emerges from the AV node, traverses the fibrous AV ring, and descends along the superior margin of the interventricular septum before bifurcating into Right and Left Bundle Branches.

  • Purkinje Fibers:

    • Specialized conducting myocardial fibers emerging from bundle branches that arborize throughout the subendocardial ventricular myocardium.

• Sequence of Electrical Propagation:     $\text{SA Node} \rightarrow \text{Atrial Myocardium} \rightarrow \text{AV Node} \rightarrow \text{Bundle of His} \rightarrow \text{Bundle Branches} \rightarrow \text{Purkinje Fibers} \rightarrow \text{Ventricular Myocardium}$

Cardiac Cycle, Hemodynamics, and Heart Sounds

• Overview and Temporal Metrics:

  • A cardiac cycle comprises the coordinated electrical and mechanical events during a single heartbeat.

  • At an average resting rate of $72\,\text{beats/min}$, total duration of one cycle is calculated as:         $\text{Cycle Duration} = \frac{60\,\text{s}}{72} \approx 0.8\,\text{seconds}$

• Chronological Phases of the Cardiac Cycle:

  • Phase 1: Joint Diastole ($0.4\,\text{s}$):

    • All four chambers are relaxed.

    • Atrioventricular (tricuspid and bicuspid) valves are open; semilunar valves are closed.

    • Venous blood flows continuously through the atria directly into the ventricles, passively accounting for approximately $70\%$ of ventricular filling.

  • Phase 2: Atrial Systole ($0.1\,\text{s}$):

    • The SA node discharges, initiating bilateral atrial myocardial depolarization and contraction.

    • Atrial systole drives the remaining $30\%$ of blood into the ventricles, completing end-diastolic filling.

    • (Note: Total atrial diastole lasts for the remaining $0.7\,\text{s}$ of the cycle).

  • Phase 3: Ventricular Systole ($0.3\,\text{s}$):

    • Action potentials traverse the AV node, His-Purkinje network, and ventricular myocardium, initiating ventricular contraction.

    • Intraventricular pressure rises sharply, immediately forcing closure of the tricuspid and bicuspid valves.

    • Further pressure elevation exceeds arterial pressures, opening the semilunar valves and ejecting blood into the pulmonary artery and aorta.

  • Phase 4: Ventricular Diastole ($0.5\,\text{s}$):

    • Ventricles repolarize and relax. Intraventricular pressure drops below arterial pressures, causing blood to recoil and snap the semilunar valves shut.

    • Ventricular pressure falls below atrial pressure, allowing AV valves to reopen and returning the system to joint diastole.

• Hemodynamic Parameters:

  • Stroke Volume ($SV$): The volume of blood pumped out by each ventricle per contraction, measuring approximately $70\,\text{ml}$.

  • Cardiac Output ($CO$): Total volume of blood ejected by each ventricle per minute:         $CO = SV \times \text{Heart Rate} = 70\,\text{ml/beat} \times 72\,\text{beats/min} = 5040\,\text{ml/min} \approx 5.0\,\text{L/min}$

  • Conditioned athletes display increased resting stroke volumes and greater maximum cardiac outputs due to physiological hypertrophy of the myocardium.

• Auscultation of Heart Sounds:

  • First Heart Sound ("LUB"):

    • Caused by the closure of the atrioventricular (tricuspid and bicuspid) valves at the onset of ventricular systole.

    • Low-pitched, booming, and of longer duration.

  • Second Heart Sound ("DUB"):

    • Caused by the closure of the semilunar valves at the onset of ventricular diastole.

    • Sharper, higher-pitched, and of shorter duration.

Electrocardiography (ECG) and Clinical Wave Analysis

• Terminology:

  • Electrocardiograph: The clinical recording machine/apparatus.

  • Electrocardiogram (ECG): The graphic tracing of myocardial electrical activity over time.

• Lead Configurations:

  • Standard 3-Lead ECG: Utilizes three electrodes attached to the left ankle and both wrists.

  • Comprehensive 12-Lead Diagnostic ECG: Employs nine to ten physical electrodes (two on wrists, two on ankles, and six precordial chest leads) to map vectors across three dimensions.

• Electrical States:

  • Polarized: Resting myocardial state (extracellular positive, intracellular negative).

  • Depolarized: Contractile excitation state following cation influx (extracellular negative, intracellular positive).

  • Repolarized: Restoration of the resting electrical gradient across the membrane.

• ECG Waveforms and Clinical Correlates:

  • P Wave: Represents atrial depolarization, preceding mechanical atrial contraction.

  • QRS Complex: Represents rapid ventricular depolarization, initiating ventricular systole. (Atrial repolarization occurs simultaneously but is masked by ventricular electrical potentials).

  • T Wave: Represents ventricular repolarization, signaling ventricular relaxation.

  • Clinical Determination of Heart Rate: Counting the number of QRS complexes within a given time interval yields the ventricular contraction rate.

Neural and Hormonal Regulation of Cardiac Activity

• Autonomic Neural Control:

  • The cardiovascular regulatory center resides within the medulla oblongata.

  • Sympathetic Nervous System (SNS):

    • Mediates the "fight-or-flight" stress response.

    • Postganglionic sympathetic fibers release noradrenaline, stimulating adrenergic receptors on the SA node and myocardium.

    • Increases heart rate, increases conduction velocity through the AV node, enhances ventricular contractile force, and elevates total cardiac output.

  • Parasympathetic Nervous System (PNS):

    • Mediates resting, basal homeostatic functions.

    • Vagal efferent fibers release acetylcholine onto the SA and AV nodes.

    • Decreases firing rate, slows conduction velocity, and reduces cardiac output.

• Endocrine and Hormonal Control:

  • Adrenal Medullary Hormones: Circulating adrenaline (epinephrine) and noradrenaline (norepinephrine) increase heart rate and contractile strength.

  • Thyroid Hormones: Circulating thyroxine ($T_4$) accelerates basal metabolic activity, increasing sensitivity to catecholamines and elevating cardiac output.

Clinical Disorders and Pathophysiology of the Circulatory System

• Hypertension (High Blood Pressure):

  • Diagnostic Criteria: Sustained resting blood pressure exceeding $140/90\,\text{mmHg}$ (where normal resting arterial pressure is defined as $120/80\,\text{mmHg}$; $120\,\text{mmHg}$ systolic and $80\,\text{mmHg}$ diastolic).

  • Pathology: Chronically elevated arterial wall tension damages the vascular endothelium, predisposing to aneurysmal rupture, cerebral vascular accidents (strokes/hemorrhages), myocardial infarction, and renal parenchymal failure.

• Coronary Artery Disease (CAD) / Atherosclerosis:

  • Characterized by luminal narrowing of the coronary arteries caused by subendothelial accumulation of lipids, cholesterol plaques, calcium deposits, and fibrous connective tissue.

  • Significantly diminishes blood and oxygen delivery to the downstream myocardium.

• Angina Pectoris:

  • Presents as acute, crushing retrosternal chest pain caused by transient myocardial ischemia when cardiac oxygen demand exceeds coronary arterial supply.

  • Most prevalent among middle-aged and elderly individuals, though inducible across age groups by physical exertion or emotional stress.

• Differential Diagnosis of Critical Cardiac Conditions:

  • Heart Failure (Congestive Heart Failure):

    • Inability of the heart to pump blood effectively to meet metabolic demands.

    • Often presents as pulmonary congestion (fluid accumulation in alveolar spaces, producing severe dyspnea).

  • Cardiac Arrest:

    • Complete cessation of rhythmic cardiac contractions (the heart abruptly stops beating entirely due to loss of functional electrical conduction).

  • Heart Attack (Myocardial Infarction):

    • Ischemic necrosis and cellular death of myocardial tissue resulting from acute, sustained occlusion of coronary arterial blood flow.