1. Overview of the Cardiovascular System

The cardiovascular system (also termed the circulatory system or blood vascular system) is an integrated organ system designed for mass transport in multicellular organisms. In order to sustain cellular homeostasis, every cell requires continuous delivery of nutrients (O₂, glucose, amino acids, fatty acids, vitamins) and removal of metabolic waste products (CO₂, urea, lactic acid, creatinine).

Figure 1.1: Structural Components of the Cardiovascular System. The system is built from three interdependent components: blood (the fluid transport medium), the heart (the muscular pump generating pressure), and blood vessels (the network distributing blood to and from every tissue).

1.1 Primary Functions

The cardiovascular system carries out three broad categories of function: transportation, regulation, and protection.

Figure 1.2: Primary Functions of the Cardiovascular System. The system's roles fall into three categories: transportation of gases, nutrients, hormones, wastes, and heat; regulation of pH, body temperature, and fluid volume; and protection through hemostasis and immune defense.

1. Transportation

• Transport
  Gases

  Oxygen from lungs to systemic tissues; carbon dioxide from tissues to lungs.

• Transport
  Nutrients

  Absorbed nutrients from the gastrointestinal tract to body tissues.

• Transport
  Hormones

  Endocrine signaling molecules from glands to target organs.

• Transport
  Metabolic Wastes

  Nitrogenous wastes and metabolic byproducts to kidneys, liver, and lungs for excretion.

• Transport
  Heat

  Thermoregulation through peripheral vasodilation and vasoconstriction.

2. Regulation

• Regulation
  pH

  Buffered by plasma proteins and bicarbonate (HCO₃⁻) ions (normal pH range: 7.35–7.45).

• Regulation
  Body Temperature

  Absorbing and redistributing heat throughout the body.

• Regulation
  Fluid Volume

  Maintaining water-electrolyte equilibrium between intravascular and interstitial compartments.

3. Protection

• Protection
  Hemostasis

  Clotting mechanisms involving platelets and plasma coagulation factors prevent blood loss upon vascular injury.

• Protection
  Immune Defense

  White blood cells (leukocytes), antibodies, complement proteins, and phagocytic activity protect against pathogens.

Summary Table: Functional Categories

2. Composition and Physiology of Blood

Blood is a specialized liquid connective tissue consisting of cellular elements (formed elements) suspended in a fluid extracellular matrix (blood plasma).

• Property
  Physical Characteristics

  Slightly alkaline (pH 7.30–7.40), viscous fluid.

• Property
  Volume

  Comprises 8% of total body mass. Average volume is 5.0–6.0 liters in adult males and 4.0–5.0 liters in adult females.

• Property
  Proportion

  Approximately 55% blood plasma and 45% formed elements.

Figure 2.1: Whole Blood Composition. Centrifuged whole blood separates into blood plasma (55%, the fluid extracellular matrix) and formed elements (45%, the cellular components: erythrocytes, leukocytes, and platelets).

2.1 Blood Plasma

When cellular components are separated by centrifugation or allowed to settle, the straw-colored supernatant fluid is blood plasma.

• Plasma
  Water (90–92%)

  Functions as a solvent and heat-absorbing transport medium.

• Plasma
  Plasma Proteins (~7% by weight)

  Synthesized mainly by hepatocytes. Albumin (~54%): smallest and most abundant plasma protein; maintains colloid osmotic (oncotic) pressure to retain water in blood vessels and transports hydrophobic molecules (fatty acids, steroid hormones). Globulins (~38%): alpha and beta globulins transport iron, lipids, and fat-soluble vitamins; gamma globulins (immunoglobulins) are antibodies produced by plasma cells derived from B lymphocytes. Fibrinogen (~7%): soluble glycoprotein key to blood coagulation, converted by thrombin into insoluble fibrin threads during clot formation.

• Plasma
  Solutes (<1%)

  Ions, glucose, lipids, metabolic wastes, and dissolved gases.

• Plasma
  Serum

  Fluid remaining after blood clots (serum = plasma minus fibrinogen and clotting factors).

2.2 Formed Elements of Blood

Figure 2.2: Formed Elements of Blood. Formed elements comprise erythrocytes, leukocytes, and thrombocytes (platelets). Leukocytes are further divided into granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes).

1. Erythrocytes (Red Blood Cells / RBCs)

• RBC
  Count

  Males: ~5.4 million/µL; Females: ~4.8 million/µL.

• RBC
  Morphology

  Biconcave discs (7.8 µm diameter, 2.0 µm thick edge). Lacks a nucleus, organelles, and mitochondria; packed almost entirely with hemoglobin (~33% of cell weight).

• RBC
  Functional Advantages of Biconcave Shape

  High surface area-to-volume ratio facilitating rapid gas exchange across the plasma membrane; flexible structure enabling RBCs to deform and squeeze through narrow capillaries (3–4 µm diameter); ability to swell in hypotonic solutions before lysis.

Figure 2.3: Erythrocyte Morphology. The biconcave disc shape (7.8 µm diameter, 2.0 µm edge thickness) maximizes surface area for gas exchange and gives the mature RBC the flexibility to pass through narrow capillaries; the cell contains no nucleus or organelles.

Hemoglobin & Transport Physiology

• Hemoglobin
  Concentration

  Normal concentration: 12–16 g/100 mL of blood. Each RBC contains ~280 million hemoglobin molecules.

• Hemoglobin
  Structure

  Tetramer consisting of 4 globin polypeptide chains (2 alpha and 2 beta) and 4 iron-containing heme groups. Each iron (Fe²⁺) atom reversibly binds 1 oxygen molecule (O₂), allowing each hemoglobin tetramer to transport up to 4 O₂ molecules.

Lifespan and Hemopoiesis

• RBC Lifecycle
  Hemopoiesis (Hematopoiesis)

  Pluripotent stem cells in red bone marrow differentiate into myeloid and lymphoid stem lines under the control of erythropoietin (EPO), produced by the kidneys.

• RBC Lifecycle
  Average Lifespan

  120 days.

• RBC Lifecycle
  Recycling

  Ruptured or senescent RBCs are phagocytosed by fixed macrophages in the spleen, liver, and bone marrow. Globin is broken down into amino acids; iron (Fe³⁺) is salvaged via transferrin and stored as ferritin; heme is converted to biliverdin, then bilirubin, and excreted in bile.

Table 2.1: Comparison of Respiratory Pigments. Hemoglobin is the iron-based respiratory pigment characteristic of annelids and vertebrates; other taxa use different metal-based pigments such as copper-based hemocyanin.

2. Leukocytes (White Blood Cells / WBCs)

• WBC
  Count

  5,000–10,000/µL of blood. Nucleated, lack hemoglobin.

• WBC
  Leukocytosis

  Normal protective increase (>10,000/µL) in response to infection, stress, or exercise.

• WBC
  Leukopenia

  Pathological decrease (<5,000/µL) impairing immunity.

Table 2.2: Types of Leukocytes. Granulocytes (neutrophils, eosinophils, basophils) contain visible cytoplasmic granules; agranulocytes (monocytes, lymphocytes) lack them. Neutrophils and lymphocytes are the most abundant leukocyte types.

Figure 2.4: Morphology of the Five Leukocyte Types. Neutrophils show a multilobed nucleus; eosinophils a bilobed nucleus with large red-orange granules; basophils a bilobed/obscured nucleus with dark granules; monocytes a kidney-shaped nucleus; lymphocytes a large, round nucleus with scanty cytoplasm.

3. Platelets (Thrombocytes)

• Platelet
  Count

  150,000–400,000/µL.

• Platelet
  Origin

  Anucleate cellular fragments pinched off from large megakaryocytes in red bone marrow under the influence of thrombopoietin (produced by liver/kidneys).

• Platelet
  Lifespan

  5–9 days; removed by macrophages in the spleen and liver.

• Platelet
  Function

  Form platelet plugs at vascular injury sites, secrete clotting factors, and release vasoconstrictors (serotonin, TXA₂).

3. Hemostasis and Blood Clotting Mechanics

Hemostasis is the sequence of responses that stops bleeding when blood vessels are injured. It involves three primary mechanisms: vascular spasm, platelet plug formation, and blood coagulation.

3.1 The Three Mechanisms of Hemostasis

• Mechanism 1
  Vascular Spasm

  Immediate reflex constriction of smooth muscle in vascular walls.

• Mechanism 2
  Platelet Plug Formation

  Platelet adhesion (binding to exposed collagen via von Willebrand factor), platelet release reaction (ADP, Thromboxane A₂, Serotonin), and platelet aggregation.

• Mechanism 3
  Blood Coagulation (Clotting)

  Transformation of blood plasma from a liquid into a fibrin gel network.

Figure 3.1: Sequence of the Hemostatic Response. Following vessel injury, vascular spasm, platelet plug formation, and blood coagulation act in sequence — and with overlap — to arrest bleeding and stabilize the site with a fibrin clot.

3.2 The Coagulation Cascade

Blood coagulation proceeds through two converging pathways — the extrinsic and intrinsic pathways — that both feed into a final common pathway culminating in a cross-linked fibrin clot.

Figure 3.2: The Blood Coagulation Cascade. The extrinsic pathway is triggered by tissue factor released from damaged tissue; the intrinsic pathway is triggered by contact activation within the blood itself. Both converge on the final common pathway: Factor X activation, assembly of the prothrombinase complex, conversion of prothrombin to thrombin, conversion of fibrinogen to soluble fibrin, and Factor XIIIa cross-linking to form a stable fibrin clot.

Clotting Factor Reference Table

Table 3.1: Blood Clotting Factors. Factor VI is no longer considered a distinct entity (it corresponds to active Factor V).

3.3 Anticoagulants

In Vivo Anticoagulants

• In Vivo
  Heparin

  Produced by mast cells and basophils; enhances antithrombin activity to block thrombin.

• In Vivo
  Coumarins (Warfarin / Coumadin)

  Vitamin K antagonists that inhibit hepatic synthesis of factors II, VII, IX, and X by blocking gamma-carboxylation of glutamate residues.

In Vitro Anticoagulants

• In Vitro
  Sodium Citrate / EDTA

  Chelate free Ca²⁺ ions, removing Factor IV from the clotting cascade.

3.4 Anemia Types

• Anemia
  Iron Deficiency Anemia

  Defective hemoglobin synthesis due to inadequate iron absorption or chronic blood loss.

• Anemia
  Megaloblastic Anemia

  Deficiency of Vitamin B₁₂ or folic acid causing impaired DNA synthesis and red marrow production of large, abnormal RBCs (megaloblasts).

• Anemia
  Pernicious Anemia

  Lack of Intrinsic Factor (produced by gastric parietal cells), preventing intestinal absorption of Vitamin B₁₂.

• Anemia
  Hemorrhagic Anemia

  Excessive loss of RBCs through acute/chronic bleeding.

• Anemia
  Hemolytic Anemia

  Premature rupture of RBC plasma membranes (malaria, sickle cell disease, transfusion mismatch).

• Anemia
  Thalassemias

  Hereditary defect in globin chain synthesis (alpha-thalassemia or beta-thalassemia), causing unstable tetramers and premature destruction.

• Anemia
  Aplastic Anemia

  Destruction of red bone marrow by radiation, toxins, or drugs.

4. Anatomy and Physiology of the Heart

The heart is a hollow, four-chambered muscular pump located in the thoracic cavity within the mediastinum, resting upon the diaphragm and tilted slightly to the left.

4.1 Pericardium and Heart Wall Layers

Figure 4.1: Pericardium and Heart Wall Layers. From outside in: the fibrous pericardium and parietal serous pericardium enclose the fluid-filled pericardial cavity; the visceral serous pericardium (epicardium) forms the outermost layer of the heart wall proper, followed by the thick myocardium (~95% of wall mass) and the innermost endocardium lining the chamber lumen.

• Outer Layer
  Fibrous Pericardium

  Tough connective tissue forming the outermost protective sac.

• Outer Layer
  Parietal & Visceral Serous Pericardium

  Parietal serous pericardium lines the fibrous sac; visceral serous pericardium (epicardium) covers the heart surface itself; the pericardial cavity between them is filled with lubricating serous fluid that reduces friction during heartbeats.

• Middle Layer
  Myocardium

  Cardiac muscle tissue, comprising approximately 95% of the heart wall; responsible for the contractile force of the pump.

• Inner Layer
  Endocardium

  Endothelium overlying a layer of connective tissue, lining the interior chambers and valves.

4.2 Heart Chambers and Internal Anatomy

The heart contains four functional chambers: two superior atria (receiving chambers) and two inferior ventricles (pumping chambers).

Figure 4.2: Blood Flow Through the Heart Chambers. Deoxygenated blood (blue) enters the right atrium from the venae cavae and coronary sinus, passes through the tricuspid valve to the right ventricle, and is pumped through the pulmonary trunk to the lungs. Oxygenated blood (red) returns via the pulmonary veins to the left atrium, passes through the bicuspid/mitral valve to the left ventricle, and is pumped through the aortic valve into the systemic circulation.

• Chamber
  Right Atrium

  Receives deoxygenated blood from the superior vena cava, inferior vena cava, and coronary sinus. Features the fossa ovalis (remnant of the fetal foramen ovale) on the interatrial septum.

• Chamber
  Right Ventricle

  Pumps deoxygenated blood through the pulmonary valve into the pulmonary trunk. Features trabeculae carneae and papillary muscles attached to the tricuspid valve cusps via chordae tendineae.

• Chamber
  Left Atrium

  Receives oxygenated blood from four pulmonary veins. Passes blood to the left ventricle through the bicuspid (mitral) valve.

• Chamber
  Left Ventricle

  Thickest chamber wall (3× thicker than the RV) to generate sufficient pressure to pump blood through the aortic valve into the systemic circulation.

4.3 Heart Valves and Unidirectional Flow

Atrioventricular (AV) Valves

• AV Valve
  Tricuspid Valve

  Located between the right atrium and right ventricle (3 cusps).

• AV Valve
  Bicuspid (Mitral) Valve

  Located between the left atrium and left ventricle (2 cusps).

• AV Valve
  Anchoring

  Chordae tendineae (80% collagen, 20% elastin/endothelium) connect valve cusps to papillary muscles. Contraction of papillary muscles during ventricular systole prevents eversion (prolapse) of AV valves into the atria under high pressures.

Semilunar (SL) Valves

• SL Valve
  Pulmonary Valve

  Located at the exit of the right ventricle into the pulmonary trunk.

• SL Valve
  Aortic Valve

  Located at the exit of the left ventricle into the ascending aorta.

• SL Valve
  Structure

  Three crescent/moon-shaped cusps that open during ventricular contraction and snap shut during ventricular relaxation due to back-pressure of blood in the arteries.

Figure 4.3: AV Valve vs. Semilunar Valve. AV valves close during ventricular systole and are anchored against eversion by chordae tendineae and papillary muscles. Semilunar valves close during ventricular diastole, when back-pressure of blood in the arteries fills their crescent-shaped cusps and forces them shut — no chordae or papillary anchoring is needed.

Table 4.1: Summary of Heart Valves. AV valves separate atria from ventricles; semilunar valves guard the exits of the ventricles into the great arteries.

5. Cardiac Conduction System and Electrocardiography

The heart possesses intrinsic autorhythmicity generated by specialized, non-contractile cardiac muscle fibers called autorhythmic fibers (pacemaker cells).

5.1 Conduction Pathway

Figure 5.1: Cardiac Conduction Pathway. The SA node initiates each heartbeat and the impulse spreads via internodal pathways to the AV node, which delays transmission before passing it through the AV bundle (Bundle of His), down the right and left bundle branches, and out through the Purkinje fibers to trigger coordinated ventricular contraction.

• Pacemaker
  SA Node (Sinoatrial Node)

  Located in the right atrial wall inferior to the opening of the superior vena cava. Spontaneously depolarizes at 70–75 beats/min (inherent rate: ~100 bpm, reduced by vagal tone).

• Delay Node
  AV Node (Atrioventricular Node)

  Located in the interatrial septum. Slows impulse propagation (~0.1 sec delay) to allow complete atrial emptying prior to ventricular systole.

• Fast Conduction
  Bundle of His & Purkinje Fibers

  Rapidly conduct electrical signals down the interventricular septum to the apex and up through the ventricular walls.

5.2 Electrocardiogram (ECG / EKG)

An electrocardiogram is a composite record of electrical action potentials produced by all heart muscle fibers during each cardiac cycle.

Figure 5.2: The ECG Waveform. The P wave reflects atrial depolarization, the QRS complex reflects ventricular depolarization, and the T wave reflects ventricular repolarization. The P-R interval measures AV node conduction time, the S-T segment reflects the plateau of ventricular depolarization, and the Q-T interval spans the total duration of ventricular electrical activation.

Table 5.1: ECG Waveform Breakdown. Each wave and interval on the ECG corresponds to a specific electrical event in the cardiac cycle and the mechanical event that follows it.

Diagnostic ECG Abnormalities

• Abnormality
  Enlarged P Wave

  Atrial enlargement / hypertrophy.

• Abnormality
  Enlarged Q Wave

  Myocardial infarction (tissue necrosis).

• Abnormality
  Enlarged R Wave

  Ventricular hypertrophy.

• Abnormality
  Flatter T Wave

  Insufficient oxygen to myocardium (ischemia / coronary artery disease).

• Abnormality
  Elevated T Wave

  Hyperkalemia (elevated plasma K⁺ levels).

6. The Cardiac Cycle and Cardiac Output

The cardiac cycle includes all electrical and mechanical events associated with a single heartbeat (0.8 seconds duration at a normal heart rate of 75 bpm).

Figure 6.1: The Cardiac Cycle Timeline. A full cardiac cycle at 75 bpm lasts 0.8 seconds: atrial systole (0.1 sec) tops off ventricular filling with a final 25 mL, ventricular systole (0.3 sec) isovolumetrically contracts and then ejects 70 mL, and the relaxation period (0.4 sec) isovolumetrically relaxes before ventricular filling resumes.

6.1 Phases of the Cardiac Cycle

Figure 6.2: Pressure Changes During the Cardiac Cycle. Left ventricular pressure rises sharply during isovolumetric contraction, exceeds aortic pressure to open the semilunar valves during ejection, then crashes during isovolumetric relaxation before rising gently as the ventricle fills. Aortic pressure stays elevated throughout diastole due to elastic recoil of the arterial walls. S1 marks AV valve closure at the start of ventricular systole; S2 marks semilunar valve closure at its end.

• Phase 1
  Atrial Systole (0.1 sec)

  Atria contract, pushing a final 25 mL of blood into the ventricles. End-Diastolic Volume (EDV) — the total volume of blood in each ventricle at the end of the relaxation phase — reaches ~130 mL.

• Phase 2
  Ventricular Systole (0.3 sec)

  Isovolumetric Contraction (0.05 sec): ventricles begin contracting; intraventricular pressure spikes, snapping the AV valves shut (First Heart Sound: S1 / "lub"). All four valves are closed and volume remains constant. Ventricular Ejection: ventricular pressure exceeds aortic/pulmonary pressure, forcing the SL valves open; the ventricles eject ~70 mL of blood (Stroke Volume, SV). End-Systolic Volume (ESV), the volume remaining in each ventricle, falls to ~60 mL.

• Phase 3
  Relaxation Period (0.4 sec)

  Ventricles relax; ventricular pressure drops below arterial pressure, causing blood backflow that closes the SL valves (Second Heart Sound: S2 / "dup"). Isovolumetric Relaxation is a brief period where all four valves are closed. Ventricular pressure then drops below atrial pressure, opening the AV valves and initiating rapid passive ventricular filling.

6.2 Cardiac Output Dynamics

Cardiac Output (CO) is the volume of blood pumped by each ventricle into the aorta or pulmonary trunk per minute.

Figure 6.3: Cardiac Output Calculation. Cardiac output equals stroke volume multiplied by heart rate. With a stroke volume of 70 mL/beat (EDV 130 mL minus ESV 60 mL) and a heart rate of 75 beats/min, resting cardiac output is approximately 5.25 L/min.

• Standard Value
  Stroke Volume (SV)

  SV = EDV − ESV = 130 mL − 60 mL = 70 mL/beat.

• Standard Value
  Heart Rate (HR)

  75 beats/min.

• Standard Value
  Cardiac Output (CO)

  CO = 70 mL/beat × 75 beats/min = 5250 mL/min ≈ 5.25 L/min.

7. Vascular Physiology and Hemodynamics

Blood vessels form a closed circuit of conduits delivering blood from the heart to tissues and back.

Figure 7.1: The Vascular Circuit. Blood leaves the heart through elastic arteries, which branch into muscular arteries, then arterioles, before reaching the capillary beds where exchange occurs. Blood then returns through venules and large veins back to the heart, completing the closed circuit.

7.1 Histological Structure of Blood Vessel Walls

Figure 7.2: Cross-Sectional Structure of a Blood Vessel. From outside in: the tunica externa provides structural support, the tunica media contains smooth muscle and elastic fibers that regulate vessel diameter, and the tunica interna's endothelium lines the lumen through which blood flows.

Table 7.1: Structural Comparison of Blood Vessels. Wall thickness and composition shift along the vascular tree: arteries and veins have the most developed walls (for pressure resistance and structural support respectively), while capillaries retain only a single endothelial layer to permit exchange.

7.2 Hemodynamics: Flow, Pressure, and Resistance

Blood Flow (F) is the volume of blood flowing through any tissue per unit time (mL/min).

Figure 7.3: Blood Flow and Vascular Resistance. Blood flow is directly proportional to the pressure gradient between arterial and venous ends and inversely proportional to resistance. Resistance is proportional to blood viscosity and vessel length, but inversely proportional to the fourth power of vessel radius — making small radius changes the dominant lever for regulating resistance.

• Resistance Factor
  Blood Viscosity (η)

  Increased in polycythemia/dehydration.

• Resistance Factor
  Total Vessel Length (L)

  Increases with body mass/adipose tissue.

• Resistance Factor
  Radius of Vessel Lumen (r)

  Resistance varies inversely with the fourth power of vessel radius (r⁴). Small decreases in arteriolar radius double or quadruple resistance.

Figure 7.4: Pulse Pressure Calculation. Pulse pressure is the difference between systolic and diastolic blood pressure, reflecting the force the heart generates with each contraction.

7.3 Regulation of Blood Pressure

Figure 7.5: Regulation of Blood Pressure. The cardiovascular control center in the medulla oblongata regulates blood pressure through neural reflexes (baroreceptors and chemoreceptors) and hormonal mechanisms (vasoconstrictors and vasodilators).

Neural Mechanisms

• Neural
  Baroreceptor Reflexes

  Stretch-sensitive receptors located in the carotid sinus and aortic arch. Increased arterial stretch signals the CV center to increase parasympathetic (vagal) output and decrease sympathetic output, lowering heart rate and causing vasodilation.

• Neural
  Chemoreceptor Reflexes

  Receptors in the carotid and aortic bodies monitor O₂, CO₂, and H⁺ concentration. Hypoxia (↓pO₂), acidosis (↓pH), or hypercapnia (↑pCO₂) stimulate the CV center to increase sympathetic tone, causing vasoconstriction and raising blood pressure.

Hormonal Mechanisms

• Hormonal
  Renin-Angiotensin-Aldosterone System (RAAS)

  Low blood flow to the kidneys causes juxtaglomerular cells to release renin, converting angiotensinogen to Angiotensin I. Converting enzyme (ACE) forms Angiotensin II, a potent vasoconstrictor that also stimulates aldosterone secretion for Na⁺ and H₂O retention.

• Hormonal
  Epinephrine & Norepinephrine

  Sympathetic stimulation of the adrenal medulla increases heart rate, contractility, and systemic vasoconstriction.

• Hormonal
  Antidiuretic Hormone (ADH / Vasopressin)

  Released from the posterior pituitary; causes kidney water reabsorption and systemic arteriolar constriction.

• Hormonal
  Atrial Natriuretic Peptide (ANP)

  Released by atrial myocytes in response to high stretch; induces renal Na⁺ and water excretion and systemic vasodilation, lowering blood pressure.