Body Fluids and Circulation

Part 1: The Three-Stage Blood Coagulation Cascade (3 Marks):
Blood clotting (hemostasis) is an amplified biochemical cascade where inactive plasma factors are sequentially activated to form an insoluble fibrin mesh:

1. Stage 1: Assembly of Thrombokinase (Prothrombinase Complex):
   Injury triggers two convergent pathways:
   • Extrinsic Pathway: Damaged tissues release Tissue Factor (Thromboplastin / Factor III).
   • Intrinsic Pathway: Platelets adhere to exposed subendothelial collagen and release Platelet Factor 3 (PF3), activating Factor XII.
   • In the presence of $\text{Ca}^{2+}$ (Factor IV), active Factor X combines with Factor V and phospholipids to form the active enzyme complex Thrombokinase (Prothrombinase).
   
2. Stage 2: Conversion of Prothrombin to Thrombin:
   Thrombokinase, in the presence of $\text{Ca}^{2+}$, cleaves the inactive plasma globulin Prothrombin (Factor II) into the active proteolytic enzyme Thrombin:
   

$$\text{Prothrombin} \xrightarrow[\text{Ca}^{2+}]{\text{Thrombokinase}} \text{Thrombin}$$

$$\text{Fibrinogen (Soluble)} \xrightarrow{\text{Thrombin}} \text{Fibrin Monomers} \xrightarrow[\text{Factor XIII}]{\text{Ca}^{2+}} \text{Fibrin Mesh (Insoluble)}$$

Part 2: Specific Roles of Calcium Ions and Vitamin K (2 Marks):
• Role of Calcium ($\text{Ca}^{2+}$, Factor IV): Acts as an indispensable inorganic cofactor at virtually every stage of coagulation. It binds negative $\gamma$-carboxyglutamate residues on prothrombin and Factor X, anchoring them to platelet phospholipid membranes to assemble the prothrombinase complex. It is also required for Factor XIII cross-linking of fibrin threads.
• Role of Vitamin K: A fat-soluble vitamin acting as an obligatory coenzyme in hepatocytes for the post-translational $\gamma$-carboxylation of glutamic acid residues on clotting factors II (Prothrombin), VII, IX, and X. Without Vitamin K, these factors cannot bind $\text{Ca}^{2+}$ and remain functionally inert, causing severe bleeding.

Part 1: Internal Anatomy of the Heart (3 Marks):
The human heart is a 4-chambered muscular organ partitioned into right (pulmonary) and left (systemic) halves by septa:

1. Chambers:
   • Two Atria (Right & Left): Superior, thin-walled receiving chambers. The interatrial septum bears the fossa ovalis. RA receives deoxygenated blood from the venae cavae; LA receives oxygenated blood from 4 pulmonary veins.
   • Two Ventricles (Right & Left): Inferior, thick-walled pumping chambers with muscular ridges (trabeculae carneae) and papillary muscles. The left ventricular wall is 3 times thicker than the right ventricle wall because it must generate high pressure (120 mmHg) to pump blood through the high-resistance systemic circuit.
   
2. Valvular Apparatus:
   • Tricuspid Valve: Three fibrous cusps guarding the right AV orifice.
   • Bicuspid (Mitral) Valve: Two cusps guarding the left AV orifice.
   • Chordae Tendineae: Inelastic fibrous cords anchoring AV cusp edges to ventricular papillary muscles, preventing cusp eversion into atria during systole.
   • Semilunar Valves: Three crescent pockets each at the base of the Pulmonary Trunk and Aorta, preventing diastolic regurgitation.
   
   Part 2: Conducting System & Why SAN is the Pacemaker (2 Marks):
   • Nodal Conduction Pathway: Sinoatrial Node (SAN) in upper right atrium $\to$ Atrioventricular Node (AVN) with 0.1-sec delay $\to$ Bundle of His $\to$ Right & Left bundle branches $\to$ Purkinje fibers spreading through ventricular walls.
   • Why SAN is the Pacemaker: Cardiac nodal cells are autorhythmic due to spontaneous unstable resting membrane potentials ("funny" sodium channels). The SAN generates action potentials at the highest intrinsic frequency (70–75 depolarizations per minute) compared to the AVN (40–60 bpm) or Purkinje fibers (20–40 bpm). Because the SAN depolarizes fastest, its electrical wave captures and overrides all other potential pacemakers, setting the master rhythm for the entire heart.

Part 1: Events of the Cardiac Cycle (Duration: 0.8 Seconds) (2.5 Marks):
At a resting rate of 72 beats/min, each cardiac cycle occupies 0.8 seconds:

1. Joint Diastole (0.4 sec): All four chambers are relaxed. AV valves are open; semilunar valves are closed. Blood flows passively from atria into ventricles, filling ~70% of ventricular volume.
   
2. Atrial Systole (0.1 sec): SAN fires; both atria contract simultaneously, forcing remaining ~30% of blood into ventricles. Total end-diastolic volume ($\text{EDV}$) reaches $120\text{ mL}$.
   
3. Ventricular Systole (0.3 sec): AV node impulse spreads through Purkinje fibers. Ventricles contract:
   • Isovolumetric Contraction: Pressure rises sharply, snapping shut AV valves (1st heart sound, LUB). All valves closed.
   • Ventricular Ejection: Pressure exceeds aortic (80 mmHg) and pulmonary (15 mmHg) pressures $\to$ semilunar valves open $\to$ each ventricle pumps out Stroke Volume ($\text{SV} \approx 70\text{ mL}$), leaving End-Systolic Volume ($\text{ESV} \approx 50\text{ mL}$).
   
4. Ventricular Diastole: Ventricles relax; pressure drops below arterial pressure, snapping shut semilunar valves (2nd heart sound, DUB).
   
   Part 2: Cardiac Output & Heart Sounds (2.5 Marks):
   • Cardiac Output (CO): The volume of blood pumped per ventricle per minute:
   

$$\text{CO} = \text{Stroke Volume (SV)} \times \text{Heart Rate (HR)} = 70\text{ mL/beat} \times 72\text{ beats/min} = \mathbf{5,040\text{ mL/min}} \approx \mathbf{5.0\text{ Liters/min}}$$

Part 1: Standard ECG Waves and Electrical Correlates (2.5 Marks):
An ECG records the composite electrical field vector of the heart from body surface leads:

1. P-Wave: Small upward deflection representing atrial depolarization spreading from the SA node across both atria, initiating atrial contraction (systole). Duration ~0.08–0.10 sec.
   
2. QRS Complex: Large triphasic deflection representing rapid ventricular depolarization. Ventricular systole begins immediately after the Q wave peak. The number of QRS complexes per unit time gives the patient's heart rate. (Atrial repolarization occurs simultaneously but is hidden by the QRS wave).
   
3. T-Wave: Broad dome-shaped upward wave representing ventricular repolarization, marking the return of ventricular muscle from excited to resting state. The end of the T-wave marks the end of ventricular systole.
   • Clinical Benchmark: Elevated ST-segment indicates acute Myocardial Infarction; depressed ST or inverted T wave indicates myocardial ischemia.
   
   Part 2: Common Cardiovascular Disorders (2.5 Marks):
   
4. Hypertension (High Blood Pressure): Persistent resting blood pressure equal to or exceeding $140/90\text{ mmHg}$ (normal is $120/80\text{ mmHg}$, where 120 is systolic, 80 is diastolic). Puts excessive workload on the heart, leading to left ventricular hypertrophy, cerebral stroke, and kidney failure.
   
5. Coronary Artery Disease (CAD) / Atherosclerosis: Characterized by the deposition of cholesterol, lipids, calcium, and fibrous tissue in the tunica intima of coronary arteries, creating atheromatous plaques that progressively narrow the lumen and impair myocardial perfusion.
   
6. Angina Pectoris: Sudden, severe crushing substernal chest pain radiating to the left shoulder and arm. Caused by acute transient myocardial ischemia (insufficient oxygen delivery) during exertion without actual myocardial cell necrosis. Relieved by rest and nitroglycerin.