🫀 Body Fluids, Circulation & Cardiac Electrophysiology

Circulation ensures continuous hydrostatic convection of oxygen, glucose, hormones, and electrolytes to peripheral tissues while transporting metabolic wastes to excretory organs.

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1. 🩸 Blood Composition & Coagulation Cascade

$$\text{mathbf{text{Blood (100text{\%})} = text{Plasma (55text{\%})} + text{Formed Elements / Cells (45text{\%})}}}$$

• Formed Elements:
  • Erythrocytes (RBCs): $5.0-5.5{\text{ million/mm}}^3$; biconcave, enucleated; lifespan $\text{a}\approx 120\text{ days}$.
  • Leukocytes (WBCs): $6000-8000/{\text{mm}}^3$. Neutrophils ($60-65\text{\%}$), Lymphocytes ($20-25\text{\%}$), Monocytes ($6-8\text{\%}$), Eosinophils ($2-3\text{\%}$), Basophils ($0.5-1\text{\%}$, secrete Histamine, Serotonin, Heparin).
  • Thrombocytes (Platelets): $1.5-3.5{\text{ lakhs/mm}}^3$; cell fragments of megakaryocytes in bone marrow.

1.1 The Enzyme Cascade of Blood Clotting

$$\text{Injured Tissue / Platelets}⟶\text{Thrombokinase (Thromboplastin)}\stackrel{{\mathbf{Ca}}^{\mathbf{2}\mathbf{+}}}{\to }\text{Inactivates Heparin}$$$$\text{Prothrombin (Inactive Plasma Protein)}\underset{\text{Thrombokinase}}{\overset{{\mathbf{Ca}}^{\mathbf{2}\mathbf{+}}}{\to }}\text{Thrombin (Active Protease)}$$$$\text{Fibrinogen (Soluble)}\underset{\text{Thrombin}}{\overset{{\mathbf{Ca}}^{\mathbf{2}\mathbf{+}}}{\to }}\text{Fibrin Monomers}⟶\text{Fibrin Insoluble Meshwork (Clot)}$$

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2. ⚡ The Cardiac Cycle & Nodal Conduction System

                        CARDIAC CONDUCTION SEQUENCE
     SA Node (Pacemaker: 70-75 bpm) ──► Inter-nodal Pathways ──► AV Node (40-60 bpm)
                                                                       │
                                                                       ▼ [0.1 sec AV Nodal Delay]
     Purkinje Fibers (Rapid conduction) ◄── Bundle Branches ◄── Bundle of His

2.1 Cardiac Cycle Timeline ($0.8\text{ seconds}$ at $75\text{ bpm}$)

$$\text{Cardiac Output }\mathbf{(}\mathbf{CO}\mathbf{)}\mathbf{=}\text{Stroke Volume }\mathbf{(}\mathbf{SV}:\mathbf{70}\text{mL}\mathbf{)}\times \text{Heart Rate }\mathbf{(}\mathbf{HR}:\mathbf{72}\text{bpm}\mathbf{)}\text{a}\approx \mathbf{5040}\text{mL/min}\text{a}\approx \mathbf{5}\text{L/min}$$

  0.0 s ──────── 0.1 s ──────────────── 0.4 s ──────────────────────── 0.8 s
  [Atrial Systole: 0.1s] [Ventricular Systole: 0.3s]   [Joint Diastole: 0.4s]
  (AV Valves Open)       (Isovolumetric Contraction ->  (AV Valves Open; Passive
                          Semilunar Valves Eject Blood)  Ventricular Filling: 70%)

• First Heart Sound (LUBB): Closure of Tricuspid & Bicuspid (Mitral) AV valves at the onset of ventricular systole. (Low-pitched, long duration).
• Second Heart Sound (DUPP): Closure of Semilunar valves at the onset of ventricular diastole. (High-pitched, sharp, short duration).

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3. 📈 Electrocardiogram (ECG) Deflection Waves

  Voltage (mV)
      ▲               R
      │               /\
      │              /  \
      │    P        /    \          T
      │   /─\      /      \        /─\
      └──/   \────Q        \──────/   \──► Time (s)
                            S

Wave / Interval	Electrophysiological Phenomenon	Clinical Correlation
P-Wave	Atrial Depolarization (SA node fires; leads to atrial contraction)	Absent in Atrial Fibrillation; Enlarged in Atrial Hypertrophy.
QRS Complex	Ventricular Depolarization (Spreads via Purkinje fibers; leads to ventricular contraction)	Wide QRS indicates Bundle Branch Block; Q-wave abnormalities indicate Myocardial Infarction.
T-Wave	Ventricular Repolarization (Ventricular relaxation from systole to diastole)	Inverted/Flat T-wave indicates myocardial ischemia; Peaked/Tall T-wave indicates Hyperkalemia.