Chapter 7 · Class 11-12 & NEET

Body Fluids & Circulation

46 topics in 16 parts

Key ideas

Conceptual Foundation & Core Theory

The Human Circulatory System employs closed double circulation comprising pulmonary and systemic circuits. The myogenic human heart contracts autonomously under impulses generated by the Sinoatrial (SA) node, commonly termed the natural pacemaker.

1. The Cardiac Cycle & Volume Dynamics

A single heartbeat corresponds to one cardiac cycle lasting approximately 0.8 seconds at a typical resting heart rate of 72 beats per minute:

Joint Diastole (0.4 s): All four chambers are in a relaxed state. The tricuspid and bicuspid (mitral) AV valves remain open, allowing blood from the pulmonary veins and vena cava to flow passively into the left and right ventricles respectively.
Atrial Systole (0.1 s): The SA node fires an action potential causing both atria to contract simultaneously, boosting ventricular blood filling by roughly 30%.
Ventricular Systole (0.3 s): The impulse conducts through the AV node, Bundle of His, and Purkinje fibers to depolarize the ventricular myocardium. Ventricular pressure rises sharply, immediately shutting the AV valves (producing the first heart sound 'LUB'). When intraventricular pressure exceeds arterial pressures, the semilunar valves open, ejecting ~70 mL of blood per ventricle (Stroke Volume).
Ventricular Diastole: As ventricles relax, falling intraventricular pressure causes arterial backflow that snaps shut the semilunar valves (producing the second heart sound 'DUB').

2. Standard Electrocardiogram (ECG) Waves

A clinical ECG graphically records electrical voltage changes across cardiac tissue over time using surface leads (Lead I, II, III):

P Wave: Represents electrical depolarization of both atria initiated by the SA node.
QRS Complex: Represents rapid depolarization of both ventricles immediately preceding ventricular systole. The number of QRS complexes per minute directly determines the heart rate.
T Wave: Represents ventricular repolarization (return of ventricles from excited to normal polarized resting state). The end of the T wave marks the end of mechanical ventricular systole.

Key Scientific Principles, Pathways & Diagnostic Points

// Hemodynamic Formulas & Quantitative Relationships
1. Stroke Volume (SV) = End-Diastolic Volume (EDV) - End-Systolic Volume (ESV)
   SV = 120 mL - 50 mL = ~70 mL per beat

2. Cardiac Output (CO) = Stroke Volume × Heart Rate (HR)
   CO = 70 mL × 72 beats/min = ~5,040 mL/min ≈ 5 Litres/min

3. Cardiac Cycle Duration = 60 seconds / Heart Rate
   For 72 bpm → Duration = 60 / 72 = 0.8 seconds:
   - Atrial Systole = 0.1 s | Atrial Diastole = 0.7 s
   - Ventricular Systole = 0.3 s | Ventricular Diastole = 0.5 s
   - Complete Joint Diastole = 0.4 s

Body Fluids & Circulation

ECG Waves & Associated Mechanical Cardiac Events

ECG Waveform	Electrical Event	Mechanical Consequence	Clinical Pathology Indication
P Wave	Atrial depolarization (SA node fire)	Atrial contraction (completes ventricular filling)	Enlarged P wave indicates atrial hypertrophy / mitral stenosis
Q-R-S Complex	Ventricular depolarization (Purkinje network)	Ventricular isovolumetric contraction & rapid ejection	Broadened QRS indicates bundle branch block / myocardial infarction
T Wave	Ventricular repolarization	Ventricular relaxation (closure of semilunar valves)	Flat or inverted T wave indicates cardiac ischemia / hypokalemia
P-R Interval	AV conduction transit time (~0.12–0.20 s)	Ensures atria empty before ventricles contract	Prolonged PR indicates first-degree AV heart block

NEET Problem: Cardiac Output Calculation under Exercise

NEET Problem: During vigorous athletic exercise, an athlete’s heart rate increases to 150 beats/min and end-diastolic volume increases to 140 mL while end-systolic volume drops to 40 mL. Calculate their cardiac output. Step-by-step Solution: 1. Calculate Stroke Volume: $SV = EDV - ESV = 140\text{ mL} - 40\text{ mL} = 100\text{ mL}$. 2. Calculate Cardiac Output: $CO = SV \times HR = 100\text{ mL/beat} \times 150\text{ beats/min} = 15,000\text{ mL/min} = 15\text{ L/min}$. Result: The athlete’s cardiac output expands threefold from 5 L/min to 15 L/min.

Checkpoint

1. In a standard clinical electrocardiogram (ECG), which feature directly correlates with ventricular depolarization and can be counted to determine an individual's heart rate?

All 46 topics in Body Fluids & Circulation

Heart Anatomy
- Chambers, Septa & Valves
- Great Vessels
ECG
- P Wave
- QRS Complex
- T Wave
- Segments & Intervals
Double Circulation
- Pulmonary Circulation
- Systemic Circulation
- Hepatic Portal System
White Blood Cells
- Granulocytes
- Agranulocytes
- Immune & Inflammatory Functions
Blood Composition
- Plasma & Serum
- Plasma Proteins
- Red Blood Cells
- Platelets
- Respiratory Gas Transport
Blood Coagulation
- Clotting Factors
- Mechanism of Coagulation
ABO Blood Groups
- Antigens & Antibodies
- Blood Transfusion Compatibility
Cardiac Conduction System
- SA Node
- AV Node
- Bundle of His & Purkinje Fibres
Cardiovascular Disorders
- Hypertension
- Coronary Artery Disease
- Angina Pectoris
- Heart Failure
Cardiac Cycle
- Atrial Systole
- Ventricular Systole
- Joint Diastole
- Heart Sounds
Cardiac Output
- Heart Rate
- Stroke Volume
Rh Factor
- Rh Incompatibility
- Erythroblastosis Foetalis
Blood Vessels
- Arteries
- Veins
- Capillaries
Lymphatic System
- Lymph Composition
- Lymph Vessels & Lacteals
- Functions of Lymph
Types of Circulatory Systems
- Open Circulatory System
- Closed Circulatory System
Cardiac Regulation
- Neural Regulation
- Hormonal Regulation