Chapter 18: Body Fluids and Circulation – Part 2

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Chapter 18: Body Fluids and Circulation – Part 2

📘 Class 11 Biology | NEET PYQs + Concept Series
🔗 Visit: Biologyatease | NEET PYQ All

🫀 Cardiac Cycle – A Rhythmic Marvel

The human heart performs a sequence of mechanical and electrical events in a rhythmic cycle known as the cardiac cycle. It consists of alternate contraction and relaxation of the heart chambers.

🔄 Phases of the Cardiac Cycle

  1. Atrial Systole (0.1 sec):

    • Atria contract, pushing blood into the ventricles.

    • AV valves (tricuspid & bicuspid) open.

  2. Ventricular Systole (0.3 sec):

    • Ventricles contract, AV valves close.

    • Semilunar valves open.

    • Blood is pumped into pulmonary artery and aorta.

  3. Joint Diastole (0.4 sec):

    • Atria and ventricles relax.

    • Blood flows passively from veins into atria and ventricles.

🕒 Total Duration of One Cardiac Cycle = 0.8 seconds
✅ Average Heart Rate = 72 beats/min

❤️ Heart Sounds

Two sounds are produced during the cardiac cycle:

  • "Lub": Closing of AV valves (start of ventricular systole)

  • "Dub": Closing of semilunar valves (start of ventricular diastole)

📌 Auscultation using a stethoscope is used to listen to these heart sounds. Abnormal sounds may indicate valve defects (murmurs).

🔋 ECG – Electrocardiogram

ECG is a graphical recording of the electrical activity of the heart during a cardiac cycle.

🧾 Components of a Normal ECG:

  1. P-wave: Atrial depolarisation (atria contract)

  2. QRS complex: Ventricular depolarisation (ventricles contract)

  3. T-wave: Ventricular repolarisation (ventricles relax)

📌 NEET Notes:

  • ECG is recorded using electrodes (leads) placed on the body.

  • A normal ECG is essential to monitor cardiac health.

🧠 Regulation of Cardiac Activity

The heart is myogenic, meaning it can initiate its own contraction.

💡 Conducting System of the Heart:

  1. SA Node (Sinoatrial Node) – Pacemaker

    • Located in right atrium.

    • Generates impulses → initiates heartbeat.

  2. AV Node (Atrioventricular Node)

  3. AV Bundle (Bundle of His)

  4. Purkinje Fibres

🔁 These components ensure coordinated contraction of atria and ventricles.

💪 Blood Pressure (BP)

Blood pressure is the lateral pressure exerted by blood on the walls of blood vessels.

  • Normal BP: 120/80 mmHg

    • 120: Systolic pressure (during ventricular contraction)

    • 80: Diastolic pressure (during relaxation)

📌 Measured using a sphygmomanometer.

🔀 Blood Groups

Blood grouping is based on the presence or absence of antigens (A and B) on RBC membranes and antibodies in plasma.

✅ ABO Blood Group System:

Blood Group Antigen on RBC Antibody in Plasma
A A Anti-B
B B Anti-A
AB A and B None
O None Anti-A and Anti-B

  • Universal Donor: O

  • Universal Recipient: AB

✅ Rh Factor:

  • Rh+ has Rh antigen; Rh– lacks it.

  • Rh incompatibility can occur in pregnancy (erythroblastosis fetalis).

🧪 NEET Tip:

  • Cross-matching is crucial before blood transfusions.

🧬 Disorders of the Circulatory System

Understanding disorders is key to mastering NEET application-based questions.

1. Hypertension (High BP)

  • Persistent BP > 140/90 mmHg.

  • Can lead to heart diseases or stroke.

2. Coronary Artery Disease (CAD)

  • Atherosclerosis (fat deposition) narrows coronary arteries.

  • Reduces blood supply to heart muscles.

3. Angina (Chest Pain)

  • Due to insufficient oxygen supply to heart muscles.

  • Occurs during physical activity or stress.

4. Heart Failure

  • Heart’s pumping capacity is impaired.

  • Can cause fluid accumulation in lungs and other tissues.

🧪 NEET PYQs – Conceptual Examples (Part 2)

  1. ECG shows absence of P-wave – which part is defective?
    → SA node

  2. Which component acts as the pacemaker of the heart?
    → SA node

  3. Rh incompatibility may occur in?
    → Rh– mother and Rh+ fetus

  4. Which valve prevents blood backflow from right ventricle to right atrium?
    → Tricuspid valve

  5. Systolic pressure is recorded during?
    → Ventricular contraction

📚 Summary of Part 2

In this part, we covered:

  • Cardiac cycle and heart sounds

  • ECG and electrical regulation of the heart

  • Blood pressure and its measurement

  • Blood groups and Rh factor

  • Common cardiovascular disorders

  • NEET PYQs related to these topics

Understanding these basics makes it easier to tackle both theory and application-based questions in NEET with confidence.

🔗 Explore More NEET Resources

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