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Q10:
Sino-atrial node is called the pacemaker of our heart. Why?
Solution :
Step 1: Anatomical and Physiological Definition of the Sino-atrial (SA) Node
The Sino-atrial (SA) node is a specialized mass of cardiac muscle fibers located in the right upper corner of the right atrium. It consists of self-excitable cells capable of generating action potentials without external neural stimulation [Per the Myogenic Nature of the Heart].
Step 2: Mechanism of Automaticity
The SA node cells possess an unstable resting membrane potential, known as the "pacemaker potential." This is characterized by a slow, spontaneous depolarization caused by a constant inward leak of $Na^+$ ions and a decreased efflux of $K^+$ ions. Once the threshold potential is reached, $Ca^{2+}$ channels open, triggering an action potential that propagates through the heart [Reference: Membrane Physiology of Autorhythmic Cells].
Step 3: Frequency of Impulse Generation
The SA node exhibits the highest rate of rhythmicity among all components of the cardiac conduction system. It generates impulses at an intrinsic rate of approximately 70-75 beats per minute in a healthy adult. Because the SA node reaches its threshold faster than other potential pacemaker sites (such as the Atrioventricular node or Purkinje fibers), it effectively "overdrives" the rest of the heart by suppressing subordinate pacemaker sites [Principle of Overdrive Suppression].
Step 4: Comparative Analysis of Conduction
| Pacemaker Site | Intrinsic Rate (BPM) | Role |
|---|---|---|
| SA Node | 70-75 | Primary Pacemaker |
| AV Node | 40-60 | Secondary (Latent) Pacemaker |
| Purkinje Fibers | 20-40 | Tertiary (Escape) Pacemaker |
Step 5: Justification for the "Pacemaker" Title
The SA node is designated the pacemaker because it functions as the primary initiator of the cardiac cycle. By setting the pace for the contraction of the entire heart (atrial depolarization followed by ventricular contraction), it ensures the rhythmic, coordinated pumping of blood. If the SA node fails, the next fastest pacemaker (the AV node) assumes control, but at a significantly slower rate [Clinical reference: Sinus Node Dysfunction].
Final Solution: The Sino-atrial (SA) node is called the pacemaker of the heart because it possesses the highest intrinsic rate of self-excitation and impulse generation among all cardiac tissues, thereby setting the rhythm and frequency of the heartbeat for the entire organ.
More Questions from Class 11 Biology Body Fluids and Circulation EXERCISES
- Q1: Name the components of the formed elements in the blood and mention one major function of each of them.
- Q2: What is the importance of plasma proteins?
- Q3: Match Column I with Column II : Column I Column II (a) Eosinophils (i) Coagulation (b) RBC (ii) Universal Recipient (c) AB Group (iii) Resist Infections (d) Platelets (iv) Contraction of Heart (e) Systole (v) Gas transport
- Q4: Why do we consider blood as a connective tissue?
- Q5: What is the difference between lymph and blood?
- Q6: What is meant by double circulation? What is its significance?
- Q7(a): Write the differences between : (a) Blood and Lymph
- Q7(b): Write the differences between : (b) Open and Closed system of circulation
- Q7(c): Write the differences between : (c) Systole and Diastole
- Q7(d): Write the differences between : (d) P-wave and T-wave
- Q8: Describe the evolutionary change in the pattern of heart among the vertebrates.
- Q9: Why do we call our heart myogenic?
- Q11: What is the significance of atrio-ventricular node and atrio-ventricular bundle in the functioning of heart?
- Q12: Define a cardiac cycle and the cardiac output.
- Q13: Explain heart sounds.
- Q14: Draw a standard ECG and explain the different segments in it.
CBSE Solutions for Class 11 Biology Body Fluids and Circulation
Chapters in CBSE - Class 11 Biology
- The Living World
- Biological Classification
- Plant Kingdom
- Animal Kingdom
- Morphology of Flowering Plants
- Anatomy of Flowering Plants
- Structural Organisation in Animals
- Cell – The Unit of Life
- Biomolecules
- Cell Cycle and Division
- Transport in Plants
- Mineral Nutrition
- Photosynthesis
- Respiration in Plants
- Plant Growth and Development
- Digestion and Absorption
- Breathing and Exchange of Gases
- Excretory Products and Elimination
- Locomotion and Movement
- Neural Control and Coordination
- Chemical Coordination and Integration
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