SolveItClass 11 · NCERT

NCERT Solutions · Class 11 Biology Body Fluids and Circulation

14 questions · 14 still being checked

Exercises 15.11–15.14 (part 2 of 2)

  1. Exercise 15.11

    What is the significance of atrio-ventricular node and atrio-ventricular bundle in the functioning of heart?

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    Together they are the only route by which the excitation generated in the atria reaches the ventricles, so they make the ventricles contract in proper sequence after the atria.
    Atrio-ventricular node (AVN) — a mass of nodal tissue in the lower left corner of the right atrium, close to the atrio-ventricular septum. It picks up the action potential generated by the SAN and conducts it to the ventricular side.
    Atrio-ventricular bundle (AV bundle) — a bundle of nodal fibres that continues from the AVN, passes through the atrio-ventricular septa, emerges on the top of the inter-ventricular septum and at once divides into a right and a left bundle.
    These branches give rise to minute fibres, the purkinje fibres, spread throughout the ventricular musculature of the respective sides.
    Through this path the bundle of His transmits the impulse to the entire ventricular musculature, causing ventricular systole, which drives blood into the pulmonary artery and the aorta.
    NCERT_Solution_Class11_Biology_Ch15_Q15-11

  2. Exercise 15.12

    Define a cardiac cycle and the cardiac output.

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    NCERT publishes no answers for this textbook, so there is nothing in the book to check this working against. It has also not yet been read through by hand.

    Cardiac cycle
    The cardiac cycle is the sequential event in the heart which is cyclically repeated, and it consists of the systole and diastole of both the atria and the ventricles.
    It runs as joint diastole → atrial systole → ventricular systole → ventricular diastole → joint diastole again.
    Since the heart beats $\displaystyle 72$ times per minute, $\displaystyle 72$ cardiac cycles are performed per minute and the duration of one cardiac cycle is $\displaystyle 0.8$ seconds.
    Cardiac output
    The cardiac output is the volume of blood pumped out by each ventricle per minute.
    \(\displaystyle \text{Cardiac output} = \text{Stroke volume} \times \text{Heart rate} \), where the stroke volume is the approximately $\displaystyle 70$ mL of blood pumped out by each ventricle during one cardiac cycle.
    \(\displaystyle 70\ \text{mL} \times 72 \approx 5000\ \text{mL} \), so it averages $\displaystyle 5$ litres in a healthy individual.
    The body can alter both the stroke volume and the heart rate, and hence the cardiac output — for example, the cardiac output of an athlete is much higher than that of an ordinary man.

  3. Exercise 15.13

    Explain heart sounds.

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    NCERT publishes no answers for this textbook, so there is nothing in the book to check this working against. It has also not yet been read through by hand.

    Two prominent heart sounds are produced during each cardiac cycle, and both are caused by the closure of heart valves. They can be easily heard through a stethoscope.
    First heart sound (lub) — associated with the closure of the tricuspid and bicuspid valves. It occurs at the start of ventricular systole, when the rising ventricular pressure forces these valves shut against the attempted backflow of blood into the atria.
    Second heart sound (dub) — associated with the closure of the semilunar valves. It occurs during ventricular diastole, when the falling ventricular pressure lets these valves shut and so prevents the backflow of blood into the ventricles.
    These sounds are of clinical diagnostic significance, since a change in them points to a defect in the valves.

  4. Exercise 15.14

    Draw a standard ECG and explain the different segments in it.

    The book prints no answer for this

    NCERT publishes no answers for this textbook, so there is nothing in the book to check this working against. It has also not yet been read through by hand.

    An electrocardiogram (ECG) is a graphical representation of the electrical activity of the heart during a cardiac cycle, obtained with an electro-cardiograph. For a standard ECG the patient is connected to the machine by three electrical leads — one to each wrist and one to the left ankle.
    What the diagram must show
    Draw a flat horizontal baseline; time runs along the horizontal axis and the electrical potential along the vertical axis.
    On the baseline draw, from left to right, one complete cycle: a small rounded upward hump, then a sharp spike, then a broader rounded upward wave. Repeat the pattern once more to show that the trace is cyclic.
    Label the small first hump P; label the sharp spike as the QRS complex — a small downward dip Q, a tall narrow upward peak R, and a downward deflection S; label the last broad rounded wave T.
    Mark the segment from P to Q along the baseline, and the segment from S to T along the baseline.
    Explanation of the segments
    P-wave — represents the electrical excitation (depolarisation) of the atria, which leads to the contraction of both the atria.
    QRS complex — represents the depolarisation of the ventricles, which initiates ventricular contraction. The contraction starts shortly after Q and marks the beginning of systole.
    T-wave — represents the return of the ventricles from the excited to the normal state (repolarisation). The end of the T-wave marks the end of systole.
    By counting the number of QRS complexes occurring in a given time period, the heart beat rate of an individual can be determined.
    ECGs from different individuals have roughly the same shape for a given lead configuration, so any deviation from this shape indicates a possible abnormality or disease — hence its great clinical significance.
    NCERT_Solution_Class11_Biology_Ch15_Q15-14