Question

1. Outline the steps of electrical conduction through the mammalian heart.

2. What is the importance of the skeletal muscle and respiratory pumps?

3. What happens to heart rate, stroke volume, cardiac output, mean arterial pressure, and patterns of blood flow at the onset of exercise in humans?

Answer 1

1.) Electrical conduction through the heart

a) Impulses are generated in the sino atrial node or AV node. The have the ability of self excitation which causes automatic rhythmical discharge and contraction.

b) These impulses pause at the atrioventricular node ( 0.1s). They are the anterior, middle and posterior internodal pathways.

c) The atrio ventricular bundle connects the atria to the ventricles. The atrial conductive system is organised so taht cardiac impulse do not travel from atria to ventrciles too rapidly. And this delay time allows the atria to empty their blood to ventricles before ventricular contraction begins.

d) Impulse after travelling through internodal pathways reaches AV node about 0.03 sec after its origin in sinus node. Then thereis another delay of .09 sec in AV node itself before impulse enters penetrating portion of AV bundle, where it passes to ventricles. A final delay of 0.04 sec in the penetrating AV bundles. Thus the total delay in AV nodal and AV bundle system is 0.13 sec. This in addition to an initial delay of 0.03 sec from sinus node to AV node makes a total delay of 0.16 sec before signal reaches contracting muscle of ventricles.

f) Impulses are carried to the interventricular septum by the bundle branches

g) Purkinje fibres depolarize the contractile cells of both the ventricles by spreading to entire ventricular mass.

AV bundle makes the action potential unable to travel backwards from venricles to atria.

2) It aids the heart in circulation of blood, and increases venous return to heart, and plays an important role in arterial blood flow. In case of respiratory pumps, inhalling pulls air to lungs and help to pull blood back to thorax. It uses the negative pressure in abdominal cavities and thoracic cavities during inspiration to pull venous blood to heart.

3) Effect of exercise

a) Heart rate - heart rate increases during exercise. Even the thought of exercise increases the heart rate. Because of the impulses from cerebral cortex to medullary centres which reduces the vagal tone. In moderate exercise heart increases by 180 beats/min. In severe exercise the heart rate increases by 240 to 260 beats/min. This increased heart rate is due to vagal withdrawal.

-Impulses in the propioreceptors present in the exercising muscles acts through higher centres and

-increased carbon dioxide tension that act through medullary centres,

-rise in body temperature which act on cardiac centres via hypothalamus, increased temperature also stimulates SA node directly.

-Another reason is circulating catacholamines secreted in large quantities.  

b) Cardiac output- increases upto 20L/min in moderate and 35L/min in severe exercise. Increase in cardiac output is directly proportional to the increase in amount of oxygen consumed during the exercise.

c) Stroke volume- increases due to increased force of contraction. Because of vagal withdrawal, sympathetic activity increases leading to increase in rate and force of contraction.

d) mean arterial pressure- increases by a small amount because cardiac output also increases.

e) blood flow- increase in blood flowing to skeletal muscles during exercise. In the resting condition the blood supply to skeletal muscles is 3 to 4 ml/ 100g of muscle/ minute. It increases upto 60 to 80ml in moderate exercise and 90 to 120ml in severe exercise. During the muscular activity, stoppage of blood flow occurs when muscles contract because of the compression of blood vessels. and in between contraction blood flow increases. Factors such as hypoxia, hypercapnea, adrenaline secreted from adrenal medulla etc.. increase the amount of blood flow to muscles by means of dilation of blood vessels of muscles.