(U2) T&EM - Circulatory System In Mammals

Question 1

Give an account of the sequence of events which result in coordination of the flow of blood through the heart during one cardiac cycle

with regard to Atrial systole (4)

Answer 1

• cardiac muscle cells are myogenic
• excitation wave is initiated at the SA-node
• electrical impulses discharged across atrial muscle triggers atrial systole
• electrical impulses cannot pass directly to the ventricle muscle (sheet of non-conductive connective tissue between atria and ventricles)

Question 2

Give an account of the sequence of events which result in coordination of the flow of blood through the heart during one cardiac cycle

with regard to ventricular systole (10)

Answer 2

• AV-node “picks-up” the impulses from the atrial muscle
• impulses pass along the Bundle of His and Purkinje fibres to the ventricle wall
• ensures ventricular systole follows atrial systole
• ventricular systole increases pressure within the ventricles
• blood pressure increases from the base of the ventricles
• blood is forced against the AV-valves which close
• the chordae tendinae prevent the AV-valves blowing inside out (so preventing reflux of blood into the atria)
• closure of the AV-valves causes the first heart sound
• semilunar valves are blown open
• blood exits the ventricles via the pulmonary artery/aorta

Question 3

Give an account of the sequence of events which result in coordination of the flow of blood through the heart during one cardiac cycle

with regard to diastole (7)

Answer 3

• ventricular diastole results in a decrease in pressure within the ventricles
• semilunar valves close/as “pockets” fill with blood/ventricular pressure is less than the pressure in the artery
• closure of the semilunar valves causes second heart sound
• reflux of blood into the ventricles is prevented
• blood returns to the atria from the venae cavae or pulmonary veins (also during ventricular systole)
• as atria fill with blood, pressure increases/the AV-valves are pushed open during atrial diastole
• blood moves from atria to ventricles

Question 4

What is the purpose of the semilunar valves closing?

When do they close?

Why?

Answer 4

• Prevents reflux of blood from aorta / pulmonary artery back into ventricles
• diastole
• ventricular pressure is less than arterial pressure