The Heart as a Pump

Question 1

Describe how the heart is two pumps in series, working together as one

Answer 1

• There is equal flow through the two circuits.
• Right and left pumps contract simultaneously
• Atria contact first, ventricles contract second

Question 2

Describe how blood movement through the heart is gated by valves

Answer 2

• Atrioventricular valves (AV) control low between the atria and ventricles
• Aortic and pulmonary valves control flow from the ventricles out to the circulatory vessels
• Valves open and close to direct blood

Question 3

Describe the cellular mechanism of cardiac contraction

Answer 3

• Actin and myosin filaments, myosin binds to actin to form a cross-bridge and then pulls on actin to shorten the sarcomere and generate force (with the shortening of the heart, the chambers get smaller, producing the pressure we need to move blood through the body)
• Ca2+ levels go up, and more Ca2+ is released from the sarcoplasmic reticulum
• Every muscle cell is activated during each heartbeat (no such thing as recruitment in cardiac muscle)

Question 4

How do we increase the force of cardiac contraction?

Answer 4

• Every cardiomyocyte is activated during each heart beat
• Extent of cross-bridges formed is not maximised at rest
  An increase of cystolic Ca2+ levels increases the number of cross bridges formed and therefore increases the force of the contraction.

Question 5

What is the cellular mechanism of cardiac relaxation?

Answer 5

• Decrease in cyclic Ca2+ levels by pumping Ca2+ back into SR
• Cross-bridges release when ATP binds to myosin
• Reduction in force means the heart can relax
• All cardiac myocytes relax each beat

Question 6

How are diastole and systole timed in the heart?

Answer 6

Atrial and Ventricular systole and diastole overlap/are offset. However, for both systole is the shortest phase and diastole is the longest phase.

Question 7

What is happening in the Atrial systole phase of the cardiac cycle?

Answer 7

atria are contracting, increasing the pressure and using it to push blood into the ventricles.
AV valves then snap shut and when this occurs you get the “lubb” sound.

Question 8

What is happening in the Isovolumetric ventricular contraction phase of the cardiac cycle?

Answer 8

• happens very fast
  all valves are shut, so volume of blood stays the same (which is what isovolumetric refers to). ventricles start to contract and pressure on blood spikes very quickly.

Question 9

What is happening in the ventricular ejection phase of the cardiac cycle?

Answer 9

when pressure in the ventricles is greater than pressure in the aorta and pulmonary artery. semilunar valves open and blood rushes out of heart into aorta and pulmonary artery. this pressure allows blood to travel all throughout the body.
semilunar valves snap shut when pressure of aorta and pulmonary artery is bigger than pressure in heart, making the “dupp” sound

Question 10

What is happening in the isovolumetric ventricular relaxation phase of the cardiac cycle?

Answer 10

• happens very fast
  AV and semilunar valves are closed so volume stays the same (again, hence isovolumetric). the heart starts to relax so pressure nosedives to basically nothing.
  AV valves open

Question 11

What is happening in the passive filling phase of the cardiac cycle?

Answer 11

• this is the longest phase your heart is in in the cardiac cycle.
  blood returns from the veins, flows through the atria and starts filling up the ventricles again.

Question 12

What are the features of a blood pressure trace to be aware of?

Answer 12

• Pulsatile change in pressure in the major arteries linked to ejection of blood
• Arteries always maintain a fairly high pressure
• Periods of systole (rising pressure) and diastole (falling pressure)
• Diastole is typically longer than systole
• Systemic arterial pressure is much higher than pulmonary arterial pressure (because distance is much larger so need higher pressure to push blood to those places)

Question 13

What are the features of a blood pressure trace that represent specific points on the graph?

Answer 13

• systolic pressure is the highest point on the trace
• disatolic pressure is the lowest point on the trace
• pulse pressure is the difference between the highest and lowest points
• mean pressure is the average across the full cycle (and is lower than ‘half’ because the heart spends longer in diastole)
• hypertension is high blood pressure, while hypotension is low blood pressure

Question 14

What are the five phases of the cardiac cycle?

Answer 14

Atrial systole, isovolumetric ventricular contraction, ventricular ejection, isovolumetric ventricular relaxation, passive filling