Cardiac Cycle

Question 1

Describe the process that leads to cardiac myocyte contraction

Answer 1

1. Signal stimulates dihydropyridine receptors which open L-type calcium channels.
2. Calcium enter the myocytes causing a calcium induced calcium release from the sarcoplasmic reticulum.
3. The calcium binds Troponin C allowing Troponin I to move revealing the binding site for actin and myosin to join and cause contraction.
4. Protein Kinase A (also stimulated by calcium) phosphorylates Phospholamban which opens the Sarcoendoplasmic Reticulum Calcium (SERCA) pump so calcium can get back into the SR.
5. A calcium ATPase and Calcium sodium exchanger also remove the calcium.

Question 2

How do hyper and hypocalcemia affect muscle contraction.

Answer 2

Hyper: increases strength due to increased calcium entering the cell.

Hypo: decreases strength

Question 3

How does hypernatremia influence cardiac muscle contraction?

Answer 3

Increases strength of contraction

• high extracellular sodium will force sodium into the myocyte due to an increased concentration gradient
• this activates the sodium calcium exchanger which will work to remove the excess sodium from the myocyte. The exchanger will remove sodium but will bring more calcium into the cell having the same effect as hypercalcemia.

Question 4

How does acidosis affect contraction?

Answer 4

Creates an overall weaker contraction
-acidosis prevents phosphorylation of phospholamban. If calcium cannot get back into the SR, some myocytes will be permanently contracted and when the time comes for another heart beat, not as many myocytes will be able to contribute because they are still hypertonic.

Question 5

What effect does increased HR have on strength of contraction?

Answer 5

Increases strength of contraction (up to a point). Beta adrenergic stimulation increases chronotropy (HR), inotropy (strength), and lusitropy (relaxation).

However, if a premature beat or fast enough HR (fib or flutter) occurs the strength of contraction will be weaker. This is because residual calcium has not had time to leave the cell to allow relaxation and also the high residual calcium decreases the gradient of calcium across the membrane slowing the speed of influx thru the L-type channels.

Question 6

Time spend in each phase of the cardiac cycle.

Answer 6

1/3 systole

2/3 diastole

Question 7

What is the significance of the central venous pressure a, c and v waves?

Answer 7

a: right atrium contraction
c: tricuspid valve closes due to ventricular contraction
v: passive filling of the atrium from the veins during systole

Question 8

What is stroke volume?

Answer 8

End diastolic volume-end systolic

affected by venous return, contractility, afterload