Outline-3, BIO 3360, Circulation II – Heart Muscle Pumping

I. Heart Muscle contraction following Action Potential –similar to skeletal muscle:

  • Contraction activated by increasing [Ca ++ ] Inside cell.
  • Ca ++ influx from the SR and from the ECF.
  • AP activates receptors in the T tubes allowing inward movement of Ca++.
  • The influx of Ca ++ from the ECF triggers the release of a larger amount of Ca ++ from the SR leading to contraction

II. Heart, as a pump, produces pressure; pressure differences result in flow. Flow is the volume of blood moving in a certain amount of time. Blood flows through the heart in this fashion: Vena cava, Right atrium, Right atrioventricular (tricuspid) valve, Right ventricle, Pulmonary semilunar valve; Pulmonary trunk & arteries; Lungs for oxygenation; Pulmonary veins, Left atrium, Left atrioventricular (bicuspid) valve, Left ventricle, Aortic semilunar valve, Aorta.  All structures listed above before the lungs carry deoxygenated blood and those after the lungs carry oxygenated blood.

A. Cardiac Cycle– one entire beat.  The 2 atria contract/relax at the same time and then the 2 ventricles contract/relax at the same time.

B. Atrium Diastole and Systole

C. Ventricle Diastole and Systole

D. Comparison of pressure in LA, LV and Aorta

E. Atrioventricular Valves & Semilunar Valve

1. AV valves open when atrial pressure is higher than ventricular pressure

2. AV valves close when ventricular pressure is higher than atrial pressure at the end of ventricular diastole

3. Semilunar valves open when ventricular pressure is higher than aortic (or pulmonary trunk) pressure

4. Semilunar valves close when aortic pressure (or pulmonary trunk) is higher than ventricular pressure at the end of ventricular systole

F. Heart sounds –S1 is closure of AV valves and S2 is closure of semilunar valves

Video of cardiac cycle

III. Cardiac Output – volume of blood pumped by each ventricle per minute.  The cardiac output is the same from right & left ventricles since it is a volume, not a pressure.

A. CO = HR x SV {Heart rate is beats per minute and stroke volume is volume ejected by the ventricle per beat; avg. human resting HR is 75 bpm and SV is 70 mls/bt) }

B. Heart Rate determined by Sympathetic and Parasympathetic stimulation

-Norepinephrine or epinephrine and SNS increase HR by increasing rate of depolarization (depolarization due to calcium and sodium influx)

-Acetylcholine and PNS decrease HR by hyperpolarizing cell and therefore increasing the time for depolarization (hyperpolarization due to potassium efflux and decreased calcium influx).  The PNS nerve that travels to the heart is called the vagus nerve.

C. Stroke Volume is the difference between End Diastolic Volume and End Systolic Volume: SV = EDV – ESV

D. End diastolic volume

1. Venous return; venous filling pressure (preload)Note: preload is the tension on the heart muscle when it begins to contract due to amount of blood in the heart chamber

2. Atrial contraction pressure

3. Distensibility of ventricle

4. Time available for filling.  With a slow heart rate you have more time for filling and with too fast of a heart rate, you lose time for blood filling those ventricles.

E. End systolic volume

1. Contractility of the muscle (a more forceful contraction decreases ESV)

– changes in EDV (preload): Frank-Starling mechanism states with increased venous return, increased filling pressure and therefore increased force of contraction

-Sympathetic stimulation (norepinephrine, epinephrine) decreases end systolic volume and Parasympathetic stimulation (ACH) increases end systolic volume

-changes in arterial pressure (afterload) – this mechanical force in the arteries opposes the ejection of blood from the ventricle and therefore decreases stroke volume; afterload is the load against which heart muscle exerts its contractile force