Cardiac Muscle, Heart as a Pump, and Function of the Heart Valves

Question 1

What some differences/ similarities between cardiac and skeletal muscles?

Answer 1

• both have actin and myosin
• cardiac muscles of intercalated discs, with low electrical resistance, allowing action potentials to rapidly go through
• cardiac muscle = syncytium of many heart muscles cells –> action potential can spread rapidly from cell to cell
• atrioventricular node = slow conducting node, exclusive pathway between the atrial and ventricular syncytium (separated by fibrous tissues)

Question 2

What are the 4 phases of action potential?

Answer 2

1. phase 0: depolarization. Opening of the voltage gated fast Na+ channel - influx of Na+, membrane potential becomes more positive
2. phase 1: initial repolarization. Closing of the Na+ channel. Opening of the fast K+ channel - efflux of K+ , membrane potential becomes more negative
3. phase 2: plateau. Closing of the fast K+ channel. Opening of the Na-Ca channel - Ca2+ influx, membrane potential plateaus
4. phase 3: rapid repolarization. Closing of the Na-Ca channel. Opening of the slow K+ channel - efflux of K+, membrane potential returns to rest

Question 3

What are the 2 causes of action potential plateau?

Answer 3

1. slow entry of the Na/Ca into the cardiac muscle
2. decrease permeability of K in the cardiac muscle cells

Question 4

What promotes muscle fiber contractions?

Answer 4

Calcium diffusion into myofibrils
- action potential –> T tubules –> release of Ca2+ from longitudinal sarcoplasmic tubules into the sarcoplasmic reticulum –> Ca2+ promotes the sliding of actin and myosin filaments –> muscle contraction

Question 5

What is an unique source of Ca2+ in cardiac muscle cells (compared to skeletal muscle)?

Answer 5

• T bules has a lot of Ca2+ stored in them
• can get Ca2+ from extracellular fluids
• Ca2+ is pumped at the end of a contraction

Question 6

Describe the cardia cycle.

Answer 6

• Each beat starts at the sinus node by the R atrium
• action potential travels through both atria and the AV node and bundle into the ventricles
• 0.13s delay @ the AV node and bundle –> allows for the atria to contract before the ventricles

Question 7

Describe each component of the ECG.

Answer 7

P wave = depolarization of the atrium –> atrial contraction
QRS wave = depolarization of the ventricles –> ventricular contraction
T wave = repolarization of the ventricles

Question 8

Where does the majority of the blood volume received by the ventricles come from?

Answer 8

80% is filled during diastole, only 20% is from atrial contraction

Question 9

Describe the 3 atrial pressure waves.

Answer 9

a wave: atrial contraction
c wave: slight back flow of blood and bulging of the AV valve during ventricular systole
v wave: in filling of the atria from venous return

Question 10

Describe the events in ventricular filling.

Answer 10

• blood fills the atrium during systole, when the AV valve is closed
• isovolumetric relaxation (relaxation of the ventricular walls) = beginning of diastole
• once the pressure in the ventricle is below that of the atrium, the AV valve opens
• during diastole, the high pressure in atrium pushes blood into the ventricle
• the first 1/3 of diastole = rapid, provides the majority of the blood volume
• during the last 1/3 of diastole, atrium contracts, provides the 20% of blood volume into the ventricle –> aka, atrial kick

Question 11

Describe the events in systole.

Answer 11

• beginning of systole, ventricles contract –> increases the pressure inside the ventricles, AV valve closes –> isovolumic contraction (no outflow of blood for 0.2-0.3s)
• when the L ventricular pressure >80mmHg or R ventricular pressure > 8mm Hg –> opening of the semilunar valves –> outflow of blood, aka, period of ejection
• first part = period of rapid ejection; second part = period of slow ejection (aortic pressure slightly > ventricular pressure)
• at the last part of systole, ventricular pressure < aortic/ pulmonary artery pressure –> closing of the semilunar valves

Question 12

Describe the 4 phases of a cardiac cycle.

Answer 12

Phase I: filling. L ventricular volume increases from end of systole to end of diastole
Phase II: isovolumic contraction. L ventricle pressure increases to 80mm Hg (same as the aortic valve)
Phase III: ejection. additional pressure from systole, and the ventricular volume decrease (the amount = stroke volume)
Phase IV: isovolumic relaxation. same volume, but the ventricular pressure decreases to its diastolic pressure level

Question 13

Define preload and afterlaod.

Answer 13

Preload = end diastolic pressure
Afterload = aorta/ pulmonary artery pressure

Question 14

How is oxygen consumption related to cardiac work?

Answer 14

• O2 consumption is proportional to the tension x the amount of time under tension
• wall tension is proportional to pressure x diameter of the ventricle (Laplace’s law)
• therefore, O2 consumption is increased with increased systolic pressure or when heart is dilated

Question 15

How does the autonomic system affect heart rate?

Answer 15

Sympathetic NS: increases heart rate and contractility - can increase stroke volume by 2-3 fold
Parasympathetic NS: dramatically decrease heart rate, slightly change in contractility - can decrease cardiac output by 50%