03 - Cardiac Physiology

Question 1

Describe a cardiomyocyte

Answer 1

striated (banded of myosin and actin), branched, contain 1-2 central nuclei

Question 2

How are cardiomyoctes connected

Answer 2

via irregular thickenings of the sarcolemma –> intercalated discs
- desmosomes
- gap junctions

Question 3

How do cardiomyocytes generate atp

Answer 3

aerobic mechanisms
- fatty acids (60%)
- glucose (35%)
- creating phosphate

Question 4

What is myoglobin

Answer 4

oxygen reserve – facilitates oxygen diffusion

Question 5

How are heart contractions generated

Answer 5

autorhythmic fibres: specialized cardiac muscle fibers that repeatedly generate action potentials

Question 6

What do autorhytmic fibers do

Answer 6

act as pacemaker cells (depolarize and generate AP)
form the cardiac conduction system (synchronization)

Question 7

What are arrhythmias

Answer 7

abnormal rythms (HB too slow, too fast, too irregular)
due to issues with autorythmic fibers

Question 8

Describe the sequence by which APs propagate through the cardiac conduction system

Answer 8

1. sinoatrial (SA) node
2. atrioventricular (AV) node
3. atrioventricular bundle
4. right and left bundle branches
5. subendocardial conducting network

Question 9

What is pacemaker potential

Answer 9

spontaneous depolarization:
autorhythmic fibers of the SA node and AV node spontaneously depolarize to threshold

Question 10

What is the natural pacemaker

Answer 10

The SA node
because fires more quickly (freq 100 bpm) than av node (freq 40-60 bpm)

Question 11

What does autonomic input do

Answer 11

slows the resting heart rate to 75 bpm

Question 12

What are the three phases to AP generation
When does contraction occur in the phases

Answer 12

1. depolarization (rapid) - sodium entry via voltage gated NA+ channels
2. plateau (maintained depolarization) - calcium influx via voltage-gated slow Ca2+ channels and potassium efflux via voltage-gated K+ chanels
3. Repolarization - closure of Ca2+ channels and potassium outflow with activation of additional voltage-gated K+ channels

Contraction - plateau and a bit of repolarization (within refractory period)

Question 13

What is an ECG

Answer 13

composite record of action potentials generated by all the heart muscle fibres during each heartbeat

detected on the surface of the body

Question 14

Describe the three waveforms of an ECG

Answer 14

P wave = depolarization of atria
QRS complex = ventricles depolarizing (atrias repolarizing)
T wave = repolarization of ventricles

Question 15

Desribe ECG intervals

Answer 15

P-Q interval = excitation of atria to excitation of ventricles (beggining of P wave to beginnning of QRS complex)

Q-T interval = excitation of ventricles to repolarization of ventricles (start of QRS complex to end of T wave) - can elongate if heart is starved for O2, can’t reset membrane potential

S-T interval = ventricles undergoing contraction (end of S wave to beginning of T wave) - elevated or depressed in heart attack, not enough O2 in cells/dying

Question 16

What are isovolumentric phases

Answer 16

all four heart valves momentarily closed (2 AV & 2 SL)
same volume

contraction or relaxation but same blood volume

Question 17

What are the key ventricular volumes

Answer 17

end diastolic volume (EDV) - ventricle fully filled
end systolic volume (ESV) - blood in ventricle
stroke volume (ml/beat) = EDV - ESV = ~70ml/beat

each ventricle expels the same volume of blood per beat

Question 18

What is cardiac output

Answer 18

volume of blood ejected from the left ventricle (or right) into the aorta (or pulmonary trunk) per minute

Cardiac output = heart rate x stroke volume

typical HR at rest is 75 beats/min and stroke volume is 75 ml/beat

Question 19

What are the factors regulating stroke volume

Answer 19

1) preload
2) contractility
3) afterload

Question 20

Describe preload

Answer 20

the degree of stretch of heart before systole

Frank-Starling law of the Heart
- more blood in heart = greater contraction to resist stretching (increase contractility = increase SV)

affected by duration of ventricular diastole and venous return

Question 21

Describe contractility

Answer 21

forcefulness of contraction of cardiomyocytes

contractility is the strength of contraction at any given preload
- the strength of contraqction is directly related to cytosolic calcium levels –> calcium influx from extracellular fluid, calcium release from the sarcoplasmic reticulum

positive inotropes –> increase contractility
negative inotropes –> decrease contractility

Question 22

Describe afterload

Answer 22

the pressure ventricles must overcome before the semilunar valves open

an increase in afterload leads to decrease in SV
affected by BP and vessel structure

Question 23

What are the factors affecting heart rate

Answer 23

autonomic regulation
chemical regulation
age
body temperature