03 - Cardiac Physiology Flashcards
(23 cards)
Describe a cardiomyocyte
striated (banded of myosin and actin), branched, contain 1-2 central nuclei
How are cardiomyoctes connected
via irregular thickenings of the sarcolemma –> intercalated discs
- desmosomes
- gap junctions
How do cardiomyocytes generate atp
aerobic mechanisms
- fatty acids (60%)
- glucose (35%)
- creating phosphate
What is myoglobin
oxygen reserve – facilitates oxygen diffusion
How are heart contractions generated
autorhythmic fibres: specialized cardiac muscle fibers that repeatedly generate action potentials
What do autorhytmic fibers do
act as pacemaker cells (depolarize and generate AP)
form the cardiac conduction system (synchronization)
What are arrhythmias
abnormal rythms (HB too slow, too fast, too irregular)
due to issues with autorythmic fibers
Describe the sequence by which APs propagate through the cardiac conduction system
- sinoatrial (SA) node
- atrioventricular (AV) node
- atrioventricular bundle
- right and left bundle branches
- subendocardial conducting network
What is pacemaker potential
spontaneous depolarization:
autorhythmic fibers of the SA node and AV node spontaneously depolarize to threshold
What is the natural pacemaker
The SA node
because fires more quickly (freq 100 bpm) than av node (freq 40-60 bpm)
What does autonomic input do
slows the resting heart rate to 75 bpm
What are the three phases to AP generation
When does contraction occur in the phases
- depolarization (rapid) - sodium entry via voltage gated NA+ channels
- plateau (maintained depolarization) - calcium influx via voltage-gated slow Ca2+ channels and potassium efflux via voltage-gated K+ chanels
- 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)
What is an ECG
composite record of action potentials generated by all the heart muscle fibres during each heartbeat
detected on the surface of the body
Describe the three waveforms of an ECG
P wave = depolarization of atria
QRS complex = ventricles depolarizing (atrias repolarizing)
T wave = repolarization of ventricles
Desribe ECG intervals
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
What are isovolumentric phases
all four heart valves momentarily closed (2 AV & 2 SL)
same volume
contraction or relaxation but same blood volume
What are the key ventricular volumes
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
What is cardiac output
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
What are the factors regulating stroke volume
1) preload
2) contractility
3) afterload
Describe preload
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
Describe contractility
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
Describe afterload
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
What are the factors affecting heart rate
autonomic regulation
chemical regulation
age
body temperature
