cardiac physiology Flashcards
define systole
contraction
define diastole
relaxtion
describe sequence of mechanical events in the cardiac cycle
- ventricular filling
- atrial contraction
- isovolumetric ventricular contraction
- ventricular ejection
- isovolumetric ventricular relaxation
what happens during ventricular filling?
low pressure due to ventricular diastole so blood flows in from venous systems
what happens during artial contraction?
extra small volume of blood pushed into ventricle
what happens during isovolumetric ventricular contraction?
- ventricles are electrically stimulated to contract
- pressure in ventricles increases but volume stays the same
- very brief period
what happens during ventricular ejection?
pressure in the ventricles is greater than in the arteries, so blood moves out of ventricles and into arteries
what happens during isovolumetric ventricular relaxation?
pressure drops but volume stays the same for a brief period
what causes heart sounds?
valves snapping shut
how can valves cause an altered sound?
valvular diseases
- stenosis - narrowing
- incompetence or regurgitation
what do intercalculated disks do?
dock and form direct contact with another muscle
identify where to listen to the heart sounds for different valves
describe process of atrial and ventricular contraction
- action potential from adjacent cell enters, triggering opening of calcium ion channels
- calcium ion influx triggers release of calcium ions from sarcoplasmic reticulum
- calcium ions bind to troponin to initiate contraction
describe process of atrial and ventricular relaxation
- calcium ions unbind from troponin
- calcium is pumped back into sarcoplasmic reticulum for storage
- some calcium is exchanged for sodium at the cell membrane
- sodium ion gradient is maintained by Na+/K+ ATPase
what does the SAN do?
sets heart rate and initiates cardiac cycle
what are HCN channels and where are they located?
hyperpolarisation-activated, cyclic nucleotide-dependent nonspecific channel
located in pacemaker cells
describe phases 0, 3 and 4 on this graph
phase 0
- upstroke of slow pacemaker a.p.
- triggered at threshold potential (-55mV)
- calcium ion influx (depolarisation)
phase 3
- calcium ion channels inactivate
- increase in potassium ion efflux
- membrane hyperpolarises
phase 4
- membrane potential never levels off and starts to drift back up
- opening of sodium ion channels
- reactivation of calcium ion channels
- when m.p. is reset, cardiac cycle restarts
describe phase 4: prepotential
- permits automaticity (ability for heart cells to spontaneously depolarise and generate an action potential)
describe what happens at each number on this graph showing ventricular myocyte action potential
0 - Na channels open
1 - Na channels close; fast K channels open
2 - Ca channels open; fast K channels close
3 - Ca channels close; slow K channels open
4 - resting potential
how does the sympathetic nervous system affect heart rate?
- activation causes releease of noradrenaline
- bind to B1 adrenoceptor on cardiac pacemaker and myocyte cell membranes
- increases opening of HCN channels in pacemaker cells, increasing Na influx
- opens Ca channels, increasing Ca influx
- increase in slope of prepotential
- heart rate increases
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how does the parasympathetic nervous system affect heart rate?
- activation causes release of acetylcholine
- binds to muscarinic cholinergic receptors
- decreases opening of HCN channels, decreasing Na influx
- slows opening of Ca channels, decreasing Ca influx
- opens additional K channels, increasing K efflux
- hyperpolarises membrane and reduces slope of prepotential
- heart rate decreases
how is an a.p. initiated and spread in the heart?
- Electrical activity initiated in SAN and spreads across SAN cells
- Rapidly moves across atria to AV node
- AV node fires, causing waves of depolarisation down the interventricular septum
- Move down Bundle of His and apex via Purkinje fibers
- Move from apex to top of hear, wave of depolarisation which apreads across the ventricular muscle
define annulus fibrosis
non-conducting layer between atria and ventricles which electrically insulates the chambers from each other
define stroke volume
vol of blood pumped by one ventricle (~75mL)
define cardiac output
vol pumped per ventricle per min (~5mL)
define venous return
vol of blood returning to the heart
how is pulmonary circulation different to systemic circulation?
- lower pressure
- more pulsatile flow
- uneven blood distribution
define iontrope
an agent that alters the force or energy of muscular contractions
define chronotrope
effects that change heart rate
