Heart as a pump Flashcards
How does cardiac muscle contract?
AP causes DHP calcium channels to contract
Only 10% of influx of calcium contributes to contraction though
DHP activation causes release of calcium from sarcoplasmic reticulum via ryanodine release channels
At resting HR the rise in IC calcium due to influx and sarcoplasmic release is insufficient to cause max contraction force
Refractory period of the heart
Cardiac twitches involve all fibres of myocardium, cannot significantly summate contractions of cardiac muscle.
Refractory period due to inactivation of sodium channels
Longer RP and longer contraction time than skeletal muscle
What percentages of blood are ejected at rapid and slow ejections?
Rapid ejection – where 70% SV ejected
Slow ejection – when 30% ejected
What are the extrinsic and intrinsic controls of SV?
Intrinsic – self-regulation, Frank-Starling mechanism (allows for automatic adjustment for small imbalances between LV and RV), increased EDV increases force of contraction
Preload – venous pressure and venous return to the heart – EDP/EDV
Afterload – aortic/pulmonary artery pressure
Extrinsic mechanisms – sympathetic nerves
Discuss ANS stimulation and contractility
Symp innervation throughout entire heart, positive inotropic effect
NA on B1 receptors – enhances calcium influx, promotes storage and release of calcium from sarcoplasmic stores (increased contractility and increased speed of relaxation)
Parasymp innervation – mostly to SAN, main effect is decreased rate
What is Fick principle for determining CO?
Rate of oxygen consumption (ml/min) / arteriovenous oxygen difference (ml/L of blood)
Discuss venous return
Sympathetic innervation
Muscle pumps
Inspiratory movements
- Diaphragm descends – inc abdominal pressure, transmitted passively to intraabdominal veins
- Decreased pressure in thorax – dec pressure in intrathoracic veins and right atrium
- Therefore inc pressure difference between peripheral veins and heart
Blood volume e.g. haemorrhage, fluid challenge
