Flashcards in Cardiovascular mechanics 1 Deck (21)
How does the heart contract?
Myogenic stimualtion leading to the influx and release of Ca2+, causing contraction
How do cardiac myocytes contribute to contraction?
Specialised cells that have adaptations pertaining to the betterment of contraction.
Presence of T-tubules to carry depolarisation to the sarcoplasmic reticulum, allowing for release of Ca2+ containing vesicles.
L-type Ca2+ channel on myocyte is stimulated, conformational change by action potential.
Ca2+ moves in and causes an intracellular release from the SR by SR Ca2+ channels.
Excess Ca2+ is mopped up by Na/Ca transporter and ATPase
What happens to force as [Ca2+] increases?
Force production increase, sigmoidal relationship
What happens to force as length of myocyte increases?
Active and passive force production
But active plateaus
Compare skeletal and cardiac muscle's length-tension relation
Cardiac is more resistant to stretch and less compliant
Because of ECM and cytoskeleton
When muscle length increases too much for both, active force decreases, but passive stays same in cardiac
What are the differences between isotonic and isometric contraction?
Isotonic - muscle fibres shorten
Isometric - muscle fibres stay same length but force is produced
What is preload?
The weight that stretches muscle before it is stimulated to contract
What is afterload?
Weight only encountered upon muscle contraction
- i.e the force from the aorta the heart has to overcome to pump blood
How can preload be measured?
End Diastolic Volume
Right arterial pressure
What determines preload?
The filling of the ventricles before ejection - thus, venous return
How can afterload be measured?
Diastolic blood pressure
What determines preload?
The aortic pressure - as it is the load against which the left ventricle must eject blood.
What might happen to muscle shortening velocity should afterload increase?
It will decrease as greater force needed to overcome and hence more time needed to force ALL blood out, against the force
Similarly, what might happen to the force of contraction if preload were to increase?
Increase to a point, then too full to be too hard
What is the Frank-Starling relationship?
Increased diastolic fibre length leads to increased ventricular contraction
What is a consequence of increased ventricular contraction?
Greater stroke volume, meaning that cardiac output will match venous return
What two reasons causs the Frank-Starling relationship to be observed?
Changes in myofilament cross bridge interaction
- less actin overlap so more cross-bridge formation
Changes in sensitivity of myofilaments
- At longer sarcomere lengths TnC (regulates cross-bridge formation) has a higher affinity for Ca2+, meaning less is needed for same force
- More stretch means probability of strong binding cross-bridges forming increases
What is stroke work?
Work done bu heart to eject blood under pressure into aorta and pulmonary artery
Use an equation to show stroke work
Stroke volume x Pressure
What is the law of LaPlace?
States that when the pressure of a cylinder is constant, the tension on the walls increases as radius does
T = PxR