2. Cardiovascular mechanics 1 Flashcards
What is needed for contraction of a single ventricular cell?
Extracellular calcium
Due to Calcium induced calcium release mechanism
Size of ventricular myocyte
100 μm long
15 μm wide
What are T tubules?
Finger-like invaginations from the cell surface
Carry surface depolarisation deep into cell
Describe the excitation-contraction coupling of the heart
Depolarisation causes the opening of L-type Ca2+ channels.
Influx of Ca2+ into the myocyte
Some Ca2+ activates myofilaments
Most Ca2+ binds to the Ryanodine receptor causing conformational change and efflux of Ca2+ from the sarcoplasmic reticulum into cell.
Ca2+ binds to troponin and activates actin-myosin interaction
How is Ca2+ returned to the SR?
Use ATP to pump Ca2+ against concentration gradient from cytoplasm back into SR via Ca2+ ATPase
How is Ca2+ removed from cell?
Na+/Ca2+ exchange system
Doesn’t use ATP, uses energy gradient of Na+ to efflux Ca2+ from cell to relax
What is the shape of the force production-intracellular calcium relationship?
SIGMOIDAL
What concentration of calcium is sufficient to generate maximum contraction?
10 micromolar
Length-tension relationship
As muscle is stretched up to an optimum point, more force is produced
Equation for total force
Active force + Passive force
Compare the length-tension relationship in skeletal and cardiac muscle.
Cardiac muscle is much more resistant to stretch and less compliant
Cardiac muscle exerts a lot more passive force.
What are the 2 types of contraction used by the heart?
Isometric: Muscle fibres don’t change length, but pressures increase in both ventricles
Isotonic: Shortening of fibres, blood is ejected from ventricles
What is Preload?
The weight that stretches the muscle before it is stimulated to contract
What is Afterload?
The weight that is not apparent to the muscle in the resting state
Only encountered once the muscle has started to contract
What is the effect of increasing preload (and stretch)?
Increases the force exerted by the muscle fibres
In vivo preload
Amount of blood that returns to the heart during diastolic interval determines how much heart fills with blood and stretches
Measures of preload
End-diastolic volume: amount of blood at end of diastole (relaxation), sets stretch
End-diastolic pressure
Right atrial pressure
In vivo afterload
Load against which left ventricle ejects blood after opening of the aortic valve
Measure of afterload
Diastolic blood pressure
What are the effects of increasing afterload?
Increasing afterload decreases the amount of shortening of muscle fibres and decreases the velocity of shortening of the fibres.
Why is hypertension a risk factor for cardiac disease?
The higher the afterload (diastolic BP) the harder the ventricle has to work to expel blood
State Starling’s Law.
Increase in diastolic fibre length increases ventricular contraction.
What is the consequence of starlings law?
Ventricles pump greater stroke volume so that, at equilibrium, cardiac output exactly balances the augmented venous return
Starling’s law is caused by what 2 factors?
Changes in the number of myofilament cross-bridges
Changes in the calcium sensitivity of the myofilaments
Describe the effects of changes in the number of myofilament cross-bridges
At shorter lengths than optimal, actin filaments overlap on themselves so reducing the number of myosin cross bridges that can be made.
As stretched, more and more, optimal interdigitation between thick and thin filaments until maximum
State 2 possible explanations for increasing calcium sensitivity of the myofilaments.
- With stretch, lattice spacing (spacing between filaments) decreases, meaning for the same amount of Ca2+, more cross bridges can be formed.
- At longer sarcomere lengths, affinity of TnC for Ca2+ is increased due to conformational change in protein
Less Ca2+ required for same amount of force
What is Stroke Work?
Work done by the heart to eject blood under pressure into the aorta and pulmonary artery.
Equation for stroke work
Stroke Volume x Pressure (at which the blood is ejected)
=SV x P
What greatly affect stroke volume and pressure?
SV: Preload and afterload
P: Cardiac structure
State the law of Laplace.
When the pressure within a cylinder is kept constant, tension on its walls increases with increasing radius. T = PR.
Equation of Law of Laplace
Wall tension = Pressure in vessel x Radius of vessel
T = P x R
Law of Laplace equation incorporating wall thickness (h)
T= (PxR) / h
What is the physiological relevance of the law of Laplace with regards to the structure of the right and left ventricles?
Left ventricle has a smaller radius of curvature than the right ventricle meaning that the left ventricle is able to generate higher pressures with similar wall tension.
What is the clinical significance of the law of Laplace?
In dilated cardiomyopathy, radius of curvature increases
Hence pressure generated decreases.
How is wall stress kept low in giraffes?
They have long, narrow, thick-walled ventricles
