Cardiovascular mechanics Flashcards
How does contraction occur in a ventricular cells?
Electrical event –> Ca ion influx –> contractile event
AP increases first, then calcium, then contraction occurs ( graph on slide )
- calcium transient effect.
What are the characteristics of ventricular cells and t-tubules?
100 micrometer long, 15 wide.
T tubules create projections causing invaginations of the cell surface
T tubules open to 200nm in diameter.
*Each T-tubule lies alongside each Z-line
How does the sarcoplasmic reticulum look surrounding Ventricular cells?
It binds around each t -tubule and creates a network across the muscle filaments.
How does Excitation-contraction coupling occur in the heart?
L-type Calcium channels in T-tubules allow Ca2+ into cytosol of cell.
Binds to SR Ca2+ ligand release channels. And stored calcium in SR is released.
Binds to troponin.
How to relax ventricular cells?
Calcium ions actively pumped into SR with SR Ca2+ ATPase
and
Na+/Ca2+ exhange system in T-tubule to remove calcium.
Allows a calcium balance in the cell itself
Is there a relationship between force production and intracellular Ca2+?
Sigmodial relationship
Is there a relationship between length of cell and force produced?
Active force production increase
There is also a passive force due to the recoil of elastic
What comparisons can be made between skeletal muscle and cardiac muscle length-tension relationship
Cardiac muscle is less resistant to stress due to the properties of extracellular matrix and cytoskeleton. So, it has a higher passive forc.
Both have similar active force
So cardiac muscle has higher total force
What are the characteristics of isometric contraction?
Muscle fibres do not change length by pressures increase in both ventricles.
What are the characteristics of Isotonic contraction?
Shortening of fibres and blood is ejected from ventricles
*heart uses both types
What is Preload?
What does a small preload?
Weight that stretches muscle before it is stimulated to contract
Small preload (shorter muscle lengths) – less force can be produced and opposing for larger preload.
- Isometric contraction, preload also known as ventricular filling before isotonic contraction expels blood
What is Afterload?
Weight not apparent to muscle in resting state; only encountered when muscle has started to contract
Larger afterload would mean less shortening and speed of shortening of muscle tissue.
*Isotonic contraction, afterload also known as pressure in the aorta
What is preload dependent upon?
How can we measure preload?
Dependent on venous blood return
Measures of preload include end-diastolic volume, end-diastolic pressure and right atrial pressure
How to measure of afterload?
Diastolic blood pressure
What is the frank-starling relationship?
( filling of the heart increased, the force of contraction also increased )
Increased diastolic fibre length increases ventricular contraction
Consequence: Ventricles pump greater stroke volume so that, at equilibrium, cardiac output exactly balances the augmented venous return
What are the two factors that affect the Frank-Starling relationship?
( cross-bridges )
Changes in the number of myofilament cross bridges that interact as muscle is stretched.
( at shorter lengths the actin are overlapping reducing the number of cross bridges that could be made at optimal length. )
What are the two factors that affect the Frank-Starling relationship?
( Calcium ion )
2 hypothesis
Changes in the Ca2+ sensitivity of the myofilaments
Hypothesis 1
At longer sarcomere lengths the affinity of Troponin C for Ca2+ is increased due to conformational change in protein
Less Ca2+ required for same amount of force ( graph shift to left hand side )
Hypothesis2
With decreasing myofilament lattice spacing ( gets thinner width wise ), the probability of forming strong binding cross-bridges increases
This produces more force for the same amount of activating calcium
What is stroke work?
Work done by heart to eject blood under pressure into aorta and pulmonary artery
influenced by preload and afterload.
*And cardiac structure
Formula for Stroke work?
Stroke work = SV x P
Volume of blood ejected during each stroke (SV) multiplied by the pressure at which the blood is ejected (P)
What is the Law of LaPlace?
When the pressure within a cylinder is held constant, the tension on its walls increases with increasing radius
How to calculate wall tension?
Wall Tension = Pressure in vessel x Radius of vessel
- Incorporating wall thickness (h), this can be amended to:
T= (PxR)/h
If right heart and left heart walls are working at different pressures how is wall tension constant?
T = P x R
Need to change Radius.
Radius of curvature of walls of LV less than that of RV allowing LV to generate higher pressures with similar wall stress
What happens in Failing hearts related to the law of LaPlace?
Failing hearts often become dilated which increases wall stress
