Cardiomechanics Flashcards
What is the major ion involved in cardiac muscular contraction
Ca2+
Describe the size of cardiac ventricular cells, the structure of t-tubules and their function
Ventricular cells are 100 μm long and 15 μm wide
T-tubules (transverse tubules) are finger-like invaginations of the cell surface
T-tubule openings up to 200 nm in diameter
Spaced (approx. 2 μm apart) so that a T-tubule lies alongside each Z-line of every myofibril
Carry surface depolarisation deep into the cell
What is the rough composition of a single cardiac muscle cell in terms of mitochondria, myofibrils, sarcoplasmic reticulum, nucleus and other material
Myofibrils:46%
Mitochondria:36%
Sarcoplasmic reticulum:4%
Nucleus:2%
Other:12%
At what rough Ca2+ conc does produced force plateau?
10uM
Describe the movement of Ca2+ in cardiac muscle contraction
Ca2+ enters the cell through an L-type Ca2+ channel
The Ca2+ then binds to a ryanodine receptor on the SR, causing its Ca2+ channel to open and cause Ca2+ to move into the muscle fibres and bind to troponin…
An Na+/Ca2+ transporter continually removes Ca2+ from the cell, allowing the muscle to relax when there is no more stimulus
Describe the length-tension relationship in muscle fibres
As muscle length increases,
-Passive force increases
-Amount of force generated increases
-Active force production increased(Isometric contraction)
How is cardiac muscle different to skeletal muscle in terms of contractility and why is this?
Cardiac muscle is more resistant and less compliant than skeletal muscle due to properties of the extracellular matrix and cytoskeleton
What is the major force difference in cardiac muscle compared to skeletal muscle?
Cardiac muscle generates much more passive force in relation to stretch
Why is only the ascending limb of the length/force graph of cardiac muscle relevant?
Because the heart is contained within the pericardium, so the heart muscle cannot stretch beyond a certain limit.
What is an isometric and isotonic contraction?
Isometric: Muscle contracts but fibre length doesn’t shorten-Pressure in ventricles increases
Isotonic: Muscle contracts and its fibres shorten-Blood is ejected out of the ventricles
What is the difference between preload and afterload?
Preload: Weight that stretches muscle before it is stimulated to contract
Afterload: Weight not apparent to muscle in resting state; only encountered when muscle has started to contract
What physiological action produces preload on the walls of the ventricles?
As blood fills the heart during diastole, it stretches the resting ventricular walls
This stretch (filling) determines the preload on the ventricles before ejection
Give the measures of preload and what preload is dependent on
Measures of preload include end-diastolic volume, end-diastolic pressure and right atrial pressure
Preload is dependent on venous return
What physiological action produces afterload on the walls of the ventricles?
Afterload is the load against which the left ventricle ejects blood after opening of the aortic valve
Any increase in afterload decreases the amount of isotonic shortening-Therefore total contraction
Give the measure of ventricular wall afterload
Diastolic blood pressure
Outline what is meant by the frank-starling relationship
Observations by Frank (1895) and later by Starling (1914) showed that as filling of the heart increased, the force of contraction also increased
Increased diastolic fibre length increases ventricular contraction
Therefore, ventricles pump greater stroke volume so that, at equilibrium, cardiac output exactly balances the augmented venous return
What are the 2 factors that are thought to cause the Frank-Starling relationship?
Changes in the number of myofilament crossbridges that interact
At shorter lengths than optimal the actin filaments overlap on themselves so reducing the number of myosin cross bridges that can be made.
Changes in the Ca2+ sensitivity of the myofilaments
Ca2+ required for myofilament activation
Troponin C (TnC) is thin filament protein that binds Ca2+
TnC regulates formation of cross-bridges between actin and myosin
At longer sarcomere lengths the affinity of TnC for Ca2+ is increased due to conformational change in protein
Less Ca2+ required for same amount of force
Precise mechanism still unclear
Stroke work def
volume of blood ejected during each stroke (SV) multiplied by the pressure at which the blood is ejected (P)
Outline the law of laplace and how this can be applied to vasculature mechanics
When a cylinders’ radius remains constant, the tension on its walls increase with increasing radius
What is different about the left ventricular walls that allow it to generate the same pressure with lower stress compared to the right ventricle
Radius of curvature of walls of LV less than that of RV
C
B
