Flashcards in Cardiovascular mechanics I Deck (26):
What is the shape of single ventricular cell?
What is needed for contraction of a single ventricular cell?
What is the approximate size of a single ventricular cell?
100micrometres long and 15micrometres wide
What is the diameter of T-tubule openings?
Describe the excitation-contraction coupling of the heart including the receptors involved.
1. Depolarisation causes the opening of L-type calcium channels.
2. Leads to the influx of calcium into the myocyte.
3. Calcium then binds to the Ryanodine receptor( sarcoplasmic reticulum calcium release channel) and leads to the release of calcium from the sarcoplasmic reticulum( store where calcium is inside)
4. After it has had its effects some of the calcium is taken back up into the SR by Ca2+ ATPase channels ( SERCA)
5. The Na/Ca exchanger effluxes out as much calcium as entered the cell in the first place (does not need energy - uses energy from the concentration gradient of sodium to expel calcium from the cell)
What is the shape of the force-calcium relationship?
What concentration of calcium is sufficient to generate maximum contraction?
Compare the length-tension relationship in skeletal and cardiac muscle and explain why cardiac muscle has this property
Cardiac muscle is much more resistant to stretch and less compliant. Cardiac muscle exerts a lot more passive force, due to properties of the extracellular matrix and cytoskeleton.
It does NOT OVERSTRETCH as it is encased in the pericardium
It is more RESISTANT to stretch than skeletal so will generate a greater passive force as the length increases (stretched)
It is LESS COMPLIANT than skeletal
It uses both ISOTONIC and ISOMETRIC contraction
What are the two types of contraction used by the heart?
Isometric: contraction resists the high pressure, there is no change in length
Isotonic: contraction is the shortening of fibres when blood is ejected from the ventricles.
what limb of the cardiac muscle is important in physiological circumstances and why is the other not?
The descending limb doesn't happen in physiological conditions because the pericardium restrict the stretching.
What is Preload?
The weight that stretches the muscle before it is stimulated to contract e.g. the filling of the ventricles with blood makes it stretch before it is stimulated
What is Afterload?
The weight that is not apparent to the muscle in the resting state and is only encountered once the muscle has started to contract.
-is the weight or mass or pressure that the muscle is trying to overcome- it only becomes apparent after contraction has been stimulated
What is the effect of increasing preload ?
Increasing preload increases the force exerted by the muscle fibres. it governs the amount of force a muscle is capable of producing.
-for the same afterload if you stretch the muscle prior to excitation it will generate more force.
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.
What are the in vivo correlates of preload?
as blood fills the ventricles during diastole it stretched the resting ventricular walls. this streching determines the preload.
so preload is measured: End-diastolic volume,
What are the in vivo correlates of afterload?
-pressure in the aorta
the load against which the left ventricle ejects blood after opening of the aortic valve i.e. it is the blood pressure that the heart must overcome to eject blood.
What is a simple measure of afterload?
Diastolic arterial blood pressure
State Starling's Law.
Increase in diastolic fibre length increases ventricular contraction.
i.e. increase in preload leads to an increase in shortening and speed of shortening.
Starling's law is caused by what factors?
-Changes in the number of myofilament cross-bridges Change of #crossbridges formed
Too much overlap = very few crossbridges
Too little overlap = very few crossbridges
Optimum overlap = maximal crossbridges
-Change in sensitivity of troponin-C to Ca2+
Longer lengths of sarcomeres causes a conformational change in troponin-C so the sensitivity to Ca2+ increases
Increased length = increased Ca2+ sensitivity
-Change of lattice spacing (space between myosin and actin)
Increased stretch causes decreased space between myosin and actin (lattice spacing) and more corssbridges to form
State two possible explanations for increasing calcium sensitivity of the myofilaments.
calcium sensitivity increases when the myofilament are stretched.
-When the muscles are stretched, the lattice spacing (spacing between filaments) decreases meaning that for the same amount of calcium, more cross bridges can be formed. When sarcomere length changes, there is a conformational change in troponin C that gives it a higher affinity for calcium.
What is Stroke Work?
Work done by the heart to eject blood under pressure into the aorta and pulmonary artery.
State the equation for stroke work.
Stroke Work = Stroke Volume x Pressure (at which the blood is ejected) Stroke volume is greatly affected by preload, afterload.
State the law of Laplace.
When the pressure within a cylinder is kept constant, tension increases with increasing radius. T = PR.
What is the physiological relevance of the law of Laplace with regards to the structure of the right and left ventricles?
The 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, the radius of curvature increases and hence the pressure generated decreases.