Cardiovascular Mechanics Flashcards
How’s does contraction of a singular ventricular cell take place? (Very simple)
Electrical event ->
Ca2+ influx, release ->
Contractile event
(Excitation contraction coupling)
(Calcium is very important in cardiac muscle (unlike in skeletal muscle))
What is the structure of a ventricular cell?
100 mu m long and 15 wide
T tubules are finger like invagination and of the cell surface
T tubules are 200nm in diameter
They are spaced around 2 mu m apart so that each T tumble lies alongside each Z line of every myofibril
This allows them to carry surface depolarisation deep into the cell
They also have a sarcoplasmic reticulum which acts as the calcium store
46% of the cell is myofibrils, 36% is mitochondria, 4% sarcoplasmic reticulum, 2% nucleus. Other 12%
How do the T tubules interact with the sarcoplasmic reticulum (channels)?
The LTCC (l type calcium channels) in the T tubules have to lie very close to Ryanodine receptors (SR calcium release channels) on the SR
How does excitation- contraction coupling in the heart occur (with relation to channels and calcium)?
In the T tubules there are many LTCCs (l type calcium channels)
When the cell is depolarised the T tubules carry this deep into the cell
The depolarisation is sensed by the LTCC. This causes it to change conformation and open, allowing calcium from outside the cell to flow down its conc gradient into the cytosol
This calcium binds to ryanodine receptors on the sarcoplasmic reticulum (SR Ca2+ release channels). These are ligand operated channels which open up in response to calcium binding to them
Calcium that is stored In the SR is then released into the cytosol
This calcium then binds to troponin on myofillamnets and activates contraction
How does relaxation of the excitation- contraction coupling in the heart occur (with relation to channels and calcium)?
The cytosolic calcium is pumped up against its concentration gradient by the SR Ca2+ ATPase back into the sarcoplasmic reticulum
Here it is ready for release at the next AP
The calcium that initially entered the cell must be effluxed. This occurs during the diastolic interval using the Na+/Ca2+ exchanger
How do the cytoplasmic Ca2+ conc and force of contraction relate?
Sigmoid curve
As Ca increases so does force but in an S shape
What is the length tension relation in cardiac muscle?
As the cardiac muscle lengthens (is stretched) the peak force produced by it is increased, as well as its baseline force
So longer cardiac muscles have more active force production
There is also some recoil similarly to in elastic bands, so there is also an increase in passive force with muscle length
This is the case in isometric contraction (no shortening)
What is the length tension relation in cardiac Vs skeletal muscle?
Cardiac muscle can produce much more passive force as it is more resistant to stretch And less compliant than skeletal muscle
This is due to properties of the extracellular matrix and cytoskeleton
The total force is the sum of active and passive force. However there is an optimum point. When the muscle is stretched beyond this point the total force decreases again as active force decreases
In cardiac muscle, only the ascending limb of this length tension relationship is important as you can’t overstretch cardiac muscle (but you can with skeletal (pulling a muscle))
What are the two forms of contraction used by the heart?
Isometric - muscle fibres do not change length but pressure increases in both ventricles
Isotonic - shortening of fibres and blood is ejected from ventricles
What are the preload and afterload?
Preload - weight that stretches the muscle before it is stimulated to contract (isometric). Associated with ventricular filling
Afterload - Weight not apparent to muscle in resting state, only encountered when muscle has started to contract (isotonic). Associated with the pressure in the aorta
How are force and preload related?
As preload (stretch) is increased, force increases
This is in isometric contraction
How are afterload and shortening of muscle related?
As afterload increases, shortening of muscle decreases
This is in isotonic contraction
However if preload is increased (so a larger stretch), more force can be produced in the afterload
What is the in Vivo version of preload?
As blood fills the heart during diastole it stretches the resting ventricular walls
This stretch determines the preload on the ventricles before ejection
Preload is dependant on venous return
Measures of preload include end diastolic volume, end diastolic pressure and right atrial pressure
What is the in Vivo version of the afterload?
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 that occurs and increases the velocity of shortening
Measures of afterload include diastolic blood pressure
What is starlings law ( frank starling relationship)?
Both frank and starling showed that as filling of the heart increased, the curve of contraction also increases
INCREASED DIASTOLIC FIBRE LENGTH INCREASES VENTRICUKR CONTRACTION
Consequence: ventricles pump greater stroke volume so that, at equilibrium, cardiac output exactly balances the augmented venous return
