2- Mechanical Properties Of The Heart Flashcards
Describe excitation contraction coupling in the heart
Depolarisation is sensed by the L-type calcium channel and calcium from outside enters the cell
• Some of this calcium can directly cause contraction
• The rest of the calcium binds to Ryanodine Receptors (also called Sarcoplasmic Reticulum
Calcium Release Channel) which causes release of calcium from the
sarcoplasmic reticulum
• After it has had its effect, some of the calcium is taken back up into the SR by
Ca ATPase channels (also called SERCA - SARCO/ENDOPLASMIC RETICULUM
CALCIUM ATPase)
• Same amount of calcium that came into the cell is effluxed by a Sodium-
Calcium Exchanger
What shape is the force/ cytoplasmic calcium concentration graph
Sigmoid
What append to the force generated in cardiac muscle as muscle length is increase
Force increases
As you keep stretching the muscle, you get to a point where further stretching DOES NOT generate more force - this is because there is not enough overlap between the filaments to produce force
Compare the length tension curves in skeletal and cardiac muscle
(Why doesn’t cardiac muscle overstretch in vivo)
If you overstretch the muscle you get a decrease in force - this is what happens in skeletal muscle when you pull a muscle
• NOTE: passive force is based on the resistance to stretch of the muscle
• Skeletal muscle has much less passive force produced but there is still a bell- shaped curve
• Cardiac muscle is MUCH MORE RESILIENT TO STRETCH than skeletal muscle so exerts more passive force
• It is more resistant to stretch due to the properties of its extracellular matrix and cytoskeleton
• ONLY THE ASCENDING LIMB OF THE LENGTH-TENSION CURVE IS IMPORTANT IN PHYSIOLOGICAL CIRCUMSTANCES IN CARDIAC MUSCLE
• The descending limb doesn’t happen in physiological conditions because the pericardium restricts the stretching
What are the 2 types of contraction in the heart
ISOMETRIC contraction resists the high pressure - there is NO CHANGE IN LENGTH but there is a change in tone (SL values haven’t yet opened)
• ISOTONIC contraction is the shortening of fibres (no change in tension) when blood is ejected from the ventricles
What is the preload and afterload
PRELOAD - the weight that stretched the muscle BEFORE it is
stimulated to contract (i.e. the filling of the ventricles with blood
makes it stretch before it is stimulated to contract)
• AFTERLOAD - weight that is NOT APPARENT to the muscle in
the resting state - only encountered ONCE MUSCLE HAS STARTED TO CONTRACT
Name 3 measures of preload
end-diastolic volume, end diastolic pressure, right atrial pressure
Name a measure of afterload
Diastolic arterial BP
What happens when you increase afterload
Less force and less velocity of contraction
Describe the frank starling relationship
Also known as Starling’s Law
Increased diastolic fibre length increases ventricular contraction
• In other words: an increase in stretching leads to an increase in shortening and speed of shortening/increase in preload leads to an increase in shortening and speed of shortening
• Consequence: when diastolic fibre length increases, ventricles pump a greater stroke volume so that, at equilibrium, cardiac output exactly balances the augmented venous return
• In other words: the amount of blood coming in to the ventricles determines the strength of the ventricular contraction and hence determines the amount of blood leaving the ventricles
What factors cause the frank starling relationship
Changes in the NUMBER OF MYOFILAMENT CROSS BRIDGES that interact
(At shorter lengths than optimal, the actin filaments overlap thus reducing the number of myosin cross bridges that can be made
• The more you stretch the muscle, the more optimum interdigitation of the actin and myosin filaments you achieve)
• Changes in the CALCIUM SENSITIVITY OF THE MYOFILAMENTS
What are the 2 possibilities why Calcium sensitivity increases when the myofilaments are stretched
Possibility ONE:
Troponin C is a thin filament protein that binds to Calcium TnC regulates the formation of cross bridges between actin and myosin
At longer sarcomere lengths, the AFFINITY OF TROPONIN C FOR
CALCIUM IS INCREASED due a conformational change in the protein So less calcium is needed for the same amount of force
• Possibility TWO:
When stretched, the space between myosin and actin filaments DECREASES
NOTE: the space between myosin and actin filaments is called lattice spacing
With decreasing myofilament lattice spacing - the probability of forming strong binding cross bridges INCREASES
This produces more force for the same amount of calcium
What is stroke work?
STROKE WORK = work done by the heart to eject blood under pressure into the
aorta and pulmonary artery
This is the work done by the heart in one contraction
Stroke work= SV x P
Describe the law of Laplace
when the pressure within a cylinder is held constant, the tension on its walls increases with increasing radius
So when you increase the radius, the force around the sides increases
• Force around the side is equal to pressure x radius
Relate the law of Laplace to circulation
Radius of curvature of the LV is LESS than the RV
• This allows the left ventricle to generate HIGH PRESSURES with similar wall stress (tension)
Failing Hearts (Dilated Cardiomyopathy) -hearts become dilated which increases wall stress
