Lecture 14 Flashcards
How is the heart remarkable?
With every contraction every myocyte contracts in unison - if not there will be trouble.
Why does the force of contraction vary?
The heart has to pump out the blood of which it receives, so venous return needs to equal cardiac output.
What is resting CO?
5L/min.
What is exercise CO?
20L/min - Heart rate can only increase 3-fold, the remainder must be stroke volume (SV).
What is a consequence of an increased heart rate?
Less time for filling and ejection.
How does the heart modulate the force of contraction?
Can increase the rate, dimensions (as the ventricles fill - the ventricular walls stretch, this will have an effect - increase force of contraction), neurotransmitters to alter rate and calcium handling and inotropic drugs (positive that will increase the force of contraction).
What is contraction force dependent on?
Intracellular calcium. This can be the calcium transient or the total calcium (calcium stored in the SR). It is a non-linear response.
How is the force modulated?
So the contraction will generate both an isometric force and an isotonic force.
What are the two main ways to change the strength of contraction?
Alter the calcium transient (amplitude or the amount of calcium released by the SR) and the myofilament calcium sensitivity.
Describe myofilament calcium sensitivity?
An increase in calcium sensitivity means force is developed by the cell or mycoyte in response to calcium release. There are many things that alter calcium sensitivity.
Would you use a drug that increased or decreased myofilament calcium sensitivity in a patient with heart failure?
If you increase calcium sensitivity in a patient with HF you will decrease the relaxation time. In somebody with diastolic heart failure (inability to relax fully - there CO is compromised), so increasing sensitivity will decrease relaxation time yet increase the force of contraction (this is ok at resting heart rate). However if you decrease sensitivity you will increase relaxation time yet decrease the force of contraction.
Describe the sarcomere length and who it modulates contractile force?
Depending on the myofilament overlap you get maximum amount of cross-bridges able to form (peak amount of force produced). If you stretch beyond that point, you can get insufficient overlap - so not that many cross bridges formed. If you decrease the overlap you can get some interference between the myofilaments so disrupting the ability for cross bridges to form.
Describe the cardiac response to stretch?
Frank-Starling’s Law of the heart. Increase in EDV will increase SV. As you increase muscle length you increase the force of contraction 9this is the rapid response to stretch). If you do a step increase in muscle length, there is a rapid force then a slow increase in force overtime (calcium transients get higher in amplitude).
Describe the force-frequency relationship?
With each contraction, there is increase of calcium and increase of sodium. From a higher frequency the sodium potassium pump doesn’t remove sodium that quickly from inside the cell as it should, at higher heart rates you will tend to get an increase in intracellular-sodium. By sodium-calcium exchange the high sodium inside will bring in calcium by the sodium-calcium exchange. As frequency increases the number of calcium transient per minute increases, this will tend to load the SR with increased calcium. In failing cardiac tissue there will be a different response. If you increase HR, the force will drop off quite quickly yet the SR calcium content stays constant.
Describe the effect of increasing rate?
As calcium comes in, the removal of calcium occurs less rapidly. There is a decrease in the average membrane potential (change in the membrane potential with increased HR, this will decrease the overall calcium efflux in the cell between beats). Increased number of APs lead to increased intracellular sodium and calcium.
