Physiology of the heart 2 Flashcards
What percentage of the left ventricular blood volume is ejected in an average beat and when would this increase?
60% and increases during exercise.
What is cardiac output
heart rate x stroke volume.
What is the ejection fraction
The percentage of volume of blood ejected with each cardiac contraction - normally around 55 - 60% at rest.
Give examples of factors effecting heart rate
Sympathetic and parasympathetic nervous system Circulating catecholamines - this endocrine effect is not as instantaneous as the neural effect. Drugs
What factors are capable of effecting stroke volume.
Intrinsic contractility Intracellular calcium is very important for the activation of the heart and contractile apparatus. Oxygen, free fatty acids and ATP
What is the preload
The pressure of blood going back to the heart is the filling pressure - Due to total volume, fluid volume and venous return.
What is the afterload
Pressure of blood leaving the heart is the resistance to ejection - this may be due to blood viscosity but is mainly due to resistance in the blood vessels. Increasing the work the heart has to do.
Does the parasympathetic or sympathetic NS have a direct effect on contractility/ stroke volume.
Sympathetic - para does not.
What is the role of calcium in contraction
When the AP reaches the myocytes it causes the opening of L-type calcium channels. Calcium floods into the cell. It acts on ryanodine receptors on the sarcoplasmic reticulum. Calcium is released from intracellular stores and increases the conc of intracellular calcium dramatically. Calcium then interacts with troponin to cause contraction.
How does calcium allow for myosin binding of actin.
Actin has myosin binding sites on it and is surrounded by tropomyosin and troponin. Calcium attaches to troponin, moves tropomyosin and exposes the mysoin binding sites. Myosin and actin come together and cross link and then dissociate in a cycle.
What is the Frank-Starling mechanism of the heart
The heart has a property where by the more it is filled and stretched, the more it will contract. This allows the more blood that has entered the heart to be pumped out. If this didnt happen then there would be a back pressure in the veins which are capacitance vessels which can absorb some of this. So we would see an increase in venous pressure and also in the blood pressure of the lungs leading to breathlessness and peripheral fluid pressure accumulation.
What is the result of increasing filling pressure in frank starlings mechanism
It will effect the left ventricular end-diastolic pressure. When the ventricle has contracted, it relaxes in diastole and increasing in size up to the point where it contracts again. The stretching determines the strength of the next beat. So if more blood comes into the left ventricle there is a higher LVEDP which increases the force of contraction leading to an increase in stroke volume and an increase in cardiac output.
What relationship does LVEDP have with stroke volume
If LVEDP is increased to a certain point SV also increases. Although at too high an LEVDP the relationship breaks down because the muscle has been stretched too far. Most people operate in the lower pressure range in the steeper part of the curve. Increases in venous return result in an increase in SV and CO. The reverse happens when venous return falls.
How will the frank starling curve shift when afterload and contractility change
The curve will shift down in a patient with an intrinsically impaired heart with a decreased contractility.
Or in a patient where the afterload is increased by constricting blood vessels.
This means that the LVEDP increases, theres a very small increase in the stroke volume.
The opposite occurs when contractility is increased or afterload is decreased - the curve moves up to the left. Here only small changes in LVEDP result in large changes in SV.
What is the effect of sympathetic stimulation on the frank sterling curve
If a heart is filled with fluid to increase the preload, as pressure increases, so does SV. An inotrope moves the curve to the left and very little increases in LVEDP will effect th stroke volume to make the heart more efficient. However more energy is needed to maintain this overall.
