Week 6 - Control of Cardiac Output Flashcards
What determines venous pressure?
Rate at which blood flows into veins
Rate at which blood flows out of the veins
What determines arterial pressure?
TPR
Cardiac output
If TPR rises, but cardiac output is constant, what happens to arterial and venous pressures?
Arterial pressure rises
Venous pressure falls
If TPR is constant, but cardiac output rises, what happens to arterial and venous pressures?
Arterial pressure rises
Venous pressure falls
How does the body respond when arterial pressure falls?
Detected by baroreceptors in the carotid sinus
Sends signals to the medulla oblongata
This sends signals to heart
Results in rise in heart rate and force of contraction, in order to raise arterial pressure
What does cardiac output depend on?
Heart rate and stroke volume
What is stroke volume?
Stroke volume is the amount of blood pumped out of the heart at each systole. It is therefore the difference between end diastolic volume and end systolic volume.
How may stroke volume be increased?
Filling the heart more during diastole
Emptying the heart more during systole
What determines end diastolic volume?
Venous pressure
Ventricular compliance - how much the ventricles stretch
How do venous pressure and ventricular pressure relate?
As the ventricles fill, the walls stretch. This increases interventricular pressure.
At the point at which ventricular pressure = venous pressure, filling ceases.
Therefore, whatever venous pressure is, ventricular pressure rises to meet it.
How does venous pressure affect end diastolic volume?
Higher venous pressure means the ventricles have to fill with a larger volume of blood to match venous pressure.
Therefore higher venous pressure = higher end diastolic volume
What is Starling’s Law of the Heart?
Higher venous pressure = increased filling = higher stroke volume
Or “more in = more out”
What is preload?
How does it affect stroke volume?
Preload is the “end diastolic stretch”
Up to a point, higher preload = greater force of contraction = higher stroke volume
Why does the Starling curve show that at very high venous pressures, stroke volume begins to decrease?
At very high venous pressures, the ventricles must fill very full to match the venous pressure. However, the pericardium is not distensible, and thus prevents the heart from overfilling. Therefore, at very high venous pressures, the heart cannot fill enough to meet venous pressure, and therefore stroke volume decreases.
What is contractility?
How does it relate to the Starling curve?
Contractility is the extent by which stroke volume increases with venous pressure.
It is the slope of the curve
What determines end systolic volume?
Afterload
Force of contraction
What is afterload, and what does it depend on?
Afterload is the force against which the ventricle ejects its contents.
For the left ventricle, afterload is arterial pressure, which in turn depends on TPR.
How does TPR affect stroke volume?
Low TPR = low arterial pressure = low afterload
Therefore, it is easy for blood to leave the ventricle, and hence stroke volume will be high.
What influences force of contraction?
Preload
Contractility
How does a fall in arterial pressure affect heart rate, in reference to changes in the ANS?
If arterial pressure decreases, heart rate will increase, due to:
Reduction in parasympathetic activity
Increase in sympathetic activity (also increase force of contraction)
What is meant by the “Bainbridge reflex”?
Changes in venous pressure are sensed in the right atrium
When venous pressure falls, parasympathetic activity in the heart is reduced, and heart rate increases.
