Lecture 28 Flashcards
Describe cardiovascular regualtion?
There can be changes in cardiac output. These changes can be:
- Physiological - exercise or change in posture.
- Pathological - heart failure, haemorrhage.
What are the four factors that determine cardiac output?
- Heart rate.
- Myocardial contractility.
- Preload.
- After load.
What is the CO formula?
CO = SV x HR (SV dependent on venous return).
CO = change in P/TPR.
As oxygen consumption increases so does cardiac output. What goes back to the heart decides cardiac output/the needs of the tissue.
Describe the cardiac output curve?
Right atrial pressure is one of the main determinants of cardiac output. As you improve venous return (right atrial pressure reflects this) you will increase the preload on the heart and increase cardiac output. Hyper effective curve is the effect of both sympathetic activity and exercise training. Heart failure will flatten the curve.
What determines venous return?
- Right atrial pressure - if RA pressure is high this will impede the flow back to the heart (as there is increased resistance).
- Degree of filling - determined by blood volume, if you lose blood CO will be decreased.
- Venous capacitance - big saggy veins will also impede the return of blood, exercising can decrease the capacitance.
- Systemic vascular resistance - if this is high it will impede the flow back to the heart.
Describe the vascular function curve?
As we change venous return this will effect the preload on the heart/also right atrial pressure. When RA pressure is high venous return is 0 (RA is 7mmHg), as the RA pressure decreases and venous return increases up to a point (at 0mmHg to -4mmHg). If a vein has low pressure it collapses and there is no flow through it. Normally there is a gradient between the aorta and the veins at 100mmHg and a CO of 5L/min. You need the delta pressure of 100mmHg to make sure you can maintain the CO of 5L/min. Need to have a pressure change between arteries and veins to maintain flow back to the heart.
What happens if there is no flow?
When you top the pump, the arterial pressure decreases and the venous pressure increases. As you get a small decrease in volume and a big drop in pressure on the arterial side. A 19mmHg drop in arterial pressure will only raise venous pressure by 1mmHg. The meeting point is 7mmHg - Mean Systemic Filling Pressure. Tight relationship between blood volume and circulatory pressure. If you increase the blood volume of sympathetic stimulation you will increase the mean circulation filling pressure.
Describe the experiment?
Heart is replaced by a pump - pressure gradient is dependent on cardiac output.
What is the main determinant of systemic filling pressure?
Blood volume. If you reduce your blood volume the systemic pressure will decrease. Reduce the CO, reduce aortic pressure, less drive to venous side (so more pooling of blood on venous side). If we increase venous return we get to a limit - when RA pressure is too low. Unless you improve venous return you can’t increase CO anymore.
Describe vascular resistance?
If we increase resistance it will make it harder for blood to get back to the heart, so this will reduce the venous return. We still plateau at the same pressure no matter what resistance. Most of this is the resistance in the venous side.
Describe the cardiac function curve?
After one or two beats cardiac output and venous return must equal. Normally we are sitting at RA pressure at 2mmHg. If we were to increase cardiac function quickly this won’t have any effect on CO on the long term.
What happens when we have an increase in blood volume?
If you increase your blood volume you will increase the mean systemic filling pressure, so a higher cardiac output (due to an increase in venous return).
How will sympathetic stimulation change cardiac output?
If we have max stimulation you will improve the heart rate and an increase in CO and improve venous return by increasing mean systemic filling pressure and a slight decrease in the slope due to an increase in resistance. There is the same RA pressure but a higher CO.
Describe how exercise effects the function of the heart?
There will be sympathetic stimulation, and an increase in cardiac output curve. If you just do something to the heart in exercise then there won’t be as much of an increase to CO at all. So there also needs to be sympathetic stimulation affecting the vasculature. Blood gets back to the heart due to the muscles pushing the blood back to the heart - increase in venous return.
Describe heart failure?
The heart is struggling to get output, so the heart isn’t pumping particularly well. You will get compensations, sympathetic activation trying to increase the curve upwards. There is also fluid retention, increase in sympathetic activity - more fluid on board (increase mean systemic filling pressure. The person has a high RA pressure however their CO is normal. Compensated Heart Failure - Able to maintain CO by increasing sympathetic tone and increasing their blood volume. This isn’t a good thing because it puts more preload on the heart and there is a high arterial pressure and more work load.
