L5: Control of Cardiac Output

Question 1

Define afterload?

Answer 1

The load the heart must eject blood against (aortic impedance).

Question 2

Define preload?

Answer 2

Amount the ventricle are stretched in diastole (ventricle filling).

Question 3

Define total periheral resistance?

Answer 3

Resistance to blood flow offered by all the systemic vasculature.

Question 4

What happens to pressure of fluid in a tube as it encounters resistance?

Answer 4

As the fluid passes through the resistance the pressure drops, this is because the amount of blood that gets through is reduced.

Question 5

How does resistance affect the pressure in the arteries and veins?

Answer 5

Constriction of the arterioles –> lumen diameter decreases–> less blood can get through–> pressure in the arteries to increase (proximal to constriction)
Once the blood has got through it then enter the venules–> lumen diameter increases –> pressure in the venous side decreases

Question 6

If CO remains the same and the TPR increases what affect does this have on the arterial and venous pressure?

Answer 6

The arterial pressure will increase as less blood can get through easily. The venous pressure will decrease as there is less blood.

Question 7

If CO remains the same and the TPR decreases what affect does this have on the arterial and venous pressure?

Answer 7

The arterial pressure will decrease as the blood can flow more easily and the venous pressure will increase (compared to before) as there will be more blood.

Question 8

What is the effect of changing the cardiac output whilst keeping the TPR constant?

Answer 8

If you increase the cardiac output and the TPR remains the same then the arterial pressure will increase- more blood ejected from the heart, and the venous pressure will decrease- heart is emptying more so there is space for venous blood.
If you decreases the cardiac output and the TPR remains the same then the arterial pressure will decrease- less blood being ejected from the heart, and the venous pressure will increase- more blood left in the heart.

Question 9

During exercise the muscle require larger amount of oxygen for aerobic respiration how does the heart meet the changes in demand for blood supply?

Answer 9

• The arterioles and precapillary sphincters dilate to decrease the TPR
• The heart needs to pump more (↑CO) –> prevent arterial pressure decreasing and the venous pressure increasing (eject blood so that venous blood has somewhere to go)
• See changes in this demand as changes in arterial blood pressure and central venous pressure
• It is controlled by intrinsic and extrinsic mechanisms.

Question 10

What is cardiac output?

Answer 10

Total amount of blood ejected from the ventricles each beat (SV) X by the number of beats per minute (HR).
Normally 5L.

Question 11

How is stroke volume calculated?

Answer 11

The end diastolic volume (EDV) - the end systolic volume (ESV)
EDV = amount of blood in the heart during diastole
ESV= amount of blood in the heart after systole

Question 12

What does a ventricular compliance curve show? What sort of relationship is it?

Answer 12

The relationship between the LV pressure and the LV volume.
As the ventricles fill with blood the pressure and volume are determined by the compliance of the ventricles.
The relationship is not linear especially at higher volumes.

Ventricles fill until the wall stretch enough to produce an IV pressure equal to venous pressure
↑venous pressure, more the heart fills
More the heart fills, ↑ LV pressure

Question 13

How can the ventricle compliance be altered?

Answer 13

In diseased states the heart can become stiffer decreasing the compliance of the ventricles resulting in an increased pressure for a given volume.
In other diseased states the heart can become stretcher increasing the compliance of the ventricles resulting in a decreased pressure for a given volume.

Question 14

What does Frank-Starlings law of the heart state?

Answer 14

The more the heart fills the harder it contracts (up to a limit).

Question 15

How does venous return affect stroke volume? Why?

Answer 15

An increase in the venous return causes the Left ventricular end diastolic pressure and volume to be increased using the principles of Starlings Law this means that the force of contraction will be increased leading to an increased stroke volume.

Question 16

What does Starling curve look like?

Answer 16

Y axis- Stroke volume (ml)

X axis- Left ventricular end diastolic pressure (mmHg)

Question 17

How can Starlings Law be explained by the sarcomere in cardiac muscle?

Answer 17

As the ventricles get stretched the actin and myosin in the sarcomere overlap less meaning there is more space for contraction. In cardiac muscle stretching also causes an increase in calcium sensitivity.

Question 18

How does Starlings law impact the heart?

Answer 18

Starlings law ensures that the increased stroke volume with increased filling of the heart.
Increase in cardiac output ensuring that both sides of the heart maintain the same output. It is an example of an intrinsic control mechanism.

Question 19

How does contractility impact Starlings curve?

Answer 19

Contractility is the force of contraction for a given fibre length.
Increase in contractility will increase the force of contraction for a given EDP resulting in a larger stroke volume.

Question 20

What can change the contractility?

Answer 20

Extrinsic factors such as sympathetic innervation and adrenaline will increase contractility.
Parasympathetic innervation will decrease contractility.

Question 21

How does increasing the arterial pressure affect the stroke volume?

Answer 21

Increase in arterial pressure (afterload- the pressure the heart has to pump against) is caused by an increase in TPR, this is turn results in decreased venous pressure and therefore reduced filling of the heart leading to reduced stroke volume.

Question 22

What factors determine the cardiac output?

Answer 22

1. How hard the heart contracts
   - Determined by how much the ventricles fill (EDV) -Starlings Law and sympathetic input
2. How hard it is to eject blood
   - determined by aortic impedance (afterload)

Question 23

How does the cardiovascular system respond after eating a meal?

Answer 23

After eating a meal the TPR decreases to allow increased blood flow to the gut, this causes the arterial pressure to decrease and the venous pressure to increase. The heart rate increases to compensate for a decrease arterial pressure and the stroke volume increases to cope with the increased venous return and to increase arterial pressure. This causes the cardiac output to increase which increases the arterial pressure and decrease the venous pressure.

Question 24

When you stand up what happens to the CVS?

Answer 24

Standing up causes the blood to pool in you feet and legs, this causes the venous pressure to decrease which causes and decrease the the stroke volume and cardiac output this means that the arterial pressure also decreases. It can no longer be controlled by intrinsic mechanisms.
Baroreceptors detect the decrease in pressure and activates the sympathetic system to increase the heart rate and increase TPR.

Question 25

How does exercise change the CVS?

Answer 25

Exercise causes an increase in the venous return due to muscular pumping. This means that the stroke volume increases. However exercise also causes the heart rate to increase (increased sympathetic drive) and contractility to increase (increased sympathetic drive) which leads to increased cardiac output.

Question 26

What does the jugular venous pulse measure?

Answer 26

Measures the central venous pressure (right atria pressure).

Question 27

What are the different stages of the jugular venous pulse?

Answer 27

It is a biphasic pulse with 3 peaks and 2 troughs that correspond to different parts of the cardiac cycle.
Peaks
1. Peak is the A wave (atrial pressure rises causes backflow into the SVC which is reflected to the right internal jugular vein)
2. Peak 2 is the C wave (tricuspid valve closes,which causes a small increase in pressure in the right atrium which is reflected in the internal jugular vein)
3. Peak 3 is the V wave (Right atrium fills with blood increasing the pressure)
Troughs
1. X descent- when the ventricle contract pulls the atria down decreasing the pressure in the atrium
2. Y descent- pressure in the atria exceeds the pressure in the ventricles the tricuspid valve opens causing the pressure to decrease.

Question 28

What can cause the jugular venous pressure to increase?

Answer 28

• Defects in pumping of the heart
• Volume overload with IV infusion
• If something impairs the filling of the heart

Question 29

How is jugular venous pressure measured?

Answer 29

Patient lies down turns head to left.
internal jugular vein cannot be seen as it lies beneath the muscle (sternocleidomastoid).
Measured by lifting patient up until external jugular vein disappears and measuring from the most cranial part draw an imaginary line horizontally to above the sternal angle and add +4cm as 4cm above right atrium.
JVP measure in cm H20 as blood is like water
normally 5-8cmH2O.