Sessiol 6 - Control of cardiac outout

Question 1

What determine venous pressure?

Answer 1

• The rate at which blood enters the veins

- The rate at which the heart pumps it out

Question 2

At a constant CO, if TPR falls, how does this effect arterial and venous pressure?

Answer 2

• Arterial pressure will decrease (less pressure needed to drive CO through the arteries)
• Venous pressure will increase (The drop of pressure over arterioles has decreases/blood enters veins easier and being pumper out the same)

Question 3

At a constant CO, if TPR rises, how does this effect arterial and venous pressure?

Answer 3

• Arterial pressure will increase (more pressure needed to drive the CO through the arterioles)
• VP will decrease (the large pressure drop over the arteries and blood cannot enter veins as easy, but is being pumped out at the same rate)

Question 4

If TPR remains the same, and CO increases, what is the effect on arterial and venous pressure?

Answer 4

• Arterial pressure will increase (more pressure needed to drive the increased CO)
• Venous pressure will fall (decreased capacitance in the veins as CO increased/large arterial drop)

Question 5

If TPR remains the same and CO decreases, what effect will it have on arterial and venous pressure?

Answer 5

• Arterial pressure will decrease (less CO being pumped through same resistance -> less pressure needed)
• Venous pressure will increase (Capacitance of veins increased)

Question 6

How does the CVS stabilise when there is an increased CO dduring exercise?

Answer 6

-Increased CO-> increases arterial pressure and decreases venous pressure-> reduced blood flow in arteries-> increased vasodilator factors -> TPR decreases-> causes a decrease in arterial pressure and an increase in venous pressure back to normal with sustained increased CO

Question 7

How does the CVS stabilise if there is a decrease in CO?

Answer 7

-Decrease in CO-> Decreased arterial pressure (less need for pressure) -> decreased TPR-> decreased pressure drop across arterioles -> increased venous pressure -> CO increases

Question 8

What happens to CO, TPR, AP and VP after eating a meal and why?

Answer 8

-Eat a meal -> gut needs more blood as tissues increase metabolic activity -> increase vasodilator metabolites dilates arterioles -> decreased TPR -> decrease in AP and increase VP -> increased CO -> AP and VP maintained -> increased TPR and bodys demand decreases -> decreased CO

Question 9

Define stroke volume

Answer 9

-The amount of blood expelled from the left ventricle per beat

Question 10

Define end diastolic voume

Answer 10

-The amount of blood in the LV at the end of diastole

Question 11

Define end systolic volume

Answer 11

-The amount of blood which remains in the left ventricle after systole

Question 12

How is resting stroke volume a proportion of the maximum stroke volume?

Answer 12

• Increasing expulsion will increase SV and decrease end systolic volume
• Increasing end diastolic volume increases SV

Question 13

What determines filling of the ventricles during diastole?

Answer 13

-Pressure in the veins determines the pressure in the atria which determines how much the ventricles fill

Question 14

Why does the pressure in the veins determine ventricular filling?

Answer 14

-Filling of the ventricles occurs until the walls stretch enough to produce an intraventricular pressure equal to venous(and thus atria) pressure so the valve closes

Question 15

What is the ventricular compliance curve?

Answer 15

-The relationship between the venous pressure and the ventricular volume

Question 16

Define pre-load

Answer 16

-The end diastolic pressure which stretches the ventricle to its greatest dimensions ie the initial stretching of the cardiomyocytes prior to contraction

Question 17

What determines pre-load?

Answer 17

-Venous pressure

Question 18

Define after-load

Answer 18

-The force necessary to expel blood into the arteries at the very beginning of systole

Question 19

How does end diastolic volume effect contraction?

Answer 19

-The greater the end diastolic volume -> the greater the stretch of the ventricular myocytes-> the harder the contraction ie the extent of the filling determines the force of contraction

Question 20

What two factors determine arterial pressue?

Answer 20

• CO

- TPR

Question 21

What is starlings law?

Answer 21

-A rise in VP will cause a rise in SV

Question 22

Draw starling’s curve and explain it

Answer 22

• Venous pressure on X axis, Stroke volume on Y axis
• As venous pressure increases so does stroke volume up to a point
• At some point stretch is resisted by the pericardium and increasing filling over this point compromises contraction and SV decreases

Question 23

Define contractility

Answer 23

-Contractility is the extent to which SV increases in relation to venous pressure (ie the slope of starlings curve)

Question 24

What point of starlings curve is reached under physiological conditions?

Answer 24

-The plateau and declining gradient is not reached, always stays on the slope

Question 25

What determines the force of contraction?

Answer 25

• End diastolic volume (starlings law)

- Contractility

Question 26

What happens to venous pressure and CO if venous return increases?

Answer 26

-VP increases -> end diastolic volume increases -> SV increases -> CO increases

Question 27

What effect does increasing VP have in SV?

Answer 27

-SV rises

Question 28

What effect does decreased AP have on SV?

Answer 28

-SV rises

Question 29

What happens to VP and CO if venous return decreases?

Answer 29

-Venous return decreases –> venous pressure falls -> end diastolic volume decreases -> SV falls -> CO falls

Question 30

What determines end systolic volume?

Answer 30

• How hard the heart contracts

- How easy it is to eject blood (TPR)

Question 31

What effect does increased after-load have on SV?

Answer 31

-Increased TPR -> Increased arterial pressure -> increased after load -> decreased SV

Question 32

What effect does decreased afterload have on SV?

Answer 32

-Decreased TPR-> Decreased arterial pressure -> decreased afterload -> increased SV

Question 33

How is autonomic input to the heart controlled?

Answer 33

-Through baroreceptors

Question 34

What are baroreceptors?

Answer 34

-A sensory system which detects pressure in the arteries and veins

Question 35

What is the carotid sinus?

Answer 35

-A swelling in the base of the internal carotid artery which cantains numerous baroreceptors for detection of arterial pressure through stretch

Question 36

-What happens when the carotid sinus senses a change in arterial blood pressure?

Answer 36

-Sends signals to medulla oblongata of brain which controls the sympathetic activity of the heart

Question 37

Describe the effects of a fall in AP detected by baroreceptors on the HR

Answer 37

-Ultimate increase in HR to counteract low ap through decreasing parasympathetic activity and increasing sympathetic activity (if want a rise above 100)
(Vagus nerve parasympathetic activity is constantly active, lowering HR from 100 to resting -> decreasing this activity will increase the HR)

Question 38

What happens to contractility when there is a fall in AP?

Answer 38

• Fall detected by baroreceptors
• Increased contractility through increasing sympathetic activity
• Increases CO to raise AP

Question 39

Why is there a limit to increasing HR to maintain AP?

Answer 39

-Increasing heart rate shortens diastole, if the HR is very high the heart will not fill adequately and the CO will fall

Question 40

Describe the effects of increasing VP on heart rate

Answer 40

• Bainbridge reflex

- VP increased -> detected by receptors in RA -> increase in CO due to decreased parasympathetic activity