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HOM: cardiovascular physiology > Lecture 5: control of cardiac output > Flashcards

Lecture 5: control of cardiac output Flashcards

1
Q

Describe the experiment by Guyton et al, 1957

What was observed?

What did they conclude

A

The hearts of dogs were replaced by high output pumps

Pumping capacity was reduced below normal CO

However, inceasing pumping capacity did not increase CO

  1. Heart rate is necessary to maintain CO
  2. The heart does not normally limit CO
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2
Q

Why doesn’t an increase in HR increase CO?

A

The circulation is a closed system and so the heart cannot increase the arteriovenous pressure gradient (PA-PV) beyond the point at which PV becomes negative because this will collapse veins and limit CO

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3
Q

What is theh central venous pressure in a healthy heart?

A

Almost 0 (even at rest)

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4
Q

What must also increase with HR to cause an increase in CO?

A

Venous return

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5
Q

What is the main determinant of cardiac output?

A

Mean systemic filling pressure (MSFP)

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6
Q

How can PA be increased if PV =0?

A

Increase MSFP

If PV was 0 then it would become negative when heart pumped causing venous collapse

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7
Q

What does the mean pressure in a system determine according to Darcy’s law?

A

The maximum flow rate for a given resistance

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8
Q

What is mean systemic filling pressure (MSFP)?

A

The mean pressure in a system (the pressure that would eventually exist everywhere if the heart stopped pumping)

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9
Q

How can MSFP be increased?

A
  • Blood transfusion
  • Drinking isotonic fluid
  • Venoconstriction
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10
Q

How is the total blood volume calculated?

A

70-80ml/Kg of body mass

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11
Q

What is the approximate total volme of blood for a 70Kg person?

A

5L

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12
Q

Why can large amounts of blood be added to empty vessels without causing an increase in pressure?

A

Pressure does not change until vessel wall begins to stretch

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13
Q

What is the unstressed volume of a circulation?

What percentage of the total volume does this acount for?

A

The volume of blood that just fills the circulation without stretching the vessel walls

80%

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14
Q

How is compliance of a vessel calculated?

A

ΔV/ΔP

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15
Q

When may a vein become stiff?

A

If it is overstretched

observed in a saphenous vein graft used to replace a blocked coronary artery

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16
Q

What does venous compliance ensure?

A

Any reduction in venous pressure with cardiac activity is relatively small

17
Q

Which has a higher compliance, veins or arteries?

18
Q

What is the averge pressure in the cardiovascular system?

19
Q

Why is arterial pressure so much higher than venous pressure?

A

Arteries have a much lower compliance which means that movement of blood into arteries causes a steep rise in pressure

20
Q

What is the value of mean arterial pressure?

21
Q

What equation connects CO, the pressure gradient and the resistance in the system?

A

CO = (ABP-RAP)/TPR

May also be seen as CO = ABP/TPR

(because RAP is so small compared to ABP)

22
Q

How is the arteriovenous pressure gradient calculated?

23
Q

How would an increase in MSFP affect RAP assuming CO remains constant?

A

Increase

(pressure would increase throughout the circulation and the greatest percentage increase would be at the right atrium where pressure was initially lowest)

24
Q

How does MSFP affect RAP?

A
  • Initial increase in RAP
  • Increased stroke volume
  • Increased CO
  • Brings RAP close to 0 (starling mechanism)
25
Q

How can afterload be increased?

A

Arteriolar vasoconstriction causing an increase in TPR

26
Q

How does increasing TPR affect CO?

A

Remains constant (does not decrease due to Starling mechanism)

Increased TPR = increased ventricular stretch = increased force of contraction = increased ABP

(SV and CO remain constant)

27
Q

How does increasing MSFP affect HR?

A

Increases HR

28
Q

Why is exercise impaired by severe dehydration?

A

Decreased MSFP = Decreased CO

29
Q

What is MSFP determined by?

A

Volume of blood and the mean tension in blood vessel walls

30
Q

How much are MSFP and CO changed by when blood volume increased by 20%?

A

MSFP doubles

(stressed volume is doubled)

CO doubles

31
Q

At what point does MSFP fall below 0 as blood is lost from the body?

A

After 40% of blood is lost

32
Q

What percentage of the blood is in venules and small veins?

33
Q

How much can sympathetic venoconstriction influence MSFP?

A

It can treble MSFP

34
Q

What effect does venoconstriction have on TPR and MSFP?

A

Increases MSFP

Does not significantly influence TPR

35
Q

What effect does arteriolar constriction have on TPR and MSFP?

A

Does not change MSFP

Increases TPR

36
Q

How much of the blood in the circulation is contained within the arterioles?

37
Q

How is RvR regulated?

A

Not specifically regulated but can decrease in exercise

38
Q

How does increased RAP affect CO?

A

CO increases

39
Q

What are the axes of the Guyton curve?

A

X: RAP
Y: VR or CO

HOM: cardiovascular physiology (7 decks)

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