The Control Of Cardiac Output

Question 1

Define afterload

Answer 1

The load the heart must eject blood against (roughly equivalent to aortic pressure)

Question 2

Define preload

Answer 2

The amount the ventricles are stretched (filled) in diastole - related to end diastolic volume or central venous pressure

Question 3

Define total peripheral resistance

Answer 3

Sometimes referred to as systemic vascular resistance - 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

The pressure that the blood exerts drops as it flows through ‘a resistance’

The arterioles offer the greatest resistance.

Constriction of the arterioles increased the resistance. This will cause pressure in the capillaries and on the venous side to fall but will cause pressure on the the arterial side to rise.

Incresing total peripheral resistance increased BP.

Question 5

What will be the effects of reducing the total peripheral resistance?

Answer 5

If total peripheral restance falls and cardiac output is unchanged then:

• Arterial pressure will fall
• Venous pressure will increase

Question 6

What will be the effects of increasing the total peripheral resistance?

Answer 6

If the total peripheral resistance increases and cardiac output stays the same then;

Arterial pressure will increase

Venous pressure will fall.

Question 7

What will happen if cardiac output increases?

Answer 7

If cardiac output increases and total peripheral resistance is unchanged then:

Arterial pressure will increase

Venous pressure will fall

Question 8

What will happen if cardiac output decreases?

Answer 8

If cardiac output decreases and total peripheral resistance stays the same then:

Arterial pressure will fall.

Venous pressure will rise.

Question 9

How does the heart meet the changes in demand for blood?

Answer 9

If the tissues need more blood, the arterioles and precapillary sphincters will dilate.

Therefore the peripheral resistance falls

The heart needs to pump more so that arterial pressure does not fall and venous pressue doesn’t rise.

The heart ‘sees’ changes in this demand as changesin arterial blood pressue (aBP) and central venous pressue (CVP)

The heart responds to changes in CVP and aBP by INTRINSIC and EXTRINSIC mechanisms.

Question 10

What is cardiac output equal to?

Answer 10

Cardiac output = Stroke Volume x Heart Rate.

Question 11

What is stroke volume equal to?

Answer 11

Stroke Volume = end diastolic volume - end systolic volume

SV = EDV - ESV

Typical stroke volume is 70ml.

This is about 2/3 of normal end diastolic volume.

You can increase SV by increasing EDV or decreasing ESV.

Question 12

How does the ventricle ‘know’ how much to fill?

Answer 12

In diastole, the ventricles communicate with the atrium and the veins but are isolated form the outflow tract.

The ventricle fills until the walls stretch enough to produce an intraventricular pressure equal to the venous pressure.

Question 13

What is the ventricular compliance curve?

Answer 13

The higher the venous pressure, the more the heart fills.

The compliance curve can be increased or decreased in diseased states.

Question 14

What is the frank starling law of the heart?

Answer 14

The law states that the stroke volume of the heart increases in response to an increase in the volume of blood in the ventricles, before contraction (the end diastolic volume), when all other factors remain constant.

Like skeletal muscle - if you stretch the fibres of the heart before contracting, it will contract harder.

An increase in venous pressure will fill the heart more.

-How much the ventricles fill depend on the compliance.

Question 15

What is the starling curve?

Answer 15

Question 16

What is the length tension curve for cardiac muscle?

Answer 16

• If sarcomere length is too short filament overlap interferes with contraction.
• In cardiac muscle also get an increase in calcium sensitivity as the muscle fibres are stretched.

Question 17

How does starlings law of the heart ensure both sides are balanced?

Answer 17

The incrased stroke volume with increased filling of the heart is an intrinsic control mechanism which ensures that both sides of the heart pump maintains the same output.

The pulmonary and systemic circulations operate in series. This means that the same volume of blood pumped to the body must also be pumped to the lungs.

Question 18

What is contractility? How is a chnage in contractility seen and what can cause this change?

Answer 18

Contractility is the force if contraction for a given fibre length.

A change in contractility is seen as a change in the slope of the starling curve.

An increase in contractility will increase the force of contraction for a given left end diastolic pressure.

Extrinsic factors such as sympathetic stimulation and circulating adrenaline can increase contractility.

Reduce sympathtic stimulation will reduce contractility.

Question 19

What is the effect of increasing arterial pressure on stroke volume?

Answer 19

After look is the pressure that the heart has to pump against.

This is the pressure in the aorta (aortic impedance)

Arterial (aortic) pressure is increased when the peripheral resistance is increased. -This makes it harder for the heart to pump out.

Increased total periphral resisant also reduces venous pressue and therefore reduces filling of the hear.

Over time you can get an inappropriate increase in arterial pressre. The heart will have to work harder.

Question 20

What factors determine cardiac output?

Answer 20

1. How hard it contracts
   
   • Determined by end diastolic volume (how much the heart fills) and contractility (increased by sympatheric drive)
2. How hard it is to eject blood.
   
   • Determined by aortic inpendance (roughly arterial pressure).

Cardiac Output = Stoke Volume x Heart Rate

Question 21

What happens if your body is more metabolically active?

Answer 21

If the metabolism of the body increases, then TPR will fall to supply more blood. This will result in a fall in arterial pressure and an increase in venous pressure. The heart will respond by pumping more.

Question 22

How does the cardiovasculalr system respond after eating a meal?

Answer 22

Question 23

How does the cardiovascular system respond to standing up?

Answer 23

• Standing up causes a ‘Pooling’ of blood in legs due to effects of gravity on a column of liquid.
• This decreases
  
  • Venous pressure
  • Cardiac output
  • Arterial pressue
• Both arterial and venous pressure have changed in the same direction so they cannot be adjusted by intrinsic mechanisms.
• Baroreceptor reflex and autonomic nervous system increase heart rate and increase TPR
• But, if the reflexes dont work then you get postural hypertension.

Question 24

How does the cardiovascular system respond to exercise?

Answer 24

• Initially muscles pumping and venoconstriction returns more blood to the heart.
• Later decreased TPR also increase venous return.
• Very early response of increased heart rate (decreased parasympatheitc drive, increased sympathetic drive)
• Increased contractility (increased sympathetic drive)

Question 25

How measure jugular venous pulse?

Answer 25

• Measures in right internal jugular vein.
• Biphasic pulse observed.
• Meaure highest visible pulsations from sternal angle.
• Pulse is seen behind sternocleidomastoid muscles.
• Normal 5-8 cm H₂O.
• Can also be measured with a central line inserted into internal jugular vein or SVC. - slows waveform to be seen.

Question 26

What conditions increase the jugular venous pulse?

Answer 26

• If the right side of the heart doesn’t pump put properly.
• Volume overload with IV infusion.
• If something impairs filling of the heart.