Control of Cardiac Output

Question 1

What is the general minimum stroke volume, heart rate, and thus cardiac output?

Answer 1

SV: ~70ml - ~140ml

HR: ~70bpm - ~200bpm

CO: ~5L/min - ~30L/min

Question 2

Describe the extrinsic and intrinsic control of stroke volume

Answer 2

Intrinsic mechanisms
– Self-regulation
– Frank-Starling mechanism
– ↑EDV ↑force of contraction

Preload
– Venous pressure and venous return to heart (end diastolic pressure, EDP)

Afterload
– Aortic/pulmonary artery pressure

Extrinsic mechanisms
– Sympathetic nerves

Question 3

What is Frank-Starling’s law of the heart?

Answer 3

The stroke volume of the left ventricle will increase as the left ventricular volume increases due to the myocyte stretch causing a more forceful systolic contraction

Question 4

In basic terms what does Frank-Starling’s law of the heart do?

Answer 4

Allows for automatic adjustment for small imbalances between the left ventricle and right ventricle.
Automatic balancing between CO from left-side of heart to volume returning to right-side.

Question 5

How is cardiac output calculated?

Answer 5

Stroke volume x heart rate

Question 6

What can increase stroke volume?

Answer 6

↑ in end-diastolic ventricular volume

↑ in sympathetic nerve activity

↑ in adrenaline

Question 7

What can increase heart rate?

Answer 7

Decrease in parasympathetic activity

↑ in sympathetic nerve activity

↑ in adrenaline

Question 8

Describe transmission and damping of pressure pulses in the arteries

Answer 8

Arterial system has a limited capacity and so systemic vascular resistance limits how fast blood can escape in systole. This means the arterial system expands to accommodate the full ventricular stroke volume

In diastole energy stored in the arterial walls during systole expansion drives blood forwards

Basically elasticity bitches

Question 9

Describe what is meant by vascular compliance

Answer 9

A noncompliant vessel is a rigid tube that resists expansion when internal pressure rises: capillaries, arterioles, actual plumbing etc

A compliant vessel is a tube with elastic walls that swell when internal pressure rises: arteries, veins, car tyres etc

Compliance decreases with age and vasoconstriction

Question 10

How is mean arterial blood pressure calculated?

Answer 10

MABP = Diastolic pressure + 1/3 pulse pressure

Where
Pulse pressure = systolic pressure – diastolic pressure

So its not realllyyy a mean which is mean

Question 11

How do you calculate pulse pressure?

Answer 11

Systolic pressure – diastolic pressure

Question 12

What factors determine the magnitude of pulse pressure?

Answer 12

Stroke volume
– Intrinsic and extrinsic factors
– Remember afterload, preload, sympathetic innervation

Speed of ejection of stroke volume

Arterial compliance
– Decreases with age
• Arteriosclerosis

Question 13

What factors affect flow of a fluid through a vessel?

Answer 13

Viscosity of fluid (increases w hematocrit - clinical use?)
Vessel length - bigger is lower ml/sec
Vessel radius - bigger is more ml/min

Question 14

What is the Poiseuille equation?

Answer 14

Flow is proportional to deltaP x (radius of vessel to the power of 4)
OVER
viscosity x length

This becomes this in physiology:
Flow = DeltaP
OVER
resistance

as viscosity and vessel length (now known as “K”) plus vessel radius are all factors that generate resistance to flow by contributing to friction between blood and the walls of vessels.

Basically:
As resistance increases, flow decreases.

Question 15

How is arterial pressure calculated in relation to total peripheral resistance?

Answer 15

Arterial pressure = cardiac output x total peripheral resistance

Question 16

How do you calculate resistance in the pulmonary and systemic circulation?

Answer 16

MABP = Diastolic pressure + 1/3 pulse pressure
Systemic = 120/75 Pulmonary = 26/8

Arterial pressure = cardiac output x total peripheral resistance

so total peripheral resistance = arterial pressure/cardiac output

Question 17

Main points

Answer 17

Cardiac output depends on venous return and can be intrinsically and extrinsically altered
Arterial pressure depends on cardiac output and total peripheral resistance
Small changes in vessel radius have big implications on blood flow

Question 18

Learning outcomes

Answer 18

To explain how cardiac output can be influenced by both intrinsic and extrinsic factors.
To describe how and why pressures within different parts of the cardiovascular system vary.
To identify the variables within the Poiseuille equation that define mean arterial blood pressure, and how these vary between systemic and pulmonary circulations.
To calculate mean arterial blood pressure based on systolic and diastolic pressures.