Cardiac Output And Blood Pressure

Question 1

What is the role of the smooth muscle in arteries ?

Answer 1

It gives rise to compliance and resistance which in turn give rise to blood pressure

Question 2

What is compliance ?

Answer 2

It is the change in volume with respect to the change in pressure in arteries

Question 3

What does compliance do ?

Answer 3

Compliance alters the arteries to act as pressure reservoirs to maintain continuous flow during diastole when the heart is refilling

Question 4

How much blood is in the left ventricle during systole ?

Answer 4

130 ml and this is known as end diastolic volume

Question 5

What happens when the left ventricle contracts ?

Answer 5

The heart pumps 70 ml of blood into the aorta
From this 70 ml, 30 ml will get trapped in the pressure reservoirs due to the compliance of the vessel and so 40 ml will flow down the cardiovascular tree

Question 6

What happens during diastole ?

Answer 6

There is elastic recoil of the aorta and the remaining blood is pushed into circulation providing pulsatile but continuous flow

Question 7

What are the factors that the arterial pressure fluctuations in the cardiac cycle are dependant on ?

Answer 7

1) rate of ejection from the ventricles
2) distension of arterial walls
3) rate of distribution - varies based on the resistance in the vessels

Question 8

Describe how distension of the arterial walls affects arterial pressure fluctuations in the cardiac cycle

Answer 8

When there is low compliance there are large pressure fluctuations due to stiff arterial walls
High compliance highlights greater elasticity leading to less pressure fluctuations

Question 9

What is the systolic phase ?

Answer 9

It is the peak pressure during the cardiac cycle
Peak of ventricular ejection
80 - 120 mmHg

Question 10

What is the diastolic phase ?

Answer 10

It is measured just before ventricular ejection
60-80 mmHg

Question 11

What is pulse pressure ?

Answer 11

It is the difference between systolic and diastolic blood pressure
~ 40 mmHg

Question 12

What is mean arterial pressure ?

Answer 12

• heart spends more time in diastole than systole
• cardiac cycle is 800 ms
• systole is 300 to 350 ms
• mean arterial pressure is a time weighted average
• approximately 93 mmHg

Question 13

What is the dicrotic notch ?

Answer 13

It is associated with the closing of the aortic valve during the falling phase of arterial pressure

Question 14

How does the lub-dub sound of the heart arise ?

Answer 14

• The closure of the valves at the end of ventricular systole creates a lub-dub sound
• The dub is the closure of the aortic valve

Question 15

How is blood pressure measured ?

Answer 15

• blood pressure is measured using the Auscultatory method
• it requires the use of a sphygmomanometer cuff around the upper arm
• attached to the sphygmomanometer is an inflation bulb
• when the brachial artery is pushed down using the inflation bulb there will be a point when the brachial artery is completely blocked
• the stethoscope is placed on the antecubital fossa so the korotkoff sounds can be heard

Question 16

What is the equation to work out blood pressure ?

Answer 16

Blood pressure = total peripheral resistance x cardiac output

Question 17

What is resistance to blood flow dependent on ?

Answer 17

• vessel radius
• vessel length
• blood viscosity

Question 18

Describe how vessel radius affects resistance

Answer 18

Vasodilation and vasoconstriction control resistance in arteries and arterioles
Vasodilation decreases resistance
Vasoconstriction increases resistance

Question 19

Describe how vessel length affects resistance

Answer 19

The longer the vessel the greater the resistance

Question 20

Describe how blood viscosity affects resistance

Answer 20

The greater the viscosity of the blood the greater the resistance

Question 21

How does the diameter of the blood vessel change ?

Answer 21

• sympathetic nervous system
• metabolites e.g. ATP and lactate
• circulating hormones

Question 22

How does the the sympathetic nervous system change the diameter of the blood vessel ?

Answer 22

Noradrenaline binds to adrenergic receptors causing vasoconstriction of the blood vessel
This increases the mean arterial pressure by increasing total peripheral resistance
Tonic vasoconstriction during rest maintains blood pressure by sympathetic innervation

Question 23

How do metabolites change the diameter of the blood vessel ?

Answer 23

H+ , ATP and lactate usually cause vasodilation of the smooth muscle cells lining the arterioles

Question 24

How do circulating hormones affect the diameter of blood vessels ?

Answer 24

Adrenaline activity could influence vasoconstriction and vasodilation
Alpha and beta adrenergic receptors are important in this process

Question 25

What is cardiac output ?

Answer 25

It is the volume of blood ejected by the left ventricle per minute (5L/min)
- it varies depending on the body’s demand e.g. in exercise it can go up to 25L/min
- total peripheral resistance and cardiac output are important in determining blood pressure

Question 26

What is the value of resting heart rate ?

Answer 26

60 - 70 bpm

Question 27

What is stroke volume ?

Answer 27

Stroke volume = end diastolic volume - end systolic volume
~ 70 ml

Question 28

What is venoconstriction ?

Answer 28

Increase in blood flow from the vena cava into the heart which increases the end diastolic volume therefore having an impact on cardiac output

Question 29

What does Starlings law suggest ?

Answer 29

It states that the energy of contraction of the ventricles is a function of the initial lengths of the muscle fibres in the walls of the heart
Increased stretch gives rise to stronger contraction

Question 30

What is the relationship between the length of the sarcomere and tension ?

Answer 30

As sarcomere length increases so does tension but this only happens until a certain point
Beyond this point actin myosin cross bridges can’t form and so there is a decrease in stroke volume and end diastolic volume

Question 31

What are the factors that affect end diastolic volume ?

Answer 31

• internal factors : venous pressure - blood pressure in the vena cava is low and this is greatly influenced by gravity
• external factors : during inspiration, there is a fall in intrathoracic pressure which promotes filling of the right ventricle therefore impacting end diastolic volume

Question 32

What is the relationship between end diastolic volume and stroke volume ?

Answer 32

A greater end diastolic volume = greater stroke volume and therefore increased contractility