Control of cardiac output

Question 1

Cardiac output equation?

Answer 1

Cardiac output= heart rate x stroke volume

Question 2

What is cardiac output?

Answer 2

The amount of blood ejected from the heart per minute

Question 3

Define heart rate?

Answer 3

How often the heart beats per minute

Question 4

Define stroke volume

Answer 4

How much blood is ejected per beat

Question 5

.Describe how cardiac output? (3pts)

Answer 5

1. Cardiac output changes according to demand. At rest it is 70 bpm.
2. During exercise heart rate increases because the strength of contraction increases. As a result stroke volume increases
3. As you get older heart rate decreases

Question 6

What is the blood flow equation?

Answer 6

Blood flow= Blood pressure / Total peripheral resistance

Question 7

What is Preload?

Answer 7

Stretching of the heart at rest. This increases stroke volume. This is due to Starlings Law

Question 8

What is Afterload?

Answer 8

Afterload opposes ejection. It reduces stroke volume. This is due to Laplace’s law

Question 9

Describe what controls heart rate? (3pts)

Answer 9

1. The SA node pacemaker
2. Sympathetic nerves
3. Parasympathetic nerves

Question 10

What increases intracellular contraction? ( 2pts)

Answer 10

The strength of contraction due to sympathetic nerves and circulating adrenaline increases intracellular contraction.

Question 11

Describe Starlings law?

Answer 11

The energy of contraction of cardiac muscle is relative to the muscle fibre length at rest

Question 12

Describe the ventricles in diastole?

Answer 12

There is greater stretch of ventricles in diastole which is when blood enters. As the ventricles fill they stretch and contract. The energy of contraction is greater when more blood enters the ventricle. As a result in systole the stroke volume is greater.

Question 13

Describe what happens when you increase venous pressure?

Answer 13

Increasing venous pressure causes more blood to enter the heart which significantly increases stroke volume.

Question 14

Describe what happens when you decrease venous pressure?

Answer 14

Reducing central venous pressure reduces stroke volume therefore less blood is ejected per beat from the heart.

Question 15

What happens when there is excess blood in the heart?

Answer 15

when there is excess blood in the heart the muscles overstretch which decreases stroke volume. As a result the heart will start to fail.

Question 16

Describe unstreched fibres in preload?

Answer 16

1. There is overlapping actin/ mysoin. ATP and calicum causes mysoin and actin to move against each other
2. Mechanical inference
3. Less cross bridge formation is available for contraction

Question 17

Describe stretched fibres in preload? ( 4pts)

Answer 17

1. There is less overlapping actin and myosin
2. There is less mechanical inference
3. There is potential for more cross bridge formation
4. There is increased sensitivity to calcium ions. As a result myosin and actin will move against each other more.

Question 18

Describe Starlings law? (2pts)

Answer 18

1. Starlings Law balances the outputs of the right and left ventricle
2. Starlings law is responsible for the fall in cardiac output during a drop in blood volume or vasodilatation e.g hemorrhage and sepsis.

Question 19

What does intravenous fluid do?

Answer 19

Intravenous fluid is injected which will restore cardiac output and stretching

Question 20

Describe Starlings law during orthostasis? (6pts)

Answer 20

1. Starlings law is responsible for the fall in cardiac output during orthostasis which is when you stand up fo a long time
2. This leads to postural hypotension and dizziness as the blood pools in the legs
3. As a result less blood returns to the heart
4. This means the heart will stretch less thus decreasing its cardiac output and less blood will be transported to the rest of the body via the left side of the heart
5. This will cause you to faint- the lack of consciousness allows blood to travel to the brain again.
6. Starlings law also causes increased stroke volume and cardiac output during upright exercise.

Question 21

Describe Laplace’s law and afterload? (4pts)

Answer 21

1. Afterload opposes the contraction that ejects blood from the heart and is determined by wall stress directed through the heart wall.
2. Stress through the wall of the heart prevents muscle contraction
3. More energy of contraction is therefore required to overcome this wall stress to produce cell shortening and ejection.
4. Increasing the pressure of the heart and ventricles increases the tension in the walls. The higher the pressure the more tension in the walls.

Question 22

How is afterload increased? (2pts)

Answer 22

1. Increasing pressure

2. Increasing radius

Question 23

How is afterload reduced? (1pt)

Answer 23

Increasing wall thickness

Question 24

Describe a small ventricle radius? (4pts)

Answer 24

1. A small ventricle has greater wall curvature
2. More wall stress is directed towards the centre of the chamber
3. As a result there is less afterload
4. There is better ejection of blood

Question 25

Describe a large ventricle radius? (4pts)

Answer 25

1. A large ventricular radius means there is less wall curvature
2. This means more wall stress is directed through the heart wall
3. Less wall stress is directed towards the centre of the chamber
4. As a result there is more afterload and less ejection of blood occurs.

Question 26

Describe the wall curvature?

Answer 26

The greater the wall curvature the more stress is directed towards the centre of the chamber and the less afterload there is.

Question 27

What does increasing the radius do?

Answer 27

1. Increases the stress of the heart

2. More stress is directed towards the wall of the heart and less stress is directed towards the centre.

Question 28

Describe the importance of Laplace’s law?

Answer 28

1. Opposes starlings law at rest
2. Facilitates ejection during contraction
3. Contributes to a failing heart at rest and during contraction

Question 29

What does Laplaces law mean?

Answer 29

Laplaces law means good ejection with small radius and bad with large radius. A large radius makes the heart less efficient at pumping blood.

Question 30

What does Laplace’s law state about increased blood pressure?

Answer 30

Laplace’s law states that increased blood pressure will increase wall stress which will increase afterload and reduce ejection.

Question 31

What does the rise in blood pressure affect? (2M)

Answer 31

1. Starlings law= increased stretch gives increased contraction of the heart and increased stroke volume. This will increase hormones such as noradrenaline.
2. This will decrease the sympathetic tone of the blood vessels causing them to dilalate. As a result blood vessels will dilate which will decrease blood pressure. (BAROREFLEX)

Question 32

Why does blood pressure need to be kept constant during exercise?

Answer 32

A high blood pressure will reduce cardiac output and the heart will have to work harder to maintain cardiac output.

Question 33

Describe heart failure?

Answer 33

Heart failure occurs when the heart does not contract properly

Question 34

What can heart failure be due to:

Answer 34

1. Myocardial infraction
2. Cardiomyopathies
3. Mitra-valve re-gurgitation- this means blood is left in the ventricle leading to volume overload and there is too much blood left in the heart.
4. Pressure overload in the chambers

Question 35

What happens when there is an increase in radius? (4pts)

Answer 35

1. Increases in radius or pressure will increase wall stress ( afterload) which opposes ejection
2. The heart compensates with ventricular hypertrophy. This means there is greater myoctye size and more sacromers are formed thus increasing wall resistance
3. This decreases wall stress per sacromere and therefore afterload so maintains stroke volume and cardiac output.
4. This however means more energy is required so more sarcomeres are used. There is greater oxygen. The amount of oxygen required continues to increase so the heart will contract less and produce more heart failure and more of the heart will become oxygen deprived in this circle.

Question 36

Define the energy of contraction?

Answer 36

Is the amount of work required to generate stroke volume

• Depends on starlings law and contractility
• Stroke volume carries out 2 functions:

1. Contracts until the chamber pressure is more than the aortic pressure
2. Ejects blood from the ventricles

Question 37

What are the 2 functions of stroke volume?

Answer 37

1. Contracts until the chamber pressure is more than aortic pressure
2. Ejects blood from the ventricles

Question 38

What do preload and afterload do to stroke volume?

Answer 38

1. Preload increases stroke volume

2. afterload opposes the stroke volume

Question 39

What happens during exercise? (2 pts)

Answer 39

1. During exercise there is increased venous return which leads to increased preload causing the heart to stretch more
2. This causes higher end diastolic volume
3. The ventricle will eject blood to the same end systolic volume so there is an increase in stroke volume shown by an increase in the width of the PV loop.

Question 40

What is hypertension? (2pts)

Answer 40

1. In hypertension there is longer time spent in volumetric contraction to increase pressure in the chamber above that in the aorta to open the valve
2. This uses more energy and lowers the force of contraction reducing stroke volume and an increasing end-systolic volume.