Control of cardiac output Flashcards
Cardiac output equation?
Cardiac output= heart rate x stroke volume
What is cardiac output?
The amount of blood ejected from the heart per minute
Define heart rate?
How often the heart beats per minute
Define stroke volume
How much blood is ejected per beat
.Describe how cardiac output? (3pts)
- Cardiac output changes according to demand. At rest it is 70 bpm.
- During exercise heart rate increases because the strength of contraction increases. As a result stroke volume increases
- As you get older heart rate decreases
What is the blood flow equation?
Blood flow= Blood pressure / Total peripheral resistance
What is Preload?
Stretching of the heart at rest. This increases stroke volume. This is due to Starlings Law
What is Afterload?
Afterload opposes ejection. It reduces stroke volume. This is due to Laplace’s law
Describe what controls heart rate? (3pts)
- The SA node pacemaker
- Sympathetic nerves
- Parasympathetic nerves
What increases intracellular contraction? ( 2pts)
The strength of contraction due to sympathetic nerves and circulating adrenaline increases intracellular contraction.
Describe Starlings law?
The energy of contraction of cardiac muscle is relative to the muscle fibre length at rest
Describe the ventricles in diastole?
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.
Describe what happens when you increase venous pressure?
Increasing venous pressure causes more blood to enter the heart which significantly increases stroke volume.
Describe what happens when you decrease venous pressure?
Reducing central venous pressure reduces stroke volume therefore less blood is ejected per beat from the heart.
What happens when there is excess blood in the heart?
when there is excess blood in the heart the muscles overstretch which decreases stroke volume. As a result the heart will start to fail.
Describe unstreched fibres in preload?
- There is overlapping actin/ mysoin. ATP and calicum causes mysoin and actin to move against each other
- Mechanical inference
- Less cross bridge formation is available for contraction
Describe stretched fibres in preload? ( 4pts)
- There is less overlapping actin and myosin
- There is less mechanical inference
- There is potential for more cross bridge formation
- There is increased sensitivity to calcium ions. As a result myosin and actin will move against each other more.
Describe Starlings law? (2pts)
- Starlings Law balances the outputs of the right and left ventricle
- Starlings law is responsible for the fall in cardiac output during a drop in blood volume or vasodilatation e.g hemorrhage and sepsis.
What does intravenous fluid do?
Intravenous fluid is injected which will restore cardiac output and stretching
Describe Starlings law during orthostasis? (6pts)
- Starlings law is responsible for the fall in cardiac output during orthostasis which is when you stand up fo a long time
- This leads to postural hypotension and dizziness as the blood pools in the legs
- As a result less blood returns to the heart
- 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
- This will cause you to faint- the lack of consciousness allows blood to travel to the brain again.
- Starlings law also causes increased stroke volume and cardiac output during upright exercise.
Describe Laplace’s law and afterload? (4pts)
- Afterload opposes the contraction that ejects blood from the heart and is determined by wall stress directed through the heart wall.
- Stress through the wall of the heart prevents muscle contraction
- More energy of contraction is therefore required to overcome this wall stress to produce cell shortening and ejection.
- Increasing the pressure of the heart and ventricles increases the tension in the walls. The higher the pressure the more tension in the walls.
How is afterload increased? (2pts)
- Increasing pressure
2. Increasing radius
How is afterload reduced? (1pt)
Increasing wall thickness
Describe a small ventricle radius? (4pts)
- A small ventricle has greater wall curvature
- More wall stress is directed towards the centre of the chamber
- As a result there is less afterload
- There is better ejection of blood