Control of Cardiac Output Flashcards
Learning outcomes
- 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.
What is stoke volume?
How can stroke volume be calculated?
- The stroke volume is the volume of blood pumped from the left ventricle per beat
- Our stroke volume can be defined as EDV – ESV (End diastolic volume subtract End systolic volume)
- End diastolic volume (EDV) is the volume of blood in the right or left ventricle before the heart contracts
- End systolic volume (ESV) is the volume left in the right or left ventricle at the end of the systolic ejection phase
What are 3 intrinsic mechanisms that affect stroke volume?
1) Contractility
2) Preload
3) Afterload
what is contractility?
- Contractility refers to the force of contraction of the heart muscle
- There is a self-regulatory method known as the Frank-Starling mechanism
- When blood flow to the heart increases, EDV increases, which results in a greater stretch in the ventricle wall that triggers the Frank-Starling mechanism
- The Frank Starling mechanisms results in greater overlap of actin and myosin, which causes an increase in force of contraction (contractility), which increases stroke volume
- The Frank Starling mechanism causes automatic balancing between the cardiac output from the left side of the heart to the volume returning to the right side (balancing of ventricles)
what is pre load?
- Preload is the volume of blood in the ventricles prior to contraction
- Preload is affected by venous pressure and venous return to the heart
- A greater flow of venous return to the heart will result in a greater End diastolic volume (EDV – volume of blood in ventricle before contraction)
- This will cause the ventricle wall to stretch more, triggering the Frank Starling mechanism
- This will cause the force of contraction to increase, which will increase stroke volume
what is after load?
- Afterload is the pressure the heart must work against to eject blood out of the ventricle during systole
- Afterload can be aortic/pulmonary artery pressure
- An increase in afterload will result in a decrease in stroke volume
- Increased afterload reduces the velocity at which the muscle fibre shortens, which reduces the velocity of blood being ejected, leading to a decrease in stroke volume
What is an extrinsic mechanism that affects stroke volume?
1) Sympathetic nerves
* An increase in sympathetic activity in the heart results in an increase in contractility of the heart
* This will cause the EDV to stay the same while the ESV decreases, as an increase in contractility leads to more blood being ejected from the heart per beat, increasing stroke volume
* This increase in contractility is changing calcium handling in order to favour greater force of contraction over shorter periods of time
What does inotropic mean?
What does chronotropic mean?
Do both these effects have to happen at the same time?
- Inotropic refers to how strong the heart beats (contractility)
- Chronotropic refers to how fast the heart beast (hear rate)
What happens if a positive inotropic effect via sympathetic innervation is added to the Frank Starling mechanism?
What has to change for the Frank Starling method to function?
How is this different for sympathetic innervation?
- When a positive inotropic effect via sympathetic innervation is added to the Frank Starling mechanism, this results in an even greater force of contraction
- EDV has to change for Frank Starling mechanism to work and for stroke volume to increase, whereas stroke volume can increase independent of EDV through sympathetic innervation through a change in Calcium handling
What is the formula for cardiac output?
- Cardiac output = stroke volume x heart rate
How is maximum cardiac output achieved (4 factors)?
- Max cardiac output is achieved through a combination of:
1) Increase in End diastolic ventricular volume
2) Increase sympathetic innervation
3) Increased release of adrenaline
4) Decrease parasympathetic activity
What can stroke volume, heart rate, and cardiac output increase to from rest?
- Stroke volume can increase from about 70ml to 140ml
- Heart rate can increase from around 70bmp to 200bpm
- Cardiac output can increase from about 5L/min to 30L/min
How is pressure in systemic and pulmonary circulation different?
What are reasons why the pressure decreases as we move down the vessel tree?
Why is this decrease in blood pressure important?
How do capillaries decrease blood pressure?
- Pressure in systemic circulation (120/80 mmHg) is significantly higher than that in pulmonary circulation (30/12 mmHg)
What are reasons why the pressure decreases as we move down the vessel tree?
Why is this decrease in blood pressure important?
- The pressure decreases as we move down the vessel tree because resistance decreases
- This decrease in blood pressure Is important as it allows smooth laminar flow through the capillaries and also prevents high pressures from damaging the capillaries
How do capillaries decrease blood pressure?
- When blood reaches the capillaries, the branching of the capillaries leads to the pressure being spread out, causing the blood pressure to decrease
What limits how fast blood can exit the left ventricle?
- Systemic vascular resistance (SVR) and limited capacity in the arterial system limits how fast blood can exit the left ventricle
What is vascular compliance in relation to blood vessels?
What happens to vascular compliance with age and vasoconstriction?
- Vascular compliance in relation to blood vessels is the relationship between the volume of blood within a vascular segment and the blood pressure which is generated by the presence of that volume
- Vascular compliance decreases with age and vasoconstriction
What is the formula for Mean Arterial Blood Pressure (MABP)?
What is the formula for pulse pressure?
- MABP = Diastolic Pressure + 1/3 Pulse Pressure
- Pulse Pressure = Systolic Pressure – Diastolic Pressure
What are 3 factors affecting flow of a fluid through a vessel?
1) Viscosity – If haematocrit (proportion of red blood cells in the blood) varies, the viscosity of blood varies
2) Vessel length – the longer the vessel, the greater the resistance due to increased friction, and the lower the flow. As resistance increases, blood pressure increases
3) Vessel radius – Something bigger in radius is subject to less forces against the external walls (flow = diameter of vessels^4)
What is Pouseille’s equation for?
How can it be simplified?
Describe how the equation is formed simplified and rearranged.
What Is the formula that this creates for arterial pressure (MABP)?
- Pouseille’s equation describes the pressure drop due to change in viscosity of fluid in the vessel and length and radius of the vessel
- These factors represent resistance in the vessel, so these factors can be simplified to ‘resistance’ in the equation
- This creates the formula Arterial pressure (MABP) = Cardiac Output x Total Peripheral Resistance
Calculating resistance in pulmonary and systemic circulation question
