26-09-22 – Exercise and Coronary Blood Flow Flashcards
Learning outcomes
- To explain the limits of cardiac perfusion and the difficulties in matching the body’s demand for cardiac output versus cardiac tissue demand.
- To predict and explain the changes in cardiovascular parameters in response to dynamic and static exercise.
- To relate the major systems and effectors of blood pressure together and explain their dependencies.
Where do the coronary arteries start? When is the window for coronary flow?
What are 3 ways we can decrease the window for coronary flow?
- The coronary arteries are found on the coronary sinuses (aka aortic sinuses) of the right and left cusps of the aortic valve
- The window for coronary flow is during diastole, as this is when blood is able to flow through the coronary arteries
- 3 ways we can decrease the window for coronary flow:
1) Heart rate increase
* Increasing the heart rate will cause the periods of systole and diastole to be shorter
* This will result in less amount of time for coronary flow, which will decrease the size of the window of coronary flow
2) Ventricular pressure increase
* An increase in ventricular pressure from incomplete emptying, resulting in a decrease in the size of the window of coronary flow
3) Aortic pressure decrease
* This is the pressure that pushes blood into the coronary arteries
* If aortic pressure drops, this will decrease the size of the window of coronary flow
What are 2 issues with coronary blood flow?
Why are these issues?
How much does work output of the heart increase during strenuous exercise?
How much of coronary artery blood flow O2 is used at rest?
What must an increase in demand be met by?
- Issues with coronary blood flow:
1) The myocardium cannot function anaerobically
* Anaerobic glycolysis causes an increase in lactic acid production
2) Arterioles of coronary arteries close mechanically during systole
* This results in a decrease in diastolic filling period during exercise, even though oxygen demand and metabolic demand increase during exercise
- Work output of the heart increased 6-9x during strenuous exercise
- 70-80% of coronary blood flow O2 is used at rest
- An increase in demand must be met by increase blood flow
What is the primary controller of coronary blood flow?
What is it in proportion with?
What does it stimulate the release of?
How does sympathetic stimulation indirectly and directly affect the coronary flow of the heart?
- The primary controller of coronary blood flow is the local metabolism, which controls coronary artery radius
- Local metabolism is in proportion to the need of the cardiac musculature for O2 e.g more local metabolism leads to greater cardiac musculature need for O2
- Local metabolism leads to the release of vasodilators (e.g adenosine - present through breakdown of ATP), which increase blood flow.
- Indirect Sympathetic stimulation of coronary blood flow:
- Sympathetic stimulation indirectly affects coronary flow by increasing heart rate and contractility, which increases metabolism and metabolic demands (O2)
- This will lead to an increase in coronary blood flow
- Direct sympathetic stimulation of coronary blood flow:
- The heart has a high degree of sympathetic innervation
- The role of this is unclear, but may contribute to pathophysiologies
- There is no reason we would want sympathetics to cause vasoconstriction of the coronary vessels, so it doesn’t make a lot of sense
Sympathetic stimulation of skeletal muscle arteries.
Where is noradrenaline released from?
What effect does noradrenaline have on skeletal muscles?
What receptors does it act on?
What is adrenaline (epinephrine)? What is it released by?
What is adrenaline part of?
What is it released in response to?
What 2 things does adrenaline act on?
How does this affect skeletal muscles?
How is vasodilation of skeletal muscle arterioles favoured during fight or flight responses?
- Noradrenaline is released predominantly from the ends of sympathetic nerve fibres
- Noradrenaline causes vasoconstriction of skeletal muscle arterioles by acting on alpha1 receptors (α1 receptors)
- Adrenaline (epinephrine) is a hormone and a neurotransmitter that is released by the adrenal medulla
- Adrenaline is part of the sympathetic nervous system and is released in response to fight or flight
- Adrenaline can act on:
1) Alpha 1 (α1) receptors, which causes vasoconstriction of skeletal muscle arterioles
2) Beta 2 (β2) receptors, which causes vasodilation of skeletal muscle arterioles - How vasodilation of skeletal muscle arterioles is favoured during fight or flight responses:
- Prolonged periods of fight or flight responses are mediated by circulating adrenaline
- Adrenaline has a higher affinity for Beta 2 receptors, which will favour vasodilation of skeletal muscle arterioles during the fight or flight
How does moderate and intense exercise affect cardiac output?
How does exercise change the distribution of the cardiac output?
How and why is this done?
- During exercise the cardiac output increases from 5L/min at rest to 15L/min at moderate intensity and 30L/min at maximum intensity for highly trained athletes
- Exercise changes the distribution of the cardiac output to:
1) The heart and muscle
* Done through active hyperaemia
* Blood is shunted from the digestive viscera and skin to the heart, brain, and skeletal muscles
* This is due to the types of receptors found on each tissues type
* e.g more beta 2 receptors present in the brain, so adrenaline will cause vasodilation to the brain during fight or flight
* e.g more alpha 1 receptors in the skin, so adrenaline will cause vasodilation to the skin during fight or flight
* Blood is directed towards
2) Skin
* The core temperature increases
* Detected by the hypothalamus
* Decreased sympathetic innervation to the skin
What is pulse pressure?
How does moderate exercise effect pulse pressure, cardiac output, and peripheral resistance?
- Pulse pressure is the difference between systolic and diastolic pressure
- How moderate exercise affects:
1) Pulse pressure (increase)
* Moderate exercise increases pulse pressure
* This is due to an increase in systolic pressure, which is caused by an increase in stroke volume and speed of ejection
2) Cardiac output (increase)
* Big Increase in HR due to decreased parasympathetic innervation to SA node and increase sympathetic innervation to the SA node
* Increase in stroke volume due to increased contractility and increases Frank Starling
3) Peripheral resistance (decrease)
What needs to happen for Cardiac output to increase during exercise?
What are 4 factors promoting venous return during exercise?
- Cardiac output can only be increased to high levels if venous return is increased to the same degree
- 4 factors promoting venous return during exercise:
1) Skeletal muscle pump activity
2) Frequency and depth of inspiration
3) Venous tone via sympathetic innervation
4) Ease of flow from arteries to veins through dilated skeletal muscle arterioles
What is dynamic exercise?
What does it initiate?
What are examples of dynamic exercises?
- Dynamic exercise is defined as rhythmic muscular activity resulting in movement
- it initiates a more appropriate increase in cardiac output and oxygen exchange
- Examples of dynamic exercise include running, swimming, cycling, weightlifting, as these all involve joint movement
What is the feedforward control during dynamic exercise?
- So far, we’ve discussed reactive changes to the cardiovascular system, but anticipatory changes to the cardiovascular system are made with the expectation of physical activity.
- Both cardiac output and total peripheral resistance are adjusted in preparation for activity, preparing the body for activity, by decreasing parasympathetic tone and increasing sympathetic activity.
- In addition, antidiuretic hormone (ADH; vasopressin) release is stimulated as part of this feedforward control to promote retention of water and decrease urine production.
- Feedforward control in exercise also allows the resetting of baroreceptors upwards, to mute the effect of increased arterial pressure on the normal reflex mechanisms they would otherwise induce.
- In other words, the cardiovascular system is primed, and then activity is modified to suit the demand of the exercise that follows.
What happens in the vasodilation of muscular arterioles during dynamic exercise?
- What happens in the vasodilation of muscular arterioles during dynamic exercise:
- Use of muscles increases their metabolic activity, resulting in the accumulation of local factors that stimulate vasodilation (active hyperaemia).
- As a result, blood flow in exercising muscles can increase up to 20x normal flow.
- The issue now is that increased arteriolar dilation in a large number of muscles will greatly decrease total peripheral resistance.
- Reducing blood flow to non-essential organs and tissues can help restrict the amount total peripheral resistance drops by as a result of increased muscle blood flow.
- However, in whole-body intensive exercise (such as running), the increased blood flow to skeletal muscle may outweigh what’s conserved by reducing blood flow to non-essential organs.
- The overall effect of exercise can therefore be a drop in total peripheral resistance from normal levels. In these situations, cardiac output thereby has to increase to maintain arterial blood pressure.
- Short-term, blood flow can be restricted to the skin.
- However, as exercise continues then heat will be generated that needs to be dissipated.
- Returning blood flow to the skin becomes essential for regulating temperature, via radiation and sweating, but will also contribute to a decrease in total peripheral resistance.
What happens to cardiac output during dynamic exercise?
- What happens to cardiac output during dynamic exercise:
- Increased sympathetic and decreased parasympathetic control of the heart causes an increase in the cardiac output by both increasing the heart rate and stroke volume (the latter via increased contractility).
- Venous return to the heart increases, due to compression of veins within the exercising muscles (combined with increased respiration) moving blood back to the heart.
- This may cause a minor increase in the end-diastolic volume (EDV), which would cause a further increase in stroke volume due to the Frank-Starling mechanism.
- Remember that an increased heart rate and increased contractility will deal with a large volume of blood returned to the heart by muscle/respiratory pump and sympathetic effects on the veins – therefore a big increase in cardiac output can be seen without necessarily a big change in EDV.
- In exercise, the contribution of the Frank-Starling mechanism to increased stroke volume may be small.
How does dynamic exercise affect the kidneys?
- How dynamic exercise affects the kidneys:
- Redistribution of the cardiac output decreases blood flow to the kidneys during exercise, which will decrease urine production due to the decreased pressure diuresis mechanisms discussed in lecture 8.
- Furthermore, feedforward controlled release of ADH further enhances fluid retention, as does increased activity of the renin-angiontensin-aldoesterone system (but probably to a lower extent than ADH).
- These effects result in the net retention of water and substantial decrease in urine production – just as well, really, because increased losses of fluids occur through sweating and increased respiration.
How does dynamic exercise affect MABP?
- How dynamic exercise affects MABP:
- During moderate dynamic exercise, mean arterial blood pressure may increase slightly but is generally not elevated greatly due to the changes in cardiac output and total peripheral resistance balancing out.
- Pulse-pressure may increase, due to increased systolic pressure (as a result of increased cardiac output into the aorta) and a mild decrease in diastolic pressure (as a result of decreased afterload due to decreased total peripheral resistance).
Other stuff to consider during dynamic exercise
- Other stuff to consider during dynamic exercise:
- During dynamic exercise, blood flow to exercising muscle becomes subject to similar problems seen in the myocardium: Contraction of skeletal muscle means blood perfusion is restricted to the time the muscle spends relaxed.
- If insufficient time is spent in the relaxed state, blood supply may not match demand. Lactic acid can also build up in skeletal muscle during anaerobic exercise
Name the changes in these factors due to dynamic exercise:
* Skeletal muscle blood flow
* Total peripheral resistance
* Stroke volume
* Heart rate
* Cardiac output
* MABP
* Systolic arterial pressure
* Diastolic arterial pressure
* Renal blood flow
* Renin release
* Vasopressin release
* Urine production
Name the changes in these factors due to dynamic exercise:
* Skeletal muscle blood flow
* Total peripheral resistance
* Stroke volume
* Heart rate
* Cardiac output
* MABP
* Systolic arterial pressure
* Diastolic arterial pressure
* Renal blood flow
* Renin release
* Vasopressin release
* Urine production
