Flashcards in Cardiovascular Responses To Aerobic Exercise And Training Deck (22)
What are the 3 major tasks of the cardiovascular system in response to exercise ?
1- pulmonary blood flow has to INCREASE to enhance gaseous exchange - provide sufficient oxygen to working muscles
2- blood flow to working muscles must INCREASE
3- a stable BP must be maintained - had to be maintained to prevent affecting cerebral perfusion
How much does oxygen consumption increase during exercise and how does this happen ?
Increases 13 fold
1.5 times in SV
3 times in HR
3 times in arteriovenous oxygen difference
How does exercise affect HR ?
Increases relatively linearly up to a max of about 180-200bpm
- caused by decrease in vagal output
- increase in sympathetic stimulation of SAN to increase frequency of pacemaker potentials
- increase in sympathetic stimulation of AVN to increase speed of conduction and reduce AVN delay
Why does HR reach a max of about 180-200bpm ?
Because any higher wouldn't provide sufficient time for the heart to fill and so this would reduce SV
Eg 200bpm means the 1 heart cycle occurs in about 0.3 seconds
How is SV increased ?
Increase in EDV
Increase in ventricular contractility - this increases EF and decreases ESV
Why does most of the increase in SV occur at low work rates ?
Because at higher work rates there is reduced ventricular filling which reduces EDV
If the autonomic nerves are denervated can the person do any exercise ?
Because there is a back up mechanism
The release of noradrenaline and adrenaline from the adrenal medulla are able to increase HR during the moderate exercise
However the HR can't reach as high a level as in sympathetic stimulation and it it also takes longer to reach its max level
Explain the effects of the distribution of blood flow ?
A 5 fold increase in CO can increase blood flow to muscles by 20fold due to redistribution of blood
- vasodilation of blood vessels where blood needs to be sent
- vasoconstriction of blood vessels to organs which don't need he blood as much
What are the 2 conflicting demands on the skin during exercise ?
At the start of exercise cutaneous vasoconstriction may occur to support BP due to metabolic vasodilation taking place at active skeletal muscles and to help in the redistribution of blood
during exercise 75% of metabolic energy consumed Appears in the form of heat which must be dissipated to prevent the core body temp rising too much as it can cause heat illness
A core body temp greater or equal to 40 is dangerous and at 42 it is fatal
How is heat lost from our skin ?
Need to increase blood flow to skin to produce a temperature gradient so heat can be lost by conduction, radiation and convection
Also cuntaneous vasodilation has to occur to increase blood flow to sweat glands so sweat can be evaporated
What is cardiac drift ?
For increased skin blood flow to reduce increase in core temp, it requires an increase in CO but due to cutaneous venodilation there is a reduction in venous return so there is a reduction in cardiac filling pressure due to FS mechanism this reduces SV so therefore HR has to increase even more
What is the problem with cardiac drift ?
In hot environments the temp gradient is less so therefore the HR starts to reach its max levels and then the blood flow is being competed for by the muscles and the skin
Normally muscle wins and this causes heat stroke
What is arteriovenous oxygen difference ?
It is the difference between the amount of oxygen which is taken up by the lungs and the amount liberated by peripheral tissue
During exercise arteriovenous difference increases by 3 fold, how does this happen ?
It's caused by a very low venous oxygen concentration forming - this is because instead of only removing one oxygen from haemoglobin (as in rest) 3 oxygen molecules are removed during exercise
What happens to arterial blood pressure during dynamic exercise ?
Systolic pressure rises a lot due to increased SV and ejection velocity
Diastolic pressure either only rises slightly or falls because it is a reflection of the total peripheral resistance
His combined effect causes a slight increase in MAP and a large increase in pulse pressure
How does the increase in MAP during exercise not cause a decrease in sympathetic stimulation by baroreceptors ?
Because there is a central resetting of map set point which prevents a counterproductive decrease by parasympathetic stimulation
Effect is caused by higher brain functions
What is the central command hypothesis ?
It proposes that's the cerebral cortex initiates voluntary muscle contraction and commands autonomic neurons in the cardiovascular centre
- HR increases before exercise as an anticipatory response
- after blocking meuromusclular junctions voluntary attempts to contract the muscle have shown INCREAES HR and BP
What is the peripheral reflex hypothesis ?
States that both proprioceptor inputs from joints and muscle and chemoreceptors inputs from muscle contribute to cardio response to exercise
- HR still increases a dew mins after exercise suggesting accumulation of metabolites could be driving the response
- if a cuff occuleds the venous drainage from muscle the metabolites are retained in the muscle after exercise and this seems to to maintain the pressor response
How is oxygen transport limited during endurance events ?
Max attainable CO
Extracellular resistance to diffusion between red blood cells and muscle myoglobin
To increase max attainable CO there are both indirect and direct changes in cardiac structure that can occur. What are they?
Increasing thickness of the ventricular wall to increase ejection fraction
Increase the size of the ventricular cavities to increase EDV
Increase myocardial vascularity
These changes increase stroke volume
How do athletes develop a max CO of 7x larger than at rest whereas a normal person can only produce a 4.5 X increase ?
Athletes have a similar resting CO but they are able to produce the CO at a lower HR due to a larger SV
This means when they exercise there starting HR is lower so there is a proptionally larger increase to max HR