Term
| What 3 tasks does aerobic exercise impose on the CV system? |
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Definition
1. pulmonary blood flow must increase to enhance gaseous exchange in the lungs
2. blood flow through the working muscles must increase
3. a reasonably stable blood pressure must be maintained |
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Term
| How much can O2 consumption increase by from rest? |
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Definition
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Term
| How is the 13-fold increase in O2 absorption by the pulmonary circulation is typically achieved? |
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Definition
• a 1.5X increase in Stroke Volume
• a 3X increase in Heart Rate
• a 3X increase in Arteriovenous O2 difference |
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Term
| Describe the O2 consumption to CO relationship? |
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Definition
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Term
| How much can CO2 increase? |
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Definition
| In an untrained adult, CO can increase from around 5l.min-1 at rest to a maximum of 20 - 25 l.min-1 (a 4 to 5 fold increase) |
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Term
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Definition
| Heart rate rises linearly with work rate up to a maximum of 180 – 200 beats.min-1 in adults |
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Term
| What causes an increase in HR? |
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Definition
1. decreased vagal (parasympathetic) inhibition
2. increased sympathetic stimulation of the pacemaker cells in the SAN |
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Term
| What does sympathetic stimulation of the AVN cause? |
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Definition
increases AP conduction rate
shortens AV delay |
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Term
| How does exercise affect chemoreceptors? |
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Definition
| increased activation due to the changing composition of blood |
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Term
| How does exercise affect baroreceptors? |
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Definition
| increased activation due to the increase in SV which leads to an increase in blood volume which leads to an increase in BP |
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Term
| What causes an increase in SV? |
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Definition
due to an increase in sympathetic outflow...
1. increases in filling pressure (increases EDV)
2. increase in ventricular contractility (which increases ejection fractin and decreases ventricular ESV) |
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Term
| When are increases in SV observed and why? |
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Definition
| Almost all of the increase in SV occurs at low work rates: at higher work loads, reduced ventricular filling time compromises ventricular EDV. There isn't enough time to fill the ventricle with an effective and required amount of blood. |
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Term
| Describe the relationship between O2 uptake and SV |
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Definition
| as o2 uptake increases, SV increases linearly, up until an O2 consumption of around 3.5Lmin-1 is reached. When this limit is reached we can deduce that the demand on the heart is so great, there is simply not enough time between successive heart beats (diastole) to fill the ventricles with enough blood required to sustain activity or exercise at this level. |
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Term
| What mainly drives the rise in HR and ejection fraction during exercise? |
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Definition
| cardiac sympathetic nerves |
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Term
| What is the back up mechanism (experienced in patients with transplanted/denervated hearts)? |
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Definition
| the rise in plasma adrenaline and noradrenaline levels that occur due to their release from the adrenal medulla (increase in sympathetic outflow is detected by the adrenal medulla) |
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Term
A 5-fold increase in CO can result in a 20-fold increase in muscle
blood flow. How? |
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Definition
| as a result of the changes in the way the blood is distributed between the various systemic vascular beds |
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Term
| What are the effects of vasodilation in the vascular beds of active muscle? |
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Definition
| (mainly as a result of metabolic autoregulation) decreases resistance and therefore increases flow |
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Term
| What are the effects of vasoconstriction in the 'non-essential' vascular beds of active muscle? give examples of 'non-essential' vascular beds |
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Definition
(renal and splanchnic)
- mediated by by sympathetic nerves
- diverts a greater proportion of the CO to active muscles |
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Term
| What occurs in the skin during exercise? |
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Definition
initially, cutaneous vasoconstrcition:
- supports blood pressure in the face of metabolic vasodilation taking place in the vascular beds of the active skeletal muscles
- to assist in the redistribution of blood flow to those vascular beds |
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Term
| Why is muscular exercise only 25% efficient at best? |
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Definition
| at least 75% of all metabolic energy consumed during exercise appears in the form of heat |
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Term
| What would happen if this excess heat is not dissipated? |
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Definition
| the body core temperature will rise and, although a modest increase is acceptable (or even desirable) during exercise, it cannot be allowed to rise unchecked or heat illness will result |
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Term
| What does physical heat transfer from the body to the environment depend on? |
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Definition
| increasing the temperature gradient between the skin and the environment |
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Term
| How is the temperature gradient between the skin and the environment increased? |
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Definition
| increase in skin temperature produced by an increase in cutaneous blood flow ie but cutaneous vasodilation |
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Term
| How else is heat loss obtained? |
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Definition
| Evaporative heat loss (sweating) also relies on a reasonable high rate of skin blood flow to deliver fluid to the sweat glands |
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Term
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Definition
- increased skin blood flow requires a further increase in CO
- cutaneous venodilation reduces cardiac filling pressure
- this reduced SV by Frank-Starling mechanism
- HR must increase even more |
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Term
| When is cardiac drift most significant? |
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Definition
| during exercise in a hot environment, where there is a smaller gradient for heat transfer between the body and its surroundings |
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Term
| What competition then arises? who wins? |
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Definition
there is a competition for the available blood between the active
muscles and the skin. Muscle wins (usually)! Hence, heat stroke (a potentially life-threatening medical emergency) is a real risk during exercise in the heat, particularly in highly-motivated athletes. |
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Term
| What does the AV O2 difference reflect? |
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Definition
| the amount of O2 (per litre of blood) that is taken up in the lungs and liberated in the peripheral tissues (primarily skeletal muscle) |
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Term
| How is the 3-fold AV O2 difference possible during maximal exercise? |
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Definition
| it is not due to increases in arterial o2 content, but is a result of the low venous o2 concentration that develops |
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Term
| How does systolic pressure change during dynamic exercise? what does this reflect? |
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Definition
| Systolic pressure rises markedly (by up to 60mmHg during maximal exercise). This reflects the increases in stroke volume and ejection velocity. |
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Term
| How does diastolic pressure change during dynamic exercise? what does this reflect? |
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Definition
| Diastolic pressure is a reflection of TPR, which may rise only slightly (or even fall) depending on the balance between metabolic vasodilation and sympathetic vasoconstriction |
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Term
| Therefore, how do PP and MAP change during dynamic exercise? |
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Definition
| As a result, there is a large increase in pulse pressure and a more modest increase in MAP (around 20mmHg). |
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Term
| If MAP increases (during exercise), would the baroreflex decrease sympathetic (and increase parasympathetic) drive to counteract effects? |
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Definition
NO
The baroreflex doesn't act to oppose an increase in systolic pressure or even a slight increase in MAP because it is now set to buffering a higher level experienced during exercise. |
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Term
| Describe central resetting |
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Definition
| prolonged exercise stimulates the body to 'reset' the blood pressure to the new mean level experienced during exercise |
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Term
| What does central resetting prevent? |
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Definition
| this prevents frequent attempts from the body to decrease blood pressure to the resting norm and thus preventing the activation many conflicting systems in the body |
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Term
| What is the one physical circulatory change brought about in exercise? |
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Definition
AV O2 difference
lower partial pressure of oxygen in the interstitial fluid then more oxygen is stripped of haemoglobin |
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Term
| What are the other factors? |
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Definition
- heart rate
- muscle blood flow
- cardiac output
- stroke volume
- MAP
- EDV
- TPR |
|
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Term
| How are changes in these factors initiated? |
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Definition
the cardiovascular centre
there are two hypothesis proposed to explain specifically |
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Term
| What are the two hypothesis proposed to explain how the responses are initiated? |
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Definition
1. central command hypothesis
2. peripheral reflex hypothesis |
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Term
| What does the central command hypothesis propose? |
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Definition
| Proposes that the cerebral cortex both initiates voluntary muscle contraction and ‘commands’ the autonomic (and respiratory) neurones of the brainstem |
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Term
| What evidence is there for the central command hypothesis? (2) |
|
Definition
1. Heart rate may increase before the onset of exercise – an ‘anticipatory response’
2. After partial neuromuscular blockade, voluntary attempts to contract the partially paralysed muscle (requiring, presumably, a bigger central command signal) produce enhanced rises in heart rate and blood pressure |
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Term
| What does the peripheral reflex hypothesis propose? |
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Definition
| Proposes that both proprioceptor inputs from joints and muscle and chemoreceptor inputs from the muscle contribute to the cardiovascular responses to exercise, particularly the pressor response (i.e. elevated MAP) |
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Term
| What evidence is there for the peripheral reflex hypothesis? (2) |
|
Definition
1. Heart rate continues to increase for 1-2 minutes after the onset of exercise, suggesting that the gradual interstitial accumulation of metabolites such as lactate and K+ ions could be driving the response
2. If a spygmomanometer cuff is used to occlude venous
drainage from exercising muscles in a limb, these metabolites are retained in the muscle after exercise ceases: in these circumstances, the pressor response is partially maintained |
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Term
| What mechanism is likely to contribute to the CV responses observed during exercise? |
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Definition
| both central and peripheral mechanisms |
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Term
| What is central command most likely to be responsible for? |
|
Definition
the initial increase in heart rate, mainly via a suppression of vagal
outflow to the sinoatrial node |
|
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Term
| What does the muscle chemoreflex contribute to? |
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Definition
| the subsequent, slower, sympathetically-mediated rises in heart rate and stroke volume, and to the peripheral vasoconstriction |
|
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Term
| What does aerobic training stimulate? |
|
Definition
| a series of circulatory adaptations that are very important for endurance athletes |
|
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Term
| What is an important performance-limiting factor in aerobic respiration? |
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Definition
| the maximal rate of O2 transport from lungs to the mitochondria of the active muscles |
|
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Term
| What is O2 transport limited by? |
|
Definition
1. The maximal attainable cardiac output
2. Extracellular resistance to diffusion between the erythrocytes and muscle myoglobin |
|
|
Term
| What has the greatest effect (directly and indirectly) on CO? |
|
Definition
| changes in the cardiac structure |
|
|
Term
| What changes happen in cardiac structure? |
|
Definition
• The ventricular wall increases in thickness
• The ventricular cavities enlarge
• Myocardial vascularity increases |
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Term
| What do changes in cardiac structure lead to? |
|
Definition
increases (both at rest and during exercise) in ventricular EDV and the ejection fraction (decreasing ESV)
these changes increase SV |
|
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Term
| How do resting CO between trained and untrained individuals compare? |
|
Definition
| The resting cardiac output is similar in trained and untrained individuals, but a trained athlete achieves this at a lower heart rates, due to the increase in resting stroke volume |
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Term
| How does an athletes heart rate differ to an average person? |
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Definition
| The athlete’s maximum heart rate is not significantly altered as a result of training, but since the resting value is lower, a proportionally larger change can occur |
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Term
| What is the maximal cardiac output achieved by an elite athlete? |
|
Definition
| 7X resting (values of more than 35 l.min-1 have been recorded in a few elite male athletes) |
|
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Term
| What adaptations occur to improve the rate of o2 diffusion from erythrocytes to the muscle mitochondria? |
|
Definition
• The development of new capillaries within the skeletal muscle vascular beds reduces the average diffusion distance
• Muscle mitochondria increase in number, especially at subsarcolemmal sites close to the capillaries
• Muscle myoglobin concentration increases |
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