CHAPTER 4 ACUTE RESPONSES TO EXERCISE

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Acute respiratory responses to exercise

The respiratory system is responsible for the delivery of oxygen to, and the removal of carbon dioxide from, the cells of the body.

At the beginning of exercise, receptors in the working muscles stimulate ventilation by sending messages to respiratory centres in the brain to increase the volume and rate of breathing.

Ventilation varies from person to person, but can be anywhere from 4 to 15 litres at rest, and increase by as a much as 30 times that during exercise.

Increase in ventilation is a result of an increase in tidal volume(TV) or respiratory rate(RR)

V(litres per minute) = TV(litres) x RR(breaths per minute)

Acute respiratory responses to exercise: ventilation

When exercise commences, RR and TV increase ventilation dramatically. This increase the volume of oxygen in the lungs, which can be transported to the working muscles.

Athletes’ bodies tend to choose a combination of increased RR and TV that seems most natural to them.

Ventilation responses to submaximal and maximal exercise

Acute respiratory responses to exercise: diffusion

Gas exchange occurs in the lungs at the alveolar-capillary interface and in the muscle at the tissue-capillary interface through diffusion.

Diffusion of gases always occurs from an area of high pressure to an area of low pressure.

In the lungs O2 concentration is high, so O2 diffuses from the alveoli in the bloodstream. CO2 levels in the blood are high, so CO2 moves for the blood into the alveoli via a diffusion path.

Acute respiratory responses to exercise: diffusion

The opposite occurs at the muscle, where blood O2 levels are high and muscle O2 levels are low

During exercise, diffusion capacity is increased, due to greater SA of alveoli and muscle tissue, so that greater amounts of O2 and CO2 can be exchanged.

Therefore, during exercise, greater amounts of O2 are available at the muscle and greater amounts of CO2 can be removed.

Cardiovascular responses to exercise

The circulatory system works to maintain certain variables at optimal levels in all conditions, including rest and exercise.

The CV system needs to deliver greater amounts of oxygen and energy substrates to the working muscles. The focus is to get more blood to the working muscles and therefore speed up the removal of CO2 and other waste products.

The CV system undergoes a number of changes to do this…

Cardiac output

Cardiac output (Q) is the product of stroke volume (SV), which is the amount of blood pumped out of the left ventricle of the heart per beat, and heart rate (HR).

Go to page 107 and answer Q1. of thinking things through. Include in your answer the differences between trained and untrained individuals.

Blood pressure

During exercise, the increase in cardiac output results in an increase in blood pressure.

Exercise using large muscle groups (most), affects systolic more than diastolic BP.

During this exercise, arterioles will vasodilate (become larger in diameter), in order to get more blood draining into the capillaries. This action minimises any changes to diastolic BP.

Venous return

Venous return is the flow of blood back to the heart. During exercise the venous return is increased due to 3 actions; the muscle pump, respiratory pump and venoconstriction.

Write a definition for each of the 3 actions…

Blood volume

During aerobic exercise, the amount of blood in the body decreases.

Plasma volume can decrease by as much as 10% during sustained endurance activities (marathon, ironman).

The amount of decreased experienced can be factored by exercise intensity, environmental factors and hydration levels.

Redistribution of blood flow

Oxygen consumption (VO2) and arteriovenous oxygen difference (a-VO2 diff)

Oxygen consumption is the volume of oxygen that can be taken up and used by the body.

As exercise intensity increases, so does O2 consumption. This is a direct result of an increase in cardiac output and an increase in the a-VO2 diff.

At rest, as little as 25% of O2 is released to muscles/organs, the remaining 75% returning to the heart.

During exercise, the working muscles extract greater amounts of O2, increasing the a-VO2 diff.

While some O2 always returns to the heart, oxygen extraction can run close to 100%.

Oxygen consumption (VO2) and arteriovenous oxygen difference (a-VO2 diff

You should have already answered Q1. Complete Q2 – Q4.

Acute muscular responses to exercise

In order for exercise to begin, the muscular contractions responsible for movement need to increase.

The type of contraction, the force, the speed of contraction are controlled by the central nervous system, i.e the brain.

There are a number of mechanisms responsible for acute responses to exercise, including increased blood flow, recruitment of motor units, lactate production and heat production.

Increased blood flow

Blood is redistributed from organs to working muscles. Skeletal capillaries open up and serve 3 main purposes;

- allow for increases in total blood flow- deliver large blood volume with minimal increase in blood flow velocity- increase surface area to increase diffusion rates.

Motor unit recruitment

A motor unit is the means by which the central nervous system talks to muscles to control muscular contractions.

During exercise, the amount of motor units recruited by the brain increases, increasing the amount of force the muscle produces.

Depending on the required strength and speed of the contraction, the number of motor units recruited, and the rate at which they are recruited can be adjusted.

A motor unit will contract maximally or not at all, all or nothing principle

Energy substrates

ATP is the immediate source of energy for all muscular contractions.

However we can only store a small amount and when used up we require energy substrates to fuel our metabolism.

Glycogen is used for aerobic and anaerobic systems. PC donates a phosphate to ADP in the ATP-PC system.

Lactate

When exercise starts, lactate is released from the muscle as part of anaerobic ATP production.

It takes time for the body to respond to the increased demand of exercise, and until oxygen supply can meet the demand the energy required must be produced anaerobically.

At sub maximal exercise, there is a sharp increase in lactate, until O2 consumption can increase to meet energy demands and lactate can be delivered for removal.

Lactate inflection point

When lactic acid is produced and removed there is obviously no accumulation of lactate.

Blood lactate level will remain elevated but will not increase exponentially until production of lactate exceeds the bodies ability to remove it, the lactate inflection point.

The increase is not a reflection of the bodies increased reliance on anaerobic ATP production, rather a sign that removal mechanisms are not coping with the rate at which lactate is being produced.

Body temperature

Heat is a by-product of converting energy substrates into movement. An increase in ATP production will be accompanied by an increase in body temperature.

The body accommodates these changes by stimulating the sweat glands to produce sweat, as well as increasing blood flow to the skin.