Acute physiological responses to exercise across the cardiovascular, respiratory and muscular systems, including the mechanisms that drive each response and how the response scales with exercise intensity and duration

What this dot point is asking

VCAA Unit 3 AoS 2 expects you to know how the cardiovascular, respiratory and muscular systems respond to the onset and continuation of exercise. The questions ask you to identify the responses, explain the mechanisms that drive them, and link each response to the functional need it serves (delivering oxygen and substrate, removing by-products, sustaining the energy systems). Acute responses are distinct from chronic adaptations (which take weeks to develop and are covered in Unit 4 AoS 2).

The answer

When exercise begins, every system in the body shifts from rest toward a working state within seconds to minutes. The shifts are coordinated by the nervous system and the endocrine system, and they scale with the intensity and duration of the exercise. Strong responses identify the response, name the mechanism, and link it to a functional need.

Cardiovascular responses

The cardiovascular system delivers oxygenated blood and substrate to working muscle and removes carbon dioxide and metabolic by-products. Acute responses maximise delivery during exercise.

Heart rate (HR).

Resting HR sits around 60 to 80 bpm in untrained adults, lower in trained endurance athletes. During exercise, HR rises rapidly toward a maximum that approximates 220 minus age (the Tanaka formula 208 minus 0.7 times age is more accurate for adults). For a 17 year old, estimated HR max sits around 196 bpm.

The mechanism for the rise has three contributions:

• Anticipatory rise from central command (the motor cortex signals an imminent muscle action and HR rises before exercise begins).
• Reduction in parasympathetic (vagal) tone, which removes the resting brake on the sinoatrial node.
• Sympathetic activation, with noradrenaline at the heart and circulating adrenaline from the adrenal medulla, both accelerating depolarisation of the sinoatrial node.

Functionally, the rise in HR is the largest contributor to the rise in cardiac output that delivers oxygen to working muscles.

Stroke volume (SV).

Resting SV is around 70 mL per beat. During exercise, SV rises and typically plateaus at around 40 to 60 per cent of VO2 max. The plateau exists because at very high HR the time available for ventricular filling is reduced.

The mechanism is twofold:

• Increased venous return, driven by the skeletal-muscle pump (rhythmic muscle contraction squeezes veins toward the heart) and the respiratory pump (pressure changes from breathing draw blood back to the heart).
• Increased ventricular contractility from sympathetic stimulation, which empties the ventricles more completely with each beat.

Functionally, the SV rise contributes alongside HR to the rise in cardiac output.

Cardiac output (Q).

Q = HR times SV. Resting Q is approximately 5 L per minute. During maximal exercise, Q can reach approximately 20 to 25 L per minute in healthy untrained adults and 35 L per minute or more in elite endurance athletes.

Functionally, the rise in Q is the central cardiovascular response. It delivers the oxygen and substrate the working muscles need and removes carbon dioxide.

Blood pressure.

Systolic blood pressure rises substantially during exercise (a healthy systolic of 120 mmHg at rest can rise to 180 to 200 mmHg or more at maximal exercise) because cardiac output rises. Diastolic blood pressure typically stays similar or falls slightly during dynamic exercise because peripheral resistance falls (vasodilation in working muscle).

In isometric or resistance exercise (sustained muscle contraction), both systolic and diastolic blood pressure can rise sharply because the contraction compresses blood vessels and raises peripheral resistance.

Blood flow redistribution.

At rest, the gut and kidneys receive a large share of cardiac output. During exercise, blood flow is redistributed away from non-essential tissues (digestive, renal) toward working skeletal muscle and skin (for heat loss).

The mechanism is local vasodilation in active muscle (driven by accumulating metabolites including carbon dioxide, lactate, adenosine, hydrogen ions, and a drop in tissue oxygen) and sympathetic vasoconstriction in inactive tissues. In maximal exercise, working muscle can receive 80 to 85 per cent of cardiac output.

Functionally, this targets the elevated cardiac output to where it is needed most.

a-vO2 difference.

The arteriovenous oxygen difference is the difference in oxygen content between arterial and venous blood. At rest, it is approximately 5 mL of oxygen per 100 mL of blood (working tissues extract about a quarter of the available oxygen). During exercise, it widens substantially because working muscle extracts more oxygen from each unit of blood that passes through. This widening allows higher oxygen uptake without requiring cardiac output to rise proportionally.

Respiratory responses

The respiratory system brings oxygen into the body and removes carbon dioxide. Acute responses match ventilation to the rising rate of metabolic gas exchange.

Ventilation rate and tidal volume.

Resting respiratory rate is approximately 12 to 16 breaths per minute. Resting tidal volume is approximately 500 mL per breath. Resting minute ventilation is approximately 6 L per minute.

During exercise, both rate and depth rise. At maximal exercise in healthy trained adults, minute ventilation can reach approximately 100 to 150 L per minute, with rates above 40 breaths per minute and tidal volumes of 2 to 3 L. At submaximal intensities the rise is more in tidal volume; at maximal intensities respiratory rate dominates.

Mechanisms of the ventilatory rise.

The rise in ventilation is driven by:

• Central command from the motor cortex (an anticipatory rise before exercise begins, similar to the HR anticipatory rise).
• Chemoreceptor feedback. Central chemoreceptors in the medulla detect rising carbon dioxide and falling pH in cerebrospinal fluid; peripheral chemoreceptors in the carotid and aortic bodies detect falling arterial oxygen, rising carbon dioxide and falling pH.
• Mechanoreceptors and proprioceptors in working muscles and joints signal mechanical activity to the respiratory centre.

Functionally, the elevated ventilation keeps alveolar oxygen pressure high and removes carbon dioxide, matching the rate of gas exchange to the rate of metabolism.

Ventilation-perfusion (V/Q) matching.

During exercise, both pulmonary ventilation and pulmonary blood flow rise. Their distribution across the lungs becomes more uniform: at rest, the bottom of the lung is preferentially perfused; during exercise, perfusion to the upper lobes rises and matches ventilation more closely. This improves gas-exchange efficiency.

Oxygen consumption (VO2).

VO2 rises from approximately 3.5 mL per kg per minute at rest (one MET) toward an individual's VO2 max. For a healthy untrained adult, VO2 max sits around 35 to 45 mL per kg per minute; for elite endurance athletes, values can exceed 70 mL per kg per minute.

VO2 rises in proportion to exercise intensity up to VO2 max, after which it plateaus while exercise intensity can still rise (the additional energy comes from anaerobic sources).

Muscular responses

The muscular system performs the work. Acute responses recruit motor units, mobilise fuel and generate force.

Motor unit recruitment.

A motor unit is one motor neuron and the muscle fibres it innervates. As the force demanded rises, motor units are recruited in size order (the size principle, sometimes called Henneman's principle). Small slow-twitch (Type I) motor units fire first for low-intensity, sustained work; larger fast-twitch (Type IIa, then Type IIx) motor units are added as force demand rises.

Functionally, this recruitment pattern matches fibre type to the demand. Low-intensity sustained work uses oxidative slow-twitch fibres; high-intensity short-duration work recruits glycolytic and fast-oxidative fibres.

Fuel mobilisation.

At the onset of exercise, the muscle's stored ATP and creatine phosphate are used first (covered in the energy-systems dot point). Within seconds, anaerobic glycolysis ramps up using muscle glycogen. Within minutes, the aerobic system contributes increasingly, fully oxidising carbohydrate and fat in the mitochondria.

Acute responses include:

• Glycogen breakdown to glucose-6-phosphate inside the muscle.
• Glucose uptake from blood (insulin-independent during exercise; muscle contraction itself increases glucose transporter trafficking to the cell membrane).
• Free fatty acid release from adipose tissue, driven by adrenaline and the drop in insulin.
• Increased blood flow to muscle (delivering fuel and oxygen, removing by-products).

By-product production and removal.

Carbon dioxide is produced in proportion to aerobic metabolism. It diffuses from muscle to blood, is carried to the lungs (largely as bicarbonate in plasma), and is exhaled.

Lactate is produced by anaerobic glycolysis. It is exported from muscle to blood and shuttled to other tissues (other muscle fibres, heart, liver) for re-oxidation or for gluconeogenesis. Lactate itself is not the fatigue agent (a common misconception); the accompanying hydrogen ions lower muscle pH and impair contraction.

Heat is produced in proportion to metabolic rate. Core temperature rises during exercise; the cardiovascular system redirects blood to the skin and sweating raises evaporative heat loss to dissipate it.

How acute responses scale with intensity and duration

The responses described above are graded with the exercise.

• Low intensity. Modest rises in HR, SV, Q, ventilation and motor-unit recruitment. Largely slow-twitch fibres, aerobic system dominant, no significant lactate accumulation.
• Moderate intensity. Larger rises across all systems. Fast-oxidative fibres engaged. Some lactate appearing in blood but cleared as fast as produced.
• High intensity. Q approaches maximum, ventilation rises sharply, fast-twitch fibres fully recruited, lactate accumulates faster than it is cleared, blood pH falls.
• Maximal. Q at maximum, VO2 at VO2 max, ventilation near maximum, all fibre types recruited, rapid fatigue.

Duration also matters. Sustained submaximal exercise eventually causes a slow rise in HR at constant workload (cardiovascular drift, driven by rising core temperature and blood being directed to skin for cooling, reducing venous return and SV). Long-duration exercise also depletes muscle glycogen and dehydrates the body, both of which limit performance independently of the cardiovascular and respiratory systems.

How this dot point applies

A typical VCAA application asks you to describe the acute responses to a named exercise scenario, or to compare the acute responses at two different intensities. Strong responses identify each response, name the mechanism, and link it to a functional need (oxygen delivery, by-product removal, force generation).

The mistake to avoid is confusing acute responses with chronic adaptations. Acute responses happen within seconds to minutes of starting exercise; chronic adaptations are the changes that develop over weeks to months of repeated training. This dot point is about the acute, not the chronic.

Examples in context

Example 1. An NRL forward making a one-off goal-line carry. When a forward braces for and executes a 10-second goal-line carry, central command from the motor cortex anticipates the contraction and HR rises before contact. Sympathetic activation accelerates HR sharply through the carry; SV rises with the rise in venous return from leg drive. Cardiac output triples to deliver oxygen to the working leg, trunk and arm muscles. Motor units are recruited rapidly through the size order, fast-twitch fibres firing within seconds for maximal force. The ATP-PC system fuels the contraction, no significant ventilatory rise has had time to develop, and blood pressure spikes (the isometric component of bracing into contact compresses blood vessels and elevates peripheral resistance). After the play, ventilation rises sharply to remove the carbon dioxide that has accumulated during the effort, and HR remains elevated as the body recovers.

Example 2. A 5 km recreational runner in the second kilometre. Two kilometres into a submaximal 5 km run, HR has stabilised at around 75 to 85 per cent of HR max, SV is at its plateau, and cardiac output is sustaining the workload. Ventilation has risen to around 60 to 80 L per minute with elevated tidal volume and rate driven by carbon dioxide accumulation detected by central chemoreceptors. Blood has been redistributed away from gut and kidney toward working muscle (now receiving 80 to 85 per cent of cardiac output) and toward the skin (for heat loss as core temperature rises). The runner is using a mix of glycogen and free fatty acids, with the aerobic system supplying most ATP. Motor unit recruitment is dominated by slow-twitch fibres with fast-oxidative fibres contributing. Lactate is being produced and cleared at similar rates; blood pH is largely stable. As the run continues, cardiovascular drift will gradually raise HR at constant workload because rising core temperature redirects more blood to the skin and reduces venous return.

Try this

Q1. Define cardiac output and state the formula that produces it. Give an approximate resting value and an approximate maximal value for a healthy untrained adult. [3 marks]

• Cue. Cardiac output is the volume of blood pumped by the heart per minute. Q = HR x SV. Resting around 5 L per minute; maximal around 20 to 25 L per minute in a healthy untrained adult.

Q2. Explain why ventilation rises at the onset of exercise, naming at least two mechanisms. [3 marks]

• Cue. Central command from the motor cortex produces an anticipatory rise; chemoreceptor feedback (central chemoreceptors detecting rising carbon dioxide and falling pH in cerebrospinal fluid, peripheral chemoreceptors detecting the same in arterial blood) drives a sustained rise; mechanoreceptors in working muscles and joints reinforce the response.

Q3. A 17 year old runner starts a 400 m race. State two acute cardiovascular responses and one acute respiratory response that occur within the first 30 seconds, and explain the functional purpose of each. [3 marks]

• Cue. HR rises (sympathetic activation, parasympathetic withdrawal) to raise cardiac output and deliver oxygen; blood is redistributed by vasodilation in working muscle and vasoconstriction in non-essential tissue to direct cardiac output where it is needed; ventilation rises (central command and chemoreceptor feedback) to remove carbon dioxide and maintain alveolar oxygen.