Acute responses to exercise (SACE Stage 2 Physical Education)

What this dot point is asking

You must explain the short-term responses of the cardiovascular, respiratory and muscular systems to one exercise session, distinguishing these immediate changes from the long-term adaptations that take weeks of training.

Cardiovascular responses

The cardiovascular system works to move more oxygenated blood to the muscles.

• Heart rate (HR) rises almost immediately, even slightly before movement begins (anticipatory rise), and increases roughly in proportion to intensity until it nears maximum.
• Stroke volume (SV), the blood ejected per beat, increases as more blood returns to the heart (venous return) and the heart contracts more forcefully. It plateaus at around 40 to 60 percent of maximum effort.
• Cardiac output (Q) is HR multiplied by SV, so it rises sharply because both components rise. This is the master variable for oxygen delivery.
• Blood pressure rises, mostly systolic pressure, as cardiac output increases.
• Blood flow redistribution shifts blood away from the gut and toward the working muscles and skin (for cooling) through vasodilation and vasoconstriction.

Respiratory responses

The respiratory system increases the rate of gas exchange.

• Respiratory rate (breaths per minute) increases.
• Tidal volume (air per breath) increases.
• Together these raise minute ventilation (the volume of air moved per minute), which can climb from about 6 L/min at rest to over 100 L/min.
• Oxygen diffusion at the alveoli and carbon dioxide removal both speed up to match the rising metabolic demand.

Muscular and metabolic responses

• Muscle temperature rises, which speeds enzyme activity and makes muscle more pliable.
• Oxygen uptake (VO2) at the muscle increases as the muscles extract more oxygen from the blood.
• Fuel use shifts: stored ATP and phosphocreatine power the first seconds, then glycogen and, at lower intensities, fats are mobilised.
• By-products such as carbon dioxide, heat and (at high intensity) hydrogen ions accumulate.

Why the responses are linked

Every acute response serves one goal: matching oxygen and fuel supply to the muscles with the demand, and clearing the waste this produces. A rise in cardiac output is useless without a rise in ventilation to oxygenate the blood, and blood flow must be redirected so the extra oxygenated blood reaches the muscles that need it.

The responses also scale with intensity, which is why data from fitness testing and diagnostics tasks is often plotted against effort. At submaximal intensities the body reaches a steady state where supply matches demand and the responses level off. As intensity climbs toward maximum, heart rate and ventilation keep rising while stroke volume has already plateaued, and the increasing reliance on anaerobic energy pathways produces a sharper rise in carbon dioxide and hydrogen ions. Reading where each variable plateaus or accelerates on a graph, and explaining it through this supply-and-demand logic, is exactly the analysis the e-investigation and source questions reward.