Cardiovascular drift is the gradual increase in heart rate and decrease in stroke volume during prolonged, steady-intensity exercise, despite the workload remaining constant. It is mainly caused by rising body temperature and fluid loss through sweating, particularly in hot conditions.
The Mechanism
During prolonged exercise, thermoregulation increases blood flow to the skin so that heat can be transferred from the body to the environment. At the same time, sweating causes water and electrolyte loss.
Sweat loss reduces blood plasma volume, lowering the total volume of blood returning to the heart. This reduced venous return decreases end-diastolic volume, so the ventricles fill with less blood before each contraction. Consequently, stroke volume falls.
Because cardiac output is calculated as , heart rate rises to compensate for the lower stroke volume and help maintain cardiac output. With substantial dehydration or heat stress, this compensation may become insufficient, causing cardiac output and exercise performance to decline.
| Factor | Effect contributing to cardiovascular drift |
|---|---|
| Sweating | Reduces body water and plasma volume |
| Loss of plasma volume | Decreases venous return and ventricular filling |
| Increased skin blood flow | Diverts more blood toward the skin for heat loss |
| Rising body temperature | Increases cardiovascular strain and heart rate |
| Electrolyte loss | Can impair fluid balance and normal muscle and nerve function |
This links directly to water and electrolyte balance: dehydration increases the cardiovascular effort required to sustain the same exercise intensity. Replacing water and sodium can support plasma volume, although replacement needs depend on sweat losses and exercise duration.
A common misconception is that cardiovascular drift occurs because exercise intensity increases. It occurs while the external workload remains constant; the body's internal physiological strain increases.
Exam Technique
In an IB SEHS response, define cardiovascular drift, then explain the causal sequence: sweating and skin blood flow reduced plasma volume and venous return reduced stroke volume increased heart rate. Link the mechanism explicitly to thermoregulation, dehydration, and water-electrolyte balance.