Describe the cardiovascular, respiratory and metabolic changes that occur during exercise. Add a note on the effects of training.

Types of Exercise

Cardiovascular Changes

• Cardiac output rises chiefly by heart rate; stroke volume plateaus at about 40–50% of maximum effort
• Redistribution — muscle flow rises from 20% to 85% of cardiac output; renal and splanchnic flow fall sharply; cerebral flow is unchanged and coronary flow rises with demand
• Isometric exercise raises diastolic BP markedly — the pressor response — because sustained contraction occludes the vessels. This is why it is avoided in hypertension and ischaemic heart disease

Respiratory Changes

• Ventilation rises from 6 L/min to 100–120 L/min
• VO2 max — 250 mL/min at rest to 3000–6000 mL/min; the best index of cardiorespiratory fitness
• Chiefly by ↑ tidal volume early, then by rate
• Arterial PO2, PCO2 and pH stay almost unchanged in moderate exercise — a remarkable regulatory achievement
• Anaerobic threshold — the point at which lactate begins to accumulate; ventilation then rises disproportionately

Mechanisms of exercise hyperpnoea

• Neurogenic (feed-forward) — collaterals from the motor cortex; ventilation rises before any chemical change
• Proprioceptors in moving joints and muscles
• ↑ Body temperature; ↑ K+ from muscle
• Lactic acidosis, in severe exercise only

Metabolic and Other Changes

• Fuel use — creatine phosphate for seconds, anaerobic glycolysis for 1–2 minutes, then oxidative metabolism. Carbohydrate is used early, fat increasingly with duration
• Temperature — core may reach 39–40 °C; sweating up to 1.5–2 L/hour
• Blood — haemoconcentration; leucocytosis
• Oxygen debt — extra oxygen consumed after exercise to restore ATP and creatine phosphate, convert lactate to glucose (Cori cycle) and replenish stores; maximum about 11.5 L

Effects of Training

• Athlete's heart — physiological hypertrophy with a large chamber and normal function; not the same as pathological hypertrophy

Recovery After Exercise

• Oxygen consumption stays raised for minutes to hours — the excess post-exercise oxygen consumption
• Used for — resynthesis of ATP and creatine phosphate, lactate → glucose by the Cori cycle in the liver, replenishing myoglobin and haemoglobin oxygen stores, and restoring temperature and hormone levels
• Active recovery (light exercise) clears lactate faster than complete rest, by maintaining muscle blood flow

Applied Aspects

• Regular aerobic exercise lowers blood pressure, improves lipid profile and insulin sensitivity, and reduces cardiovascular mortality — among the best-established preventive measures in medicine
• Exercise is central to the management of type 2 diabetes because it recruits GLUT-4 independently of insulin
• Exercise stress testing unmasks coronary insufficiency not evident at rest
• Sudden cardiac death in young athletes — usually hypertrophic cardiomyopathy; a reason for pre-participation screening
• Overtraining syndrome — fatigue, poor performance, disturbed sleep and immune suppression; requires rest
• Female athlete triad — low energy availability, menstrual dysfunction and reduced bone density
• Exercise prescription — 150 minutes of moderate aerobic activity a week is the usual public health target