Exercise Physiology — The Body Under Load

Exercise is one of the greatest physiological stress tests the body ever faces. Within seconds of the first step of a run, almost every organ system adjusts in a coordinated response to the sudden demand for energy and oxygen. Exercise physiology is the study of these adjustments — and it ties together the cardiovascular, respiratory, muscular, nervous, and endocrine systems into a single, elegant story. The fundamental challenge is this: working muscles suddenly need far more ATP, oxygen, and fuel, and they generate far more heat and carbon dioxide as by-products. The body must increase supply and clear waste simultaneously, all while defending homeostasis — keeping core variables like blood pH, temperature, and blood glucose within their normal ranges. The response unfolds in layers, on three different timescales: - Immediate (seconds): the nervous system raises heart rate and breathing rate before the muscles even strictly need it — an anticipatory response — and redirects blood flow toward the working muscles. - Short-term (minutes): hormones such as adrenaline mobilise fuel from the liver and fat stores; breathing and circulation settle into a steady state matched to the workload. - Long-term (weeks to months): with repeated exercise, the body remodels itself structurally — the heart, muscles, and even bones become stronger and more efficient. This is the physiological basis of physical training. Studying exercise physiology reveals how the systems you’ve studied separately in earlier lessons actually work together as a single integrated machine.

Muscles need a constant supply of ATP to contract, but they only store enough for a few seconds of activity. To keep going, the body regenerates ATP continuously using three energy systems, which switch on at different speeds and sustain different durations of effort. | Energy system | Duration | Oxygen needed? | By-product | |---|---|---|---| | ATP-PC (phosphocreatine) | ~5–10 seconds | No | None | | Anaerobic glycolysis | ~10 seconds – 2 minutes | No | Lactic acid | | Aerobic respiration | Minutes to hours | Yes | CO₂ + water | 1. The ATP-PC system — immediate and very short-lived. Phosphocreatine stored in the muscle rapidly donates a phosphate group to regenerate ATP, powering an all-out sprint or a heavy lift for about 5–10 seconds. No oxygen is needed and there are no fatiguing waste products — but the fuel runs out almost instantly. 2. Anaerobic glycolysis — fast but limited. Glucose is broken down without oxygen to produce ATP quickly, powering intense efforts lasting roughly 10 seconds to 2 minutes (a 400 m sprint, for example). This pathway produces lactic acid (lactate) as a by-product; its build-up contributes to the burning sensation and fatigue felt during hard effort. 3. Aerobic respiration — slower to switch on, but sustainable. Glucose and fats are fully broken down using oxygen in the mitochondria, yielding far more ATP per glucose molecule than glycolysis. It powers prolonged, moderate-intensity activity — jogging, cycling, anything lasting more than a couple of minutes — and produces only carbon dioxide and water as waste, making it sustainable for hours provided fuel and oxygen keep arriving. In practice these three systems overlap and blend continuously, but the balance shifts with intensity and duration: sprinters lean on the first two systems, while marathon runners depend almost entirely on the aerobic system.

To deliver oxygen and fuel and clear waste fast enough, the heart, blood vessels, and lungs all ramp up dramatically during exercise. Cardiovascular changes: - Heart rate rises from around 70 beats per minute at rest to 180 or more during intense exercise. - Stroke volume (the blood pumped per beat) increases as the heart fills and contracts more forcefully. - Cardiac output — heart rate × stroke volume, the total blood pumped per minute — can increase five-fold or more, from roughly 5 litres/min at rest to 20–30 litres/min in trained athletes. - Blood flow is redistributed: vessels supplying the working muscles vasodilate, while those to the gut and kidneys vasoconstrict, diverting blood to where it’s needed most. - Blood pressure rises moderately to drive the increased flow. Respiratory changes: - Breathing rate and depth both increase, raising ventilation — the volume of air moved per minute — from about 6 L/min at rest to over 100 L/min during hard exercise. - This boosts oxygen uptake and carbon dioxide removal at the alveoli. - The main drive to breathe harder comes from rising CO₂ levels detected by chemoreceptors, not from falling oxygen — the same principle covered in the respiratory physiology lesson. VO₂ max is the maximum rate at which a person can take up and use oxygen, and it is the gold-standard measure of aerobic fitness: the higher it is, the more sustained, intense exercise a person can perform. Endurance training raises VO₂ max significantly, which is one reason trained athletes can sustain a much harder pace than an untrained person of similar size.

Exercising muscle is only about 20–25% efficient — the rest of the energy it uses is released as heat, which the body must shed to avoid a dangerous rise in core temperature. Temperature regulation: Blood is diverted to the skin, where heat is lost to the surrounding air, and sweating increases sharply. As sweat evaporates from the skin it carries heat away — evaporative cooling — the body’s main defence against overheating during exercise. In hot or humid conditions, cooling becomes much harder: humid air slows evaporation, and heat exhaustion (dizziness, nausea, heavy sweating) can progress to heat stroke, a medical emergency in which the cooling system fails altogether and core temperature climbs dangerously. Fluid and electrolyte balance: Sweating loses both water and electrolytes, especially sodium. Significant fluid loss reduces blood volume, forcing the heart to work harder for the same cardiac output and impairing performance — this is dehydration. Drinking only plain water during very prolonged exercise can dangerously dilute blood sodium (exercise-associated hyponatraemia), which is why endurance athletes favour electrolyte drinks over water alone during long events. Fatigue is the decline in performance during sustained effort, and it has multiple causes working together: depletion of fuel stores (muscle glycogen), accumulation of metabolic by-products, rising body temperature, dehydration, and even protective signals from the brain that deliberately limit effort. Far from being a weakness, fatigue is protective — it stops the body being pushed to the point of serious harm.

The remarkable thing about exercise is that the body adapts to it. Repeated training stresses each system just enough that it rebuilds stronger — the core principle underlying all physical training. Cardiovascular adaptations: the heart muscle strengthens and the left ventricle enlarges, increasing stroke volume — a trained athlete’s resting heart rate can fall below 50 beats per minute because each beat pumps more blood. Capillary density in muscle increases, improving oxygen delivery, and total blood volume rises. Muscular adaptations: resistance (strength) training increases muscle fibre size (hypertrophy) and strength; endurance training increases the number and size of mitochondria and the muscle’s capacity for aerobic respiration. Respiratory and metabolic adaptations: the respiratory muscles strengthen and breathing becomes more efficient, and the body becomes better at using fat as fuel, sparing glycogen for when it’s really needed. Skeletal adaptations: weight-bearing exercise increases bone density, helping to prevent osteoporosis later in life. Health benefits: regular exercise reduces the risk of heart disease, type 2 diabetes, obesity, several cancers, and depression, while improving blood pressure, cholesterol, insulin sensitivity, mood, and sleep. Few interventions in medicine have benefits this broad — which is why "exercise is medicine" has become a guiding principle in modern healthcare.