Exercise Physiology
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Direct answer
Cardiac output climbs from about 5 litres per minute at rest to over 20 L/min in an untrained young adult during maximal dynamic exercise, and above 30 L/min in endurance athletes — the central engine of the whole-body response. Oxygen uptake follows the Fick principle (VO2 equals cardiac output times the arterial-venous oxygen difference), and its ceiling, VO2 max, is the gold-standard index of cardiorespiratory fitness: about 35–40 mL/kg/min in untrained men (roughly 10% lower in women), rising to 70 and beyond in the trained, with elite endurance athletes reported at 85–90. Above the lactate threshold, metabolism tips anaerobic and exhaustion is timed, not willed.
What you must remember
- VO2 max is limited centrally by cardiac output delivery, not by muscle oxygen extraction, which is already near-maximal in most people.
- Maximal heart rate is approximately 220 minus age (years); stroke volume plateaus at about 40–50% of VO2 max in untrained subjects, so beyond that, output rises by rate alone.
- Arteriovenous oxygen difference widens from about 5 mL/dL at rest to 15–16 mL/dL at maximum — extraction triples.
- Muscle blood flow rises up to 20-fold (from roughly 4 to 80 mL per 100 g per minute), taking up to 80% of cardiac output, while splanchnic and renal flows fall to a fraction of resting values.
- Minute ventilation rises from 6 to beyond 100 L/min; the ventilatory threshold marks hyperventilatory buffering of lactate.
- Lactate (anaerobic) threshold occurs near 55–60% of VO2 max in the untrained and 70–80% or more in the trained — the trainable variable that endurance performance actually tracks.
- Fuel and fibres: type I slow oxidative red fibres sustain marathon work; type IIx fast glycolytic white fibres power sprints; the first seconds of intense effort run on phosphocreatine.
- Static (isometric) exercise raises blood pressure steeply through a pressor response — hazardous in cardiac patients — whereas dynamic exercise raises cardiac output with modest pressure change.
- Training adaptations: resting bradycardia, higher stroke volume, greater capillary and mitochondrial density, and a lower heart rate at any given workload; 1 MET equals 3.5 mL/kg/min.
From the sofa to the treadmill — a graded test reasoned
Put a sedentary 40-year-old on a treadmill and raise the grade every three minutes. In the first minute, heart rate and stroke volume rise, muscle arterioles dilate on local metabolites, and splanchnic vessels constrict — blood pressure rises only modestly because total systemic conductance widens. Below the lactate threshold, he is in steady state: phosphocreatine has handed over to aerobic metabolism, oxygen uptake matches demand, and he could continue for an hour. Push past 55–60% of maximum and lactate accumulates, hydrogen ions drive ventilation upward (the ventilatory threshold), and the clock starts.
At his VO2 max, the system is saturated: heart rate near 180 (220 minus 40), stroke volume plateaued, extraction tripled — and the test terminates on exhaustion. Stop him and he breathes hard to repay the oxygen debt: rebuilding ATP and phosphocreatine, clearing lactate via hepatic gluconeogenesis (the Cori cycle) and oxidation in heart and muscle. Now train him for six months. His resting pulse falls to the 50s from vagal tone and a larger, more compliant ventricle; his lactate threshold drifts from 60% towards 75% of a now-higher VO2 max; and his muscle shows more mitochondria, myoglobin and capillaries — he runs the same pace at a lower fraction of maximum, which is what fitness means.
Where students slip
Two reversals catch candidates. First, what limits VO2 max: it is cardiac output (oxygen delivery), not the muscle's ability to extract — a trained muscle cannot extract what never arrives. Second, static versus dynamic: lifting a heavy weight can push systolic pressure above 300 mmHg because sustained contraction compresses the vessels and the pressor reflex fires — this is why isometric exercise is prescribed cautiously after myocardial infarction while graded dynamic walking is encouraged. A third favourite: why do athletes have bradycardia — higher vagal tone plus a larger stroke volume, not heart disease.
Frequently asked questions
What does the Fick principle state about oxygen uptake?
VO2 equals cardiac output multiplied by the arteriovenous oxygen difference, so maximal uptake is raised by increasing delivery, extraction, or both.
What is the lactate threshold and how does training change it?
The workload at which blood lactate rises steeply; it occurs near 55–60% of VO2 max in untrained subjects and can be pushed to 70–80% with endurance training.
Why does stroke volume stop rising beyond moderate exercise in untrained people?
Beyond about 40–50% of VO2 max, filling time shortens as heart rate climbs, so further rises in cardiac output come almost entirely from rate.
Why is static exercise more dangerous than dynamic exercise in cardiac patients?
Isometric contraction compresses muscle vessels and evokes a strong pressor response, spiking blood pressure and afterload, whereas dynamic exercise mainly raises output.
What is the clinical unit of exercise intensity?
The metabolic equivalent: 1 MET equals an oxygen uptake of 3.5 mL/kg/min, the resting value used to grade activities and prescribe rehabilitation.