Physiology · Temperature, exercise and metabolism

Exercise and training physiology !

Regulation
Exam priority: high

During exercise the heart pumps more, the lungs move more air and muscle pulls more oxygen; training improves every step.

Updated 2026-11-01

Also called: VO₂ max, Lactate threshold

See it in 3D: Left ventricle · Right lower lobe · Left lower lobe

What it is

Exercise can raise muscle oxygen demand 15–20-fold. The body answers with more cardiac output, redistribution of flow to muscle, more ventilation and greater O₂ extraction in tissue. Endurance training raises stroke volume and mitochondria; strength training enlarges muscle fibres.

How it works, step by step

The motor cortex drives muscle and the cardiovascular centre in parallel (central command); muscle ergoreceptors add to it.
Sympathetic output raises HR and contractility; the muscle pump boosts venous return and stroke volume.
Local metabolites (K⁺, adenosine, CO₂, H⁺, NO) dilate muscle arterioles; viscera and inactive skin constrict.
In muscle capillaries heat, CO₂ and acid shift the hemoglobin curve right: more O₂ is unloaded.
The first seconds run on ATP and phosphocreatine; then glycolysis and, in long efforts, oxidation of fat and glucose.
After exercise, extra O₂ use persists (EPOC) to restore phosphocreatine, clear lactate and cool down.

Oxygen–hemoglobin dissociation curve

P50 38.5 mmHgArterial blood 93 %Mixed venous blood 53 %O₂ given up to the tissues 40 %

Right shift (P50 38.5 mmHg): Hb has less affinity and gives up more O₂ in the tissues. Acid, CO₂, heat and 2,3-BPG cause it.

Current curve
Normal curve (pH 7.40 · 37 °C)
Arterial blood (PO₂ 100)
Mixed venous blood (PO₂ 40)
O₂ given up to the tissues

In working muscle, acid, CO₂ and heat release more O₂. Compare with «Normal».

Equations

Fick principleVO₂ = CO × (CaO₂ − CvO₂)

CO: cardiac output
CaO₂ − CvO₂: arteriovenous O₂ difference

At rest the difference is ≈5 mL O₂/dL; at peak exercise it can triple.

Approximate values (young adult)

Parameter	Value	Note
Cardiac output	≈5 L/min at rest → 20–25 L/min	elite athletes >30 L/min
Muscle blood flow	≈1 L/min → up to ≈20 L/min
Ventilation	≈6 L/min → 100–150 L/min
VO₂	≈250 mL/min (3.5 mL per kg per minute = 1 MET)	VO₂ max ≈35–45 mL per kg per minute if sedentary
Maximum HR	≈220 − age (rough)

Approximate values for a healthy adult at rest; each laboratory sets its own ranges.

Clinical notes

Stable angina: exercise raises HR and shortens diastole, when the left ventricle is perfused; the plaque prevents coronary flow from rising.
Beta-blockers blunt exercise HR: the maximum-HR formula does not apply to these patients.
Regular exercise raises insulin sensitivity (GLUT4 via AMPK, insulin-independent) and lowers resting BP.
In Bogotá VO₂ max is lower than at sea level for the first days; acclimatized athletes compensate with more hemoglobin.

Test yourself

Think of the answer, then tap the question to see it.

Why does a marathon runner have a resting HR of 45/min with the same cardiac output as a sedentary person?

Training enlarges the ventricle and its stroke volume; with more volume per beat fewer beats are needed (higher vagal tone).

What happens to PaCO₂ during moderate aerobic exercise, and why?

It barely changes: ventilation rises in step with CO₂ production. It falls only above the lactate threshold, when breathing increases to compensate for acidosis.

Why does exercise lower blood glucose in someone with insulin resistance?

Contraction activates AMPK and moves GLUT4 to the membrane by an insulin-independent route, so muscle takes up glucose.

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See it in 3D

The structures of Durgical's 3D anatomy where it happens.

Stable angina

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Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: 10.6 Exercise and Muscle Performance (OpenStax (Rice University), 2022); 22.6 Modifications in Respiratory Functions (OpenStax (Rice University), 2022); Benefits of Exercise: MedlinePlus (MedlinePlus, US National Library of Medicine, 2026); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 85). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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Representative model, for educational purposes only.