During exercise, muscles work harder — contracting more frequently and with more force.
This increased muscle activity requires:
MORE GLUCOSE — the fuel for respiration.
MORE OXYGEN — for aerobic respiration.
FASTER REMOVAL of carbon dioxide and lactic acid — waste products.
The body must respond to these increased demands almost immediately.
Three systems work together to meet the demands of exercise:
The HEART — pumps blood faster and harder.
The LUNGS — breathe faster and deeper.
The BLOOD VESSELS — redistribute blood flow to muscles.
Changes During Exercise
The body makes several rapid adjustments when exercise begins:
HEART RATE INCREASES:
The heart beats faster (higher rate) and contracts more strongly (higher stroke volume).
More blood circulates per minute → more O₂ and glucose delivered to muscles.
More CO₂ and waste removed per minute.
BREATHING RATE AND DEPTH INCREASE:
You breathe faster and take deeper breaths.
More O₂ inhaled per minute.
More CO₂ exhaled per minute.
Maintains the concentration gradients in the alveoli.
VASODILATION — blood vessels to muscles widen:
More blood is redirected to active muscles.
Skin flushes (more blood near surface for cooling).
Blood flow reduced to non-essential organs (e.g. digestive system) temporarily.
GLYCOGEN BREAKDOWN:
Glycogen stored in the LIVER and MUSCLES is converted back to glucose.
Increases the blood glucose supply to muscles.
Lactic Acid and Oxygen Debt
During very intense exercise (e.g. sprinting), the body cannot supply oxygen fast enough.
Muscles switch to ANAEROBIC RESPIRATION:
glucose → lactic acid
LACTIC ACID ACCUMULATES in muscles:
Lowers the pH of the muscle tissue.
Disrupts enzyme activity.
Causes the burning, aching sensation during intense effort.
Leads to muscle fatigue.
OXYGEN DEBT (also called EPOC — Excess Post-Exercise Oxygen Consumption):
After exercise stops, the body continues to breathe faster than normal.
This extra oxygen is used to process the accumulated lactic acid.
Lactic acid is transported in the blood to the LIVER.
In the liver, lactic acid is converted BACK into glucose — this requires oxygen.
The amount of extra oxygen needed after exercise to clear the lactic acid = the OXYGEN DEBT.
The harder the exercise, the more lactic acid produced, the larger the oxygen debt, and the longer recovery takes.
Why Training Improves Performance
Regular exercise training causes adaptations that improve the body's response:
HEART:
Cardiac muscle becomes stronger — heart pumps more blood per beat (larger stroke volume).
Resting heart rate falls (a sign of cardiovascular fitness).
MUSCLES:
More mitochondria develop in muscle cells — greater capacity for aerobic respiration.
Larger glycogen stores — more fuel available.
Improved blood vessel density — more efficient oxygen delivery.
LUNGS:
Larger lung capacity.
More efficient gas exchange.
BLOOD:
Higher red blood cell count in trained athletes — more haemoglobin → more O₂ carried per litre of blood.
⚠️ Common Mistake
Oxygen debt is NOT the lack of oxygen in the blood during exercise. It is the EXTRA oxygen needed AFTER exercise to convert the lactic acid that accumulated. Students often say 'you breathe harder during exercise to repay oxygen debt' — you breathe harder DURING exercise to supply O₂, and continue breathing hard AFTER to repay the debt.
📌 Key Note
Exercise → more O₂ and glucose needed → heart rate up, breathing rate up, vasodilation. Intense exercise → anaerobic → lactic acid → muscle fatigue. After exercise → oxygen debt → lactic acid converted to glucose in liver.
🎯 Matching Activity — Match the Exercise Response to its Purpose
Match each change to why it happens during exercise. — drag the symbols on the right to match the component names on the left.
Heart rate increases
Drop here
Breathing rate and depth increase
Drop here
Vasodilation in muscles
Drop here
Glycogen breakdown in liver
Drop here
Continued heavy breathing after exercise
Drop here
Repaying oxygen debt — extra O₂ to convert lactic acid back to glucose
Takes in more O₂ and expels more CO₂ per minute
Delivers more O₂ and glucose to muscles, removes CO₂ faster
More blood redirected to active muscles — better O₂ supply
Converts stored glycogen to glucose to fuel muscles
🎯 Test Yourself
Question 1 of 3
1. Why does heart rate increase during exercise?
2. Why do you continue to breathe heavily for several minutes after stopping intense exercise?
3. What is oxygen debt?
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