All subjects · Biology · WBI15 (Unit 5)

Lactate after exercise and respiratory quotient

Lactate made during hard exercise must be dealt with afterwards. The respiratory quotient tells us which fuel an organism is respiring.

What happens to lactate

During intense exercise, muscles respire anaerobically and lactate builds up, lowering pH and contributing to fatigue.

After exercise, extra oxygen is needed (the oxygen debt, or excess post-exercise oxygen consumption). Lactate is carried in the blood to the liver, where it is:

oxidised back to pyruvate, which can enter the Krebs cycle and be respired aerobically, or
converted back to glucose and stored as glycogen.

Extra oxygen is also used to resynthesise ATP and creatine phosphate and to reload myoglobin.

Respiratory quotient

RQ = volume of CO2 produced / volume of O2 used

RQ depends on the substrate:

Substrate	Typical RQ
Carbohydrate (glucose)	1.0
Protein	about 0.8 to 0.9
Lipid	about 0.7

Using RQ

For glucose: C6H12O6 + 6O2 → 6CO2 + 6H2O, so RQ = 6/6 = 1.0.

Lipids contain less oxygen relative to carbon and hydrogen, so more O2 is needed to oxidise them: RQ is lower.

An RQ above 1 suggests some anaerobic respiration (for example yeast producing ethanol, which releases CO2 without using O2).

An RQ of about 0.8 at rest suggests a mixture of substrates.

Worked example

Stearic acid is respired: C18H36O2 + 26O2 → 18CO2 + 18H2O. Calculate the RQ.

RQ = CO2 produced / O2 used
= 18 / 26 = 0.69

Answer: RQ = 0.69, typical of a lipid.

Key idea: Lactate goes to the liver, where it is oxidised to pyruvate or converted to glucose and glycogen, using extra oxygen after exercise. RQ = CO2 produced / O2 used: carbohydrate 1.0, lipid about 0.7; above 1 means some anaerobic respiration.

Check yourself

An organism produces 4.2 cm³ of CO2 and uses 5.0 cm³ of O2 in the same time. Calculate its RQ.
Show answer
0.84. 4.2 / 5.0 = 0.84.
Tripalmitin is respired: 2C51H98O6 + 145O2 → 102CO2 + 98H2O. Calculate the RQ to 2 decimal places.
Show answer
0.70. 102 / 145 = 0.70.
Where is most lactate converted back to pyruvate or glucose after exercise?
- Kidney
- Lungs
- Liver
- Heart valves
Show answer
Liver. Blood carries lactate to the liver.
Why does breathing stay fast for a while after hard exercise?
- Carbon dioxide levels fall to zero
- Extra oxygen is needed to oxidise lactate and resynthesise ATP and creatine phosphate
- Muscles respire only anaerobically after exercise
- The heart stops pumping
Show answer
Extra oxygen is needed to oxidise lactate and resynthesise ATP and creatine phosphate. This is the oxygen debt.
An RQ greater than 1 suggests that
- only lipid is being respired
- no carbon dioxide is being made
- only protein is being respired
- some anaerobic respiration is happening
Show answer
some anaerobic respiration is happening. Extra CO2 without O2 use comes from anaerobic pathways such as fermentation.
Why do lipids have a lower RQ than carbohydrates?
- They release more CO2 per O2
- They contain less oxygen relative to carbon and hydrogen, so need more O2 to oxidise
- They are respired anaerobically
- They contain no carbon
Show answer
They contain less oxygen relative to carbon and hydrogen, so need more O2 to oxidise. More O2 used per CO2 released lowers the ratio.
Which substrate has an RQ of 1.0?
- A triglyceride
- Stearic acid
- A typical protein
- Glucose
Show answer
Glucose. 6 CO2 produced for 6 O2 used.

Practise with instant feedback

Sources

Last updated 7 November 2026. Spotted a mistake? Tell us.