Respiratory quotient

Key takeaways

Respiratory quotient belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Respiratory quotient to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Respiratory quotient from memory before moving on to harder problems.

Reference excerpt

The respiratory quotient (RQ or respiratory coefficient) is a dimensionless number used in calculations of basal metabolic rate (BMR) when estimated from carbon dioxide production. It is calculated from the ratio of carbon dioxide produced by the body to oxygen consumed by the body, when the body is in a steady state. Such measurements, like measurements of oxygen uptake, are forms of indirect calorimetry. It is measured using a respirometer. The respiratory quotient value indicates which macronutrients are being metabolized, as different energy pathways are used for fats, carbohydrates, and proteins. If metabolism consists solely of lipids, the respiratory quotient is approximately 0.7, for proteins it is approximately 0.8, and for carbohydrates it is 1.0. Most of the time, however, energy consumption is composed of both fats and carbohydrates. The approximate respiratory quotient of a mixed diet is 0.8. Some of the other factors that may affect the respiratory quotient are energy balance, circulating insulin, and insulin sensitivity. It can be used in the alveolar gas equation.

Respiratory exchange ratio The respiratory exchange ratio (RER) is the ratio between the metabolic production of carbon dioxide (CO2) and the uptake of oxygen (O2). The ratio is determined by comparing exhaled gases to room air. Measuring this ratio is equal to RQ only at rest or during mild to moderate aerobic exercise without the accumulation of lactate. The loss of accuracy during more intense anaerobic exercise is among others due to factors including the bicarbonate buffer system. The body tries to compensate for the accumulation of lactate and minimize the acidification of the blood by expelling more CO2 through the respiratory system. The RER can exceed 1.0 during intense exercise. A value above 1.0 cannot be attributed to the substrate metabolism, but rather to the aforementioned factors regarding bicarbonate buffering. Calculation of RER is commonly done in conjunction with exercise tests such as the VO2 max test. This can be used as an indicator that the participants are nearing exhaustion and the limits of their cardio-respiratory system. An RER greater than or equal to 1.0 is often used as a secondary endpoint criterion of a VO2 max test.

Calculation The respiratory quotient (RQ) is the ratio: RQ = CO2 eliminated / O2 consumed where the term "eliminated" refers to carbon dioxide (CO2) removed from the body in a steady state. In this calculation, the CO2 and O2 must be given in the same units, and in quantities proportional to the number of molecules. Acceptable inputs would be either moles, or else volumes of gas at standard temperature and pressure. Many metabolized substances are compounds containing only the elements carbon, hydrogen, and oxygen. Examples include fatty acids, glycerol, carbohydrates, deamination products, and ethanol. For complete oxidation of such compounds, the chemical equation is CxHyOz + (x + y/4 - z/2) O2 → x CO2 + (y/2) H2O and thus metabolism of this compound gives an RQ of x/(x + y/4 - z/2). For glucose, with the molecular formula, C6H12O6, the complete oxidation equation is C6H12O6 + 6 O2 → 6 CO2 + 6 H2O. Thus, the RQ= 6 CO2/ 6 O2=1. For oxidation of a fatty acid molecule, namely palmitic acid:

23 O 2 + C 16 H 32 O 2 → 16 C O 2 + 16 H 2 O + 129 A T P {\displaystyle 23\ \mathrm {O} _{2}+\mathrm {C} _{16}\mathrm {H} _{32}\mathrm {O} _{2}\to 16\ \mathrm {CO} _{2}+16\ \mathrm {H} _{2}\mathrm {O} +129\ \mathrm {ATP} }

R E R = V C O 2 V O 2 = 16 C O 2 23 O 2 ≈ 0.7 {\displaystyle \mathrm {RER} ={\frac {\mathrm {VCO} _{2}}{\mathrm {VO} _{2}}}={\frac {16\ \mathrm {CO} _{2}}{23\ \mathrm {O} _{2}}}\approx 0.7}

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Respiratory quotient

Start with the simplest possible case. Write down what Respiratory quotient claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Respiratory quotient before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Respiratory quotient ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Respiratory quotient

In research: Respiratory quotient appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Respiratory quotient in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Respiratory quotient is common in secondary-school and first-year university syllabi. It links to neighbouring topics Biochemistry methods, Energy conversion, Exercise biochemistry, so understanding it makes those chapters shorter.
In everyday life: Look for Respiratory quotient outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Respiratory quotient in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Respiratory quotient means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Respiratory quotient out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Respiratory quotient in simple terms?

The respiratory quotient (RQ or respiratory coefficient) is a dimensionless number used in calculations of basal metabolic rate (BMR) when estimated from carbon dioxide production. It is calculated from the ratio of carbon dioxide produced by the body to oxygen consumed by the body, when the body i…

Why does Respiratory quotient matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Respiratory quotient?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Respiratory quotient.