Understanding Respiratory Quotient (RQ)

The question asks for the term defining the ratio between the volume of carbon dioxide (CO$_2$) expired and the volume of oxygen (O$_2$) consumed over a specific period.

Defining Respiratory Quotient

The Respiratory Quotient (RQ) is specifically defined as this ratio. It is a dimensionless quantity used in physiology and biochemistry to indicate the relative amounts of carbon dioxide produced and oxygen consumed by an organism during respiration.

The formula is expressed as:

$ RQ = \frac{Volume\;of\;CO_2\;expired}{Volume\;of\;O_2\;consumed} $

The value of RQ varies depending on the metabolic substrate being oxidized (e.g., carbohydrates, fats, proteins). For carbohydrates, RQ is typically 1, while for fats, it's around 0.7.

Analyzing Other Options

The other options provided are distinct physiological measurements:

• Inspiratory quotient: This term is not standard in respiratory physiology and seems incorrect. It might be confused with Respiratory Quotient or relate to the ratio of inspired gases, but it's not the definition sought.
• Vital capacity: This refers to the maximum volume of air a person can exhale after a maximal inhalation. It measures lung volume, not gas exchange ratios.
• Total lung capacity: This is the total volume of air in the lungs after a maximal inhalation. It also relates to lung volume, not the ratio of expired CO$_2$ to consumed O$_2$.

Therefore, the correct term for the ratio of CO$_2$ expired to O$_2$ consumed is the Respiratory Quotient.