Respiratory Volumes and Capacities: Spirometer Measurements

Respiratory volumes and capacities are the measurable amounts of air that move in and out of the lungs during different levels of breathing. A machine called a spirometer records these values and helps doctors judge how healthy the lungs are.

These measurements matter because many lung diseases first show up as changed volumes long before a person feels severe symptoms. A simple breathing test can distinguish blocked airways from stiff lungs and track whether treatment is working.

Key Concepts

Tidal volume (TV) is air in a normal quiet breath, about 500 mL. Inspiratory reserve volume (IRV) is the extra air you can inhale after a normal inhalation, about 2,500 to 3,000 mL. Expiratory reserve volume (ERV) is the extra air you can exhale after a normal exhalation, about 1,000 to 1,100 mL. Residual volume (RV) is air always left in lungs after maximum exhalation, about 1,100 to 1,200 mL, and cannot be measured by a simple spirometer. A capacity is the sum of two or more volumes.

• Vital capacity (VC): maximum air exhaled after maximum inhalation, TV + IRV + ERV, about 3,500 to 4,600 mL.
• Inspiratory capacity (IC): TV + IRV, about 3,000 to 3,500 mL.
• Functional residual capacity (FRC): ERV + RV, about 2,100 to 2,300 mL.
• Total lung capacity (TLC): VC + RV, about 5,800 mL.

Formulas

The capacities are simple additions. Vital Capacity = Tidal Volume + Inspiratory Reserve Volume + Expiratory Reserve Volume, or VC = TV + IRV + ERV. Total Lung Capacity = Vital Capacity + Residual Volume, or TLC = VC + RV. Inspiratory Capacity = TV + IRV and Functional Residual Capacity = ERV + RV. All volumes are in millilitres (mL) at body conditions. A spirometer measures TV, IRV, ERV and VC directly from the traced curve, while RV, FRC and TLC need special gas dilution or body box methods because residual air never leaves the lungs.

Worked Example

A student has TV of 500 mL, IRV of 3,000 mL, ERV of 1,100 mL and RV of 1,200 mL. Vital capacity is 500 plus 3,000 plus 1,100, which equals 4,600 mL. Total lung capacity is 4,600 plus 1,200, which equals 5,800 mL. Functional residual capacity is 1,100 plus 1,200, which equals 2,300 mL. If another student with asthma has the same vital capacity but needs much longer to blow it out, the doctor suspects airway obstruction rather than small lungs. The calculation separates size from airflow speed.

Common Misconceptions

• Misconception: Vital capacity is the total air in lungs. Correction: Residual volume always remains, so total lung capacity is larger than vital capacity.
• Misconception: A spirometer measures every volume. Correction: It cannot measure residual volume because that air is never exhaled.
• Misconception: Bigger vital capacity always means healthier lungs. Correction: Values must be compared with predicted values for height, age and sex, and airflow timing also matters.

Real-World Applications

Spirometry diagnoses asthma and chronic obstructive pulmonary disease and monitors workers exposed to dust. Forced expiratory volume in one second (FEV1) from the same test guides treatment. Athletes use lung volumes to assess fitness, and anaesthetists use them to set ventilator tidal volumes safely. Post-surgery breathing exercises aim to restore vital capacity.

Volumes are the building blocks and capacities are their sums. Measured with a spirometer, they convert invisible breathing into precise numbers that reveal lung size, strength and disease.