📊 Respiratory System Lesson

TV, IRV, ERV, RV: the key lung volume measurements

Several distinct volume measurements combine in specific ways to describe lung capacity — and one of them can't even be measured by standard spirometry.

~500mL

IRV

~3000mL

ERV

~1200mL

📖 Full Breakdown

Four base volumes that combine into two clinically important capacities

Vital Capacity and Total Lung Capacity are both DERIVED values, built by adding together the base volumes.

Tidal Volume (TV)

~500 mL

The amount of air moved in one normal, relaxed breath.

Inspiratory Reserve Volume (IRV)

~3000 mL

The extra air you CAN inhale beyond a normal breath, if you choose to breathe in as deeply as possible.

Expiratory Reserve Volume (ERV)

~1200 mL

The extra air you CAN exhale beyond a normal breath, if you choose to breathe out as forcefully as possible.

Residual Volume (RV)

~1200 mL

The air that ALWAYS remains in the lungs even after maximum forced exhalation — critically, this volume CANNOT be measured by spirometry, since it can never actually be exhaled.

Vital Capacity (VC)

TV + IRV + ERV ≈ 4700 mL

A derived capacity representing the maximum air that can be moved in a single breath cycle — reduced specifically in restrictive lung diseases.

Total Lung Capacity (TLC)

VC + RV ≈ 5900 mL

The absolute maximum volume the lungs can hold, combining everything that can be measured (VC) with the portion that can never be exhaled (RV).

🩺 Clinical / Exam Application

A pulmonary function test shows a patient's Vital Capacity is reduced, but the technician needs Total Lung Capacity to fully characterize the disease pattern, and this can't be measured by spirometry alone. Because spirometry can only measure air that is actually exhaled, and Residual Volume by definition can never be exhaled, TLC (which includes RV) must be calculated using other methods like body plethysmography — a limitation that traces directly back to the basic definition of what Residual Volume actually is.

⚠️ Exam Alert

A frequently tested detail: Residual Volume cannot be measured by standard spirometry, which is why Total Lung Capacity (which depends on RV) requires alternative measurement techniques — this limitation is a favorite specific fact on exams covering pulmonary function testing.

🚧 Common Trap

Don't assume all these volumes are directly measured, independent values. Vital Capacity and Total Lung Capacity are both CALCULATED by adding together the more basic volumes (TV, IRV, ERV, and RV) — they are derived capacities, not separately measured quantities.

✅ Quick Check

Why can't Total Lung Capacity be measured directly by standard spirometry, even though several of its component volumes can be?

📝 Exam Prep

Common Exam Questions

❓ What is Vital Capacity and how is it calculated?

✅ Vital Capacity is the maximum amount of air that can be moved in a single breath — calculated as Tidal Volume + Inspiratory Reserve Volume + Expiratory Reserve Volume (TV + IRV + ERV), approximately 4700 mL.

❓ Why can't Residual Volume be measured by spirometry?

✅ Residual Volume is the air that always remains in the lungs even after maximum forced exhalation — since it can never actually be exhaled, spirometry (which measures exhaled air) cannot capture it directly.