Pulmonary Capacities
நுரையீரல் கொள்ளளவுகள்
Most students confuse 'Volumes' with 'Capacities'. Volumes are individual measurements; capacities are the sum of two or more volumes. If you cannot distinguish between Vital Capacity and Total Lung Capacity, you are losing 4 marks in the exam.
The idea
Pulmonary capacities are derived by adding specific respiratory volumes. Inspiratory Capacity (IC) is TV + IRV. Expiratory Capacity (EC) is TV + ERV. Functional Residual Capacity (FRC) is ERV + RV. Vital Capacity (VC) is ERV + TV + IRV, representing the maximum air one can breathe out after a forced inspiration.
Why it matters
Capacities provide a comprehensive view of lung health and the total volume of air an individual can utilize for gas exchange.
NEET lens
NEET frequently tests the formulas for IC, EC, FRC, and VC. Memorize the definitions, but derive the sums logically to avoid errors. Watch out for questions involving 'Total Lung Capacity' (TLC), which includes the Residual Volume (RV). Never confuse VC with TLC; VC excludes RV.
Clinical significance
In clinical practice, FRC is critical because it represents the air remaining in the lungs after a normal expiration. In conditions like Emphysema, the loss of alveolar elasticity increases the residual volume, leading to an abnormally high FRC. This traps air and prevents efficient gas exchange.
Introduction
Pulmonary capacities are combinations of two or more lung volumes that represent functionally meaningful measures of lung performance. Key capacities include IC, FRC, VC, and TLC.
Core concept
Four capacities: (1) Inspiratory Capacity (IC) = TV + IRV = 500 + 2500 = 3000 mL. Total air that can be inhaled after normal expiration. (2) Expiratory Reserve Capacity (FRC) = ERV + RV = 1100 + 1100 = 2200 mL. Air remaining after normal expiration. (3) Vital Capacity (VC) = IRV + TV + ERV = 2500 + 500 + 1100 = 4100 mL. Maximum air that can be moved in a single breath. (4) Total Lung Capacity (TLC) = IRV + TV + ERV + RV = 2500 + 500 + 1100 + 1100 = 5200 mL (NCERT value). All the air lungs can hold.
Go deeper
Vital capacity (VC) is clinically most important: reduced in restrictive lung disease (fibrosis, pneumonia — lungs stiffer). FEV₁ (forced expiratory volume in 1 second) reduced in obstructive lung disease (asthma, COPD — airways narrowed). FEV₁/FVC ratio distinguishes obstructive vs restrictive disease.
From Class 10
Lung capacities are combinations of two or more lung volumes. They give a more complete picture of lung function than individual volumes alone.