Respiratory Volume

Respiratory volumes refer to the amount of air inhaled or exhaled during different types of breathing. These volumes are measured in milliliters (mL) or liters (L) and can vary depending on age, sex, physical fitness, and activity level.

1.0Types of Respiratory Volumes

Tidal Volume (TV)

• The tidal volume (TV) is the amount of air inhaled or exhaled during a normal, quiet breath. On average, the tidal volume for a healthy adult is approximately 500 mL. 
• This is the volume of air that moves in and out of the lungs during normal, relaxed breathing.

Inspiratory Reserve Volume (IRV)

• The inspiratory reserve volume (IRV) is the additional amount of air that can be forcibly inhaled after a normal inspiration (inhalation). 
• It's the "extra" air you can take in beyond your usual breath. The average IRV is about 2500-3000 mL.

Expiratory Reserve Volume (ERV)

• The expiratory reserve volume (ERV) is the additional amount of air that can be forcibly exhaled after a normal expiration (exhalation). 
• It represents the "extra" air you can push out of your lungs after breathing out normally. The average ERV is approximately 1000-1100 mL.

Residual Volume (RV)

• The residual volume (RV) is the volume of air that remains in the lungs even after a maximum forceful expiration. This air can never be completely exhaled. 
• The residual volume is crucial because it keeps the alveoli inflated, preventing the lungs from collapsing. The average RV is about 1100-1200 mL.

2.0Respiratory Capacities: Combinations of Volumes

Respiratory capacities are formed by combining two or more respiratory volumes. They help in understanding the total air a person can move in and out of their lungs.

Inspiratory Capacity (IC)

• The inspiratory capacity (IC) is the total volume of air a person can inhale after a normal expiration. 
• It is the sum of the tidal volume and the inspiratory reserve volume.
• IC=TV+IRV
• Average IC: 500mL+2500−3000mL=3000−3500mL

Functional Residual Capacity (FRC)

• The functional residual capacity (FRC) is the volume of air that remains in the lungs after a normal expiration. 
• It's the sum of the expiratory reserve volume and the residual volume.
• FRC=ERV+RV
• Average FRC: 1000−1100mL+1100−1200mL=2100−2300mL

Vital Capacity (VC)

• The vital capacity (VC) is the maximum volume of air a person can breathe out after a maximum forceful inspiration. 
• It is the sum of the expiratory reserve volume, tidal volume, and inspiratory reserve volume.
• VC=ERV+TV+IRV
• Average VC: 1000−1100mL+500mL+2500−3000mL=4000−4600mL

Total Lung Capacity (TLC)

• The total lung capacity (TLC) is the total volume of air a person's lungs can hold after a maximum forceful inspiration. 
• It is the sum of all four respiratory volumes: tidal volume, inspiratory reserve volume, expiratory reserve volume, and residual volume. 
• It can also be calculated as the sum of vital capacity and residual volume, or inspiratory capacity and functional residual capacity.
• TLC=TV+IRV+ERV+RV
• Average TLC: 500mL+2500−3000mL+1000−1100mL+1100−1200mL=5100−5800mL This is the maximum amount of air your lungs can possibly hold.

3.0Measurement of Respiratory Volumes

The spirometer is the most commonly used instrument for measuring respiratory volumes and capacities.

• Procedure: The subject breathes through a mouthpiece connected to the spirometer.
• Data Obtained: Tidal volume, inspiratory and expiratory reserve volumes, and vital capacity.
• Significance: Used in clinical diagnosis of conditions like asthma, COPD, and restrictive lung diseases.
• Boyle's Law: In a laboratory setting, this principle is used in conjunction with pressure measurements and gas concentrations to derive lung volumes, notably the functional residual capacity (FRC). 

4.0Factors Affecting Respiratory Volumes

1. Age: Lung capacity decreases with age.
2. Sex: Males generally have higher lung volumes than females.
3. Physical Fitness: Athletes often have larger tidal and vital capacities.
4. Health Conditions: Respiratory diseases can reduce lung volumes.
5. Body Size: Taller individuals have greater lung volumes.

5.0Normal Values of Respiratory Volumes in Adults