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Lung Volumes and Capacities and Transport of Respiratory Gases

Inspiratory reserve volume (IRV), Expiratory reserve volume (ERV), Residual volume (RV), Inspiratory capacity (IC), Total lung capacity (TLC).

Volume and capacity of the lungs

A lung volume measurement (respiratory volume measurement) is required to determine lung capacity by monitoring how much gas is in the mungs at a given time, it calculates how much gas is present in it. The lung capacity is determined by summing the lung volumes. Male adults have a total lung capacity of about 6 liters of air on average. The pulmonary function test includes the measurement of lung volumes. The volume of these fluids tends to vary with breathing depth, ethnicity, gender, age, body composition, and disease. In addition to measuring lung volumes with spirometry, many other variables can be measured, including tidal volume, expiratory reserve volume, and inspiratory reserve volume. The capacity, volume, and residual capacity of the lungs can be determined by various methods. Plethysmography and nitrogen washout are both effective methods of determining this.

Lung volumes

In one respiratory cycle, we can inhale or exhale a certain amount of air. All parts of the respiratory system are shown here, including the respiratory centers, the lungs, and the chest wall. Healthy adults, produce 10% of their vital capacity. A healthy adult produces roughly 300-500 milliliters per day (6-8 milliliters for every kilogram), but when they exercise, they can produce 50%.

Inspiratory reserve volume (IRV)

An individual's reserve volume is the amount of air they can bring into their lungs for forcible inhalation after they have completed a normal tidal volume. When used for deep breathing, the IRV is normally held in reserve. 1900-3300 ml is the normal value for an adult.

Expiratory reserve volume (ERV)

A simple explanation for expiratory reserve volume is- It is nothing more than the amount of gas you exhale upon forcefully exhaling after the tidal exhalation has taken place. An adult normally exhales 700-1200ml. In obese people or those suffering from ascites or after upper abdominal surgery, ERV is reduced.

Residual volume (RV)

The amount of gas in your lungs after you exhale forcefully is known as the residual volume. The FRC and ERV values cannot be measured through spirometry in adults. Hence, it is indirectly measured by the summation of FRC and ERV.

As a result of incomplete lung drainage and air entrapment, RV is often markedly elevated in patients with obstructive lung diseases. For patients who require surgery and mechanical ventilation, the RV value should not exceed 140% because of the increased risk of barotrauma, pneumothorax, infection, and venous return. During the postoperative period, patients who are at higher risk require a higher inflation pressure.

Lung capacities

Inspiratory capacity (IC)

A person can inhale the most air possible when resting. An inspiratory reserve volume is calculated based on the sum of tidal volume and inspiratory reserve volume.

Total lung capacity (TLC)

During inhalation, total lung capacity is nothing more than the amount of air that enters the lungs. It is not the volume of the air that lungs can hold but the volume of air that the lungs hold after the maximal inhalation. About 6,000mL (4-6L) is the normal value. A total lung capacity (TLC) can be calculated by adding together the four primary lung volumes (ERV, TV, RV, IRV). If a patient has an obstruction like emphysema or kyphoscoliosis, their TLC can be higher or lower.

Vital capacity (VC)

Immediately following maximal inhalation, the exhaled air determines vital capacity. Depending on their age and weight, adults need approximately 4800mL of liquid per day. Adding tidal volume can be used to calculate many different types of calculations, including inspiratory reserve volume and expiratory reserve volume. Since VC measures the strength of the inspiratory and expiratory muscles, it should be three times larger than TV during coughing. In obstructive disorders, VC may be reduced, but always in restrictive disorders.

VC = TV + IRV + ERV.

Function residual capacity (FRC)
The residual air in the lungs at the end of a normal exhalation is measured by the tidal volume. Residual volume is added to expiratory reserve volume to calculate expiratory reserve volume. About 1800 – 2200 mL is the normal value.


Resting position for FRC is achieved by relaxing muscles and balancing inner and outer elastic recoils. Restrictive disorders reduce FRC. An increased FRC over a rising TLC indicates hyperinflation. The ratio of FRC to TLC can reach up to 80%.

Transport of respiratory gases

A period of time will pass after the respiratory gases diffuse in the lungs, and the blood will be O2 rich and CO2 will be exhaled before the O2 rich blood is transported to the tissues that require it. It is imperative to introduce new batches of oxygen-rich blood to the lungs before the process begins. The gas is carried from one part of the body to the other by the bloodstream, according to the accompanying image of the cardiovascular system (heart and blood vessels). Blood rich in oxygen leaves the lungs through the pulmonary veins and is pumped to the rest of the body by the left ventricle via the aorta. Hemoglobin (Hb) determines how much oxygen can be carried by the blood. With CO2, H+ ions, or high temperatures, O2's ability to bind to Hb is also reduced. A lack of oxygen results in less energy being produced through aerobic metabolism, which decreases performance. The capillaries deliver oxygenated blood into the tissues, while the pulmonary arteries return carbon dioxide-filled blood to the heart. The lungs, after passing through the alveoli, emit CO2 after the gas diffuses into the alveoli from the capillaries.

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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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