Lung Volumes and Lung Capacities in Health and Respiratory Diseases
The changes in the volumes of the lungs occur in a predictable manner during quiet breathing and deep breathing. Four such measurable volumes have been described and based on the four volumes four capacities have been defined.
Tidal volume (TV) – volume inhaled or exhaled during quiet breathing
Inspiratory Reserve Volume (IRV) – volume inhaled in excess of the TV in a deep inspiration
Expiratory Reserve Volume (ERV) – volume exhaled in excess of the TV and IRV in a deep expiration
Residual Volume (RV) – volume that remains in the respiratory tract following a deep expiration
Inspiratory Capacity (IC) = TV + IRV
Vital Capacity (VC) = IRV + TV + ERV
Functional Residual Capacity (FRC) = ERV + RV
Total Lung Capacity (TLC) = IRV + TV + ERV + RV
Lung volumes depend on the age, sex, ethnicity and built....
The lung capacities and volumes tend to increase as the size of the body increases and are higher in males compared to females. As the age increases after the third decade, the residual volume and the functional residual capacity increases due to the stiffening of the lungs as the elastic recoil forces tend to decrease with ageing. The tidal volume and the expiratory reserve volume decreases but the total lung capacity remains relatively constant.
Alterations of lung volumes with the position.....
In the supine position, functional residual capacity decreases as a result of a decrease in the expiratory reserve volume, compared to an erect position. However, the decrease in the functional residual capacity results in an increase in the inspiratory reserve volume. Since the venous return to the thorax increases in supine position, the vital capacity and the total lung capacity may decrease.
Lung Volumes and Capacities in Pregnancy
In pregnancy, as the uterus enlarges and the abdomen gets distended, the diaphragm is pushed upwards. This results in a decline in the total lung capacity due to a reduction in the residual volume, inspiratory reserve volume and the expiratory reserve volume, sparing the tidal volume. Therefore, the vital capacity and the functional residual capacity tend to decrease. Despite the normal tidal volume, to meet the increased demand of oxygen, the respiratory rate increases resulting in an increase in the minute ventilation.
Lung Volumes in Restrictive Lung Disease
Lung volumes and capacities also tend to be affected in different types of lung diseases. In restrictive lung disease such as fibrosing alveolitis, the alveoli tend to get fibrosed and as a result become stiffer. Thus, the lungs become less expandable resulting in a reduction in all the volumes and the capacities. The pathophysiology of restrictive lung disease seen in neuromuscular diseases such as myasthenia gravis, severe Guillain Barre Syndrome and phrenic nerve palsy is similar. To compensate for the decreased tidal volume in such conditions, the rate of respiration is increased so that the minute ventilation (i.e. the tidal volume X respiratory rate) could be maintained at a level closer to a normal individual. Unlike in fibrosing alveolitis, which is a generalized process, lung fibrosis can involve only certain foci as seen in tuberculosis. In such conditions the reduction in the volume in the involved segments is usually compensated by hyper-expansion of the healthy lung segments. However, as the disease progresses, the increased respiratory drive fails to compensate for the loss of volume and results in hypoxia and hypercapnoea. With further deterioration, the patients with such diseases tend to undergo ventilator failure which is also known as type II respiratory failure.
Lung Volumes in Obstructive Airway Disease
In certain other respiratory diseases such as bronchial asthma, the airways become narrowed and results in difficulty in inspiration and expiration. Since the negative intra-thoracic pressure during inspiration helps to maintain the airways open during inspiration, the impact of the disease is more during expiration than during inspiration. This causes trapping of air inside the lungs causing the residual volume and hence the functional residual capacity to be increased. The inspiratory reserve volume is relatively constant but the expiratory reserve volume tends to reduce. In chronic obstructive pulmonary disease (COPD), this phenomenon is exaggerated as the connective tissue in the lung parenchyma is destructed in addition to the airway narrowing. Therefore, the residual volume increases further resulting in a barrel shaped chest. This rise in residual volume also decreases the vital capacity and to compensate the tidal volume becomes deeper and the respiratory rate becomes slower.
Learn more about lung volumes and capacities...
- Lung Volumes and Capacities
Breathing (inspiration and expiration) occurs in a cyclical manner due to the movements of the chest wall and the lungs. The resulting changes in pressure, causes changes in lung volumes.
Multiple Choice Questions on Lung Volumes in Health and Disease
view quiz statistics
Learn more about respiratory physiology...
- Respiratory Physiology - Introduction
The respiratory physiology is on the process of incorporation of oxygen in the environment for the utilization of energy from the organic compounds and for the elimination of carbon dioxide