Define Maximum Breathing capacity

it is the largest volume of air that can be moved into andout of the lungs in one minute by maximum voluntary effort. Normal: 90-170 L/min (average 100 L/min).

Define Lung compliance

The change in lung volume per unit change in airway pressure (AV/AP) is the ‘distensibility’ (compliance) of the lungs and the chest wall.
Where V = volume of the lung
P = airway pressure distending Pressur
A = the difference.
It is expressed as litre/cm H2O.

Define Residual volume

It is the volume of air which remains in lungs after a maximal expiration.
Normal: 1200 mL.

Define FEV1 and FVC

FEV1:- (Forced Expiratory Volume in 1 sec) i.e. volume of FVC expired in 1st sec of exhalation.
Normal: 80% of FVC.
FVC:- FVC stands for Forced Vital Capacity, which is the maximum amount of air a person can exhale forcefully and completely after taking a deep breath. It is measured during pulmonary function tests and is an important indicator of lung health and function. FVC is often used in diagnosing and monitoring conditions such as asthma, chronic obstructive pulmonary disease (COPD), and pulmonary fibrosis.

What is Surfactant

Surfactant is a mixture of phospholipids, proteins & ions

The major phospholipid is dipalmitoyl-phosphotidylcholine (DPPC) The proteins are SPA, SPB, SPC and SPD

Secreted by type II alveolar epithelial cells

  • Primary function:-

-Reduces the surface tension of intra alveolar fluid by reducing the attraction between water molecules

  • Secondary functions
  1. Stabilizes alveolar size during inspiration and expiration. During inspiration, surfactant layer becomes thin. This can not reduce the surface tension. The surface tension opposes further expansion of alveoli during inspiration. During expiration, surfactant layer becomes thick. This reduces surface tension. This prevents alveolar collapse during expiration. Thereby the alveolar size is stabilized
  2. Prevents pulmonary edema
    Since surface tension of intra alveolar fluid is one of the causes for pulmonary edema, by reducing surface tension the surfactant prevents pulmonary edema
  3. Increase in compliance
  • Increases compliance by decreasing the elastic recoiling of the lungs which tend to collapse the lung
  • Reduces the work of breathing by causing easy expansion of alveoli
  • Facilitates the reopening of collapsed airway & alveoli
  • Facilitates phagocytosis of micro-organisms by alveolar macrophages
Factors affecting surfactant
Surfactant decreases due to:
1. long-term inhalation of 100% O2, (as occurs during cardiac surgery)
2. occlusion of main bronchus,
 3. occlusion of one pulmonary artery.
4. cigarette smoking, 
 5. cutting both the vagi.
Surfactant increases due to:
1. thyroid hormones, increase the size and number of inclusion bodies in type II alveolar lining epithelial cells, and
2. glucocorticoids, speed up the maturation of surfactant.

Define Lung compliance

  • Lung compliance refers to the ability of the lungs to expand and contract.
  • It is a measure of the change in lung volume that occurs for a given change in pressure across the lung.
  • It is influenced by the elasticity of lung tissue, surface tension of the fluid lining the inside of the lungs, and resistance to airflow in the airways.

Muscle Actions Responsible For (A) Normal Expiration (B) Forced Expiration

(a)Muscle actions responsible for normal expiration:

  • Passive relaxation of the diaphragm and external intercostal muscles
  • Elastic recoil of the lung tissue and chest wall

(b) Muscle actions responsible for forced expiration:

  • Contraction of the internal intercostal muscles to pull the ribs downward and inward
  • Contraction of the abdominal muscles to increase pressure in the abdominal cavity and push the diaphragm upward
  • Contraction of the quadratus lumborum muscle to stabilize the ribcage and prevent it from moving upward during exhalation.

Dead space

Dead space air:

The amount of the air present in respiratory system which do not take part in gaseous exchange process and thus becomes a wastage is known as Total Dead space or Physiological dead space air. It is constituted by two types of dead space air.

(1) Anatomical dead space air -The amount of air which is confined in the respiratory passages and thereby are not available for gaseous exchange process is known as anatomical dead space air

Normal value :150 ml
It can be measured by N2 Washout technique

(i) Alveolar dead space air:

In normal subjects all of the alveoli are not equally perfused. The apical alveoli are adequately ventilated but due to poor blood supply most of the ventilation in these alveoli is wasted This amount of air which is not utilized from the alveoli are called alveolar dead space air

It can be measured as per the basis of Bohr’s equation as follows:
ADS= Expiratory air volume (PCO2 in alveolar air PCO2 in expiratory air)/ PCO2 in alveolar air

Physiological dead space air= Anatomical dead space air + alveolar dead space air.


1. Physiological
(i) Sex: DS is more in males.
(ii) Age: DS increases with age, because inflated lungs pull the airways thereby increasing the airway diameter.
(iii) Body height: DS increases in proportion with increase in body height.

2. Pathological
(i) Emphysema: loss of elasticity of lungs in emphysema decreases elastic recoil, this produces hyperinflation of lungs to cause increase in DS.
(ii) Bronchiectasis: It is associated with dilated bronchi, thereby causes DS to increase.
(iii) Pulmonary embolism which produces regional decrease in pulmonary vascular bed.

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