movement of air between the atmosphere and the lungs is known as ventilation or
breathing. Inspiration and expiration are the two phases of breathing.
Inspiration is the movement of atmospheric air into the lungs and expiration is
the movement of alveolar air that diffuse out of the lungs. (Figure 6.4)
not contain muscle fibres but expands and contracts by the movement of the ribs
and diaphragm. The diaphragm is a sheet of tissue which separates the thorax
from the abdomen. In a relaxed state, the diaphragm is domed shaped.
Ribs are moved by the intercostal muscles. External and internal intercostal muscles found between the ribs and the diaphragm helps in creating pressure gradients. Inspiration occurs if the pressure inside the lungs (intrapulmonary pressure) is less than the atmospheric pressure likewise expiration takes place when the pressure within the lungs is higher than the atmospheric pressure.
is initiated by the -contraction of the diaphragm muscles and external
intercostal muscles, which pulls the ribs and sternum upwards and outwards and
increases the volume of the thoracic chamber in the dorso–ventral axis, forcing
the lungs to expand the pulmonary volume. The increase in pulmonary volume and
decrease in the intrapulmonary pressure forces the fresh air from outside to
enter the air passages into the lungs to equalize the pressure. This process-
is called inspiration.
of the diaphragm allows the diaphragm and sternum to return- to its dome shape
and the internal -intercostal muscles contract, pulling the ribs
reducing the thoracic volume and pulmonary volume. This results in an increase
in the intrapulmonary pressure slightly above the atmospheric -pressure causing
the expulsion of air from the lungs. This process is called expiration.
12–16 times/minute. An instrument called Spirometer is used to measure the
volume of air involved in breathing movements for clinical assessment of a
person’s pulmonary function.
volume of air present in various phases of respiration is denoted as
• Tidal Volume (TV) Tidal
volume is the amount of air inspired or expired with each normal breath. It is approximately 500 mL., i.e. a normal human
adult can inspire or expire approximately 6000 to 8000mL of air per minute.
During vigorous exercise, the tidal volume is about 4–10 times higher.
Reserve volume (IRV) Additional volume of air a person can inspire by forceful inspiration is
called Inspiratory Reserve Volume. The normal value is 2500–3000 mL.
Reserve volume (ERV) Additional volume of air a person can forcefully exhale by forceful
expiration is called Expiratory Reserve Volume. The normal value is 1000–1100
Volume (RV) The volume of air remaining in the lungs after a forceful expiration. It is
approximately 1100–1200 mL.
capacity (VC) the maximum volume of air that can be moved out during a single breath following a
maximal inspiration. A person first inspires maximally then expires maximally.
capacity (IC) The total volume of air a person can inhale after normal expiration. It includes
tidal volume and inspiratory reserve volume. IC=TV+IRV
capacity (EC) The total volume of air a person can exhale after normal inspiration. It
includes tidal volume and expiratory reserve volume. EC=TV+ERV
Lung Capacity (TLC) The total volume of air which the lungs can accommodate after forced
inspiration is called Total Lung Capacity. This includes- the vital capacity
and the residual volume. It is approximately 6000mL. TLC=VC+RV
Respiratory Volume The amount of air that moves into the respiratory passage per minute is
called minute respiratory volume.
= 500mL; Normal respiratory rate = 12 times/minute
minute respiratory- volume = 6 Litres/minute (for a -normal healthy man).
the inspired air never reaches the gas exchange areas but fills the respiratory
passages where exchange of gases does not occur. This air is called dead space.
space is not involved in gaseous exchange. It amounts to approximately 150mL.