In general, the word ‘ventilation’ refers to circulation of replacement of air or gas in a space. In respiratory physiology, ventilation is the rate at which air enters or leaves the lungs. Ventilation in respiratory physiology is of two types:

1. Pulmonary ventilation

2. Alveolar ventilation.


Pulmonary ventilation is defined as the volume of air moving in and out of respiratory tract in a given unit

of time during quiet breathing. It is also called minute ventilation or respiratory minute volume (RMV).

Pulmonary ventilation is a cyclic process, by which fresh air enters the lungs and an equal volume of air

leaves the lungs.


Normal value of pulmonary ventilation is 6,000 mL (6 L)/minute. It is the product of tidal volume (TV) and

the rate of respiration (RR). It is calculated by the formula:

Pulmonary ventilation

= Tidal volume × Respiratory rate

= 500 mL × 12/minute

= 6,000 mL/minute.


Alveolar ventilation is the amount of air utilized for gaseous exchange every minute.

Alveolar ventilation is different from pulmonary ventilation. In pulmonary ventilation, 6 L of air moves

in and out of respiratory tract every minute. But the whole volume of air is not utilized for exchange of

gases. Volume of air subjected for exchange of gases is the alveolar ventilation. Air trapped in the respiratory passage (dead space) does not take part in gaseous exchange.


Normal value of alveolar ventilation is 4,200 mL (4.2 L)/ minute.

It is calculated by the formula:

Alveolar ventilation

= (Tidal volume – Dead space) x Respiratory rate

= (500 – 150) mL × 12/minute

= 4,200 mL (4.2 L)/minute.


Dead space is defined as the part of the respiratory tract, where gaseous exchange does not take place. Air present in the dead space is called dead space air.


Dead space is of two types:

1. Anatomical dead space

2. Physiological dead space.

Anatomical Dead Space

Anatomical dead space extends from nose up to terminal bronchiole. It includes nose, pharynx, trachea,

bronchi and branches of bronchi up to terminal bronchioles. These structures serve only as the

passage for air movement. Gaseous exchange does not take place in these structures.

Physiological Dead Space

Physiological dead space includes anatomical dead space plus two additional volumes.

Additional volumes included in physiological dead space are:

1. Air in the alveoli, which are non-functioning. In some respiratory diseases, alveoli do not function because of dysfunction or destruction of alveolar membrane.

2. Air in the alveoli, which do not receive adequate blood flow. Gaseous exchange does not take place

during inadequate blood supply. These two additional volumes are generally considered as wasted ventilation. Wasted ventilation and wasted air Wasted ventilation is the volume of air that ventilates

physiological dead space. Wasted air refers to air that is not utilized for gaseous exchange. Dead space air is generally considered as wasted air.


Volume of normal dead space is 150 mL. Under normal conditions, physiological dead space is equal to

anatomical dead space. It is because, all the alveoli are functioning and all the alveoli receive adequate blood flow in normal conditions. Physiological dead space increases during respiratory

diseases, which affect the pulmonary blood flow or the alveoli.


Dead space is measured by single breath nitrogen washout method. The subject respires normally for few minutes. Then, he takes a sudden inhalation of pure oxygen.

Oxygen replaces the air in dead space (air passage), i.e. the dead space air contains only oxygen and it

pushes the other gases into alveoli. Now, the subject exhales through a nitrogen meter.

Nitrogen meter shows the concentration of nitrogen in expired air continuously.

First portion of expired air comes from upper part of respiratory tract or air passage, which contains only

oxygen. Next portion of expired air comes from the alveoli, which contains nitrogen. Now, the nitrogen meter shows the nitrogen concentration, which rises sharply and reaches the plateau soon. By using data obtained from nitrogen meter, a graph is plotted. From this graph, the dead space is calculated.

The graph has two areas, area without nitrogen and area with nitrogen. Area of the graph is measured

by a planimeter or by computer. Area without nitrogen indicates dead space air.

It is calculated by the formula:


without N2 Volume of

Dead space = × expired air Area Area

with N2

+ without N2

For example, in a subject:

Area with nitrogen = 70 sq cm

Area without nitrogen = 30 sq cm

Volume of air expired = 500 mL

Dead space = × 500

70 + 30


= × 500


= 150 mL.

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