1 Ventilation

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.

2 Pulmonary Ventilation

Definition

It is also called respiratory minute volume (RMV).

Normal Value and Calculation

Normal pulmonary ventilation: 6 L/minute.

It is the product of tidal volume (TV) and the rate of respiration (RR).

3 Alveolar Ventilation

Definition

Alveolar ventilation is different from pulmonary ventilation.

In pulmonary ventilation, 6 L of air moves in and out of lungs in 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 that is trapped in the respiratory passage (dead space) does not take part in gaseous exchange.

Normal Value and Calculation

Normal alveolar ventilation: 4,200 mL (4.2 L)/minute.

4 Dead Space

Definition

Air present in the dead space is called dead space air.

Types of Dead Space

Dead space is of two types:

1. Anatomical dead space.
2. Physiological dead space.

Anatomical Dead Space

Anatomical dead space includes nose, pharynx, trachea, bronchi and branches of bronchi up to terminal bronchioles.

Physiological Dead Space

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

1. Air in the alveoli, which are nonfunctioning: In some respiratory diseases, alveoli do not function due to 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.

Wasted Ventilation

Wasted ventilation is the volume of air that occupies physiological dead space.

Wasted air is the air that is not utilized for gaseous exchange.

Therefore, it is generally considered as wasted air.

Normal Value of Dead Space

Under normal conditions, physiological dead space is equal to anatomical dead space.

It is because all the alveoli are functioning and all alveoli receive adequate blood flow in normal conditions.

Volume of normal dead space: 150 mL.

In respiratory disorders, which affect pulmonary blood flow or alveoli, the dead space increases.

It is associated with reduction in alveolar ventilation.

5 Measurement of Dead Space

Dead space is measured by single breath nitrogen wash-out method.

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 determines 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, nitrogen meter shows increasing nitrogen concentration, which rises sharply and reaches plateau soon.

By using data obtained from nitrogen meter, a graph is plotted.

From this, the dead space is calculated.

Calculation of Dead Space

Example

6 Ventilation-Perfusion Ratio

Definition

It is expressed as V_A/Q.

V_A is alveolar ventilation and Q is the blood flow (perfusion).

Normal Value and Calculation

Normal ventilation-perfusion ratio: About 0.84.

Significance of Ventilation-Perfusion Ratio

Ventilation-perfusion ratio signifies the gaseous exchange.

It is affected if there is any change in ventilation or in blood flow.

Variations in Ventilation-Perfusion Ratio

Physiological Variation

1. Ratio increases, if ventilation increases without any change in blood flow.
2. Ratio decreases, if blood flow increases without any change in ventilation.

Pathological Variation

In chronic obstructive pulmonary diseases (COPD), ventilation is affected because of destruction of alveolar membrane.

So, the ventilation-perfusion ratio reduces greatly.

Wasted Air and Wasted Blood

Ventilation-perfusion ratio is not perfect because of existence of two factors on either side of alveolar membrane:

1. Physiological dead space, which includes wasted air.
2. Physiological shunt, which includes wasted blood.

7 Inspired Air

Composition of inspired air is given below.

Alveolar Air

It is collected by Haldane-Priestley tube.

Importance of Alveolar Air

1. Alveolar air is different from the inspired air or atmospheric air. Alveolar air is partially replaced by atmospheric air during each breath.
2. Oxygen diffuses from alveolar air into pulmonary capillaries constantly.
3. Carbon dioxide diffuses from pulmonary blood into alveolar air constantly.
4. Dry atmospheric air is humidified, while passing through respiratory passage just before entering the alveoli.

8 Renewal of Alveolar Air

Alveolar air is constantly renewed.

Rate of renewal is slow during normal breathing.

During each breath, out of 500 mL of tidal volume, only 350 mL of air enters the alveoli and the remaining 150 mL (30%) becomes dead space air.

Hence, the amount of alveolar air replaced by new atmospheric air with each breath is only about 70% of total alveolar air.

Slow renewal of alveolar air is responsible for prevention of sudden changes in concentration of gases in the blood.

9 Expired Air

It is a combination of dead space air and alveolar air.

Expired air is collected by using Douglas bag.

Concentration of gases in expired air is somewhere between inspired air and alveolar air.

Composition of Inspired Air, Alveolar Air and Expired Air