Pulmonary Ventilation Physiology Notes

Ventilation

Pulmonary Ventilation

Pulmonary Ventilation Definition:

• Pulmonary ventilation is a cyclic process, by which fresh air enters the lungs and an equal volume of air leaves the lungs.
• It is the volume of air moving in and out of the lungs per minute in quiet breathing. It is also called respiratory minute volume (RMV).

Normal Value And Calculation:

Normal Value: Normal value of pulmonary ventilation is 6000 mL (6 liters)/minute.

Read And Learn More: Medical Physiology Notes

Calculation:

Pulmonary ventilation 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 x 12/minute

= 6,000 mL/minute

Alveolar Ventilation

Alveolar Ventilation Definition:

The 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 the lungs in every minute. But the whole volume of air is not utilized for the exchange of gases.
• The volume of air subjected to the exchange of gases is alveolar ventilation.
• The air trapped in the respiratory passage (dead space) does not take part in gaseous exchange.

Normal Value And Calculation:

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

Calculation:

It is calculated by the formula given below. Alveolar ventilation

= (Tidal volume – Dead space volume) × Respiratory rate

= (500- 150) x 12 = 4,200 mL (4.2 liters)/minute

Dead Space

Dead Space Definition:

Dead space is defined as the part of the respiratory tract, where gaseous exchange does not take place.

• The air present in the dead space is called dead space air.
• The parts of the respiratory tract, which form the dead space are the 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.

Types Of Dead Space:

Dead space is of two types:

1. Anatomical dead space
2. Physiological dead space.

Anatomical Dead Space:

It is the volume of respiratory tract from the nose up to the terminal bronchiole.

Physiological Dead Space:

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

1. These two additional volumes are generally considered as wasted ventilation.
2. The additional volumes included in physiological dead space are:
3. The air in the alveoli, which are non-functioning.
4. In some of respiratory diseases, alveoli do not function because of dysfunction or destruction of the alveolar membrane
5. The air in the alveoli, which do not receive adequate blood flow. Gaseous exchange does not take place during inadequate blood supply.

Wasted air:

• The dead space air is generally considered wasted air.
• Wasted air refers to air that is not utilized for gaseous exchange.

Normal Value Of Dead Space:

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

• It is because all the alveoli are functioning and all alveoli receive adequate blood flow in normal conditions.
• The volume of normal dead space is 150 mL.
  In respiratory disorders, which affect the pulmonary blood flow or the alveoli, the dead space increases.
• It is associated with a reduction in alveolar ventilation.

Measurement Of Dead Space- Nitrogen Washout Method:

The dead space is determined by the single breath nitrogen washout method.

• The subject respires normally for a few minutes. Then, he takes a sudden inhalation of pure oxygen.
• The oxygen replaces the air in dead space (air passage), i.e. the dead space air contains only oxygen, * pushes the other gases into alveoli.
• Now. the subject exhales through a nitrogen meter. TTogsrr meter shows the concentration of nitrogen in Ine expired air continuously.
• The first portion of expired air comes from the upper part of the respiratory tract or air passage which only oxygen.
• The next portion of expired air comes from the alveoli, which contain nitrogen.
• Now, the nitrogen meter shows the nitrogen concentration, which rises sharply and reaches the plateau soon.
• By using data obtained from the nitrogen meter, a graph is plotted. From this graph, the dead space is calculated.
• The graph has two areas, the area without nitrogen and the area with nitrogen. The area of the graph is measured by a planimeter.

The area without nitrogen indicates dead space air. It is calculated by the formula:

For example, in a subject:

1. Area with nitrogen = 70sq.cm.
2. Area without nitrogen = 30sq.cm.
3. The volume of air expired =500mL

Ventilation – Perfusion Ratio

Ventilation – Perfusion Ratio Definition:

The ventilation-perfusion ratio is the ratio of alveolar ventilation and the amount of blood that perfuses the alveoli.

It is expressed as V_A/Q:

• V_A is alveolar ventilation
• Q is the blood flow (perfusion)

Normal Value And Calculation:

Normal Value: The normal value of the ventilation-perfusion ratio is about 0.84.

Calculation: Alveolar ventilation is calculated by the formula:

Alveolar ventilation

Alveolar ventilation = (Tidial volume – Dead space) x Respiratory rate

= (500-150) x 12

= 4,200 mL/minute

Blood flow through alveoli (Pulmonary blood flow)

= 5,000 mL/minute

Therefore, the Ventilation-perfusion ratio

= 4,200/5,000

= 0.84

Significance Of Ventilation – Perfusion Ratio:

The ventilation-perfusion ratio signifies the gaseous exchange. It is affected if there is any change in alveolar ventilation or in blood flow.

Wasted Air And Wasted Blood:

• The ventilation-perfusion ratio is not perfect because of the existence of two factors on either side of the alveolar membrane.
• The two factors are physiological dead space and physiological shunt.
• Physiological dead includes wasted air (see above) and physiological c birr includes wasted blood.

Variations In Ventilation – Perfusion Ratio:

Physiological Variation:

• The ratio increases, if ventilation increases without any change in blood flow
• The ratio decreases if blood flow increases without any change in ventilation
• In a sitting position, there is a reduction in blood flow in the upper part of the lungs (zone 1) than in the lower part (zone 3).
• Therefore, in zone 1 of the lungs ventilation-perfusion ratio increases three times.
• At the same time, in zone 3 of the lungs, because of increased blood flow.
• The ventilation-perfusion decreases.

Pathological Variation:

In chronic obstructive pulmonary disease (COPD), ventilation is affected because of obstruction and destruction of the alveolar membrane. So, the ventilation-perfusion ratio reduces greatly.