Alveolar Air Vs Inspired Air

INSPIRED AIR

Inspired air is the atmospheric air, which is inhaled during inspiration.

ALVEOLAR AIR

Alveolar air is the air present in alveoli of lungs.

Alveolar Air Vs Inspired Air

Alveolar air is different from inspired air in four ways: 1. Alveolar air is partially replaced by the atmospheric air during each breath

2. Oxygen diffuses from the 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 before entering the

Alveoli.

RENEWAL

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 quantity of 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 the total alveolar air.

Thus,

350

Alveolar air = × 100 = 70%

500

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

in the blood.

METHOD OF COLLECTION

Alveolar air is collected by using Haldane-Priestely tube. This tube consists of a canvas rubber tube, which is 1 m long and having a diameter of 2.5 cm. It is opened on both ends.

A mouthpiece is fitted at one end of the tube. Near the mouthpiece, there is a side tube, which is fixed

with a sampling tube. Mouthpiece and the side tube are interconnected by means of a three-way cock.

By keeping the mouthpiece in the mouth, the subject makes a forceful expiration through the mouthpiece.

Alveolar air is expired at the end of forced expiration. So, by using the three-way cock, the last portion of expired air (alveolar air) is collected in the sampling tube.

EXPIRED AIR

Expired air is the amount of air that is exhaled during expiration. It is a combination of dead space air and

alveolar air.

COMPOSITION

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

METHOD OF COLLECTION

Expired air is collected by using Douglas bag

VENTILATION-PERFUSION RATIO

DEFINITION

Ventilationperfusion ratio is the ratio of alveolar ventilation and the amount of blood that perfuse the

alveoli. It is expressed as VA/Q. VA is alveolar ventilation and Q is the blood flow (perfusion).

NORMAL VALUE AND CALCULATION

Normal Value

Normal value of ventilationperfusion ratio is about

0.84.

Calculation

Alveolar ventilation is calculated by the formula:

Alveolar ventilation Ventilationperfusion ratio = Pulmonary blood flow

Alveolar ventilation = (Tidal volume – Dead space) ×

Respiratory rate

= (500 – 150) mL × 12/minute

= 4,200 mL/minute

Blood flow through alveoli

(Pulmonary blood flow) = 5,000 mL/minute

Therefore, 4,200

Ventilationperfusion

ratio =

5,000

= 0.84

SIGNIFICANCE OF VENTILATIONPERFUSION

RATIO

Ventilation-perfusion ratio signifies the gaseous exchange. It is affected if there is any change in alveolar

ventilation or in blood flow. Ventilation without perfusion = dead space

Perfusion without ventilation = shunt

WASTED AIR AND WASTED BLOOD

Ventilationperfusion ratio is not perfect because of existence

of two factors on either side of alveolar membrane.

These factors are:

1. Physiological dead space, which includes wasted air

2. Physiological shunt, which includes wasted blood.

VARIATIONS IN VENTILATIONPERFUSION

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

3. In sitting position, there is 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 lungs ventilationperfusion

ratio increases three times. At the same time, in zone 3 of the lungs, because of increased blood flow ventilation-perfusion ratio decreases.

Pathological Variation

In chronic obstructive pulmonary diseases (COPD), ventilation is affected because of obstruction and destruction of alveolar membrane. So, ventilationper Fusion ratio reduces greatly.

 

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