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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