Mechanism of ventilation/breathing :

Respiratory System of Class 11

Mechanism of ventilation/breathing :

Pressure within pleural cavity is called intrapleural pressure. It is normally negative (–5 cm.). if atmospheric pressure (760 mm Hg) is considered to be zero (or base) than the 755 mm Hg intrapleural pressure is said to be negative i.e. – 5 mm. Hg. During inspiration, negativety increases and during expiration it decreases. In quiet respiration intrapleural pressure varies between – 3.5 mm Hg (at the end point of inspiration) to +0.7 mm Hg (at then end of expiration) In very deep inspiration intrapleural pressure may be – 30 mm Hg. In violent expiration, especially with glottis closed the intrapleural pressure may be +20 mm Hg. In clinical practice, the intrapleural pressure is measured in cm H2o. (1 cm H2o= .7 mm Hg). Pressure within lungs is assumed to be equal intrapleural pressure. In diseases in which breathing is difficult (i.e. asthma, emphysema) patients are most comfortable on sitting up, leaving forwards and fixing the arms. During inspiration fresh air follows following path external nares → nasal chambers → internal nares → pharynx → glottis → larynx → trachea → bronchi → bronchioles → alveolar ducts → alveoli. Advantage of negative pressure breathing : Mammals have negative pressure breathing, i.e. the lungs draw air due to reduction in pressure within them. This allows them to eat and breathe at the same time. If air were to be forced into the lungs, it might carry food particles into the trachea and block it. Negative pressure breathing gently moves air which is less likely to carry food particles into the wind pipe.

Positive pressure breathing : Frog closes the mouth, opens the nares and lowers the throat. This enlarges the buccopharyngeal cavity, where reduced pressure draws fresh air via nares. This part of breathing occurs on the negative pressure principle. The frog then closes the nares and raises the throat, forcing the air into the lungs. This is positive pressure breathing. After exchange of gases in the lungs, frog opens the nares and expels foul air by contracting abdominal muscles.


It takes place in the alveoli by simple diffusion due to difference of pressure of CO2 and O2 in the alveoli and the perialveolar blood capillaries.

Partial pressure of oxygen (Po2) in inspired air is 158 mmHg, but in alveolar air is 100 mmHg (2 1%) the Po2, in venous blood is 40 mm Hg (16%).

Partial pressure of CO2 (Pco2) in alveolar air is about 36 mm Hg while in venous blood it is about 46 to 50 mmHg and in atmospheric air it is 0.03%.

Gas Atmospheric air Inspired air Alveolar air Expired air

N2 597.0 (78.62%) 563.4 (74.09%) 569.0 (74.9) 566.0 (74.5%)

O2 159.0 (20.84%) 149.3 (19.67%) 104.0 (13.6%) 120.0 (15.7%)

CO2 0.3 (0.04%) 0.3(0.04%) 40.0 (5.3%) 27.0 (3.6%)

H2O 3.7 (0.5%) 47.0 (6.2%) 47.0 (6.2%) 47.0 (6.2%)

760.0 (100%) 760.0 (100%) 760.0 (100%) 760.0 (100%)


Blood is the medium to carry O2 from lungs to tissue and CO2 from tissue to lungs.

Transport of O2

Being less soluble in water, only 0.3 ml of O2 per 100 ml. of blood is transported as dissolved in plasma, this is only a negligible amount. Mainly transported in bound form through the respiratory pigment, haemoglobin (Hb).

Haemoglobin (Hb)

A conjugated protein, consists of 4 globin chains (protein moiety) each with 1 haem group (an Fe-porphyrin) as non-protein part.

It is a large molecule of quarternary structure and the molecular weight 66800 – 68000 D.

Globin chains are of four—  α,β,γ and δ types.

Depending upon the number of different types of chains there are many types of Hb:

(1) Embryonic or foetal Hb (HbF)— contains 2α + 2γ  globin chains (α2γ2) .

(2) Adult Hb (HbA) i.e. after birth is of two types:

(i) HbA-l has2α + 2β chains (α2β2) .

(ii) HbA-2 has 2α + 2δ chains(α2δ2).

In our body 98% Hb is of Hb A-1 type and 2% of Hb A-2 type.

In Hb molecule Fe remains in Fe++ state and it is the site for O2 combine

1 Fe atom combines with 1 molec. of O2

1 molec. Hb combines with 4 molec. of O2

66800 gm (gm mol. wt.) of Hb combines with 4 × 22.4 litre O2

1 gm of Hb will combine with 1.34 ml O2

15 gmHb (l00 ml blood) will carry 15 × 1.34 ml O2 = 20 ml. O2.

To carry about 20 ml O2 in 100 ml blood (20% of its volume) is called as total respiratory capacity of blood.

Combining with O2 it forms oxyhaemoglobin (HbO2) — an unstable compound which dissociates at lower pressure of O2 in the tissue undergoing metabolism.

Hb + O2 Mechanism of ventilation/breathing HbO2 (Oxyhaemoglobin)

This is no oxidation reduction reaction as Fe remains in Fe++ state on both sides.

However, in higher nitrogen oxide level in the medium, Hb is oxidised and forms methemoglobin (or ferrihemoglobin). This causes a pollution disease, methemoglobinemia in child, where nitrate level is higher in underground (or drinking) water.

The rate of association and dissociation depends upon Po2, the graph plotted as Hb-O2 dissociation curve is typically a segmoid curve.

At 60 mm Hg Po2 there is more than 80% saturation of Hb and the rate of reaction below it is faster, this is called as critical pressure of O2

50% saturation takes place only at 30 mm Hg of Po2 (P50 value of O2 is 30 mm Hg)

Maximum saturation of Hb even at 100 mm Hg Po2 is 97.8%.

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