252 ^|

A Handbook of Biology

MËÇHÅÑÏSM ØF BRËÅTHÏÑG (ÏÑSPÏRÅTÏØÑ & ËXPÏRÅTÏØÑ)

ÏÑSPÏRÅTÏØÑ

Active intake of air from atmosphere into lungs

These changes reduce pressure inside the thorax

causing the expansion of lungs. Thus, pulmonary

volume increases resulting in decrease of intra-

pulmonary pressure, less than the atmospheric

pressure. So, air moves into lungs. [Odisha NEET 2019]

There is a negative pressure in the lungs with respect to

atmospheric pressure.

During this, the diaphragm contracts (flattens)

causing an increase in vertical volume (antero-

posterior axis). [NEET 2020]

Contraction

of

external

intercostal

muscles

(muscles found between ribs) lifts up the ribs and

sternum causing an increase in thoracic volume in

the dorso-ventral axis

EXPIRATION



Passive expelling of air

from the lungs.



During this, intercostal

muscles & diaphragm

relax

causing

a

decrease in thoracic

volume and thereby

pulmonary

volume.

This

leads

to

an

increase

in

intra

pulmonary pressure to

slightly above atmospheric pressure. So, air moves out.

Air entering

lungs

Air expelled

from lungs

Ribs and

sternum

raised

Ribs and

sternum

returned to

original

position

Ribs cage

Volume of thorax

increased

Volume of

thorax

decreased

Inspiration

Expiration

Diaphragm

contracted

Diaphragm

relaxed

and arched

upwards