Lecture 8: Mechanics of Breathing

Boyle’s Law

At any constant temperature, the pressure exerted by a gas varies inversely with the voume of gas

Respiratory Mechanics

Air tends to move down a pressure gradient hence why air flows in and out of lungs

If intra-alveolar pressure is less than atmospheric pressure…

air flows into lungs

If intra-alveolar pressure is more than atmospheric pressure…

air flows out of lungs

Inspiration

the diaphragm contracts and flattens and the external intercostal muscles contract, the volume of thorax and lungs increases, the pressure inside the lungs decreases to below atmosphere pressure and air flows into the lungs

Expiration

diaphragm relaxes, lungs recoil, thorax volume decreases, lung volume decreases and inside the lungs the pressure rises to 3mm Hg and air moves out along pressure gradient

What happens during forced inspiration & expiration?

external intercostal muscles and diaphragm contract, principle is the same but much larger changes in pressure

What is the role of pleural membranes?

they surround lung tissues as a double membrane fluid filled bag and creates a vacuum round the lungs to stop the lungs collapsing

Pleural membrane: during inspiration

diaphragm contracts, thorax expands, intrapleural pressure becomes more sub-atmospheric, transpulmonary pressure increases, lungs expand, alveolar pressure decreases, air flows in

Pleural membrane: during expiration

lungs must be attached to ribcage but several layers between, stopping lungs collapsing and detaching from thorax

Neuronal control of breathing

regulated by neurones in the medulla oblongata and PONS, I neurons control inspiration and E neurons regulate expiration, recipricol inhbition

Reciprocal inhibition

when one set of neurons fire, it inhibits the second neurones, generating the basic rhythm of breathing

Medulla oblongata

contains the dosal respiratory group and the ventral respiratory group, signals from these centres are sent to the intercostal muscles and diaphragm

PONS

aphuestic centre sends impulses to I neurones to keep inspiration going and pneumotaxic centre sends impulses to the DRG that switches off I neurones limiting the time of inspiration