5B.3: Breathing + gas exchange

Bony anatomy of the thoracic cage

Thoracic muscles

Contraction of the diaphragm _____ thoracic volume

increases

pressure changes

Pressure changes → respiration

* breathing is literally all about pressure change

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Dependant upon the volume of the thoracic cage

increasing volume = inspiration

Decreasing volume = expiration

Fundamental mechanics

Lungs are under tension (interstitium is primary elastic)

* naturally want to collapse
* stuck to visceral pleura
* Alveolar pressure = atmospheric pressure

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Pleura has parietal & visceral layers creating a sac

* intrapleural pressure = 4 mmHg below atmosphere

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When the thoracic cage expands (muscles contraction), so does the intrapleural pressure)

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The lungs follow suit, decreasing the alveolar pressure

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Air flows in

Breathing diagram

pressure = A

Pressure in the intrapleural space = A - 4

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__**Breath in:**__

* ribcage expands
* pulls on parietal layer of pleura
* diaphragm drops
* pressure in intrapleural space decreases
* lungs expand
* drops pressure in lungs
* air flows in from periphery

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__**Breath out:**__

* everything happens in reverse

Pneumothorax

* puncture to pleural membrane causes air (pneumo) in pleural space

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Intrapleural pressure = atmospheric pressure

* doesn’t change with thoracic cage expansion
* lung is no longer under tension + collapses

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==Blood== in this space is called a ==hemothorax==

treatment:

* reseal the hole + get lung to expand

BUT
* this can be dangerous because if the pressure in the intrapleural space returns to normal, this can shift the positions of the organs in the thorax = very bad😔

What happens during pneumothorax

disrupt the pleural membrane = disrupt the pressure

* air flows in
* now the pressure in both the lung and intrapleural space is the same
* now we now longer have oppositional force → **lung collapse**

Gas exchange

Oxygen in

CO2 out

Pulmonary vs Bronchial circulation

__**Pulmonary**__

* lungs oxygenate blood

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__**Bronchial**__

* systemic
* supplies lung tissue

Vasculature of the tracheobronchial tree

* arteries in centre of bronchopulmonary segment

What is gas exchange?

Goal:

* oxygenate blood + remove carbon dioxide

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Diffusion of oxygen + carbon dioxide between alveoli + pulmonary capillaries

Gas has to pass through 3 zones:

* pulmonary capillary
* fused basement membrane
* alveoli

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==Oxygen== moves:

alveoli → fused membrane → pulmonary capillary

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^^CO2^^ moves:

Pulmonary capillary → fused membrane → alveoli

Ventilation vs perfusion

* gas exchange depends upon the relationship between ventilation (air in alveoli) + perfusion (blood flow through capillaries)

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Va/Q = alveolar ventilation/ cardiac output

Shunt

__**= adequate perfusion, but no ventilation**__

causes:

* pulmonary edema
* asthma
* COPD
* pneumothorax
* gas trapping

Dead space

__**= Adequate ventilation, but no perfusion**__

\*opposite to shunt

causes:

* hemorrhage
* dehydration
* pulmonary embolism
* (basically anything that prevents blood from getting to the capillaries)

Pulmonary edema

Usually secondary to heart failure

* blood not effectively pumped from L ventricle leads to back up in pulmonary veins + lungs

Swelling and eventual leaking of pulmonary capillaries = **fluid accumulation + increased pressure in interstitium**

* increases pressure around alveoli + respiratory bronchioles, which may lead to collapse + shunting because air becomes trapped
* Diffusion (and thus oxygenation) becomes difficult

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Fluid may leak into the pleural cavity (pulmonary effusion) or mediastinum

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Alveolar flooding is possible (very problematic)