Lung Mechanics

Know what Lung compliance is

Compliance: how easily the lungs stretch/ expand

High compliance: lungs are easy to inflate

Low compliance: lungs are stiff and hard to inflate

What affects compliance?

• Connective tissue: affects how stretchy the lungs are

• Alveolar surface tension: water pulls the alveoli inward making them harder to expand

Know the effects of lung diseases on pulmonary compliance and airway resistance

• Pulmonary Fibrosis: lungs become stiff & hard to expand = LOW complaince

• Emphysema/COPD: Elastic tissue is destroyed, so lungs are too stretchy = HIGH COMPLIANCE

• Asthma: Airways constrict + thick mucus block airflow= HIGH AIRWAY RESITANCE

• Chronic bronchitis: inflamed airways + excess mucus narrow the airway

Know the different functions of lung surfactant

Lung Surfactant: substances made by type 2 alveolar cells that helps keep alveoli open

3 main jobs

1. Surface tension goes down: reduces the inward pull of water in the alveoli

2. Lung compliance goes up: makes the lungs easier to expand

3. Prevents alveolar collapse: especially keeps small alveoli form collapsing into larger ones

Surfactant = less tension → easier breathing → keeps alveoli open.

Understand atelectasis, Law of Laplace, and how surfactant can prevent atelectasis

Laplace’s Law: P= 2T/R smaller alveolus—> smaller radius—> higher pressure and higher pressure causes air to move from small alveolus to large alveolus

Small alveolus can eventually collapse——> atelectasis

Surfactant (prevents alveolar collapse): becomes more concentrated in smaller alveoli —→ lowers their surface tension—> lowers their pressure which they need the surfactant the most bc of tht

Know all Lung Volumes and Capacities and what Volumes and Capacities we can measure by spirometer

Volumes= individual amounts of air

Capacities= combinations of volume.

Lung Volumes

Volume	Amount	Simple meaning
TV	~500 mL	Air in/out during a normal breath
IRV	~3000 mL	Extra air you can forcefully breathe IN
ERV	~1200 mL	Extra air you can forcefully breathe OUT
RV	~1200 mL	Air left in lungs after forcing all the way out

RV= remains

Know the Dead Space Volume

Dead space: air that doesn’t participate in gas exchange

Anatomic dead space: air in the conducting airways (trachea, bronchi) where no gas change occur

Alveolar dead space: air in alveoli that don’t have enough blood flow, so gas exchange can’t happen

Physiological dead space: Anatomical + alveolar dead space

Dead space = wasted ventilate bc no exchange between o2 & co2

Understand the differences between Alveolar and Total ventilation and how to calculate them

Minute ventailation: the total air you breathe in/out per minute

Alveolar ventilation: the fresh air that actually reaches the alveoli for gas exchange

Respitory rate: if you take more breaths inc in RR but 150 ml wasted on every breakth

TV inc id you take deeper breaks to get past the dead space

Rate = more breaths
Depth = more useful air per breath

Know what partial pressure is

the pressure caused by one specific gas in a mixture of gases, so pressure from one gas

Know the Atmospheric, Alveoli and Systemic arterial Po2 & PCO2

Location	O₂	CO₂
Atmospheric air	160 mmHg	0.3 mmHg
Alveoli	104 mmHg	40 mmHg
Systemic arterial blood	100 mmHg	40 mmHg

The difference in partial pressures is what drives gas movement:

• O₂ moves from alveoli → blood

• CO₂ moves from blood → alveoli

Know the effects of alveoli ventilation on Alveoli and Systemic arterial PO2 and PCO2

If alveolar ventilation increases:

• More O₂ gets into the alveoli → PO₂ ↑

• More CO₂ is removed → PCO₂ ↓

• Blood follows → arterial PO₂ ↑ and arterial PCO₂ ↓

If alveolar ventilation decreases:

• Less O₂ gets into the alveoli → PO₂ ↓

• CO₂ builds up → PCO₂ ↑

• Blood follows → arterial PO₂ ↓ and arterial PCO₂ ↑

More ventilation = O₂ ↑, CO₂ ↓

Less ventilation = O₂ ↓, CO₂ ↑