L45 | Gas Exchange

Where does gas exchange occur within lung?

• diffusion between air in alveoli and blood in pulmonary capillaries

  • O₂ diffuses into pulmonary capillaries

  • CO₂ diffuses into alveoli

Where does gas exchange occur within systemic tissues?

• diffusion between gases in blood in systemic capillaries and surrounding cells

  • O₂ diffuses → tissues

  • CO₂ diffuses → systemic capillaries.

What is minute ventilation (V_E)?

volume of air moved into respiratory system each minute

Why doesn’t all the air that is moved into the respiratory system reach the respiratory zone?

because we have TIDAL BREATHING  (air comes in & then goes out from same pathway) & a conducting zone.

What is alveolar ventilation (V_A)?

volume of air moved into respiratory zone (site of gas exchange) each minute

Alveolar Ventilation (V_A) is always __________ Minute Ventilation (V_E) because of __________________

Alveolar Ventilation (V_A) is always less than Minute Ventilation (V_E) because of anatomical dead space (V_D).

What is barometric pressure (atmospheric pressure) comprised of?

N₂ (79%), O₂ (21%), & CO₂ (0.04%)

Barometric pressure ___________ as elevation increases

Barometric pressure decreases as elevation increases

Partial pressure individual gas = ___________________ x ____________

• Partial pressure is indicated by “P” before gas: i.e. PO₂

Partial pressure individual gas = total pressure (mmHg) x (% of gas in mixture)

• Partial pressure is indicated by “P” before gas: i.e. PO₂

How does gas exchange happen during breathing?

• Every breath fills the alveoli with fresh air → increases O₂ partial pressure

• Because O₂ pressure is higher in alveoli than in blood, O₂ diffuses into the blood

• Because CO₂ pressure is higher in blood than in alveoli, CO₂ diffuses into the alveoli

How does alveolar ventilation change Alveolar O₂ and CO₂?

• alveolar ventilation INCREASES alveolar PO₂

  • Arterial PO₂ will increase as alveolar PO₂ increases

• alveolar ventilation DECREASES alveolar PCO₂              

• Increase alveolar ventilation → ↑ PO₂ and ↓ PCO₂

• Decrease alveolar ventilation → ↓ PO₂ and ↑ PCO₂

Why will P_AO₂ will never exceed 150 mmHg?

PO₂ OF HUMIDIFIED, INSPIRED AIR = 150 mmHg

______________ PO₂ & PCO₂ Determine __________ PO₂ & PCO₂

Alveolar PO₂ & PCO₂ Determine Arterial PO₂ & PCO₂

What is hypoventilation?

• LESS CO₂ expired (exhaled) than produced in tissues

  • decrease in alveolar ventilation (V_A) sufficient to increase­ arterial PCO₂ ABOVE its normal range.

  • Decrease gradient between mixed venous blood & alveoli

  • Less diffusion of CO₂ out of pulmonary capillaries

  • P_aCO₂ increases → hypercapnia/hypercarbia (­increase arterial PCO₂)

What is hyperventilation?

• MORE CO₂ expired (exhaled) than produced in tissues

  • increase in alveolar ventilation (V_A) sufficient to decrease arterial PCO₂ BELOW its normal range.

  • Increase gradient between mixed venous blood & alveoli

  • More diffusion of CO₂ out of pulmonary capillaries

  • P_aCO₂ decreases → hypocapnia/hypocarbia (decrease arterial PCO₂)

What are the changes in arterial blood gases during hypoventilation?

↑ PaCO₂ and ↓ PaO₂

What are the changes in arterial blood gases during hyperventilation?

↓ PaCO₂ and ↑ PaO₂