Diffusion of Respiratory Gases

Pulmonary Gas exchange and transport steps (6+photo)

1. O₂ enters the blood at alveolar capillary interface

2. O₂ transported in blood is dissolved in plasma or bound to haemoglobin inside RBCs

3. O₂ diffuses into cells

4. CO₂ diffuses out of cells

5. CO₂ transported is dissolved / bound to hemoglobin / as HCO₃^-

CO₂ enters alveoli at alveolar-capillary interface

What does cellular respiration determines?

Metabolic CO₂ production

Gas exchange in the lung definition

Occurs across a pressure gradient to reach an equilibrium, allowing O₂ to enter the blood and CO₂ to be expelled

Gas exchange in the lungs results in (3)

• Increased O₂ concentration in the blood

• Diffusion of gases (O₂ and CO₂) across the alveolar and capillary membranes

• Decreased CO₂ concentration in the blood

Gas exchange is promoted by + reason (3+3)

• Large surface area

  • due to numerous small alveoli (reduced by the destruction of alveolar walls in emphysema)

• Thin single-cell layer

  • for efficient and quick diffusion, strengthened by collagen proteins for structural integrity

• Elastin fibers

  • aid in the alveoli's ability to recoil after expansion

Pressure Gradient for Gas Exchange depends on

the partial pressure of the gas in alveolar air and plasma (dissolved gas)

Structure of Alveoli - ANATOMY (2)

• consists of clusters resembling a honeycomb at the ends of respiratory bronchioles

• single outpouchings along the respiratory bronchioles

Alveoli main types of cells - HISTOLOGY (2)

• Type I Alveolar Cells

• Type II Alveolar Cells

Type I Alveolar Cells (2)

• Make up 95-97% of lung cells

• Responsible for gas exchange across a thin basement membrane

Type II Alveolar Cells (2)

• Secrete surfactant to reduce surface tension

• Reabsorb sodium (Na⁺) and water (H₂O) to prevent fluid build-up in the lungs

Detailed anatomy of respiratory membrane (photo)

Rate of Diffusion depends on with examples (4+4)

• Surface area (alveoli)

  • Fx.: Emphysema (reduced surface area)

• Barrier permeability (thin basement membrane)

  • Fx.: Fibrotic scars (decreased permeability)

• Distance of Diffusion

  • Fx.: Pulmonary Oedema (increased diffusion distance)

• Concentration gradient of gas

  • Fx.: COPD (decreased alveolar ventilation)

Factors/diseases influencing Diffusion of Gases (4+photo)

• Emphysema

• Fibrotic Lung Disease

• Pulmonary oedema

• Asthma

Maximum amount of gas dissolved in a fluid depends on (3)

• Solubility of the gas in the fluid

• Temperature of the fluid

• Partial pressure of the gas

Henry's Law

Since solubility and temperature in blood are constant, the concentration of a gas in plasma depends on the partial pressure of the gas

Dalton’s Law

Total Pressure of a Gas Mixture = ∑ of the pressures of each gas in the mixture

Partial Pressure definition

the pressure that a particular gas in a mixture exerts independently

Partial Pressure Equation

Partial pressure = total pressure x fraction of that gas in the mixture

Change in Air Constituents as It Moves Into the Lungs (5)

• External Air

• Atmospheric Air

• Air in Anatomical Dead Space

• Inspired Air - in respiratory zone

• Alveolar Air

External Air composition - Air Constituents

PN₂ + PO₂ + PCO₂

= Atmospheric Air

Atmospheric Air value - Air Constituents (2)

P_{dry atmosphere} = PN₂ + PO₂ + PCO₂ = 760 mmHg

PO₂ = 21% of 760 mmHg = 159 mmHg

Increase in Altitude of Atmospheric Air (3)

• Atmospheric pressure decreases

• PO₂ is reduced

  • can affect the oxygen available for gas exchange in the lungs

Air in Anatomical Dead Space - Air Constituents (4)

• No gas exchange occurs (nose, mouth, larynx, trachea, bronchi, and bronchioles)

• Composition: PN₂ + PO₂ + PCO₂ + P water vapor

• Less PO₂ and more PCO₂ due to gas exchange at the alveoli

• Conducting Zone

=inspired air

Inspired Air - Air Constituents (4)

P_{wet atmosphere} = PN₂ + PO₂ + PCO₂ + PH₂O

P_{water vapor} at 37°C = 47 mmHg

The air becomes saturated with water vapor at body temperature (37°C), adding 47 mmHg of pressure to the total

PO₂_{(sea level)} = 21% of (760 mmHg - 47 mmHg) = 150 mmHg

Alveolar Air equation - Air Constituents

PN₂ + PCO₂ (inc.) + PO₂ (dec.) + P water vapor + Temperature 37°C

Alveolar Air Composition (4)

• Increase in PCO₂ due to gas exchange at the alveoli

  • PO₂ decreases

• Temperature is constant at 37°C

• Saturated with water vapor (100% humidity)

• PO₂ is diminished to about 105 mmHg at sea level due to gas exchange in the alveoli

Alveolar Gas Exchange influenced by (12-photo)

Oxygen Electrode (4)

• produces an electric current in proportion to the concentration of dissolved oxygen in the plasma

• not measure oxygen bound to haemoglobin

• Useful for assessing lung function (gas exchange)

• PO₂ in systemic blood is typically 5 mmHg lower than in alveolar air (105 mmHg at sea level)

Oxygen Content in Blood (5)

• At normal PO₂ (100 mmHg), whole blood contains almost 20 mL of O₂ per 100 mL of blood.

• Plasma contains 0.3 mL of O₂.

• Red Blood Cells (as oxyhemoglobin) contain 19.7 mL of O₂.

• Breathing 100% oxygen increases the amount of oxygen dissolved in the plasma

  • not significantly change the total oxygen content in the blood

Oxygen Diffusion to Tissues (2)

• Increasing plasma PO₂ concentration at normal PO₂ → increases the rate of oxygen diffusion to tissues

• Oxygen bound to haemoglobin in RBCs must first dissolve in plasma before diffusing into cells

Pulse Oximeter (6)

• measures oxyhemoglobin saturation

• Non-invasive method

• Clipped to the ear pinna or finger

• Works using two LED lights (red and infrared).

• Oxyhemoglobin and deoxyhemoglobin absorb light differently

• Sensors and microprocessors determine the percentage of oxyhemoglobin in the blood

Classification of Hypoxia + causes (photo)

Normal Blood Values in Pulmonary Medicine (photo)

Ventilation-Perfusion Mismatch Types (2)

• Dead Space Ventilation

• Shunt

Due to high altitude

Dead Space Ventilation (5)

• Left

• Good ventilation

• No CO2 reaches

• No perfusion (blood flow) to the alveoli

V/Q > 1

Shunt (5)

• Right

• Bronchus is blocked

• No airflow reaching the alveoli

  • low PO₂

• No CO2 is exchanged

V/Q = 0

Ventilation/Perfusion Ratio (V/Q Ratio)

V (Volume of Air ventilated, L/min) / Q (Volume of Blood flow, L/min)

During standing (5)

• Blood flow to the base of the lungs ↑ due to gravity

• At the apex of the lungs, the intrapleural pressure is lower - fluid

  • keeping alveoli more open but less compliant

  • resulting in less efficient ventilation (smaller volumes of air exchanged)

• If this effect is not in the same proportion → leading to overventilation (underperfusion) at the apex

V/Q mismatch

occurs when there is a disruption in ventilation, gaseous exchange, or blood circulation

Conditions that cause V/Q mismatch and it’s effect on V/Q (3+3)

• Pneumonia

  • V/Q: Decreases

• Pulmonary embolism

  • V/Q increases

• Pulmonary oedema

  • V/Q: Decreases

Pneumonia causes

Decreased ventilation & gaseous exchange due to inflammation and mucus

Pulmonary embolism causes

Disrupted blood circulation due to a blockage

Pulmonary oedema causes

Decreased gaseous exchange due to fluid in the lungs

Pulmonary Circulation (5)

• Pulmonary arterioles constrict when PO₂ is low and dilate when PO₂ is high

• This response is opposite to that of systemic circulation

• The purpose is to match ventilation to perfusion:

  • Blood flow is reduced in areas with low ventilation (low PO₂)

  • Blood flow is increased in well-ventilated alveoli

Pulmonary Circulation in Adult (4)

• Low pressure (10 mmHg) compared to systemic circulation (100 mmHg)

• Low vascular resistance helps protect against pulmonary oedema by maintaining lower filtration pressure

• Left ventricular heart failure→ lead to pulmonary hypertension

  • increasing filtration pressure and potentially causing pulmonary oedema

Pulmonary Circulation in the Fetus (6)

• Lungs are partially collapsed - not involved in ventilation

• High vascular resistance in the fetal lungs

  • not yet functioning for gas exchange

• Blood shunting occurs:

  • From the right atrium to the left atrium through the Foramen Ovale

  • From the pulmonary artery to the aorta through the Ductus Arteriosus

Blood shunts function

These shunts help bypass the non-functional lungs and direct blood to the rest of the body, allowing fetal circulation to operate without relying on the lungs.

Pulmonary Circulation After Birth (6)

• Vascular resistance falls due to:

  • Physical stretching of the lungs during inspiration

  • Dilatation of pulmonary arterioles in response to increased alveolar PO₂

• Blood flow through pulmonary vessels increases sharply as a result

• Foramen ovale and ductus arteriosus close

  • redirecting blood flow through the lungs for oxygenation

Hyperbaric Oxygen Therapy (Hyperbaric Unit) (2)

• 100% oxygen is administered at 2-3 atmospheres pressure

• Discontinued in premature infants due to the risk of fibrotic deterioration of the retina, which can lead to blindness

Hyperbaric Oxygen Therapy used to treat (3)

• Decompression sickness

• Severe traumatic injury (e.g., crush injury)

• Very poor peripheral circulation

Conditions caused by High Partial Pressures of Gases (3)

• Oxygen Toxicity

• Decompression Sickness (the Bends)

• Nitrogen Narcosis

Oxygen Toxicity (6)

• Breathing 100% oxygen at 2-3 atmospheres pressure

  • can be safely tolerated for a few hours

• Higher partial pressures of oxygen can cause:

  • Oxidation of enzymes

  • Damage to the nervous system → coma and death

• Divers use gas mixtures with inert gases

  • (e.g., nitrogen, helium) to avoid oxygen toxicity

Decompression Sickness (the Bends) (7)

• Nitrogen dissolves in plasma as pressure ↑

• Rapid ascent (after diving or flying) causes dissolved nitrogen to form gas bubbles

  • can block small blood vessels

• This blockage leads to:

  • Muscle and joint pain

  • Other damage from obstructed blood flow

• Also occur if an airplane descends too rapidly from high altitudes (even with a pressurized cabin)

Nitrogen Narcosis (5)

• Nitrogen is physiologically inert - becomes harmful under high pressures

• Large amounts of dissolved nitrogen can cause symptoms similar to alcohol intoxication:

  • dizziness, extreme drowsiness, and a condition known as "rapture of the deep"

• Symptoms develop after prolonged exposure (more than an hour)

  • nitrogen takes time to dissolve into the body

Breathing 100% Oxygen (3)

• Reduces body's response to PCO₂

• Chemoreceptor sensitivity is blunted

• Ventilation rate decreases