Functional Divisions of the Respiratory System:
Conducting portion: the air passage conduit

• Trachea
• Bronchi
• Bronchioles

Respiratory portion: the interface for gas (oxygen and carbon dioxide) exchange.

• Respiratory bronchioles
  – Smooth muscle in respiratory bronchiole wall contracts to control airflow into alveolar sacs.
• Alveolar ducts: connect bronchioles to thin-walled alveolar sacs.
• Alveoli: grape-like sacs with a large surface area for gas exchange.
  – Dense capillary network on alveolar sac maximizes surface area and gas exchange efficiency.

Alveolar sac

• Functional unit of the lung
• Specialized alveolar cells:
  – Type I cells: simple squamous epithelial cells that form alveolar walls → very thin, allowing easy oxygen and carbon dioxide diffusion; most abundant.
  – Type II cells: secrete surfactant.
  – Alveolar macrophages: clear debris and perform immune functions; circulate within alveolar lumen.
  – Interstitial fluid bathes the alveolus and capillaries that surround it.
  – Elastin fibers in interstitial fluid assist in pulmonary elastic recoil and efficient expiration of air.

Surface Tension in Lungs & Surfactant

• Water produces surface tension via its cohesive properties (hydrogen bonding).
• Surfactant = lipid- and protein-rich fluid secreted from Type II cells.
  – Covers Type I cells.
  – Reduces surface tension.
  – Prevents alveolar collapse upon expiration.
  – Without surfactant, surface tensions causes alveoli collapse.

Clinical Correlation: Newborn Respiratory Distress Syndrome

• Premature infants are unable to produce surfactant.
• Causes increased alveolar surface tension.
• Alveoli collapse upon expiration.
• Treated with surfactant replacement until their Type II cells produce surfactant, themselves.

The Basics of Gas Exchange

• Gas exchange occurs across the respiratory portion of the lungs via diffusion.
• Diffusion = movement of solutes (CO2 and O2) along their concentration gradient, from areas of high concentration to areas of low concentration.
• Circulation:
  – The heart delivers deoxygenated blood to the lung capillary via the pulmonary arteries.
  The pulmonary arteries are the only arteries that carry deoxygenated blood, and the lungs are the only tissue to receive deoxygenated blood.
  – Lung capillaries return oxygenated blood to the heart via the pulmonary veins.
  – Concentration gradients differ in the lungs and the peripheral tissues → determine direction of diffusion.

Gas Exchange in the Alveoli

Concentration gradients:

• Alveolar lumen: high O2, low CO2 concentration.
  – We breathe in oxygen-rich air!
• Alveolar capillary: low O2, high CO2 concentration.
  – Most of blood oxygen has been used in peripheral tissue.
  – Blood accumulates metabolic wastes (CO2) as it circulates.

Creates concentration gradient → allows diffusion of O2 and CO2

Diffusion of O2 and CO2 down their concentration gradients:

• O2 diffuses from the alveolar lumen → across the interstitial space → into the capillary, picked up by hemoglobin.
• CO2 diffuses from capillary → across interstitial space → into alveolar lumen.

Gas Exchange in the Peripheral Tissues

The systemic circuit:

• Heart delivers O2-rich blood to a peripheral capillary via the aorta.
• Peripheral tissues return deoxygenated blood to the heart via the venae cava, which concludes the systemic circuit.
  • CO2 and O2 diffuse down their gradients just like in the lungs.

Concentration gradients:

• Peripheral Tissue: low O2, high CO2.
  – CO2 accumulates as a metabolic byproduct.
  – Peripheral Capillary: high O2, low CO2.

Creates concentration gradient → allows diffusion of O2 and CO2

Diffusion of O2 and CO2 down their concentration gradients:

• O2 diffuses from capillary →peripheral tissues.
• CO2 diffuses from the peripheral tissues → capillary.
• Blood returns to the lungs → CO2 diffuses out of the blood → CO2 exhaled.

Summary:

• Oxygen diffuses into alveolar capillaries and out of the peripheral capillaries.
• Carbon dioxide diffuses out of alveolar capillaries and into the peripheral capillaries.