Anatomical Review:
Conducting portion comprises a set of passages where air is "conducted" from the external environment to the internal.
Respiratory portion comprises specialized structures for gas exchange with the pulmonary blood supply.
Alveoli comprise the following specialized cells:
Type I Cells are thin epithelial cells that form the alveolar walls; these are the most abundant cells in the alveolus.
Type II Cells secrete surfactant and play important roles in alveolar regeneration and immune functioning.
Alveolar macrophages are specialized macrophages that clear debris.
Surfactant is a fluid rich in lipids and proteins; by covering the inner wall of the alveoli, it reduces surface tension and prevents alveolar collapse during expiration.
Water produces surface tension via its cohesive properties, which arise from its tendency to maximize the number of hydrogen bonds. So, without surfactant, alveoli exhibit more surface tension and collapse.
Interstitial fluid bathes the alveolus and the capillaries that surround it (we'll show the capillaries soon).
Indicate that the interstitium and septal walls comprise fibroelastic components, which assist in pulmonary elastic recoil and the efficient expiration of air.
Clinical Correlation: Newborn respiratory distress syndrome
Occurs when a birth is premature, before the lungs can produce surfactant. This causes an increase in alveolar surface tension, and the alveoli collapse upon expiration.
We can treat these infants with surfactant replacement until their Type II cells produce surfactant, themselves.
Gas Exchange: Diffusion
Diffusion is the movement of solutes down their concentration gradients; thus, differing oxygen and carbon dioxide gradients in the lungs and tissues drives gas exchange.
Pulmonary Circulation:
Pulmonary arteries deliver deoxygenated blood to the pulmonary capillaries, where it picks up oxygen. Oxygenated blood returns to the heart via the pulmonary veins.
The heart is the bridge between the pulmonary and systemic circulations.
The aorta and other systemic arteries deliver oxygenated blood to the peripheral capillaries.
In the peripheral capillaries, oxygen leaves the blood; the deoxygenated blood returns to the heart via the venae cavae.
Oxygen and carbon dioxide move down their concentration gradients to drive gas exchange in the lungs.
When we inhale oxygen-rich air, we increase the oxygen concentration in our alveoli;
Conversely, the blood entering our pulmonary capillaries has a low oxygen concentration because it's just "dropped off" its oxygen in the peripheral tissues.
Alveolar oxygen moves down this concentration gradient: it travels across the alveolar wall, interstitial space, and capillary wall to enter the blood, where hemoglobin binds it.
Now, show that the blood entering our pulmonary capillaries is high in carbon dioxide, which it picked up in our peripheral tissues.
Conversely, the inhaled air in our alveoli has a lower carbon dioxide concentration.
Thus, carbon dioxide moves down this concentration gradient and enters the alveoli; it takes the reverse path of oxygen.
Peripheral Tissue Gas Exchange
Blood arriving to the tissues has high oxygen concentration from passing through the lungs;
Conversely, the peripheral tissues have a low oxygen concentration because oxygen was consumed for metabolic functions.
As a result of this gradient, oxygen will diffuse from the capillary to the tissues.
Blood arriving to the tissues has a low carbon dioxide concentration because CO2 was removed in the lungs;
Conversely, show that the peripheral tissues have a high carbon dioxide concentration because CO2 accumulates as a metabolic biproduct.
Thus, carbon dioxide moves down its concentration gradient from the tissues to the capillary blood.
When this blood returns to the lungs, carbon dioxide diffuses out of the blood and is exhaled from the alveoli.
Oxygen diffuses into alveolar capillaries and out of the peripheral capillaries.
Carbon dioxide diffuses out of alveolar capillaries and into the peripheral capillaries.
As a final point, consider that the concentrations of carbon dioxide and oxygen actually vary over the length of the capillary; we show a simplification of their concentrations, here.
