Respiratory System Structure and Gas Exchange (3B) - MCAT Biological and Biochemical Foundations of Living Systems

Conducting: air transport; Respiratory: gas exchange (alveoli). Conducting zone conditions and transports air, while respiratory zone enables oxygen and carbon dioxide diffusion.

Carbonic anhydrase. It accelerates CO2 hydration to carbonic acid, enabling bicarbonate formation for transport.

Oxygenation of hemoglobin decreases $CO_2$ carrying capacity, promoting $CO_2$ unloading. It describes how oxygen binding reduces hemoglobin's CO2 affinity, facilitating release in lungs.

Increased $CO_2$ or $H^+$ decreases hemoglobin $O_2$ affinity (right shift). It shifts the oxygen-hemoglobin dissociation curve rightward, aiding O2 release in acidic tissues.

Internal intercostals and abdominal muscles. They contract to decrease thoracic volume, increasing pressure to expel air forcefully.

As volume increases, pressure decreases; as volume decreases, pressure increases. Boyle's law dictates inverse relationship between volume and pressure in a closed system at constant temperature.

Negative relative to atmospheric pressure. This subatmospheric pressure opposes lung recoil, keeping alveoli inflated.

It becomes more negative. Thoracic expansion during inspiration further lowers intrapleural pressure to inflate lungs.

$P_{tp}=P_{alv}-P_{ip}$. It represents the pressure gradient that maintains lung expansion against elastic recoil.

Mucociliary escalator clears particles and pathogens from airways. Cilia beat to move mucus-trapped debris upward for expulsion, protecting lower airways.

$C=frac{Delta V}{Delta P}$. It quantifies lung distensibility as volume change per unit pressure change.

Type II pneumocytes (type II alveolar cells). These cells secrete surfactant to lower alveolar surface tension and prevent collapse.

Nasal cavity pharynx larynx trachea bronchi bronchioles alveoli. This sequence follows the path of inspired air through the respiratory tract to the site of diffusion.

Diaphragm. Its contraction flattens and enlarges the thoracic cavity, reducing pressure for air inflow.

Type I pneumocytes (type I alveolar cells). Their thin, squamous structure minimizes diffusion distance for efficient gas exchange.

Alveolar macrophages. They phagocytose inhaled particles and microbes to maintain alveolar sterility.

Decreases surface tension and reduces alveolar collapse. Surfactant lowers surface tension proportionally more in smaller alveoli, stabilizing them per Laplace's law.

C-shaped hyaline cartilage rings. These rings provide rigid support to maintain airway patency while allowing esophageal expansion.

$P=frac{2T}{r}$ (Laplace law). It shows pressure required to prevent collapse is inversely proportional to radius and directly to tension.

$O_2$ diffuses from alveoli into pulmonary capillary blood. Gases diffuse down partial pressure gradients, from higher alveolar to lower blood levels.

Pseudostratified ciliated columnar epithelium with goblet cells. This specialized epithelium traps debris in mucus and uses cilia to propel it out of the airways.

$CO_2$ diffuses from blood into alveoli. Diffusion follows the partial pressure gradient from higher in blood to lower in alveoli for exhalation.

Increased area increases diffusion rate. Fick's law states diffusion rate is directly proportional to surface area and inversely to membrane thickness.

Bicarbonate ($HCO_3^-$) in plasma. Most CO2 is converted to bicarbonate for soluble transport, minimizing blood pH changes.