Basic and Clinical Sciences Examination (BCSE) Practice Exam 2025 – Comprehensive All-in-One Guide for Exam Success

The blood-gas barrier is essential for efficient gas exchange in the lungs, and its primary function is to facilitate the transfer of oxygen and carbon dioxide between the air in the alveoli and the blood in the capillaries. The fused basal lamina plays a critical role in this barrier. It is formed by the merging of the basal laminae from the alveolar epithelium and the endothelial cells of the capillary walls. This close approximation reduces the distance that gases must diffuse, thus enhancing the rate of gas exchange.

The fused basal lamina not only serves as a structural element but also contributes to the selective permeability of the barrier, allowing gases to pass through while preventing the passage of larger molecules and pathogens. This characteristic is crucial for maintaining the integrity and function of the respiratory system.

In contrast, other choices such as pulmonary arteries, type II pneumocytes, and type A macrophages do not primarily serve as components of the blood-gas barrier. Pulmonary arteries are responsible for carrying deoxygenated blood from the heart to the lungs, type II pneumocytes are involved in surfactant production and do not form the barrier themselves, and type A macrophages are part of the immune system in the lungs, involved in the defense