The 63-year-old patient came

to the doctor due to a dry cough lasting for many months, accompanied by

shortness of breath on exertion and fatigue. In addition, he had recently lost

a lot of weight. On physical examination, crepitus (a specific listening

phenomenon - like unzipping a Velcro strap) is present in the lungs. Decreased

oxygen saturation is also measured. In addition, the patient has typical mallet-shaped

fingers with clubbing nails in the shape of an hour glass. To confirm the

diagnosis suspected by his physician, a HRCT of the lungs (high-resolution CT

scan) is required, followed by a lung biopsy and histological examination of

the sample taken. The examination confirm the diagnosis of idiopathic pulmonary

fibrosis. It is probably a uniform pathological response of lung tissue to

infectious and non-infectious agents, but the cause of this disease is unknown.

In this disease, fibrosis and thickening of the blood-air barrier occurs. What

physiologically forms the blood-air barrier?