Pulmonalkapillare

Pulmonalkapillare, or pulmonary capillaries, are the network of tiny blood vessels in the lungs that surround the alveoli and form the principal site of gas exchange in the pulmonary circulation. They receive blood from the pulmonary arteries and, after gas exchange, return oxygenated blood to the left atrium via the pulmonary veins.

Anatomy and histology: Pulmonary capillaries are extremely thin-walled structures composed mainly of a single layer of endothelial cells with a surrounding basement membrane. They lie intimately with the alveolar epithelium, particularly the type I pneumocytes, forming the alveolar-capillary membrane. This arrangement creates a very short diffusion distance between the gas in the alveolar air and the red blood cells.

Physiology and gas exchange: The primary function of the pulmonalkapillare is to facilitate diffusion of oxygen into the blood and carbon dioxide out of it. Oxygen diffuses from alveolar air through the alveolar-capillary membrane into plasma and erythrocytes, while CO2 diffuses in the opposite direction. The process is rapid due to the thin barrier and large surface area, and it is influenced by regional ventilation-perfusion matching and capillary perfusion.

Regulation and clinical relevance: Blood flow through pulmonary capillaries is modulated to optimize gas exchange. Hypoxic vasoconstriction redirects flow away from poorly ventilated regions. Conditions such as pulmonary edema or inflammatory injury can thicken the alveolar-capillary barrier and impair diffusion, while chronic changes in the capillary bed contribute to pulmonary hypertension or right-ventricular strain. Acute lung injury and ARDS involve capillary leak and disruption of the barrier, with impaired gas exchange as a hallmark.