Microcirculation in the lungs: special features of construction and dynamics

In this study we investigated microcirculation in the lungs in their in situ physiological location inside the thorax. The study was performed with the use of a system of contact optics. A 'window', 4 × 4 mm in size, was made in thorax tissues and pleura of an anaesthetized rat. The lung collapsed and then was filled with oxygen or hypoxic gas mixture under the pressure of 10-15 cm H(2)O through a tracheostomic canula. This almost excluded the respiratory movements of the lung. Then, the lung was brought in contact with a lens (1.7 mm aperture). We showed that there is a whole system of wide microvessels (20-30 μm in diameter) which run between the alveoli; the finding contradicting the hitherto notion that each alveolus is supplied with blood via the thinnest (5-10 μm in diameter) lung arterioles. The microvessels we visualized surround each alveolus almost from all sides. In this way, each alveolus receives a maximum amount of blood. Such a structure of lung circulation accounts for a substantial blood flow through the lungs (up to 6 l per min in humans) and for a rapid saturation of the blood with oxygen (about 100 ml per second). The alveoli saturate the blood with oxygen and subsequently the microvessels form the lung veins entering the left auricle. The photographs and video films of the alveoli at a high magnification were presented, demonstrate the special features of the structure and circulation in the alveoli. The plausible mechanisms of rapid saturation of the blood with oxygen are discussed.