An ultrastructural study of pulmonary capillary vessels in blood volume-overloaded rat.

Blood volume overload to pulmonary capillaries was experimentally induced by arterio-venous fistula between the left common carotid artery and the left external jugular vein in 42 S-D rats. The animals were sacrificed 1,2,4,8 and 24 weeks after the operation. Just after anastomosis, blood flow index of the shunted rats was about 4 times larger than in the controls and the high value was kept throughout the experiment (p less than 0.05). The relative right ventricular weight was about 1.5 times larger than in the controls (p less than 0.05). The capillary endothelium of the alveolar septum showed nuclear enlargement, swelling of the cytoplasm with microvillous projection, and increases of pinocytotic vesicles and microfilaments. In some cases, focal edema was seen accompanied with an increase of immature stromal cells. The basement membrane showed vague in contour in edematous area, especially in the epithelial side of the alveolar septum. These changes appeared most predominantly in 2 weeks and subsided almost completely in 24 weeks. Number of endothelial nucleus of the shunted rats increased significantly in 2 and 4 weeks and was normalized in 24 weeks. The surface area and volume of the capillaries also increased significantly in 2 and 4 weeks of the shunted animals. These findings showed that the capillary endothelium reacted and then was adapted to the blood volume overload.