Relations between capillary dimensions and transmural pressure in the turtle lung.

The interalveolar capillaries of the turtle lung were treated as a vascular sheet. Dimensional changes in the vascular sheet with transmural pressure were examined in the most vascular portion (anterior and medial segments) of the lung. The sheet thickness increased with transmural pressure by an average value of 0.082 mum . cm H2O-1, but the vascular density (proportion of alveolar surface area occupied by capillaries) did not change. The compliant behaviour of the interalveolar capillaries is compared with respiratory capillaries in other vertebrates and is discussed in terms of its potential effect on pulmonary blood flow.