Comparative mathematical analyses of freezing in lung and solid tissue.

In contrast to most organs of the body, the lung is composed 80% of air and 20% of tissue. Because freezing of the lung is fundamental to cryomicroscopy, cryopreservation, and cryosurgery, mathematical analyses of freezing in lung and solid tissue were performed to determine differences in freezing behavior resulting from differences in tissue composition. A comparison of the cooling rates of these tissues is presented. At the microscopic level ultrarapid solidification is more rapid in the subpleural region of the lung than in the same region of solid tissue. In this region, the air insulates the lung tissue and prevents transfer of heat from surrounding regions. Cooling rates on the order of 10(6) K/s can be achieved in the pleura and outermost alveolar wall because their aggregate thickness is less than 5 microns in the rat. At the macroscopic level and after steady-state freezing has occurred, one-dimensional analyses show that freezing front propagation in the lung and solid tissue differs by less than 10%. This occurs even though the lung is less conductive than solid tissue; however, by having less heat storage capacity, the thermal diffusive property of lung is similar to that of solid tissue.