This study was aimed at investigating the current knowledge on the similarities between pulmonary and portal circulation to try to improve the diagnostic capabilities of functional radiology in these two districts. These two organs are similar both from an anatomical and a functional viewpoints, sharing the same microarchitecture and a double vascular system with similar hemodynamic characteristics. In the past, the parameters to evaluate pulmonary flow and pressure consisted in the analysis of the distribution, diameter and number of vessels with conventional radiology, but today, HRCT permits the regional assessment of perfusion and air volume, using density values. Dynamic density changes (expiratory and prone scanning), together with the morphological features of peripheral bronchial and vascular structures, play a fundamental diagnostic role in differentiating small airway conditions from normal and hemodynamic changes. When HRCT shows a "mosaic" pattern--i.e., regions with different density values--reduced perfusion can be distinguished, because in this case hypodense areas are vascularized by fewer, and smaller, vessels. Expiratory scanning can exclude abnormal ventilation. Hyperperfusion is characterized by higher density areas vascularized by more, and bigger, vessels. Doppler US and MRA show, once again, their limitations in calculating the absolute values of flow velocity and flow volume in the splanchnic district; in clinical studies, only the semiquantitative data yielded by Doppler US are considered reliable. Therefore, also in this district, relative data must be preferred to absolute ones; for instance, it is interesting to analyze the hemodynamic changes occurring in patients under different physiologic or experimental conditions. We believe that, in the near future, technological progress and growing operators' skills will make functional radiology a major tool helping the clinician approach and treat these patients correctly.