PORTAL CIRCULATIONS AND THEIR RELATION TO COUNTERCURRENT SYSTEMS

TLDR

Portal circulations are generally two serial capillary beds, either connected by larger vessels (convergent) or not (continuous), with the former being more common. Convergent portal circulations, exemplified by hepatic, placental, hypophysial, renal, ovarian, testicular, and lymphatic systems, often include countercurrent concentrating mechanisms within their primary or secondary beds. Our review shows that portal circulations are more widespread in animals than previously thought.

Abstract

We have reviewed the distribution of portal circulations throughout the animal body; they are commoner than is generally supposed. Most portal circulations consist of two serial capillary beds connected by one or more larger vessels. We have called these 'convergent' portal circulations: examples are hepatic portal, placental, hypophysial, renal, ovarian and testicular circulations, as are parts of the lymphatic circulation. A second type of portal circulation, which is less common, consists of two serial capillary beds that are not connected by larger vessels. These we have called 'continuous' portal circulations: adrenal and pancreatic circulations are examples of this type. When a countercurrent concentrating mechanism exists in the body it is always part of the primary or secondary bed of a convergent portal circulation, though some convergent portal circulations are not associated with countercurrent mechanisms.