Anatomy of the Rete Mirabile  

The retea mirabile are capillary organs located on the dorsal surface of the eel swim bladder. They function in countercurrent exchange of blood gasses and efficiently deliver oxygen to the swim bladder giving the eel neutral bouyancy.
The capillaries are close-packed unbranched segments representing two separate circulations in which blood flows in counterparallel directions. The segments in large eels may be as long a 1 cm. Each rete consists of about 200,000 capillaries.

 

Each capillary bed consists of different capillary types, thick-walled, small diameter arterial segments and thin-walled large diameter venous segments arranged in close-packed hexagonal arrays. .
The arterial segments are of the continuous type and are have pericytes attached to their outer surfaces. The venous segments are thin walled and fenestrated.

 

Scanning electron microscopy reveals that pericyte coverage of arterial capillaries is extensive with processes enveloping their outer surfaces.