rete mirabile
A rete mirabile (Latin for 'wonderful net'; plural retia mirabilia) is a complex of arteries and veins lying very close to each other, found in some vertebrates. The rete mirabile utilizes countercurrent blood flow within the net (blood flowing in opposite directions. ) It exchanges heat, ions, or gases between vessel walls so that the two bloodstreams within the rete maintain a gradient with respect to temperature, or concentration of gases or solutes.