Collateral circulation is when an area of tissue or an organ has a number of different pathways for blood to reach it. This is often as a result of anastamoses

Circulatory anastomosis

A circulatory anastomosis is a connection between two blood vessels, such as between arteries , between veins or between an artery and a vein . Anastomoses between arteries and between veins result in a multitude of arteries and veins, respectively, serving the same volume of tissue...

 - branches formed between adjacent blood vessels.

An example of the usefulness of collateral circulation is a systemic thrombo-embolism in cats. This is when a thrombus

Thrombus

A thrombus , or blood clot, is the final product of the blood coagulation step in hemostasis. It is achieved via the aggregation of platelets that form a platelet plug, and the activation of the humoral coagulation system...

 lodges above the external iliac artery

External iliac artery

The external iliac arteries are two major arteries which bifurcate off the common iliac arteries anterior to the sacroiliac joint of the pelvis. They proceed anterior and inferior along the medial border of the psoas major muscles...

(Common Iliac Artery), blocking the external and internal iliac arteries and effectively shutting off all blood supply to the hind leg. Even though the main vessels to the leg are blocked, enough blood can get to the tissues in the leg via the collateral circulation in order to keep them alive.