Scope and limitations of collateral circulation. Presidential address.

The human body, possessing no true end arteries, is capable of furnishing some collateral vessels for any arterial, venous, or lymphatic obstruction. In places, only capillary anastomoses are immediately available and ischemia may be profound. We have learned to support organs like the heart, brain, or liver while small communications enlarge to increase the flow of blood or lymph. Even in the retina, some success has been obtained by retrograde perfusion through the supra-orbital artery. Supplemental flow, ie, blood varying in content or pressure--such as bronchial blood substituting for pulmonary blood--carries some detriment in the form of diminished function or the rupture of thin-walled vessels.