Physiology and pathophysiology of the colonic circulation.

There is growing evidence that colonic blood flow is controlled by both intrinsic and extrinsic factors. The existence of intrinsic vascular control mechanisms is evidenced by pressure-flow (and oxygen uptake) autoregulation, reactive hyperemia, vascular responses to acute venous hypertension, and a functional hyperemia. Although myogenic factors have long been considered to be solely responsible for the intrinsic ability of the colon to regulate its blood flow, recent developments indicate that metabolic mechanisms may be of equal importance in this regard. Both parasympathetic and sympathetic nerves play an important role in regulating colonic blood flow. The influence of circulating vasoactive agents and ischemia on colonic oxygenation are largely explained in terms of the relationship between oxygen uptake and blood flow. Colonic vascular dysfunction appears to be a major factor in the pathogenesis of inflammatory bowel diseases, chronic portal hypertension, and neonatal necrotizing enterocolitis. Future progress in this area will require the development of techniques for the measurement of colonic blood flow in man.