The blood circulation plays an important role in the support of intestinal functions, such as propulsion of chyme and assimilation of ingested nutrients. Intrinsic regulatory mechanisms allow the intestine to adjust the distribution of blood flow between the muscular and mucosal layers in accordance with local metabolic needs. An extensive network of collateral channels within and external to the gut wall helps to ensure adequate intestinal blood flow.
Intestinal blood flow accounts for 10 to 15 percent of the resting cardiac output (500 to 750 mL/min) in the adult human. There appears to be an oral-to-anal gradient in blood flow (mL/g tissue) along the small intestine. In the resting state, approximately 65 percent of the total intestinal blood flow is directed to the mucosa, 25 percent to the muscularis, and the remainder to the submucosa. This distribution of flow within the bowel wall is usually attributed to the greater metabolic demand of the mucosa. Stimulation of epithelial transport processes favors improved mucosal perfusion while enhanced motor activity redistributes blood flow toward the muscle layers.
Extrinsic control of intestinal blood flow is exerted by neural and humoral factors. Activation of parasympathetic nerves usually results in vasodilation (increased blood flow) mediated by acetylcholine. Sympathetic nerve stimulation elicits vasoconstriction (decreased blood flow) that is mediated by norepinephrine. This a-adrenergic vasoconstriction is short-lived because intestinal arterioles escape from the constrictor influence of norepinephrine, resulting in partial restoration of normal blood flow (autoregulatory escape). Local release of adenosine appears to mediate this autoreg-ulatory escape. Hormones such as VIP, cholecystokinin, and secretin can induce vasodilation and increase blood flow, while angiotensin II and vasopressin are potent constrictors of intestinal arterioles. Indeed, a large proportion of basal vascular tone in the intestine can be attributed to circulating angiotensin II and vasopressin.
Intrinsic control of intestinal blood flow is mediated by both metabolic and nonmetabolic factors. Ingestion of a meal results in an increase in both intestinal oxygen consumption and blood flow. The postprandial hyperemia is directly coupled to the increase in intestinal oxygen con sumption. For any given increase in oxygen consumption, a greater initial oxygen extraction results in a larger postprandial hyperemic response. If the initial oxygen extraction is low, then the postprandial hyperemia is minimal and the increased oxygen demand is met primarily by an increase in oxygen extraction. The opposite holds if the initial oxygen extraction is high.
The postprandial hyperemia is confined to that segment of intestine directly exposed to chyme; segments distal to the bolus of chyme have normal resting blood flow. Of the hydrolytic products of food digestion, luminal glucose and oleic acid are capable of eliciting an intestinal hyperemia. Intraluminal glucose presumably elicits a hyperemia due to stimulation of absorptive processes, since 2-deoxyglucose (which is not absorbed) does not elicit a hyperemia. The glucose-induced hyperemia is mediated by metabolic factors, such as low tissue po2, and adenosine release. The same metabolic factors contribute to the oleic acid-induced functional hyperemia; however, a portion of the hyperemia can be attributed to oleic acid-induced irritation of the mucosa, which is linked to local release of vasoactive intestinal pep-tide. The importance of active transport of nutrients to the postprandial hyperemia is best exemplified by the differential responses of the jejunum and ileum to luminal bile or bile salts. In the jejunum, bile does not elicit a hyperemia, whereas in the ileum (where bile salts are actively transported), luminal bile produces a profound hyperemic response.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.