A quarter of cardiac output at rest, arranged largely in series through the portal vein
Expansion
- Receives about 25 per cent of cardiac output at rest, rising after a meal
- Arranged largely in series: gut and spleen drain into the portal vein and then through the liver, so the liver receives blood already partly deoxygenated
- Holds about 20 per cent of total blood volume, acting as a major reservoir
Postprandial hyperaemia increases flow by 30 to 100 per cent for several hours, mediated by gastrointestinal hormones, local metabolites and increased metabolic activity.
In shock, intense sympathetic vasoconstriction diverts blood away from the splanchnic bed and autotransfuses the reservoir centrally. This protects the heart and brain but produces gut mucosal ischaemia, which is implicated in bacterial translocation and multiple organ failure.
The villus countercurrent arrangement, with arteriole and venule running in opposite directions, allows oxygen to short-circuit at the base of the villus, leaving the villus tip relatively hypoxic and the first area to infarct in low flow states.