portal system
Summary
A portal system sends blood through two capillary beds in series, joined by a portal vein, before it returns to the heart. This is a biology topic, not organic chemistry. Testable point: in the hepatic portal system, most absorbed nutrients reach the liver before the general circulation.
Detail
Meaning: In most circulation, blood crosses one capillary bed between an artery and a vein. In a portal system, blood leaves one capillary bed, travels through a portal vein, and enters a second capillary bed before heading back to the heart. The second organ therefore receives substances from the first at high concentration, before they are diluted in the general circulation.
The hepatic portal system carries blood from capillaries of the stomach, intestines, spleen and pancreas through the hepatic portal vein to the liver sinusoids. Hepatic veins then drain to the inferior vena cava. The liver can store glucose as glycogen, process amino acids, and detoxify absorbed substances. The hypothalamic-hypophyseal portal system carries releasing and inhibiting hormones from the hypothalamus to the anterior pituitary.
On the MCAT: Expect to trace the path of glucose or an ingested toxin after a meal, or of a hypothalamic hormone to the anterior pituitary. A question may ask what happens if portal flow is blocked or bypassed.
Common confusion: Not every absorbed nutrient takes the portal route. Most dietary fats are packaged into chylomicrons, enter lacteals, and travel in lymph to the bloodstream, bypassing the liver at first. Also, the posterior pituitary is not portal: hypothalamic neurons extend into it and release ADH and oxytocin directly.
Sources
- Campbell Biology
- Human Physiology: An Integrated Approach
- Biology (Raven)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.