Define a Portal System. Describe the anatomy and function of three portal systems in the body.

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College Answer

A portal system is an arrangement by which blood collected from one set of capillaries passes 
through a large vessel or vessels, to another set of capillaries before returning to the systemic 
circulation. The three portal systems are the - 
1) system of blood vessels that link the hypothalamus and the anterior 
pituitary in the brain, which allows endocrine communication between the two structures. 
2) within the liver, whereby venous blood from the GI tract drains into the superior and 
inferior mesenteric veins; these two vessels are then joined by the splenic vein to form the 
portal vein which enters the liver, drains into the hepatic sinusoids and then eventually 
into the hepatic veins which join the inferior vena cava, with the purpose of defending 
against by breaking down and metabolising most of what has been absorbed from the 
gastrointestinal tract (including an immunoprotective action). 
3) within the kidney, whereby blood from the afferent arterioles enters the glomerulus (first 
capillary network), followed by the efferent arterioles, then the peritubular network 
(second capillary network) and eventually the venous system, with the purpose of 
stronger re-absorptive capacity for water from within long Loops of Henle that go deep 
within the renal medulla

Recommended sources: Ganong Review of Medical Physiology Chps 18, 38, 29