Question 9

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 

Discussion

Definition of  a portal circulation:

  • 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.
  • A portal circulation has five structural components:
    • Feeding artery
    • Primary capillary bed
    • Portal vessel
    • Secondary capillary bed
    • Draining vein

Portal Systems in Human Circulation

System Anatomy Function
Hepatic

Feeding artery: SMA, IMA, coeliac trunk

Primary capillary bed: intestinal capillaries

Portal vessel: the portal vein

Secondary capillary bed: hepatic sinusoids

Draining vein: hepatic veins

Portal blood undergoes metabolic and immune modifications in the hepatic sinusoid, which allow for the biotranformation of drugs or metabolic substrates and the clearance of pathogens. 
Pituitary

Feeding artery: superior  and inferior hypophyseal arteries

Primary capillary beds: 

  • hypothalamic capillaries
  • posterior pituitary capillaries

Portal vessel: the long portal vessels and short portal vessels

Secondary capillary bed: capillaries of the anterior pituitary

Draining vein: hypophyseal veins, which variably drain into the cavernous sinuses

To efficiently present hypothalamic regulatory hormones to the pituitary gland in high concentration (rather than releasing them into the systemic circulation)

Renal

Feeding artery: afferent arteriole

Primary capillary bed: glomerular capillaries

Portal vessel: efferent arterioles

Secondary capillary beds: 

  • Peritubular capillaries
  • Vasa recta

Draining vein: Renal vein

To reclaim solutes from the glomerular ultrafiltrate fluid.

To deliver solutes fr active excretion by the proximal tubule

To maintain concentration gradients in the renal medulla, (to reabsorb water).

Placental

Feeding artery: umbilical arteries

Primary capillary bed: foetal placental capillaries

Portal vessel: umbilical vein

Secondary capillary beds: foetal hepatic capillaries

Draining vein: foetal hepatic vein

To allow the exchange of nutrients and gases between the mother and the foetus

References

Henderson, J. R., and P. M. Daniel. "Portal circulations and their relation to countercurrent systems." Quarterly Journal of Experimental Physiology and Cognate Medical Sciences: Translation and Integration 63.4 (1978): 355-369.

Page, R. B. "The pituitary portal system." Morphology of Hypothalamus and its Connections. Springer, Berlin, Heidelberg, 1986. 1-47.

Daniel, Peter M. "The blood supply of the hypothalamus and pituitary gland." British medical bulletin 22.3 (1966): 202-208.