Viva G4(ii)i

This viva tests Section G4(ii) of the 2017 CICM Primary Syllabus, which expects the exam candidate to "describe the distribution of blood volume and flow in the various regional circulations and explain the factors that influence them, including autoregulation. These include, but not limited to, the cerebral and spinal cord, hepatic and splanchnic, coronary, renal and uteroplacental circulations."

Specifically, this viva is all about portal systems in the regional circulation.

What is a "portal system"?

"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."

Essential structural elements of a portal system are:

  • Feeding artery
  • Primary capillary bed
  • Portal vessel
  • Secondary capillary bed
  • Draining vein
What portal systems are present in the human circulation?
  • Hepatic
  • Pitutary
  • Renal
  • Placental
Describe the hepatic portal system

Portal circulation of the liver

  • Feeding artery: mesenteric vessels: SMA, IMA, coeliac trunk
  • Primary capillary bed: intestinal (villous) capillaries
  • Portal vessel: the portal vein, confluence of mesenteric and splenic veins
  • Secondary capillary bed: hepatic sinusoids
    • Anastomosis with hepatic artery capillaries occurs here
  • Draining vein: hepatic veins
Describe the pituitary portal system

portal circulation of the pituitary

  • Feeding artery: superior hypophyseal artery
    • Occasionally, a medial hypophyseal artery is mentioned, but either not everybody has one, or not every anatomist agrees that it exists.
  • Primary capillary bed: hypothalamic capillaries and posterior pituitary capillaries
  • Portal vessel: the long portal vessels and short portal vessels
  • Secondary capillary bed: capillaries of the anterior pituitary
    • Unlike the liver, the anterior pituitary gland does not have any direct arterial supply, and relies on the portal network for its oxygenation, which makes it susceptible to ischaemia and infarction.
  • Draining vein: hypophyseal veins, which variably drain into the cavernous sinuses
What is the purpose of the pituitary portal system?
  • The function of the pituitary portal circulation is to act as a conduit for the quick transport of hypothalamic hormones into the anterior pituitary gland. 
  • This is necessary because:
    • Hypothalamic hormones are quite large molecules
    • They are rapidly degraded in the bloodstream
    • They need to be present in relatively high concentration (as without a decent concentration gradient these substances would never cross the membranes)
    • There are no systemic targets for these hormones, so it would make no sense for them to enter the systemic circulation
Describe the renal portal system

portal circulation of the mammalian kidney

  • Feeding artery: afferent arteriole
  • Primary capillary bed: glomerular capillaries
  • Portal vessel: efferent arterioles
  • Secondary capillary beds: 
    • Peritubular capillaries
    • Vasa recta
  • Draining vein: Renal vein

The functional purpose of this portal circulation is to reclaim water and solutes from the glomerular ultrafiltrate fluid.

Describe the uteroplacental portal system

portal circulation of the placenta

  • 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

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.

Popa, Gregor, and Una Fielding. "A portal circulation from the pituitary to the hypothalamic region." Journal of anatomy 65.Pt 1 (1930): 88.