2011 Second Sitting Paper 1 Question 9 2011-2-9 Define a Portal System. Describe the anatomy and function of three portal systems in the body. Portal System - System where two separate capillary beds are supplied in consecutive series, before blood returns to the heart Hepatic portal system - Anatomy Arterial blood perfuses the GI tract (from stomach to proximal rectum) via the Coeliac trunk, superior mesenteric arteries and inferior mesenteric arteries. Capillary blood from GI tract drain to superior mesenteric and inferior mesenteric veins which converge to form the portal vein Portal vein perfuses the liver as part of the portal triad Hepatic venous blood drains into the inferior vena cava before returning to the heart - Function Protects systemic circulation from blood from portal venous blood Toxins absorbed from GI tract (ammonia) metabolized by liver before reaching systemic circulation Pathogens encounter leukocytes (e.g. macrophages) in space of Disse in liver Fluctuation in blood constituents due to GI absorptions buffered by liver (e.g. glucose) Renal portal system - Anatomy Renal arterial blood perfuses the renal cortex via afferent arterioles glomeruli Glomeruli drain blood into efferent arterioles Efferent arteriolar vessels drain into peritubular capillary network Proportion of efferent arteriolar vessels form vasa recta which supply blood to renal medullary interstitium via vasa recta drains to renal venules and Sakurai 2016 2011 Second Sitting Paper 1 Question 9 veins before returning to heart - Function As peritubular capillary network has lost Na and H 2O to glomerular filtrate increases reabsorptive capacity of renal tubules due to concentration gradients Hypothalamic-Pituitary Axis - Anatomy Arterioles from superior hypophysial artery (branch of circle of willis) supply hypothalamus via primary plexus drains to portal hypophysial vessels Portal veins supply anterior pituitary Blood returns to heart via internal jugular vein - Function Delivery of tropic hormones to anterior pituitary for signalling CRH TRH GnRH GRH PRH Examiners Comments: 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 Sakurai 2016 2011 Second Sitting Paper 1 Question 9 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. Sakurai 2016 .