The sinusoid is the smallest caliber blood vessel in the liver. Functionally, it connects the portal veinules (inflow) to the hepatic vein (outflow). Kupffer cells, the resident macrophages of the liver, dwell within the sinusoid and send signals to nearby cells when pathogens or certain toxins (ie. alcohol) are detected. Stellate cells lie just outside the sinusoid, in Disse’s space, and produce collagen (ie. fibrosis) when activated. A great deal of hepatopathology, including portal pressure determination, comes at the level of the sinusoid.
As a helpful reminder, when thinking about causes of portal hypertension, you should think of etiologies in relation to the sinusoid, ie. presinusoidal (portal vein thrombus, primary biliary cirrhosis), sinusoidal (cirrhosis) or postsinusoidal (Budd-Chiari syndrome).