portal vein

What makes a vein a portal vein?

Ordinary veins run one way: capillaries collect into small veins, small veins into large ones, and blood returns to the heart. A portal system interrupts that path. Blood leaving one capillary bed is routed into a second capillary bed somewhere else before it is allowed to head back. The arrangement lets an organ receive a concentrated delivery. The liver gets absorbed nutrients and swallowed drugs at full strength this way, before dilution in the general circulation.

Is there more than one in the body?

The pituitary example works the same way for a different purpose. Releasing hormones made in the brain travel a short private route to the gland below and arrive undiluted, so tiny quantities are enough.

Why does pressure in it matter?

Portal veins carry no valves and normally run at low pressure. When the liver becomes scarred, blood struggles to get through it, and pressure backs up along the whole system. This is portal hypertension. Blood diverts into small connecting channels that were never built for the volume, and they stretch. Veins at the base of the gullet and around the navel become visibly enlarged, the spleen swells because its own outflow is obstructed, and fluid weeps into the abdominal cavity. Measurements of the pressure difference across the liver are used to grade how advanced the problem is, taken through a catheter threaded into a hepatic vein. The enlarged spleen traps and destroys platelets as they pass, so a falling platelet count is often the first laboratory clue that pressure has risen. Both the hepatic blood vessel and the pressure inside it can be assessed with ultrasound that tracks the direction of flow, and flow that has reversed is a late sign.