PORTAL VEIN

Short, direct answers first

Where does the portal vein drain into?

👉 The portal vein drains into the liver sinusoids.

Does portal blood enter the liver sinusoids instead of the heart?

👉 Yes.
The portal vein does NOT drain directly into the heart.

Step-by-step: what actually happens to portal blood

1⃣ Origin of the portal vein

• Formed by the union of:

  • Superior mesenteric vein

  • Splenic vein

• Carries nutrient-rich, deoxygenated blood from:

  • Intestines

  • Spleen

  • Pancreas

2⃣ Entry into the liver (key point)

• The hepatic portal vein enters the liver

• Inside the liver it divides into:

  • Right and left portal veins

  • Smaller branches → portal venules

👉 These portal venules open directly into the hepatic sinusoids

📌 This is the defining feature of a portal system:

3⃣ What are hepatic sinusoids?

• Wide, fenestrated capillary channels

• Blood from:

  • Portal vein (75%)

  • Hepatic artery (25%)

• Mix here and bathe hepatocytes

👉 This is where:

• Nutrients are absorbed

• Drugs/toxins are detoxified

• Metabolism occurs

4⃣ Exit from the liver

After passing through sinusoids:

• Sinusoids → central veins

• Central veins → hepatic veins

• Hepatic veins → inferior vena cava

• IVC → right atrium of the heart

So the correct flow is:

Intestines → portal vein → Hepatic sinusoids → Central vein → Sublobular vein → Hepatic vein → Inferior vena cava

Why the highlighted sentence is correct

Because:

• All absorbed nutrients (except most fats) go:

  • Intestines → portal vein → liver sinusoids

• The liver sees and processes them first

• Only after processing does blood reach the heart

One-line exam answer