**Core Concept**
The portal venous system is a unique circulatory pathway that plays a crucial role in detoxifying the blood and facilitating nutrient uptake by the liver. Unlike other venous systems, it lacks valves, which are essential for preventing blood reflux and ensuring one-way flow.

**Why the Correct Answer is Right**
The absence of valves in the portal venous system is a distinctive feature that allows for the free flow of blood from the gastrointestinal tract, spleen, and pancreas into the liver. This characteristic is essential for the liver's ability to detoxify the blood and process nutrients. The lack of valves also facilitates the formation of portal hypertension, a condition that can lead to complications such as variceal bleeding and ascites. The hepatic portal vein is formed by the confluence of the superior mesenteric vein and the splenic vein, but valves are not present within the portal vein itself.

**Why Each Wrong Option is Incorrect**
**Option A:** The junction of the superior mesenteric artery and the splenic artery is not related to the valve structure of the portal venous system. These arteries are involved in arterial supply, not venous drainage.

**Option B:** The portal vein contains valves, but they are not present within the entire portal venous system. The absence of valves is a characteristic of the entire system, not just the portal vein itself.

**Option D:** The intrahepatic portion of the portal vein does not contain valves. However, the correct answer is that the whole system is valveless, not just the intrahepatic portion.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the absence of valves in the portal venous system contributes to the development of portal hypertension. This condition can lead to complications such as variceal bleeding and ascites, making it crucial for clinicians to be aware of this unique anatomical feature.

**✓ Correct Answer: C. The whole system is valveless**