This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Anatomy
The poal venous system contributes approximately 75% of the blood and 72% of the oxygen supplied to the liver. Which of the following is TRUE about valves in poal venous system? 
Select and explain. 
A. Present at the junction of superior mesenteric aery with the splenic aery, B. Within the poal vein only, C. The whole system is valveless, D. In the intrahepatic poion of poal vein
C: The whole system is valveless
Answer: C: The whole system is valveless 
Explanation: The poal venous system is without valves and drains blood from the spleen, pancreas, gall bladder, and abdominal poion of the alimentary tract into the liver. Tributaries of the poal vein communicate with veins draining directly into the systemic circulation. These communications occur at the gastroesophageal junction, anal canal, falciform ligament, splenic venous bed and left renal vein, and retroperitoneum. Ref: Geller D.A., Goss J.A., Tsung A. (2010). Chapter 31. Liver. In F.C. Brunicardi, D.K. Andersen, T.R. Billiar, D.L. Dunn, J.G. Hunter, J.B. Matthews, R.E. Pollock (Eds), Schwaz's Principles of Surgery, 9e.