Final answer:
The lesser omentum provides a pathway for the blood vessels entering and leaving the liver, specifically at the porta hepatis, where the hepatic artery and the hepatic portal vein enter.
Step-by-step explanation:
The structure that provides access for blood vessels entering and leaving the liver is the lesser omentum. The porta hepatis, also known as the "gate of the liver", is where vessels such as the hepatic artery and hepatic portal vein enter the liver. These vessels, along with the common hepatic duct, traverse behind the lateral border of the lesser omentum. The hepatic artery carries oxygenated blood from the heart to the liver, while the hepatic portal vein delivers nutrient-rich and partially deoxygenated blood from the small intestine. After the liver processes this blood, it releases necessary nutrients back into the circulation and blood exits the liver through the hepatic vein to join the inferior vena cava.