Please choose the correct ending to complete the given sentence: Health: [header] How to study the functions of the liver in humans [title] Learn about the structure of the liver. [step] Its structure is unusual in the sense that it has dual blood supply from two different sources. One of them is through the hepatic artery that supplies blood that is rich in oxygen to the liver tissue.

Ending1: The other is the pancreas which is located at the liver's back. In an ordinary person, the pancreas walls within the liver consist of neurons and receptors that secrete oxygen to the kidneys. Ending2: The other blood source is from the portal vein which originates in the digestive system in its various parts. [title] Study the role of the blood supply to the liver. Ending3: The other is through the liver itself which in turn are calcium ion and phosphate ions. One of these ions is slower in processing by compounds than phosphate ions. Ending4: The other is through the hepatic vein that carries blood to the liver and ventricles. The two primary physical structures that to study in humans are the liver and blood vessels in the liver.

Answer format: ending1/ending2/ending3/ending4