The hepatic portal vein connects the capillaries of the small intestine to the capillaries of the liver. For example, if too much sugar enters the liver via the hepatic portal vein, the liver will remove excess sugar from the blood and store it as glycogen. If the blood entering the liver is too low in glucose, the liver will convert stored glucose to glycogen. In this way, the concentraiton of blood leaving the liver is regulated in terms of its glucose content. Compare and contrast the structure and function of arteries and veins.
Arteries go away from the heart and veins go toward the heart.
Arteries carry oxygenated blood in the systemic circuit and deoxygentated blood in the pulmonary circuit. Veins carry deoxygenated blood in the systemic circuit and oxygentated blood in the pulmonary circuit. Arteris and veins have walls made of three layers but the middle layer of smooth muscle is thicker in arteries than in veins. Since the walls of veins are thinner, they can expand to a greater extent. Veins have blood forced through them by skeletal muscles contracting.