Pathways of Blood Circulation in the Body

In order to understand how the circulatory system delivers substances from one part of the body to another, it is necessary to have some understanding of how blood vessels are connected to each other. In each individual vessel, blood flows, of course, only in one direction. The head and brain are supplied with blood by the carotid arteries, and the outflow of blood from them occurs through the jugular veins.

In addition, the brain is served by a second pair of pathways - the vertebral arteries and veins passing near the spinal cord (not shown in the diagram). At the base of the brain, the carotid and vertebral arteries communicate with each other; therefore, if one of the vessels is cut or blocked, the brain will receive enough blood. The only exception to the rule that veins always carry blood to the heart is the portal vein of the liver, which collects blood from the spleen, stomach, pancreas and intestines and carries it to the liver.

Here the portal vein breaks into capillaries, which reconnect to form the hepatic vein, which drains blood from the liver into the inferior vena cava. Thanks to this device, called the hepatic portal system, all blood going to the heart from the spleen, stomach, intestines and pancreas must first pass through the liver. Thus, nutrients absorbed in the intestines go directly to the liver, where they can be stored.