The circulatory system includes all the blood vessels. It resembles a vast network of major highways and smaller roads that carries oxygen-rich blood from the heart to the tissues and organs of the body and returns oxygen-poor blood to the heart. The four-chambered heart lies at the center of the circulatory system.

There are two major types of blood vessels that carry blood throughout the body:

Arteries and veins are well suited to their tasks. For instance, the walls of arteries are relatively thick to withstand the high pressure of blood flow coming from the heart. Veins, on the other hand, have thinner walls that allow the blood (now under less pressure) to flow easily back to the heart.

To understand how the circulatory system works, it is helpful to know that the circulatory system, including arteries and veins, can be divided into different regions, depending on which part of the body is involved:

• Systemic circulation refers to the network of arteries that carries oxygen-rich blood from the left ventricle to the body and the veins that return oxygen-poor blood back to the right atrium. Most of the volume of blood in the body is contained in the systemic circulation. The systemic circulation includes the body’s major arteries, such as the aorta, and the body’s major veins, such as the inferior vena cava and superior vena cava (venae cavae).
  
  
• Pulmonary circulation refers to the network of arteries and veins that carry blood from the heart to the lungs and back. This is not connected to the systemic circulation. Oxygen-poor blood from the right ventricle is carried through the pulmonary arteries to the lungs, where it is cleansed of carbon dioxide and infused with fresh oxygen. The oxygen-rich blood then travels back to the left atrium through the pulmonary veins.
  
  
• Hepatic-portal circulation refers to the arteries that carry oxygen-rich blood to the intestines, spleen and liver, and the veins that carry oxygen-poor blood from these organs (the portal venous system) to the liver, where it is detoxified. The detoxified blood then reenters the systemic circulation on its way to the right side of the heart.
  
  
• Coronary circulation refers to the arteries and veins that supply the heart with oxygen-rich blood. Although the coronary arteries branch off from the root of the aorta, this is considered a discrete circulation. Oxygen-poor blood is drained by the coronary sinus directly into the right atrium, meaning it is not part of the systemic circulation.