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.

The systemic circulation is the largest system of arteries and veins in the body. Systemic circulation begins with the aorta – the major artery that travels from left ventricle down the length of the chest and abdomen. About the size of a person’s thumb, the aorta begins by extending upward from the heart. This section is called the ascending aorta. Then it forms a “u,” known as the aortic arch. Smaller arteries branch off from the aortic arch and supply blood to the upper chest area, including the:

From the aortic arch, the aorta travels downward through the chest and abdomen as the descending aorta. The descending aorta, which includes the thoracic aorta (down the length of the chest) and abdominal aorta (down the length of the abdomen), supplies blood to the chest and lower portion of the body. The descending aorta splits off into two smaller iliac arteries that provide blood to the pelvis and lower limbs.

The smaller arteries that branch off from the aorta are like smaller roads branching off from the main highway. These smaller arterial branches carry oxygen-rich blood into the regions furthest from the heart, such as the fingertips, toes and scalp. As the branches continue to narrow (into arterioles), they develop into the smallest blood vessels in the body, capillaries. A capillary is so narrow that red blood cells must pass through it one at a time. There are more than a billion capillaries in the body, with a total surface area of about 1,000 square miles.

Capillaries play a vital role in circulation because they are the sites where the actual exchange of nutrients and waste products takes place at the cellular level. Each red blood cell nourishes the body’s cells with oxygen, water and glucose, and then carries waste products away. The blood (now oxygen-poor and full of waste products) travels from the capillaries into the venules and then into larger veins, such as the saphenous veins in the legs and hips.

Veins in the lower part of the body connect with the inferior vena cava, and veins in the upper part of the body connect with the superior vena cava. Both of these venae cavae empty into the upper-right chamber (right atrium) of the heart, thus completing the systemic circulation.

Pulmonary circulation refers only to the network of arteries and veins that carry blood from the heart to the lungs and back. It is not considered part of the systemic circulation.

Pulmonary circulation begins when oxygen-poor blood is pumped from the right ventricle into pulmonary arteries. This is the only instance in which arteries carry oxygen-poor blood. The blood is taken into the lungs, where it enters a series of gradually smaller arteries until it passes into pulmonary capillaries. These capillaries have the same function as ones in systemic circulation: they allow for the exchange of waste products and oxygen. In this case, the blood cells shed their waste products (including carbon dioxide) and pick up fresh oxygen.

After the blood is freshly oxygenated in the lungs, it travels through the pulmonary veins back to the left atrium. This is the only instance in which veins carry oxygen-rich blood. Once in the left atrium, the oxygen-rich blood drains into the left ventricle, where it is pumped into the systemic circulation.

Preparing questions in advance can help patients to have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions related to the circulatory system:

1. How do I know if my circulatory system is functioning properly?
   
   
2. Am I at risk for any circulatory problems?
   
   
3. What tests are used to evaluate the circulatory system?
   
   
4. What signs or symptoms indicated a possible problme with my circulatory system?
   
   
5. What can I do to protect my circulatory system?
   
   
6. Am I currently taking any medications that might affect the way my circulatory system operates?
   
   
7. Can problems with my lungs affect the way my circulatory system functions?
   
   
8. What type of medical conditions affect my circulatory system?
   
   
9. Can circulatory problems be caused by some aspect of my diet or lifestyle?
   
   
10. Are there any medications available to me that would improve my particular circulatory problem?
    
    
11. Does pregnancy affect the way my circulatory system operates?
    
    
12. Can problems with my circulatory system affect my unborn child?
    
    
13. What type of heart problems can result from an impaired ciculatory system?
    
    
14. If I have ciculatory problems, will my children be at higher risk for the same problems?