The Heart

• Chambers of the Heart
  
  Your heart has four chambers: right and left atrium (upper chambers) and right and left ventricle (bottom chambers). The heart beat contracts (squeezes) twice--"lub DUB"--with each pulse beat. The blood-filled atria contract (lub), pushing blood into the ventricles. Then, the larger more muscular ventricles contract (DUB) to pump the blood out of the heart. The blood from the right ventricle goes out the pulmonary artery to the lungs to pick up oxygen and from the left ventricle out the aorta and throughout your body. Air exchange (carbon dioxide for oxygen) occurs in your lungs through small capillaries (tiny thin-walled vessels). As the ventricles contract, blood flows into the atria. The right atrium fills with returning carbon dioxide-laden blood from the vena cava (a large vein) and the left atrium from freshly oxygenated blood from the pulmonary veins and the lungs.
  
  

Arteries

• Capillaries
  
  Arteries leave the heart and, except for the pulmonary artery (which takes blood to the lungs to pick up oxygen), carry oxygenated blood to all the cells of the body. Arteries branch into smaller arterioles and then tiny capillaries. Arteries are thick-walled, muscular and elastic, and their walls contain nerves that stimulate the arteries to constrict or dilate, controlling the blood flow. As the left ventricle pumps blood into the arteries, the elasticity allows the lumen (opening) of the arteries to expand slightly as blood pulses through. Blood flows first to your heart's coronary arteries, then the brain, internal organs and the rest of your body. Because the lumen is narrow, fatty plaques (atherosclerosis) or blood clots can easily block an artery, cutting off blood supply to part of your body. The arterial blood flows into the tiny capillaries that feed the cells, exchanging oxygen for carbon dioxide. As the capillaries become filled with carbon dioxide, they become part of the venous system, returning the blood to the heart.
  
  

Veins

• Artery & Vein
  
  The carbon-dioxide laden venous capillaries carry your blood to venules, larger veins and into the superior vena cava (from the head and arms) and inferior vena cava (from the internal organs and legs) to your heart. Only the pulmonary veins (bringing blood from the lungs to the heart) carry oxygenated blood. Veins are thin-walled with a narrow band of muscle, so they are able to expand more than arteries and hold a greater volume of blood. While veins can constrict and dilate, they do so less effectively than arteries because of the small muscle. Therefore, the veins that carry blood against gravity (such as the leg veins) have one-way valves that open to let blood flow through, but close to prevent backflow. Veins, especially in your limbs, depend on contractions of surrounding muscles to keep the blood moving, so if you are sedentary or gain weight, the blood may pool, dilating the veins and damaging the valves, allowing backflow and causing varicose veins, swelling, and/or decreased circulation.