by Alicia M Prater PhD

Created on: May 18, 2008   Last Updated: September 25, 2009

The circulatory system is a complex network of muscular and elastic tubes, called vessels, that connect all of the tissues to a central pump - the heart. Life is dependent on nutrients and oxygen getting to each and every tissue and cell within the human body. The circulatory system, also known as the cardiovascular system, is responsible for carrying out this necessary process. Oxygen and carbon dioxide are exchanged when blood passes through the lungs, and other wastes are removed when blood passes through the kidneys.

What is Blood?

Red blood cells contain a molecule called hemoglobin that can bind to and carry oxygen through the blood vessels. The red blood cells are mixed with plasma, which contains nutrients and water, and white blood cells and other immune system components as they circulate through the blood vessels. Together, this mixture is known as blood.

Large Blood Vessels

The larger blood vessels are named according to the direction in they carry blood, either to the heart or away from the heart. Arteries carry blood away from the heart (a way to remember it is "A" for artery and "A" for away). Blood pumped out of the heart to the body is oxygenated and travels through the largest blood vessel, the aorta. From the heart to the lungs, the blood travels through the pulmonary artery, the only arterial vessel to carry deoxygenated blood. Veins carry blood to the heart. From the body, deoxygenated blood enters the heart through the largest veins, the inferior and superior vena cava. From the lungs, oxygenated blood travels through the pulmonary vein, the only vein to carry oxygenated blood.

Capillaries

Blood vessels vary in size and are made of different tissue depending on their proximity to the heart. The smallest blood vessels are found in the tissues and act as a doorway between the arteries and veins. These vessels are wide enough to allow the passage of a single red blood cell and are called capillaries. The tissue surrounding a capillary, called the capillary bed, is where gas, oxygen, and nutrient exchange occurs between the blood and tissues. A red blood cell carrying oxygen will exchange the molecule as it passes through the capillary, and then it will enter the venous system to return to the heart, where it passes again through the lungs to receive an oxygen molecule and begin its journey again.

Types of Arteries

There are three main types of arteries: elastic (large), muscular (medium), and arterioles (small). As the names imply, elastic

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