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.
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 arteries are the largest vessels and include the aorta and pulmonary artery. They expand when blood is pumped out of the heart and the resistance to relax pushes the blood out into the muscular arterial branches. The muscular arteries have a layer of smooth muscle that contracts to pump the blood into the peripheral system. This type of artery includes the aortic branches, such as the coronary and renal arteries.
The small arteries and arterioles feed into the tissues and organs. They are also made of smooth muscle to regulate pressure and flow. However, these are the most influential type of blood vessel because their size and placement in the system determine blood flow into the capillary beds. This, in turn, controls the exchange of gases and nutrients in the tissues.
The Venous System
Veins also occur in varying sizes and get larger as they get closer to the heart, but their contraction is dependent on the skeletal muscle system (such as the leg muscles), which squeezes the vessels during movement. A lack of skeletal muscle movement, for example in the legs during a long intercontinental flight, can result in venous pooling and clots. Blood flow through the venous system also depends on the arterial blood pressure and integrity of the vein walls. Valves in the venous vessels prevent the backflow of blood as it makes its way to the heart for recirculation.