the heart and the blood, the arteries and veins make up what is called the circulatory, or cardiovascular, system. The circulatory
system works to provide the body with four main services; the transportation of nutrients and wastes, the circulation of hormones,
the transportation of oxygen and carbon dioxide, and finally the maintenance of body temperature.
heart is the pump which circulates the blood that supplies these nutrients, the arteries and veins are the roadways that carry
the blood throughout the body to all its tissues and cells.
branching from the aorta leaving the heart, are the blood vessels that carry blood away from the heart to the tissues of the
body. The arteries are composed of three layers; an outermost layer of connective tissue, a layer of smooth muscle and elastic
tissue, and finally a layer of endothelium tissue. The hollow center of the artery where the blood travels is called the lumen.
Arteries have the capacity to dilate and constrict; the major arteries leaving the heart expand and collapse slightly in response
to the surge of blood caused by the pumping heart. Also, the arteries can expand and constrict in response to nervous stimulation.
Nerve impulses control the size of the arteries in response to changes in blood pressure and activity. The arteries travel
into different cavities and tissue of the body, branching off into smaller arterioles and finally tiny capillaries.
capillary beds, made of endothelial cells, are found in all tissues and work to deliver oxygen to cells, while harvesting
carbon dioxide. The capillaries leaving
the tissues grow in size, becoming venules, and eventually veins.
the blood vessels that carry deoxygenated blood from the tissues to the heart, where it can be pumped into the lungs, oxygenated,
and eventually be sent back to the tissues via the arteries. Because the pumping action of the heart has little effect on
the blood traveling through the veins, the walls of the veins are much thinner than that of the arteries, and its lumen is
larger. The lower blood pressure in the veins also makes it more difficult for blood to travel back towards the heart against
the force of gravity. In order to accommodate this difficulty, the veins have one-way valves to prevent backflow of the blood
away from the heart. Deoxygenated blood enters the heart via the superior and inferior vena cava.