A blood vessel is a tube that carries blood. Oxygen rich blood leaves the left side of the heart and enters the aorta. The aorta branches into arteries, which eventually branch into smaller arterioles. Arterioles carry blood and oxygen into the smallest blood vessels, the capillaries.

Capillaries are so small they can only be seen under a microscope. The walls of the capillaries are permeable to oxygen and carbon dioxide. Oxygen moves from the capillary toward the cells of the tissues and organs. Carbon dioxide moves from the cells and into the capillaries.

Blood leaves the capillary and enters the small venules. These venules become progressively larger vessels called veins. The vena cava are the two largest veins that carry blood into the right upper chamber of the heart (the right atrium). The superior vena cava carries blood from the brain and arms into the top of the right atrium. The inferior vena cava carries blood from the legs and abdominal cavity into the bottom of the right atrium. The vena cava are also called the "central veins". Central venous catheters are inserted with the tip in or close to the superior of inferior vena cava.

Blood is pumped from the right side of the heart into the blood vessels of the lung. When blood enters the small capillaries of the lung (called the pulmonary capillaries), fresh oxygen enters the blood and carbon dioxide is removed. This is called "gas exchange" or "respiration". Because this is the exchange of gases between the atmosphere and the bloodstream, it is also called "external respiration".

When freshly oxygenated blood reaches the capillaries of the tissue, oxygen moves from the blood toward the tissues, and carbon dioxide moves from the tissues toward the blood. This gas exchange that occur between the blood and the cells of the tissues and organs is called "internal respiration".

Blood vessels have a muscle layer that is able to relax or contract. When we need to increase our blood pressure, the muscle layer contracts and makes the blood vessel diameter smaller. This is called "vasoconstriction".

When the muscle layer of a blood vessel relaxes, the blood vessel diameter becomes larger. This is called "vasodilation". Vasodilation lowers the blood pressure.

Drugs that cause the diameter of the blood vessels to change are called vasoactive drugs. Drugs that make blood vessels constrict are used to treat low blood pressure and are called vasocontrictors. Drugs that relax the blood vessels and make them relax are used to treat high blood pressure. They are called vasodilators.

Blood Vessels


Image 1: Blood Vessels. Arteries (in red) carry oxygen rich blood from the left side of the heart to the tissues and organs. After oxygen leaves the blood and moves into the tissues, the level of oxygen in the blood becomes low. The veins (in blue) carry blood that has a low level of oxygen back to the right side of the heart. Blood from the veins is pumped from the right side of the heart through the blood vessels of the lungs, where new oxygen is picked up. This oxygen rich blood flows from the lungs to the left side of the heart.




Last Reviewed: October 23, 2014



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Last Updated October 23, 2014 | © 2007, LHSC, London Ontario Canada