Blood vessels consist of arteries, arterioles, capillaries, venules, & veins. Blood flows through the blood vessels from the heart to arteries, arterioles, capillaries (where gas exchange occurs), venules, veins, & back to the heart.
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Walls of Arteries & Veins
Artery & vein walls are composed of three main layers: the tunica interna (intima), tunica media, & tunica externa (adventitia). The tunica interna consists of simple squamous epithelial cells & the basement membrane (termed "endothelium"). The tunica media consists of loose connective (areolar) tissue & smooth muscle. The tunica externa consists of more connective tissue that contains many collagen & elastic fibers. The connective tissue helps anchor the blood vessel to surrounding organs.
Arteries contain a thin layer of elastic fibers between each of these layers. Between the tunica interna & media is the internal elastic lamina & between the tunica media & externa is the external elastic lamina.
The amount of smooth muscle & elastic fibers changes as the vessel moves further from the heart. Close to the heart, there is less smooth muscle & more elastic fibers. Further from the heart, there is more smooth muscle & less elastic fibers.
Both arteries & veins can be classified into three groups: Large, Medium, & Small. Large arteries are sometimes called elastic arteries & medium arteries are sometimes called muscular arteries.
Differences Between Arteries & Veins
The tunica media is typically the largest layer in arteries, whereas the tunica externa is typically the largest layer in veins.
The lumen (opening) is typically larger in veins than in arteries; however, the walls are typically thicker in arteries than in veins.
Medium veins contain valves that help prevent backflow of blood, while arteries do not contain valves.
In the systemic circuit (heart to the body & back), arteries carry oxygenated blood to the cells of the body, where oxygen will be dropped off & carbon dioxide will be picked up. This blood is often depicted as red in illustrations. Veins carry deoxygenated blood back to the heart. This blood is often depicted as blue in illustrations, but is not actually blue, but a deeper, darker red than arterial blood. Remember, blood is always red!
In the pulmonary circuit (heart to lungs & back), arteries carry deoxygenated blood to the lungs to pick up oxygen & release carbon dioxide. Veins carry oxygenated blood to the heart to be pumped to the body. Therefore, arteries & veins are not defined by what they carry, but by the structure of their walls.
Despite these differences, arteries always carry blood away from the heart, whereas veins always carry blood towards the heart.
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Capillaries are known as the exchange vessels as this is where oxygen & carbon dioxide are exchanged between the blood & either the cells of the body or the cells of the lungs. To aid in gas exchange, the walls of capillaries consist of the tunica interna only. There are three types of capillaries: Continuous, Fenestrated, & Sinusoid (Discontinuous).
Continuous capillaries form a continuous tube with very little passing through the walls, except very small molecules, such as glucose. These are found in skeletal muscle, fat, & the nervous system.
Fenestrated capillaries have numerous small holes in the walls allowing larger molecules to pass through. These are found in organs specialized for filtration or rapid absorption (kidneys, small intestines, endocrine glands).
Sinusoid capillaries have many holes and these holes are larger than those of fenestrated capillaries. Small & large molecules can pass through the holes. These capillaries follow the contours of the organs they are located in, such as the spleen, bone marrow, & liver.
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