Answer
The arteries have thick walls made of smooth muscle. These large vessels carry blood away from the heart to parts of the body. All arteries carry oxygen-rich blood except the pulmonary artery, which carries oxygen-poor blood to the lungs. Arteries keep branching out into smaller and smaller arteries until they finally become capillaries.
The capillaries are the smallest blood vessels in the body. They are so tiny that red blood cells travel single-file through them. Their walls are thin, allowing nutrients, wastes, and gases to pass freely through them.
The veins are blood vessels that return blood to the heart. Capillaries unite to form veins. With the exception of the pulmonary vein, which carries oxygen-rich blood from the lungs to the heart, all veins carry oxygen-poor blood. Veins have valves that keep blood from flowing backward because in many veins, blood tends to flow against the force of gravity.
Work Step by Step
The arteries have thick walls made of smooth muscle. These large vessels carry blood away from the heart to parts of the body. All arteries carry oxygen-rich blood except the pulmonary artery, which carries oxygen-poor blood to the lungs. Arteries keep branching out into smaller and smaller arteries until they finally become capillaries.
The capillaries are the smallest blood vessels in the body. They are so tiny that red blood cells travel single-file through them. Their walls are thin, allowing nutrients, wastes, and gases to pass freely through them.
The veins are blood vessels that return blood to the heart. Capillaries unite to form veins. With the exception of the pulmonary vein, which carries oxygen-rich blood from the lungs to the heart, all veins carry oxygen-poor blood. Veins have valves that keep blood from flowing backward because in many veins, blood tends to flow against the force of gravity.
