The Blood Vessels: is a tube-like muscle structure, transport blood to all the parts and organs of the body, based on their structure and function, blood vessels are classified mainly into: arteries, capillaries and veins.carry oxygenated blood and nourishment from the heart to the tissues of the body, Blood is pumped from the ventricles into large elastic arteries that branch repeatedly into smaller and smaller arteries until the branching results in microscopic arteries called arterioles.Medium and large veins have venous valves, similar to the semilunar valves associated with the heart, that help keep the blood flowing toward the heart.From the venules, it flows into progressively larger and larger veins until it reaches the heart.Capillaries The smallest and most numerous of the blood vessels, form the connection between the vessels that carry blood away from the heart (arteries) and the vessels that return blood to the heart (veins).After blood passes through the capillaries, it enters the smallest veins, called venules.