This blood has a reduced oxygen content because the oxygen has been used for metabolic activities in the tissue cells. Systemic veins transport blood from the body tissue to the right atrium of the heart. This blood has a high oxygen content because it has just been oxygenated in the lungs. In the pulmonary circuit, the pulmonary veins transport blood from the lungs to the left atrium of the heart. From the venules, it flows into progressively larger and larger veins until it reaches the heart. After blood passes through the capillaries, it enters the smallest veins, called venules. Smooth muscle cells in the arterioles where they branch to form capillaries regulate blood flow from the arterioles into the capillaries. About 5 percent of the total blood volume is in the systemic capillaries at any given time.
#Small caliber vessel meaning skin
The epidermis of the skin and the lens and cornea of the eye completely lack a capillary network. Other tissues, such as connective tissue, have a less abundant supply of capillaries. Tissues such as skeletal muscle, liver, and kidney have extensive capillary networks because they are metabolically active and require an abundant supply of oxygen and nutrients. The primary function of capillaries is the exchange of materials between the blood and tissue cells.Ĭapillary distribution varies with the metabolic activity of body tissues. CapillariesĬapillaries, 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). The connective tissue in this layer is quite dense where it is adjacent to the tunic media, but it changes to loose connective tissue near the periphery of the vessel. This layer is connective tissue with varying amounts of elastic and collagenous fibers. The outermost layer, which attaches the vessel to the surrounding tissue, is the tunica externa or tunica adventitia. It not only provides support for the vessel but also changes vessel diameter to regulate blood flow and blood pressure. The middle layer, the tunica media, is primarily smooth muscle and is usually the thickest layer. The innermost layer, the tunica intima (also called tunica interna), is simple squamous epithelium surrounded by a connective tissue basement membrane with elastic fibers. The wall of an artery consists of three layers. About 10 percent of the total blood volume is in the systemic arterial system at any given time. The arterioles play a key role in regulating blood flow into the tissue capillaries. 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. Systemic arteries transport oxygenated blood from the left ventricle to the body tissues.
Pulmonary arteries transport blood that has a low oxygen content from the right ventricle to the lungs. ArteriesĪrteries carry blood away from the heart. Based on their structure and function, blood vessels are classified as either arteries, capillaries, or veins. The other system, the systemic vessels, carries blood from the left ventricle to the tissues in all parts of the body and then returns the blood to the right atrium. One system, the pulmonary vessels, transports blood from the right ventricle to the lungs and back to the left atrium. The vessels make up two closed systems of tubes that begin and end at the heart. Acknowledgements Classification & Structure of Blood Vesselsīlood vessels are the channels or conduits through which blood is distributed to body tissues.
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