Blood Vessels

Blood vessels are hollow tubes that carry blood. Interesting Facts If you could take all the vessels in a child and line them up they would stretch about 60,000 miles. Adult blood vessels could reach about 100,000 miles.

Blood vessels carry blood between the heart, different tissues, and organs of the body Three types of blood vessels: arteries, veins, and capillaries. Vessels have the ability to expand to allow more blood to flow through them and to contract to help control the flow of blood.

Sequence of blood flow Blood flows from the big arteries to the smaller arterioles then to the capillaries. The capillaries connect to the smallest veins called venules. Blood flows from the venules to the large veins and back to the heart.

Arteries are elastic, muscular tubes that carry blood from the left ventricle of the heart to the capillaries. The walls of the arteries are thicker than other vessels because of the pressure. Fact: The diameter of the aorta is about the same as a garden hose.

Walls of the arteries are three layers thick Inner layer: Tunica Intima consists of single flattened cells called endothelium Middle layer: Tunica Media thicker part of arterial wall Outer layer: Tunica Externa (or Adventia) composed of fibrous tissue that gives strength to the wall of the artery.

Do you remember that we said that arteries had a lot of elasticity Do you remember that we said that arteries had a lot of elasticity? Here you can clearly see the internal elastic membrane of an artery.

Veins Veins carry blood from capillaries to the heart. Increase in size as they progress toward the heart. Veins that connect to capillaries are venules. Follow a path parallel to arteries. More veins than arteries Veins at any one moment carry 70% of the blood.

Veins like arteries have three layers but are not as thick.

The only vein that does not have valves is the vena cava. Located within the veins are valves that allow blood to flow toward the heart but does not allow the blood to flow backward. The only vein that does not have valves is the vena cava. Valves in the legs sometimes breakdown and allow the blood to flow backwards. This is called varicose veins.

Artery-Vein Pair Artery is on left, vein on right

Capillaries Smallest working unit in the vascular system Connect arterioles to venules Only 1 cell thick to allow for nutrient and gas exchange

Longitudinal section of a vein Longitudinal section of a vein. Note that this cannot be a capillary since its diameter is more than 1-cell wide.