BLOOD VESSELS (ARTERIES, VEINS AND CAPILLARIES)

The Circulatory System is known as a CLOSED SYSTEM because the blood is contained within either the heart or blood vessels at all times. The blood vessels that are part of the closed circulatory system of humans from a vast network to help keep the Blood flowing in one direction. The Blood Vessels that form this network and are part of the CIRCULATORY SYSTEM ARE THE ARTERIES, CAPILLARIES, AND VEINS.

5. With the exception of Capillaries and tiny Veins, Blood Vessels have WALLS made of THREE LAYERS OF TISSUE, that provides for a combination of Strength and Elasticity: A. THE INNER LAYER IS EPITHELIAL TISSUE. B. THE MIDDLE LAYER IS SMOOTH MUSCLE TISSUE. C. THE OUTER LAYER IS CONNECTIVE TISSUE.

ARTERIES AND ARTERIOLES (SMALL ARTERIES) 1. Arteries carry blood from the HEART TO CAPILLARIES AND THE REST OF THE BODY. 2. The Walls of Arteries are generally THICKER than those of Veins.

3. The Smooth Muscle Cells and Elastic Fibers that make up the Walls help make Arteries Tough and Flexible. This enables Arteries to withstand the high pressure of blood as it is pumped from the Heart. The force that blood exerts on the walls of blood vessels is known as BLOOD PRESSURE.

4. EXCEPT FOR THE PULMONARY ARTERIES, ALL ARTERIES CARRY OXYGEN-RICH BLOOD.

5. The Artery that carries Oxygen-Rich Blood from the LEFT VENTRICLE to all parts of the body, EXCEPT THE LUNGS, is the AORTA. 6. THE AORTA WITH A DIAMETER OF 2.5 cm, IS THE LARGEST ARTERY IN THE BODY. 7. As the Aorta travels away from the Heart, it branches into smaller Arteries so that all parts of the body are supplied.

8. THE SMALLEST ARTERIES ARE CALLED ARTERIOLES.

CAPILLARIES 1. ARTERIOLES BRANCH INTO NETWORKS OF VERY SMALL BLOOD VESSELS CALLED CAPILLARIES. 2. IT IS IN THE THIN-WALLED (ONE-CELL IN THICKNESS) THAT THE REAL WORK OF THE CIRCULATORY SYSTEM IS DONE.

3. The Walls of the Capillaries consist of only one layer of cells, making it easy for Oxygen and Nutrients to DIFFUSE FROM THE BLOOD INTO THE TISSUE. 4. Forces of Diffusion drive CO2 and waste products from the tissue into the Capillaries. 5. Capillaries are extremely NARROW; Blood Cells moving through them must pass in Single file.

VEINS 1. THE FLOW OF BLOOD MOVES FROM CAPILLARIES INTO THE VEINS. (Figure 47-6) 2. Veins form a system that COLLECTS Blood from every part of the Body and CARRIES it Back to the HEART. 3. The smallest Veins are called VENULES.

4. LIKE ARTERIES, VEINS ARE LINED WITH SMOOTH MUSCLE. Vein walls are thinner and less elastic than Arteries. Veins though are more FLEXIBLE and are able to stretch out readily. 5. This flexibility reduces the Resistance the flow of blood encounters on its way back to the Heart.

6. Large Veins contain Valves that maintain the one direction flow of Blood. This is important where Blood must flow against the Force of Gravity. 7. The flow of Blood in Veins is help by Contractions of Skeleton Muscles, especially those in the legs and arms. When muscles contract they squeeze against Veins and help force Blood Toward the Heart.