1. Vessels of the Circulatory System Source: AP Biology Textbook

2. Layers of the Blood Vessels

3. Endothelium Surrounding the cavity is a layer called the endothelium The endothelium’s thin surface area allows for blood to travel smoothly

4. Smooth muscle Smooth Muscle is the second layer, between the endothelium and connective tissue Smooth Muscle in the Artery is thicker, and allows for strength as blood pressure pumps blood Smooth muscle in Veins is much thinner due to less pressure in blood flow

5. Connective Tissue Connective tissue is the outermost layer of blood vessels Connective tissue in arteries is, again, thicker due to the need for strength In veins, it is much thinner

6. The Blood Vessels

7. Arteries Pulmonary Arteries take oxygen-rich blood from the heart wherever it has to go Arteries have walls three times thicker than veins, , because it controls blood flow from the heart Signals from the nervous system, along with hormones, help the smooth tissue regulate blood flow Arteries have all three layers Contractions of the various tissues in the walls of arteries lead to the pulse

8. Capillaries Capillaries are the thinnest and most abundant of vessels in the human body Capillaries exchange blood contents between arteries and veins Most of the oxygen loss happens here  blood from the artery loses oxygen and is transferred to the veins Capillaries only have the endothelium and basal lamina layers

9. Veins Veins carry oxygen-poor blood back to the heart for re-pumping Veins have all three layers, but are thinner The valves in veins keep a single-directional flow of blood Veins have thinner walls because there is less velocity and pressure in the blood