Homostasis - Circulatory System Part 3 - 28 and 2

Structural Similarities of Arteries and Veins

The walls of both arteries and veins have three similar layers. On the outside, a layer of connective tissue with elastic fiber allows the vessel to stretch and recoil. A middle layer has smooth muscle and more elastic fibers. Lining the lumen of all blood vessels is an endothelium, a single layer of flattened cells that minimizes resistance to blood flow. 

Adaptations of Structure to Functions - Arteries

• Carry blood away from the heart to the tissues
• Carry oxygenated blood EXCEPT pulmonary artery
• Thick muscular walls provide strength to accommodate blood pumped rapidly and at high pressure by the heart and their elasticity (elastic recoil) helps maintain blood pressure even when the heart relaxes.

Adaptations of Structure to Functions - Veins 

• Carries blood TOWARDS the heart from the tissues 
• Carry deoxygenated blood EXCEPT pulmonary vein
• The thinner-walled veins convey blood back to the heart at low velocity and pressure. Blood flows through the veins mainly because skeletal muscle contractions squeeze blood in veins.
• Have valves to prevent backflow of blood hence blood flow only towards the heart.

 Adaptation of Structure to Functions - Blood Capillaries  

• Very thin-walled 
• Very small in diameter
• The exchange of substances (mainly diffusion) between the blood and interstitial fluid that bathes the cells takes place across the thin endothelial walls of the capillaries.  

Functional Comparison Between Arteries and Veins

 Arteries

• Transport blood away from the heart
• Transports blood under higher pressure (than veins)
• Carry Oxygenated Blood (except in the case of the Pulmonary Artery) 

Veins

• Transport blood towards the heart
• Transports blood under lower pressure (than arteries)
• Carry De-oxygenated Blood (except in the case of the Pulmonary Vein)

Structural Comparision Between Arteries and Veins

Arteries 

• Have relatively more muscle/elastic tissue 
• Have relatively narrow lumens
• Do not have valves (except for the semi-lunar valves of the pulmonary artery and the aorta)

Veins

• Have relatively less muscle/elastic tissue
• Have relatively wide lumens
• Have valves throughout the main veins of the body. These are to prevent blood flowing in the wrong direction, as this could (in theory) return waste materials to the tissues.