Characteristic features of blood vessels

• The vessels form a closed transport system which starts and finishes at the heart. 
• The blood flows from the heart to body tissue and back to the heart. 
• This is called systemic circulation.
• In a separate circulation known as pulmonary circulation blood flows from the heart to the lungs to expel carbon dioxide and take in oxygen and then return it back to the heart. 

Arteries and arterioles 

• Arteries carry blood away from the heart. 
• Blood leaves the ventricles of the heart and enters thick elastic arteries under high pressure. 
• Artery lumen is small and arteries have thick walls that contain collagen, a fibourous protein, to help maintain the shape and volume of the arteries under pressure. 
• Artery walls also contain elastic tissues to help them expland and recoil. 
• Arteries are lined with smooth endothelium tissue, reducing the friction as the blood flows through the lumen. 
• As blood flows further from the heart, these large elastic arteries become smaller muscular…