Blood Vessels
There are three types of blood vessels. They are:
1) Arteries: Arteries carry blood from heart to different organs. They have thick wall.
2) Veins: Veins are comparatively thin walled and have valves. Veins carry back blood from different organs back to the heart.
3) Capillaries: Capillaries are very small and thin walled blood vessels. Through their thin walls, oxygen, carbon dioxide, glucose etc. which are carried by blood can diffuse from blood to tissues.