Artery:
- Arteries are blood vessels that carry oxygenated blood away from the heart to the various parts of the body.
- They have thick, elastic, and muscular walls that allow them to withstand the high pressure generated by the pumping action of the heart.
- Arteries branch into smaller vessels called arterioles, which further divide into capillaries.

Vein:
- Veins are blood vessels that carry deoxygenated blood from the various parts of the body back to the heart.
- They have thinner walls compared to arteries, as the blood pressure is lower in veins.
- Veins contain valves that prevent the backflow of blood and help in maintaining proper blood flow towards the heart.
- Veins merge to form larger vessels, such as the superior vena cava and inferior vena cava, which bring blood back to the heart.

Capillary:
- Capillaries are the smallest blood vessels and connect arterioles to venules.
- They have extremely thin walls, consisting of a single layer of endothelial cells, which enables the exchange of oxygen, nutrients, and waste products between the blood and tissues.
- Capillaries are present in large numbers in organs and tissues, allowing for efficient exchange of substances.
- They help in maintaining the balance of fluids between the blood and tissues.

Differences:
1. Structure:
- Arteries have thick, elastic, and muscular walls, while veins have thinner walls.
- Capillaries have extremely thin walls, consisting of a single layer of endothelial cells.
- Arteries and veins have three layers in their walls, whereas capillaries have only one layer.

2. Blood Flow:
- Arteries carry oxygenated blood away from the heart, while veins carry deoxygenated blood back to the heart.
- Blood flow in arteries is pulsatile and under high pressure, while blood flow in veins is continuous and under lower pressure.
- Capillaries allow for the exchange of substances between the blood and tissues.

3. Valves:
- Arteries and capillaries do not have valves.
- Veins have valves that prevent the backflow of blood.

4. Oxygenation:
- Arteries carry oxygenated blood, while veins carry deoxygenated blood.
- Oxygen is exchanged between the blood and tissues in capillaries.

5. Location:
- Arteries are located deeper within the body, closer to the heart.
- Veins are located closer to the surface of the body.
- Capillaries are present throughout the body, connecting arterioles and venules.

In summary, arteries carry oxygenated blood away from the heart, veins carry deoxygenated blood back to the heart, and capillaries facilitate the exchange of substances between the blood and tissues. Arteries have thick walls, veins have valves, and capillaries have extremely thin walls.