Describe the structure and functions of arteries, veins and capillaries.

8.2 Transport in Humans

Arteries

Arteries are the “high‑speed highways” that carry oxygen‑rich blood away from the heart. They have thick, muscular walls that can withstand the high pressure generated by each heartbeat. Key features:

• Thick elastic & muscular layers (tunica media) – keeps the vessel strong.
• High pressure: $P_{\text{arterial}} > P_{\text{venous}}$.
• Red colour in diagrams (because of oxygenated blood).
• Valves are usually absent – blood flows forward only.

Functions:

• Deliver oxygen & nutrients to tissues.
• Carry away metabolic waste (CO₂, urea).
• Maintain blood pressure through elastic recoil.

Veins

Veins are the “return roads” that bring de‑oxygenated blood back to the heart. They have thinner walls and contain valves that prevent back‑flow, especially in the limbs where gravity works against the return of blood. Key features:

• Thin walls with less muscle and elastic tissue.
• Lower pressure compared to arteries.
• Blue colour in diagrams (because of de‑oxygenated blood).
• Valves in limbs keep blood moving upward.

Functions:

• Return oxygen‑poor blood to the heart.
• Transport waste products to the liver and kidneys.
• Act as a reservoir for blood volume.

Capillaries

Capillaries are the “city streets” where the real exchange happens. They are extremely thin (only one cell thick) and allow substances to move between blood and tissues. Key features:

• Single‑layered endothelial cells.
• Very small diameter – blood flow is slow.
• No valves or valves are not needed.
• High surface area for diffusion.

Functions:

• Oxygen & nutrient delivery to cells.
• Removal of CO₂, waste, and toxins.
• Exchange of electrolytes and water.

Comparison Table