IGCSE Biology 0610 - 8.2 Transport in Humans

Structure and Function of Arteries, Veins and Capillaries

This section explores the relationship between the structure and function of the three main types of blood vessels: arteries, veins, and capillaries. The unique structural features of each vessel are directly adapted to their specific roles in the circulatory system.

Arteries

Arteries are blood vessels that carry blood away from the heart. Because blood needs to be pumped with force, arteries have a thick, elastic wall.

Structure of Arteries

• Thick, muscular walls: This allows the arteries to withstand the high pressure of blood being pumped from the heart.
• Elastic fibres: These fibres stretch and recoil with each heartbeat, helping to maintain blood pressure and smooth out the flow of blood.
• Lumen: The inner space of the artery, allowing blood to flow freely.

Function of Arteries

1. Carry oxygenated blood (except pulmonary artery): Arteries transport blood rich in oxygen to the body's tissues.
2. High blood pressure: The thick walls are essential for handling the high pressure generated by the heart.
3. Elasticity: The elastic fibres allow arteries to expand and contract with each heartbeat, maintaining a relatively constant blood flow.

Veins

Veins are blood vessels that carry blood back to the heart. Since blood returning to the heart is at lower pressure, veins have thinner walls than arteries.

Structure of Veins

• Thin walls: Veins have thinner walls compared to arteries because the blood pressure is lower.
• Less elastic fibres: Fewer elastic fibres are present.
• Valves: Veins contain valves that prevent the backflow of blood, ensuring that blood flows in one direction towards the heart.
• Smaller lumen: Generally, veins have a smaller lumen than arteries of comparable size.

Function of Veins

1. Carry deoxygenated blood (except pulmonary vein): Veins transport blood that has returned from the body tissues and is now deoxygenated (except for the pulmonary vein, which carries oxygenated blood from the lungs).
2. Low blood pressure: The thinner walls are suitable for the lower blood pressure in veins.
3. Valves: The valves prevent backflow of blood, which is important for efficient return of blood to the heart, especially against gravity (e.g., in the veins of the legs).

Capillaries

Capillaries are the smallest blood vessels in the body. They connect arteries and veins and are the site where the exchange of substances (oxygen, carbon dioxide, nutrients, and waste products) occurs between the blood and the body's tissues.

Structure of Capillaries

• Single-walled: Capillaries consist of a single layer of endothelial cells, allowing for easy diffusion of substances.
• Thin walls: The walls are very thin (only one cell thick), minimizing the diffusion distance.
• Small diameter: The narrow diameter of capillaries forces blood cells to pass through in single file, maximizing the surface area for exchange.

Function of Capillaries

1. Exchange of substances: Capillaries facilitate the exchange of gases (oxygen and carbon dioxide), nutrients, and waste products between the blood and the body's cells.
2. Large surface area: The vast network of capillaries throughout the body provides a large surface area for efficient exchange.
3. Facilitates diffusion: The thin walls and single layer of cells allow for rapid diffusion of substances.

Comparison Table

Feature	Arteries	Veins	Capillaries
Wall Thickness	Thick	Thin	Very Thin (single layer)
Elastic Fibres	Many	Few	Absent
Lumen Size	Small	Large	Very Small
Valves	Absent	Present	Absent
Blood Pressure	High	Low	Low
Blood Carried	Oxygenated (except pulmonary artery)	Deoxygenated (except pulmonary vein)	All (oxygenated and deoxygenated)