IGCSE Biology 0610 - 8.2 Transport in Humans

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

1. Arteries

Arteries are blood vessels that carry blood away from the heart. The blood in arteries is typically oxygen-rich, with the exception of the pulmonary artery which carries deoxygenated blood to the lungs.

Structure of Arteries

• Tunica intima: The innermost layer, composed of endothelium. It is smooth and allows for easy blood flow.
• Tunica media: The middle layer, containing smooth muscle and elastic fibres. This allows the artery to constrict (narrow) and dilate (widen), regulating blood flow and blood pressure.
• Tunica adventitia: The outermost layer, composed of connective tissue. It provides support and helps anchor the artery in place.

Function of Arteries

The primary function of arteries is to transport blood under high pressure away from the heart to the rest of the body. The elastic fibres in the tunica media allow arteries to stretch and recoil with each heartbeat, maintaining a continuous flow of blood.

2. Veins

Veins are blood vessels that carry blood back to the heart. The blood in veins is typically oxygen-poor, with the exception of the pulmonary vein which carries oxygenated blood from the lungs to the heart.

Structure of Veins

• Tunica intima: Similar to arteries, with an endothelial lining.
• Tunica media: Thinner than in arteries, with less smooth muscle and elastic fibres.
• Tunica adventitia: The thickest layer, containing a lot of connective tissue.
• Valves: Veins often contain valves within their walls. These valves prevent the backflow of blood, ensuring that blood flows in one direction towards the heart.

Function of Veins

The main function of veins is to return blood to the heart. Because the blood pressure in veins is much lower than in arteries, the valves are essential to prevent backflow, especially in the limbs where blood has to travel against gravity.

3. 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) between the blood and the body's tissues takes place.

Structure of Capillaries

• Capillaries have a very thin wall, consisting of only one layer of endothelial cells. This single layer allows for easy diffusion of substances.
• The walls of capillaries are very narrow, typically just wide enough for a red blood cell to pass through in single file.

Function of Capillaries

The primary function of capillaries is to facilitate the exchange of gases, nutrients, and waste products between the blood and the surrounding tissues. Oxygen and nutrients diffuse from the blood into the tissues, while carbon dioxide and waste products diffuse from the tissues into the blood.

Summary Table

Feature	Arteries	Veins	Capillaries
Direction of Blood Flow	Away from the heart	Towards the heart	Between arteries and veins
Blood Oxygen Content	Usually oxygen-rich (except pulmonary artery)	Usually oxygen-poor (except pulmonary vein)	Both oxygen-rich and oxygen-poor
Wall Thickness	Thick (multiple layers)	Thinner than arteries	Very thin (single layer)
Smooth Muscle & Elastic Fibres	Abundant in tunica media	Less than arteries	Absent
Valves	Absent	Present (especially in limbs)	Absent
Function	Carry blood under high pressure	Return blood to the heart	Exchange of substances between blood and tissues