3.5.6 Blood Vessels

Main Blood Vessels Connecting the Heart:

1. Aorta:

• Connected to the left ventricle.
• Carries oxygenated blood to all parts of the body except the lungs.

2. Pulmonary Artery:

• Connected to the right ventricle.
• Carries deoxygenated blood to the lungs for oxygenation and carbon dioxide removal.

3. Pulmonary Vein:

• Connected to the left atrium.
• Brings oxygenated blood back from the lungs.

4. Vena Cava:

• Connected to the right atrium.
• Brings deoxygenated blood back from the body tissues (excluding the lungs).

Structure and Function of Blood Vessels:

1. Arteries:

• Carry blood away from the heart under high pressure.
• Structure:
• Thick muscular walls: To withstand high pressure.
• Elastic tissue: Allows stretching and recoiling, smoothing blood flow.
• Smooth endothelium: Reduces friction, ensuring smooth blood flow.
• Narrow lumen: Maintains high pressure.
• Function:
• Transport oxygenated blood, except the pulmonary artery, which carries deoxygenated blood to the lungs.

2. Arterioles:

• Branch off from arteries and connect to capillaries.
• Structure:
• Thinner walls than arteries with less elastic tissue and muscle.
• Function:
• Regulate blood flow into capillaries by constricting or dilating.

3. Capillaries:

• Smallest blood vessels and the site of metabolic exchange.
• Structure:
• One cell thick walls: Short diffusion pathway for faster exchange.
• Narrow diameter: Slows blood flow, allowing efficient exchange.
• Function:
• Facilitate the exchange of oxygen, nutrients, carbon dioxide, and waste between blood and tissues.

4. Venules:

• Connect capillaries to veins.
• Structure:
• Larger lumen than capillaries.
• Function:
• Collect blood from capillaries and deliver it to veins.

5. Veins:

• Carry blood back to the heart under low pressure.
• Structure:
• Thin walls: Blood pressure is low.
• Wide lumen: Reduces resistance to blood flow and maximises blood volume carried.
• Valves: Prevent backflow of blood.
• Little elastic tissue or muscle: No need for stretching and recoiling.
• Function:
• Transport deoxygenated blood, except the pulmonary vein, which carries oxygenated blood from the lungs.

Comparison of Blood Vessels:

Feature	Arteries	Veins	Capillaries
Wall Thickness	Thick, muscular, and elastic	Thin, with little muscle or elastic	One cell thick
Lumen Size	Narrow	Wide	Very narrow
Valves	Absent (except in the heart)	Present to prevent backflow	Absent
Pressure	High	Low	Low (facilitates exchange)
Function	Transport blood away from the heart	Transport blood to the heart	Exchange of gases and nutrients

Key Definitions:

• Arteries: Vessels that carry blood away from the heart under high pressure.
• Veins: Vessels that carry blood to the heart under low pressure.
• Capillaries: Smallest blood vessels where exchange occurs.
• Valves: Structures in veins that prevent backflow of blood.