Biology – The circulatory system | e-Consult

The two vessel types depicted are capillaries and arteries/veins, respectively. The capillary slide shows a single layer of flattened epithelial cells, which are endothelial cells, surrounding a narrow lumen. This structure is characteristic of a capillary. The artery/vein slide shows a thick tunica media composed of smooth muscle and elastic fibres, and a distinct tunica externa. This is characteristic of an artery or vein.

Structural Differences and Functional Relationships:

• Capillaries: Single layer of endothelial cells. This thin wall allows for efficient diffusion of gases, nutrients, and waste products between the blood and the surrounding tissues. The narrow diameter facilitates rapid exchange.
• Arteries: Thick tunica media with abundant smooth muscle and elastic fibres. This allows arteries to withstand high blood pressure generated by the heart and to expand and recoil with each heartbeat, maintaining a relatively constant blood flow. The elastic fibres allow for compliance.
• Veins: Thinner tunica media compared to arteries, with less smooth muscle and elastic fibres. The walls are thinner and less elastic. Veins have valves to prevent backflow of blood, especially in the limbs, as blood pressure in veins is much lower.