Capillaries

Capillaries are the smallest blood vessels in the body, with a diameter ranging from 5 to 10 micrometers. They connect arterioles (small arteries) and venules (small veins) and play a crucial role in the exchange of oxygen, carbon dioxide, nutrients, and waste products between the blood and the surrounding tissues.

Structure of Capillaries

Capillaries are composed of a single layer of endothelial cells, which are the thinnest blood vessels in the body. This thinness allows for the efficient exchange of substances between the blood and the tissues. Capillaries have a large surface area and are highly branched, enabling them to come into close contact with almost every cell in the body.

Function of Capillaries

The primary function of capillaries is the exchange of gases, nutrients, and waste products between the blood and the tissues. Oxygen and nutrients are delivered to the tissues, while carbon dioxide and waste products are picked up from the tissues by the blood for removal from the body. This exchange occurs through the thin walls of the capillaries via processes such as diffusion and filtration.

Types of Capillaries

There are three main types of capillaries: continuous, fenestrated, and discontinuous (sinusoidal) capillaries. Continuous capillaries have a complete endothelial lining with tight junctions between cells, while fenestrated capillaries have small pores or fenestrations in the endothelial cells, allowing for increased permeability. Discontinuous capillaries have large gaps between endothelial cells, allowing for the passage of larger molecules and cells.

Capillary Beds

Capillaries are organized into networks called capillary beds, which are the sites of exchange between the blood and the tissues. Precapillary sphincters, small bands of smooth muscle, control the flow of blood into the capillary beds and regulate blood flow based on the metabolic needs of the tissues.

Study Guide

1. What are the primary functions of capillaries in the body?
2. Describe the structure of capillaries and how it facilitates their function.
3. Explain the different types of capillaries and their characteristics.
4. How are capillaries organized within the body, and what controls the flow of blood into capillary beds?
5. Discuss the importance of capillaries in the exchange of substances between the blood and the tissues.