kapillarer

Kapillarer are the smallest blood vessels in the body, forming a network that connects arterioles (small arteries) and venules (small veins). Their extremely narrow diameter, typically 5 to 10 micrometers, is just wide enough for red blood cells to pass through in single file. This small size is crucial for their primary function: facilitating the exchange of oxygen, nutrients, carbon dioxide, and waste products between the blood and the body's tissues.

The walls of capillaries are composed of a single layer of endothelial cells, making them incredibly thin. This thinness, combined with a large total surface area due to their vast numbers, optimizes the diffusion of substances. Oxygen and nutrients from the blood pass into the surrounding tissue fluid and then into the cells, while carbon dioxide and waste products move from the cells into the blood.

Capillary networks, also known as capillary beds, are found in virtually all tissues of the body. The flow of blood through these beds is regulated by precapillary sphincters, which are rings of smooth muscle that can contract or relax to control the amount of blood entering the capillaries. This regulation ensures that blood is directed to tissues that require it most, such as during exercise when muscles need more oxygen.

There are three main types of capillaries: continuous, fenestrated, and discontinuous (or sinusoidal). Continuous capillaries are the most common and have an uninterrupted endothelial lining. Fenestrated capillaries have small pores in their walls, allowing for more rapid exchange of fluids and small molecules, found in organs like the kidneys and intestines. Discontinuous capillaries have larger gaps between endothelial cells, enabling the passage of larger molecules and even cells, found in the liver and bone marrow.