What are capillaries and what is their primary function?

Capillaries are microscopic, thin-walled blood vessels that connect arterioles and venules. Their primary function is to facilitate the exchange of gases, nutrients, and waste products between the blood and body tissues, a process known as microcirculation.

What are the main types of capillaries and where are they found?

There are two main types of capillaries: continuous capillaries and fenestrated capillaries. Continuous capillaries have tight junctions and are found in the skin, muscles, lungs, and brain. Fenestrated capillaries have pores for greater permeability and are found in the pancreas, endocrine glands, intestinal villi, choroid plexus, and ciliary processes.

What are sinusoids and how do they differ from capillaries?

Sinusoids are large, irregular vascular channels that replace capillaries in certain organs, allowing slow blood flow for enhanced exchange. They differ from capillaries in key ways: sinusoids have wider, irregular lumens (30–40 µm) and thin walls with large, open pores and phagocytic cells, while capillaries have small, regular lumens (5–8 µm) and walls with only endothelial cells and small, diaphragm-covered pores.

Where in the body are sinusoids typically located?

Sinusoids are found in specific organs that require enhanced exchange or filtration, including the liver (with Kupffer cells), spleen, bone marrow, suprarenal glands, and parathyroid glands.

What is the role of capillary pores and how do they vary?

Capillary pores facilitate the transport of fluids and small molecules. They vary by type: in fenestrated capillaries, pores are covered by a mucoprotein diaphragm, allowing greater permeability, while in sinusoids, pores are large and open, enabling slow filtration and immune activity.

How do capillaries and sinusoids differ in their connections within the circulatory system?

Capillaries typically connect arterioles to venules, forming a direct pathway in microcirculation. In contrast, sinusoids may connect arterioles to venules or venules to venules, depending on the organ, allowing for more complex and slower blood flow patterns.

Capillaries & Sinusoids – Types and Key Differences