Capillaries
Introduction to blood vessels
- The circulatory system of the human body contains several different types of blood vessel:
- Arteries
- Arterioles
- Capillaries
- Venules
- Veins
- Each type of blood vessel has a specialised structure that relates to the function of that vessel
Blood vessels diagram
The circulatory system includes several blood vessels, each specialised to carry out its function
Adaptations of capillaries for exchange of materials
- Capillaries provide the exchange surface in the tissues of the body through a network of vessels called capillary beds
- The wall of a capillary is made from a single layer of endothelial cells
- Being just one cell thick reduces the diffusion distance for oxygen and carbon dioxide between the blood and the tissues of the body
- The thin endothelium cells of some capillaries have gaps between them called fenestrations which allow blood plasma to leak out and form tissue fluid
- Tissue fluid surrounds the cells, enabling exchange of substances such as oxygen, glucose, and carbon dioxide
- Tissue fluid contains oxygen, glucose and other small molecules from the blood plasma
- Large molecules such as proteins usually can't fit through the fenestrations into the tissue fluid
- The permeability of capillaries can vary depending on the requirements of a tissue
- Capillaries form branches in between the cells; this is the capillary bed
- These branches increase the surface area for diffusion of substances to and from the cells
- Being so close to the cells also reduces the diffusion distance
- Capillaries have a lumen with a small diameter
- Red blood cells squeeze through capillaries in single-file
- This forces the blood to travel slowly which provides more opportunity for diffusion to occur
- It also reduces the diffusion distance as red blood cells are brought in close contact with the capillary wall
- The wall of a capillary is made from a single layer of endothelial cells
Capillary structure diagram
Capillaries have a narrow lumen and walls that are one cell thick to increase the rate of diffusion between the blood and cells