Capillaries are the smallest blood vessels in the body, and are the blood vessels responsible for actually delivering oxygen and other nutrients to the tissues.
Capillaries are very small. They are so small that you cannot see them without a microscope. In most cases, the walls of capillaries are only one or two cells thick, and they are so narrow that blood cells have to line up in single file to pass through them.
Depending on where they are in the body, some capillaries actually have microscopic holes. These holes are too small to allow blood to pass through, but large enough to allow passage of other molecules, like proteins, to feed tissues.
Capillaries are the site where oxygen and other nutrients in the blood are actually delivered to the tissues of the body. Capillaries are so small that these substances actually pass right through them via a process known as diffusion.
While diffusion might seem like a new concept, it is a concept that everyone is already familiar with. If you spray perfume on one side of a room, in few minutes you can smell it on the other side of the room, too. That's diffusion, and it's how cells get food.
The properties of capillaries are highly varied depending on where they are.
- Kidney capillaries have lots of microscopic holes so that substances may easily pass through them and be excreted
- Liver capillaries look almost like coiled snakes, making many loops back and forth in liver tissue to allow ample time to filter harmful materials from the blood
- Brain capillaries are very tight with almost no holes to protect fragile brain tissue
This variability in structure is because capillaries play many roles besides delivery of oxygen, and these roles are differ from organ to organ.
Continue Reading: Part Five: Veins