The walls of the capillaries are so thin that the molecules can diffuse through the walls of the capillaries to the membranes of the cells that surround the capillaries.


The pulmonary capillaries allow oxygen to diffuse into the blood, while carbon dioxide is able to diffuse outward into the lungs. The arteries and veins have thick walls that do not allow the cells or molecules of the blood to spread in the body cavities.



The capillaries

Capillaries are very small blood vessels, so small that a single red blood cell can barely pass through them. They help to connect your arteries and veins, in addition to facilitating the exchange of certain elements between the blood and the tissues.


 The tissues are very active, such as muscles, liver, and kidneys, have a large number of capillaries. Metabolically less active tissues, like certain types of connective tissue, do not have as many.



Types of capillaries

Continuous capillaries

These are the most common types of capillaries. They contain small gaps between their endothelial cells that allow the passage of things like gases, water, sugar (glucose) and some hormones. However, continuous capillaries in the brain are an exception.


Types of capillaries


These capillaries are part of the blood-brain barrier. It helps protect your brain by allowing only the most essential nutrients to cross. That is why the continuous capillaries in this area do not have spaces between the endothelial cells, and the surrounding basement membrane is also thicker.



Fenestrated capillaries

Fenestrated capillaries are more “leaky” than continuous capillaries. They contain small pores, in addition to small gaps between the cells, in their walls that allow the exchange of larger molecules. This type of capillary is found in areas that require a lot of exchange between blood and tissues.


Examples of these areas include the small intestine, where nutrients absorb from food. The kidneys, where the waste products filtered from the blood.


Sinusoid capillaries

These are the rarest and “most important” capillaries. Sinusoid capillaries allow the exchange of large molecules, including cells. They can do this because they have many larger holes in their capillary wall, in addition to pores and small holes. The surrounding basement membrane is also imperfect with openings in many locations.


These types of capillaries are found in certain tissues, including those in your liver, spleen, and bone marrow. For example, in your bone marrow, these capillaries allow newly produced blood cells to enter the bloodstream and begin to circulate.



What are the functions of the capillaries?

The capillaries connect the arterial system, which includes the blood vessels that carry blood from your heart to your venous system. Your venous system includes the blood vessels that carry blood back to your heart.


The exchange of oxygen, nutrients, and waste between the blood and tissues also occurs in their capillaries. This happens through two processes:


Passive diffusion, This is the movement of a substance from an area of ​​greater concentration to an area of ​​lower concentration.

Pinocytosis, This refers to the process through which the cells in your body take small molecules, such as fats and proteins.


The walls of the capillaries are formed by a layer of thin cells. It called the endothelium that is surrounded by another thin layer called the basement membrane.


Its one-layer endothelial composition, which varies between distinct types of capillaries, and the surrounding basement membrane makes capillaries a little more “leaky” than other types of blood vessels.


This allows oxygen and other molecules to reach the cells of your body more easily. In addition, the white blood cells of your immune system can use capillaries to reach sites of infection or other inflammatory damage.