Capillaries: What are they? Capillary Structure, Types and Functions

They are fragile blood vessels that were discovered for the first time in the lungs of frogs in 1661.

They transport nutrients and oxygen to the tissues and eliminate the waste products.

Capillaries are a minor type of blood vessel in the body. Your job is to allow the exchange of substances between the blood and the surrounding tissues.

Capillaries are found in every square inch of the body, from the skin to the deeper tissues in the body cavities.

The blood vessels extend throughout the human body, depending on the size and weight of the person, and most of these are capillaries.

Hair structure

Capillaries are the smallest blood vessels within the human body. They form a capillary network that connects arteries and veins.

Its walls are fragile to allow substances to pass through them and spread quickly and easily.


Capillaries are much thinner than arteries and veins because their walls are formed by a single layer of endothelial cells, the fat cells that line all the blood vessels.

The capillaries are selectively permeable, which means they allow the passage of some substances but not others.

Their permeability allows them to carry out their work, and they are more or less porous according to the organ or tissue in which they are found.

Types of capillaries

The capillaries are divided into three main types, according to their pore structure:

  • Fenestrados.
  • Continuous
  • Discontinued

In the fenestrated capillaries, the endothelium varies in thickness, and in some areas, it is fragile and interrupted by circular fenestrations or numerous pores of various sizes.

They are found in the endocrine glands, the renal glomeruli, and certain vessels of the gastrointestinal tract.

The small intestine walls have fenestrated capillaries to allow digested food molecules to be transported into the blood.

Continuous capillaries are found in the muscles, heart, lungs, skin, and central nervous system.

The permeability of these capillaries is regulated by transport through and between the endothelial cells.

The narrowing that exists in the joints is the structural basis for a relative impermeability for large molecules such as plasma proteins.

In this type of capillary, the endothelium is a skinny and uninterrupted layer found in the entire circumference of the vein.

In the discontinuous capillaries, the endothelium and the basal lamina are sporadic due to fenestrations without diaphragms.

This allows the exchange between the blood and the tissue to be increased, the passage of the cells through their walls taking place.

These capillaries are found in the spleen, liver, bone marrow, specific lymphoid organs and endocrine glands.


The capillaries transfer nutrients and oxygen to the tissues and eliminate the byproducts of cellular reactions, such as carbon dioxide and water.

Capillaries that carry oxygenated blood to tissues and organs are called arterial capillaries, and when they mobilize blood without oxygen to the heart, they are called venous capillaries.

Substances such as nutrients, hormones, and oxygen are transported within the blood vessels, and the capillaries are responsible for transferring the necessary materials to the various cells of the body.

The substances are transported through the blood vessels in the blood plasma, the extracellular fluid surrounding the blood cells.

When the substances reach the capillaries, they are transferred from the plasma through the capillary membrane to the extracellular fluid and finally to the cell’s interior.

The procedures involved depend on the properties transported and occur through diffusion, osmosis, or active transport.

Diffusion is a process that allows substances with small molecules such as amino and fatty acids, vitamins, some mineral salts, and oxygen and carbon dioxide gases to travel through the capillary membrane to the extracellular fluid and finally to the interior of the cell.

These substances diffuse along a concentration gradient, which means that they always travel from an area with a higher concentration of the same meaning to a place of ​​lower concentration.

An example of diffusion is the exchange between carbon dioxide and oxygen through the capillary membrane inside the lungs.

The capillary network surrounds the alveoli walls, tiny sacs of air inside the lungs.

Both alveoli and capillary walls consist of a fragile membrane of flattened epithelial cells, allowing diffusion.

The level of carbon dioxide is higher inside the capillaries than in the alveoli; therefore, carbon dioxide crosses both membranes and enters the alveolus.

Oxygen enters the capillary blood of the alveolus since the concentration of oxygen is more significant inside the alveoli.

Osmosis works in the same way as diffusion; however, the term is only used to refer to water and its movement through the capillary and cellular membranes along a gradient of water potential.

This means that water molecules will pass through the membrane from an area of ​​higher concentration of water to a place of lower concentration.

Osmosis and diffusion are passive forms of transport, which means that no other energy is needed for the action to take place.

The term active transport implies that more energy is needed. This is necessary when the materials must move against a concentration gradient.

This applies to the movement of substances such as the mineral salts of sodium and potassium.

One example is that the intracellular fluid has a higher concentration of potassium than the extracellular fluid; therefore, for potassium to cross the capillary membrane and finally to the cell’s interior, it must move against a concentration gradient.

The extra energy needed for the transfer involved in active transport is provided by adenosine triphosphate, a high-energy molecule within cells. It allows substances to be pumped through the cell and capillary membranes in both directions.

Cell waste products do not cross with the capillary blood; They are transported inside the lymphatic capillaries, although some waste materials are eventually returned to the bloodstream.

Capillaries located in the dermis of the skin help maintain average body temperature.

Each time there is an increase in body temperature, the capillaries dilate, allowing the blood to cool as it circulates.

In conclusion, capillaries play an important role in circulation because they join arteries and veins, help maintain body temperature and transfer the necessary substances to the appropriate cells.