3 Types of Capillaries (Plus Interesting Facts)

Capillaries are the smallest types of blood vessels. They have a diameter of around five to 10 micrometers and the thickness of the endothelial lining is only one layer of cell. When all blood vessels are combined, they reach a length of almost 100,000 km (60,000 miles) with the capillaries making up 85 percent of the length.

Capillaries link the venules and arterioles and help the body in conducting the exchange of carbon dioxide, oxygen, and water as well as waste and nutrients.

Read on to learn more about capillaries and its types.

Types

Continuous Capillaries

By “continuous,” this definition describes endothelial cells that have a lining which is uninterrupted.

Endothelial cells line the inside of lymphatic vessels and blood vessels, and, in continuous capillaries, the only things that are allowed to get through them are very small molecules, including ions and water, which flow through the intercellular clefts of the capillaries.

It is also possible for molecules that are lipid-soluble to diffuse through the membranes of the endothelial cells. The tight junctions, which are there to prevent water and solutes from leaking out, occasionally let out certain other materials to get through, such as small anions and cations. Only vertebrates have tight junctions, and they include two subtypes:

1. Tight junctions found in fingers, skeletal muscles, and skin, which have a number of different transport vesicles.
2. Tight junctions found in the central nervous system and which have very few vesicles; these capillaries are part of the blood-brain barrier, which is actually formed by endothelial cells found in the capillary walls.

Continuous capillaries can also be found in the lungs and various fat and muscle tissues, and both the endothelium and basal lamina, or the area where the endothelium sits, are continuous. These capillaries also have pinocytotic vesicles and fine filaments.

This is what a continuous capillary looks like this.

Fenestrated Capillaries

Derived from the Latin word for “window,” fenestrated capillaries have actual pores within the endothelial cells which let small molecules and small amounts of protein diffuse.

Found mostly in the glomeruli of the kidney, intestines, pancreas, and the endocrine glands, fenestrated capillaries have two types of blood vessels and can be found with or without a diaphragm that allows this protein diffusion to occur.

Fenestrated capillaries are also found in the intestinal mucosa, and they contain fenestrae or small pores which penetrate the endothelium. The pores may or may not close with the help of a diaphragm which is very thin in size.

In addition to the above locations, fenestrated capillaries can be found in intestinal villi and in some glands, which can contain tiny, microscopic pores that come in various sizes.

The fenestrated capillaries are especially important in the kidneys because they are an important part of the blood’s filtration when urine is being formed.

A fenestrated capillary looks like this.

Sinusoid (Discontinuous) Capillaries

Sinusoid capillaries are a special type of capillary, with fairly large, open pores found in the endothelium. These vessels let both red and white blood cells pass through, as well as many different serum proteins, which are assisted through the function of a discontinuous basal lamina.

Sinusoid capillaries have no pinocytotic vesicles, which means that the transfer between various endothelial cells is made through the cell junctions’ large gaps.

Sinusoid blood vessels are found in places such as the lymph nodes, adrenal glands, and even in bone marrow. Some have no tight junctions in between the cells, and this type is usually found in the liver and the spleen.

Unlike the continuous and fenestrated types of capillaries, these small blood vessels known as sinusoids have open pores. Fenestrated capillaries have structures called diaphragms that cover the pores, as opposed to sinusoid capillaries, which have no diaphragm and merely have an open pore that is exposed.

These open pores increase the permeability of the capillary, and this permeability lets small- and medium-sized proteins, including albumin, enter and exit the bloodstream.

In addition to bone marrow, lymph nodes, and the adrenal gland, sinusoid capillaries can also be found in the spleen, liver, and endocrine organs.

Proteins can pass through the wall of this type of capillary much in the same way as colanders strain foods through the device. In addition, they are different from the sinusoids found in parts of the placenta.

Here is a photograph of a sinusoidal capillary.

What are Capillaries?

Capillaries are one type of blood vessel, the other two being veins and arteries. The capillaries can be found in between the veins and the arteries. They connect these two things and can be considered an exchange stops.

Capillaries remove nutrients and oxygen from the blood found in the arteries and distribute them to the tissues throughout the body. They are also there to place cellular waste into the veins, including carbon dioxide.

Capillaries are essential to the circulatory system. In fact, if you press a pink area of your skin, blood is pressed out of the capillaries, which causes blanching. Capillaries, therefore, are involved in hundreds of actions such as these.

Comparisons

Arteries, Veins, and Capillaries

• Arteries – carry blood away from the heart; this is oxygenated blood except for the pulmonary artery.
• Veins – carry blood to the heart; this is deoxygenated blood except for the pulmonary vein.
• Capillaries – deliver oxygen to the cells and connects the arteries and veins; carry blood from the branches of the arteries to the branches of veins.

Structures of the Different Blood Vessels

• Arteries – an outer layer of thick collagen, the middle layer of elastic fiber layers, and the inner layer of squamous epithelium layer
• Veins – an outer layer of thin collagen and elastic fiber layers, and an inner layer of squamous epithelium layer
• Capillaries – a single layer of squamous epithelium cells; no elastic fibers or collagen

What are They Made Of?

• Arteries – thick muscular walls; small passageways for blood (called internal lumen); contain blood under high pressure
• Veins – thin walls; small passageways for blood (internal lumen); contain blood under low pressure
• Capillaries – narrow with thin walls; very low blood pressure; microscopic (one cell thick)

What is Capillary Action?

Capillary action is the spontaneous flow of a liquid of any type through either a porous material or even a narrow tube. The action is not dependent upon gravity; in fact, the movement can occur in spite of and in opposition to gravity. Examples of capillary action include:

• Water moving through sand
• Water is absorbed in plaster and paper, which are both porous
• When paint wicks through the hairs of a paintbrush
• When you place a stalk of celery in a glass with water and food coloring in it and the celery begins to turn the same color as the food coloring

Capillary action needs two things to occur: adhesive forces, which are found between the tube material and the liquid; and cohesive forces, which are found in the liquid itself.

Labeled as types of intermolecular forces, adhesion and cohesion pull the liquid into the tube; however, for wicking to occur (as in the example with the paintbrush), the tube must be very small in diameter.

Interesting Facts about Capillaries

• There are over 10-billion capillaries throughout your body.
• Capillaries are the smallest type of blood vessel. They are so small, in fact, that they are only visible when you view them under a microscope.
• Capillaries are so tiny that even the largest type of capillary is thinner than a strand of hair – roughly .2 mm in width.
• Capillaries were discovered in 1661, so they’ve been familiar to us for a very long time.
• Most capillaries are between .5mm to 1mm in length.
• Since capillary walls are just one cell thick, it is easy for certain chemicals to pass through them. In fact, through these walls is where your blood passes food, oxygen, and waste to and from each of the other cells in your body.
• Capillaries are found everywhere, including ligaments and tendons; however, there are more in areas such as liver, kidneys, and various muscles than there are in ligaments and tendons.
• Capillaries are able to supply either more or less blood, depending on your specific needs, making them both versatile and important. They can save heat when you are cold and carry less to keep blood away from the surface whenever you are warm.
• If you took all of the veins, arteries, and capillaries in your body and laid them end to end, they would go around the earth more than two full times, because there are roughly 60,000 miles of them.
• Because of a capillary’s small size, red blood cells must pass through it single-file; although some are actually smaller than the red blood cells, meaning the latter has to distort its shape to fit through the capillary.
• The bigger the body, the slower its heart beats. A blue whale’s heart only beats around five times a minute, while the average human has a heartbeat of around 75 beats per minute.
• The heart continues to beat even if it isn’t inside a body, as long as it receives oxygen.
• Because the circulatory system is so complex, medical doctors actually followed an incorrect model of it for roughly 1500 years.
• Red blood cells die after about 120 days, even though your bone marrow immediately starts to make more of them after that time.
• There really is a way to “break” a heart. Stress cardiomyopathy can cause the heart to weaken temporarily and quickly, even causing symptoms that mimic a heart attack, such as an achy feeling in the arm, shortness of breath, and even chest pain. This can happen when you suddenly lose a loved one or even break up with someone.
• Blood can be various colors – except blue. The blood that goes through your capillaries and veins is actually bright-red in color, and it becomes a darker red when it races back to your heart. Veins may make your blood appear to be blue, but that is due to the way various types of light penetrate your skin and reflect the eyes.
• Having said that, some animals do actually have bluish-colored blood, including mollusks and some arthropods.

You might also like our article about “Different Types of Neurons”

Glossary of Vascular Terms

Artery: A blood vessel which carries blood from the heart to the rest of the body. If an artery is diseased, the organ that is supposed to be receiving the blood can become damaged because there are no nutrients or oxygen supplied to it.

Buerger’s Disease: This disease involves inflammation in the arms and legs and is mostly found in people who smoke.

Brachial Artery: This artery is found in the arm and is usually used in dialysis to create AV fistulas.

Calcified Vessels: Calcified vessels occur when arteries harden due to deposits of calcium in the wall. They directly affect the ability to make certain pressure measurements in the legs, and they are very often seen in diabetic patients.

Carotid Artery: This is a major artery whose function is to provide blood from the heart to the brain. Located in the front of the neck, it can become diseased if enough plaque develops in the neck area. This action causes the carotid artery to sep