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.
Table of Contents
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:
- Tight junctions found in fingers, skeletal muscles, and skin, which have a number of different transport vesicles.
- 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.
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.
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.
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 separate from the artery, called an embolism, and travel upwards to the brain, which can cause a stroke in some instances.
Celiac Artery: This is the artery that supplies blood to various internal organs, particularly the stomach.
Deep Vein Thrombosis (DVT): This occurs with there is clotting found inside of a vein, which is caused by either immobility or even an injury. Usually found in the leg, DVT can also occur due to abnormal clotting factors.
Diffusion: If molecules spread into an available space, this is called diffusion, and this tendency occurs because of the heat which is found in all molecules that have temperatures above absolute zero.
Endothelial Cells: These are cells that form the inner lining of two structures – the lymphatic system and the cardiovascular system. The inner layers of numerous organs also contain endothelial cells, including the heart, skin, brain, and lungs. They are responsible for creating new blood vessels.
Endovascular: This is a treatment that involves either reconstructing or repairing an artery and usually includes techniques that are minimally invasive.
False Aneurysm: This refers to when the layers of the artery are injured, which causes blood to leak outside of the artery and which is then held onto by the tissues that surround it.
Femoral Artery: The femoral artery is very large and located in the leg, extending from the hip to the knee. When physicians are performing bypass grafts, they usually start around the femoral artery.
Fibromuscular Dysplasia (FMD): This term refers to the narrowing of a particular artery and is usually caused by a spasm in the muscle layers of the artery wall, which is exacerbated by the fact that the layers are so thin.
Hypertension: Also known as high blood pressure, this occurs when the pressure within the arteries remains above a certain level, a level that is recommended to be healthy. Consistent high blood pressure, if left untreated, can increase a person’s risk of heart attacks and strokes.
Iliac Artery: The iliac artery is found deep down inside of the abdomen and carries the blood to the legs and pelvic area from the aorta.
Jugular Vein: The jugular vein is located in the neck and is very large; it is also used to place the catheters used in dialysis.
Leg Bypass: In a leg bypass, either a plastic tube or an actual vein gets sewn in place so that blood flow can be redirected around another blood vessel in the leg.
Leg Ulcers: Leg ulcers are essentially the breakdown of an area of skin, caused when there is an impairment of blood flow in open areas. They usually affect the feet and toes if they are found in the arteries and from the ankles to the knees if they are venous ulcers.
Lymphedema: This refers to a type of swelling that happens when the lymphatic vessels in the limbs are blocked, which has various causes. The usually recommended therapy involves some type of compression.
Mesenteric Artery: This is an artery that supplies blood to the intestines.
Percutaneous Access: This refers to taking a needle or catheter through the skin or order to access a vein or artery.
Peripheral Arterial Disease (PAD): A common disorder, this occurs when the linings of the artery wall become more narrow because of clots formed by the buildup of plaque or cholesterol. With PAD, you can suffer from a stroke or heart attack, or even die. It occurs in the arteries of the circulatory system, in particular, the brain, pelvis, neck, and legs.
Phlebitis: This is inflammation found in a vein, which is usually the result of some type of clot.
Plaque: A material made up of fatty substances and cholesterol, which can build up on the inner lining of an artery.
Popliteal Artery: This artery is found behind your knee and supplies blood to the lower part of the leg.
Post-Phlebitic Syndrome: This syndrome combines skin infections, leg pain, darkening of the skin due to its hardening, and open areas, all of which are the result of abnormal pressure in the veins.
Pulmonary Embolism: This term refers to a blood clot which breaks free from its attachment and travels to the lungs and heart. It is a serious condition that can cause an immediate heart attack.
Renal Artery: An artery which supplies blood to the kidney (renal area).
Renal Artery Stenosis: This refers to the narrowing of the artery that goes to the kidney.
Revascularization: This is a surgical procedure performed to restore blood flow within an artery.
Ruptured Aneurysm: A ruptured aneurysm occurs when an enlarged vessel leaks, and it typically happens in the abdominal aorta.
Saphenous Ablation: This is a surgical procedure that includes special catheters being inserted to heat-seal the vein. It is usually used for treating varicose veins and is minimally invasive.
Sclerotherapy: This is a treatment to improve or eliminate small veins, including spider and varicose veins. It usually involves injecting something so the veins can be sealed, or using a laser for the same results.
Spider Vein: Spider veins are very small varicose veins and are also called “cosmetic” veins.
Stenosis: Stenosis means that a type of blood vessel, i.e., a vein or artery, narrows or becomes thinner.
Stents: Stents are tubes made of wire mesh that is placed surgically within an artery using a catheter that is threaded through that artery. The stents are then opened and allow the artery to remain open, hopefully preventing a recurrence of the clogged artery.
Subclavian Artery: The subclavian artery branches off the aorta in the chest and supplies blood to the arm.
Superficial Phlebitis: This type of phlebitis involves Inflammation in a superficial vein, causing symptoms such as redness, pain, and a hard lump. It is usually caused by a blood clot.
Thrombosis: Thrombosis refers to any blockage in a vein or artery
Ultrasonic Duplex Scanning: This is a procedure that tests for PAD and can produce actual images of veins or arteries on a monitor, which are the result of a type of ultrasound equipment. The test can locate arteries that are blocked and even measure their size.
Ultrasound Test: A common test, ultrasounds can be used to diagnose vascular disease and can evaluate and measure anything located underneath the skin, such as kidneys, aneurysm arteries, and so on.
Varicose Veins: These veins are exceptionally large and located in the legs. They can be quite unattractive and even heavy and painful.
Vascular Medicine: This branch of medicine deals with various vascular diseases.
Vein Stripping: This is a surgical procedure that removes diseased veins through several small incisions made in the skin.
Veins: These are types of blood vessels and carry the blood to the heart from the body.
Venous Insufficiency: When a valve function deteriorates around the veins of the leg. This causes the blood to pool all around the ankle, resulting in a heavy feeling and even swelling.
Vertebral Artery: Located in the back of the neck, the vertebral artery provides extra blood to the brain. If the veins are blocked, vertebrobasilar insufficiency can result, and this can cause symptoms that are very similar to a stroke.
Vessels: These are tube-shaped structures in the circulatory system. They help circulate the blood all over the body. There are three main types of vessels: veins, arteries, and lymphatics. Capillaries are tiny structures which connect veins and arteries in the tissues.
VNUS Closure: This is a surgical procedure that is minimally invasive and designed to treat varicose veins. It is performed with a special catheter which is used to heat-seal the veins.