The endothelium is a layer of cells that lines the blood vessels and lymph vessels of the body. Although you probably don’t know it exists, your epithelium is vital to keeping you alive! It includes all blood-and-lymph carrying vessels, including the chambers of the heart and the glomeruli, or “filters,” of the kidneys.
Endothelial cells are a type of epithelial cell – “epithelium” being the fourth major type of animal tissue, along with connective tissue, muscle tissue, and nervous tissue.
Epithelial cells are good at selectively secreting and absorbing substances, transporting substances between cells, and sensing changes to the cellular environment. These properties make endothelial cells an excellent choice to line blood and lymph vessels.
For blood and lymph to function properly, white blood cells, water, nutrients, and other substances must be carefully balanced and maintained; the cells of the endothelium can sense changes to blood or lymph chemistry and instruct the body to react accordingly.
The endothelial cells that line blood vessels play roles including assisting with the kidney’s filtration of waste products into the urine, the release of hormones by the glands, the movement of immune cells, and the tightening or dilating of blood vessels in response to hormones like adrenaline.
Endothelial cells are squamous cells – a type of cell that is flat and has a “squashed” appearance. Squamous cells were originally named from the Latin “squama” meaning “scale,” because scientists thought these flat cells looked like the scales of a fish or snake.
An illustration of squamous cells, like those of the endothelium, can be seen here:
Function of Endothelium
You may have heard blood described as a “river of the body” along which messages and supplies between cells and organs are sent.
What you might not realize is that this river has to be tightly regulated. It can’t be allowed to get too low or too high, and decisions have to be made about which materials and messages are sent along it.
The functioning of the entire body depends on this, since cells and organs can only function if their needs are met and if they receive the proper messages through the blood.
The endothelium might not look like much if you take a look at it during a dissection or under a microscope. It lines the blood vessels, and looks a bit like cellophane plastic wrap.
But at the molecular level, the epithelium performs many important tasks that are vital to maintaining the health of our bodies.
Some of those tasks are:
- Monitoring and controlling the release of electrolytes and water into the blood.
- Controlling blood pressure, by relaxing or contracting in response to hormonal and neural signals.
- Prevents blood from clotting inside the vessels. The cells actually contain and release anti-clotting factors as needed!
- Selective permeability – it keeps things that aren’t supposed to be in the blood out, while it keeps things that are supposed to be in the blood inside the vessels. The endothelium will let white blood cells pass between the tissues and blood vessels, for example, but not bacteria.
- Growth and repair of blood vessels.
- Immune and inflammatory responses.
“Endothelial dysfunction” – when your endothelium doesn’t do some of those jobs properly – can be a symptom and cause of many diseases.
Endothelial dysfunction can play a role in high blood pressure, if the endothelial cells are not releasing proper anti-clotting factors. Endothelial problems can also lead to inflammatory immune responses that damage organs, and the formation of blood clots in the arteries.
Doctors often see endothelial dysfunction in people with cardiovascular disease, diabetes, high blood pressure, people with high cholesterol, and people who smoke.
Examples of Endothelium
Artery and Vein Lining
Arteries and veins are the major blood vessels of our bodies. Arteries are the “fast lane,” pumping huge volumes of blood at high pressures to supply our organs with the oxygen they need; veins are the slow “return lane,” where blood is returned to the heart after its oxygen has been used up.
There are some important differences between arteries and veins. Arteries have thick, muscled walls to allow their pumping action; they’re also located deep under the skin, to avoid injury. Veins on the other hand have thinner walls and are located close to the skin because injuring a vein is not as catastrophic.
Despite their differences, both have endothelial linings that monitor and regulate the chemistry of the blood. Endothelial, cells, for example, can be responsible for “telling” arteries to contract or relax depending on the body’s needs. They also control the release of water, electrolytes, and other substances into the blood.
The “endocardium” is the epithelial lining of the heart. Its name literally comes from the Greek words for “inside” and “heart.” It plays all the same important roles inside the heart as other areas of endothelium do; but it has special importance, as it protects the organ that makes the rest of the body’s life possible.
Scientists think that the endocardium controls the flow of substances between the blood and the heart, preventing harmful substances like bacteria from the blood from damaging this vital organ.
The Glomeruli of the Kidneys
The endothelial cells that help the kidneys to filter blood also have a unique role to play. Because it’s the job of the kidneys to let waste products pass out of the blood, these cells allow water, salts, and proteins to pass through freely from the blood into the urine collection system.
This is in fact why drinking a lot of water or eating a lot of salt can increase our urine output. The more water or salt in our blood, the more passes through the endothelium into the urine collection system.