Capillary (human physiology)
Capillaries are the smallest blood vessels in the human body, crucial for the exchange of nutrients, oxygen, and waste between blood and tissues. They are so narrow that red blood cells often deform to pass through them individually. Comprising a single layer of endothelial cells, capillaries are abundant yet fragile, forming extensive networks that connect arteries and veins within the circulatory system. These vessels can be categorized into three types: continuous, fenestrated, and discontinuous. Continuous capillaries, found in areas like the brain and lungs, have tightly overlapping endothelial cells, making them the least permeable and forming part of the blood-brain barrier. Fenestrated capillaries, located in the kidneys and intestines, possess small openings that facilitate the exchange of substances more easily. Discontinuous capillaries, or sinusoidal capillaries, found in organs like the liver and spleen, have larger gaps, allowing larger cells and proteins to pass through. Through these mechanisms, capillaries play a vital role in maintaining the body’s homeostasis by efficiently transporting essential elements to tissues while removing waste products.
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Capillary (human physiology)
Capillaries are the smallest blood vessels in the body. Most capillaries are so tiny that red blood cells must bend and deform themselves just to manage to squeeze through the vessels one at a time. Capillary walls are just one endothelial cell thick. Although they are the most abundant blood vessels in the body, they are also the most fragile. The capillaries are the sites where the blood and body tissues exchange waste, nutrients, and oxygen.
Overview
The capillaries are the smallest of the body's blood vessels, which also include arteries and veins. Together, these vessels make up a major part of the circulatory system. Arteries carry oxygenated blood away from the heart, while veins bring low-oxygen blood back to the heart. The arteries become smaller as they fan out throughout the body, forming networks of capillaries. The capillary networks bring oxygen and other necessary elements directly to the body's tissues and cells. Then, the capillaries merge again to form veins, which bring the depleted blood back to the heart to start the cycle over again. Many experts compare the body's blood vessels to roadways. If the arteries and veins are like highways that transport people to major destinations, then the capillaries are like the streets that lead to people's tiny neighborhoods.
There are three types of capillaries: continuous, fenestrated, and discontinuous. Continuous capillaries are found in the central nervous system (the brain and spinal cord), the lungs, the skin, and the muscles. In these capillaries, the endothelial cells overlap one another, leaving no spaces. This makes continuous capillaries the least permeable of all three types. This is an especially important feature in the central nervous system, where continuous capillaries are part of the blood-brain barrier. This barrier prevents harmful substances that may be present in the blood from passing into the brain tissue and spinal fluid.
Fenestrated capillaries are found in tissues such as the kidneys, the small intestine, and the endocrine glands. These capillaries have patterns of endothelial cells that are grouped together more loosely, creating spaces. They also have tiny openings known as fenestrae, which make them much more permeable than continuous capillaries. This allows for an easier exchange of nutrients and wastes.
Discontinuous capillaries are sometimes called sinusoidal capillaries. These capillaries are found in the spleen, liver, and bone marrow. They have much larger openings, allowing large red blood cells and plasma proteins to pass through them. These vessels are the most permeable of all the capillaries, allowing water and solutes to enter and exit with ease.
While blood flows through the capillaries, it brings oxygen, nutrients, and other chemicals to the body's tissues. Some of these elements, such as oxygen, pass through the endothelial cell walls of the capillaries through diffusion. Others, depending on their size and the type of capillary, may pass through tiny holes in the vessel. At the same time, waste products, such as carbon dioxide, enter the blood stream through the capillaries. Eventually, after the capillaries merge to form larger veins, this blood goes back to the heart and lungs to rid itself of waste products and pick up oxygen.
Bibliography
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