Simple squamous epithelium refers to a single layer of thin, flat cells that line body surfaces. One side of the surface opens to the environment while the other is anchored to the underlying cells. These cells provide a thin membrane that allows for the passage of small molecules into the body, which occurs for example when air diffuses in the lungs; when blood is filtered to form urine in the kidneys; and when nutrients diffuse from the blood into body tissues in minute blood vessels called capillaries.
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A simple squamous epithelium, called endothelium, lines the inner surfaces of arteries, veins and capillaries. In arteries and veins, endothelium reduces friction and allows for smooth blood flow. Endothelial cells in arteries and veins also aid in the constriction or dilation of the blood vessels, which regulates blood flow and pressure.
The walls of capillaries are composed of a single layer of endothelial cells. This allows for the easy exchange of nutrients and oxygen into the body tissues, and the removal of waste products.
Simple squamous epithelium lines the air sacs, or alveoli, of the lungs. The alveoli are sites where air is exchanged in the lungs. Simple squamous epithelial cells in the alveoli allow oxygen from the air to enter the blood in the capillaries of the lung. Carbon dioxide, a waste product, passes across the epithelium of the alveoli to be removed from the body.
Simple squamous epithelial cells in the kidney enable rapid filtration of the blood and diffusion of small molecules. This process allows the kidneys to remove waste products and excess water from the body in the urine.