A simple squamous epithelium is a single layer of flat cells in contact with the basal lamina

Basal lamina

The basal lamina is a layer of extracellular matrix secreted by the epithelial cells, on which the epithelium sits. It is often confused with the basement membrane, and sometimes used inconsistently in the literature, see below....

 (basement membrane) of the epithelium. This type of epithelium is often permeable and occurs where small molecules pass quickly through membranes via filtration

Filtration

Filtration is commonly the mechanical or physical operation which is used for the separation of solids from fluids by interposing a medium through which only the fluid can pass...

 or diffusion

Diffusion

Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...

. Simple squamous epithelia are found in capillaries, alveoli, glomeruli

Glomerulus

A glomerulus is a capillary tuft that is involved in the first step of filtering blood to form urine.A glomerulus is surrounded by Bowman's capsule, the beginning component of nephrons in the vertebrate kidney. A glomerulus receives its blood supply from an afferent arteriole of the renal...

, and other tissues where rapid diffusion is required.