What epithelium does
Lining a surface sounds passive. Four of these five functions are anything but.
Lining and protecting
By lining the external and internal surfaces of the body, it protects the internal tissues from pressure, friction and micro-organisms.
Absorption
The epithelial tissue in the alimentary canal absorbs the products of digestion.
Receiving stimuli
The epithelial tissue in the tongue and the nose receives the stimuli of taste and of smell.
Secretion
The epithelial tissue lining the respiratory system secretes mucus.
Filtration
The epithelial tissue in Bowman’s capsule of the nephrons filters the blood.
The pattern behind all five
Every one of them happens at a boundary. Something crosses, or is stopped from crossing, or is detected as it arrives. That is what a surface tissue is for.
The same tissue, shaped differently each time
Absorbing needs a thin lining with a huge surface; protecting needs many tough layers; filtering needs a lining thin enough to leak on purpose. All three are epithelium, and the difference between them is the cell shape and the number of layers — which is exactly what epithelia are classified by.
Five jobs, all of them at a boundary
Epithelial tissue lines and protects, absorbs digested food, receives stimuli of taste and smell, secretes mucus, and filters blood in the nephron.