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Epithelial tissues exist as covering or lining epithelia and glandular epithelia. Covering or lining epithelia function as gatekeepers, covering exposed body surfaces and lining internal organ tracts while allowing selective substances to enter and exit. Glandular epithelia are specialized structures arranged into glands that produce and release secretory substances like mucus, sweat, and hormones.

Epithelial tissues form protective sheets of closely-packed cells covering body surfaces and lining internal tracts, shielding the body from pathogenic invasion, fluid loss, and mechanical injuries. They act as the body's first line of defense against physical, chemical, and biological wear and tear. All substances entering the body must cross an epithelium, making it a critical barrier.

Epithelial tissues are richly supplied with sensory nerves that quickly relay information between external and internal environments, producing immediate responses. For example, a mosquito bite instantly stimulates nerves under the skin epithelium, triggering the body to swat at the location. This rapid sensory feedback helps protect the body from threats.

Many epithelial cells secrete mucus or other chemical compounds with specific functions. The intestinal epithelium releases digestive enzymes to help break down food, while the respiratory tract lining secretes mucus to trap incoming microorganisms and particles. These secretions support both nutrient processing and pathogen defense.

Epithelial tissues act as gatekeepers by allowing selective transfer of materials across a physical barrier. For instance, the epithelial lining of kidney tubules allows reabsorption of nutrients from urine while releasing waste products from nearby capillaries. This selective permeability ensures the body retains essential substances while eliminating waste.

Glandular tissue consists of single or multiple secretory cells that yield a variety of secretions. These glands can be unicellular or multicellular structures that discharge secretions either on the epithelial surface or into the bloodstream to reach target cells. Common secretions include mucus, sweat, digestive enzymes, and hormones.

Epithelial tissues cover all body surfaces, both internal and external, including skin, airways, the digestive tract, urinary system, and reproductive system. Hollow organs and body cavities not connected to the exterior, such as blood vessels and serous membranes, are lined by specialized epithelial tissue called the endothelium. This extensive coverage ensures comprehensive protection and selective transport.