Epithelial tissue plays a crucial role in the body, serving diverse functions that can be broadly categorized into two main groups: covering and lining epithelium, and glandular epithelium. Covering and lining epithelium is essential for protecting body surfaces and lining internal cavities and ducts. This type of epithelium has three primary functions: providing protection, regulating transport, and allowing sensations.

The protective function of covering and lining epithelium acts as a physical barrier against mechanical stresses, microorganisms, and temperature extremes. This tissue serves as a boundary, safeguarding underlying tissues from potential harm. In terms of regulating transport, covering and lining epithelium is selectively permeable, meaning it controls the movement of substances across its surface. This selectivity is vital for processes such as absorption, excretion, and filtration. For instance, in the digestive system, epithelial tissue absorbs nutrients like glucose, while excretion involves the release of waste products through urine. Filtration, particularly in the kidneys, refers to the separation of substances, ensuring that essential components are retained while waste is eliminated.

Additionally, covering and lining epithelium is involved in sensation. It connects the body to the external environment, detecting stimuli such as touch. When pressure is applied to the skin, the epithelial tissue can sense this change and activate the nervous system to process the sensation, highlighting the interplay between epithelial and nervous tissues.

The second major functional group, glandular epithelium, is specialized for secretion. Glands, formed by glandular epithelial tissue, produce and release various substances, including sweat, saliva, hormones, and enzymes. These secretions play vital roles in bodily functions, from regulating temperature to facilitating digestion.

In summary, epithelial tissue is integral to maintaining the body's integrity and functionality through its protective, regulatory, and sensory roles, as well as its ability to produce essential secretions.