Tissues in Action · Lesson 5 of 11
Epithelial Tissues - Structure and Functions
“Your body’s linings have more job descriptions than most office teams.”
• Identify epithelial tissue as a covering or lining tissue. • Relate epithelial thickness and cell shape to function. • Compare epithelial tissues used for exchange, protection, secretion, sensation and absorption. • Connect each structural form with a location in the body. • Explain why animal tissues show functional specialisation.
Animal Tissues
Blink your eyes, take a breath, or swallow some food. These simple actions happen because different parts of the body are covered or lined by special layers of cells. Some of these layers form the outer surface of the body, such as the skin, while others line internal organs and passages such as the mouth, food pipe, lungs, blood vessels and digestive tract.
Although all of these surfaces are made from epithelial tissue, they do not all need to perform the same job. The lining of the lungs, for example, must be extremely thin so that oxygen can quickly pass into the blood and carbon dioxide can move out. In contrast, the skin must be strong and protective because it is constantly exposed to rubbing, pressure, dust, microorganisms and minor injuries.
The lining of the intestine has another requirement. It must help absorb digested nutrients efficiently. In some organs, such as parts of the respiratory tract, the surface must also help move mucus and trapped dust particles. This means that one single arrangement of epithelial cells would not be suitable for every part of the body.
To meet these different needs, epithelial tissue occurs in several forms. In some places, the cells form a single thin layer. In others, they are arranged in many layers for extra protection. The cells may also be flat, cube-shaped, tall and column-like, or may have tiny hair-like structures called cilia on their surface.
Therefore, epithelial tissue is not just one simple covering. It is a group of closely packed tissues whose shape and arrangement change according to the function they need to perform. These different forms are known as the different types of epithelial tissue.
Animal cells form specialised tissues for protection, connection, movement and communication. Four broad groups appear in this chapter: epithelial, connective, muscular and nervous tissues. Their usefulness depends on how their cells and surrounding material are arranged.
Epithelial Tissue
Epithelial tissue forms the outer covering of the body and lines internal organs and passages.
Epithelial cells are closely packed with very little space between them. This continuous arrangement limits the entry of germs, reduces water loss and separates one region from another. In other locations, a thin epithelial layer allows materials to cross quickly, or specialised cells release substances and receive sensations.
A single layer of thin, flat epithelial cells provides a short pathway for rapid diffusion of liquids and gases.
Rapid diffusion requires a short path. A single layer of thin, flat cells lines exchange surfaces in the lungs and blood vessels. Gases and liquids can cross quickly because they do not have to pass through many layers.
Several layers of tightly packed epithelial cells protect underlying tissues from friction, injury and microorganisms.
Skin, the mouth and the food passage face friction, injury and microorganisms. Several layers of tightly packed cells form a stronger barrier. Surface cells can be lost without immediately exposing the tissues below.
Specialised epithelial cells produce and release substances needed by the body.
Some epithelial cells are specialised to produce and release substances such as mucus, enzymes, hormones, sweat or saliva. They may be cuboidal or columnar and occur in salivary glands, sweat glands and the stomach lining.
Specialised epithelial receptor cells detect changes connected with smell, taste, sound and balance.
Specialised receptor cells detect changes connected with smell, taste, sound and balance. Hair-like structures can respond to movement or contact. Such epithelial cells occur in places including the nostrils, taste buds and inner ear.
Tall epithelial cells with increased surface area take up nutrients and water efficiently.
The lining of the small intestine must take up digested nutrients and water efficiently. A single layer of tall, pillar-like cells, often with many tiny projections, provides a large surface through which absorption can occur.
| Function | Suitable structure | Example location |
|---|---|---|
| Exchange | One thin layer of flat cells | Lungs and blood vessels |
| Protection | Many tightly packed layers | Skin, mouth and food passage |
| Secretion | Cells specialised for producing and releasing substances | Glands and stomach lining |
| Sensation | Specialised receptor cells, sometimes with hair-like structures | Nostrils, taste buds and inner ear |
| Absorption | One layer of tall cells with increased surface area | Small intestine |
Problem
Would a many-layered protective surface be the best design for gas exchange in the lungs?
- 1.Gas exchange depends on rapid movement across the surface.
- 2.Many layers would increase the distance gases must travel.
- 3.A thin, single layer provides a shorter route.
- 4.Therefore, the protective design suited to skin would interfere with the main job of the exchange surface.
Quiz
Which description best matches Epithelial Tissue?
Which description best matches Exchange?
Which term matches this description: Epithelial tissue forms the outer covering of the body and lines internal organs and passages.
Which term matches this description: A single layer of thin, flat epithelial cells provides a short pathway for rapid diffusion of liquids and gases.
Which statement is a key takeaway from this lesson?
Practice Problems
- Why are epithelial cells usually closely packed?
- Relate a thin epithelial layer to diffusion.
- Why does skin benefit from several layers?
- Match secretion, sensation and absorption with one suitable body location each.
- Explain why one epithelial design cannot perform every surface function equally well.
Key Takeaways
• Epithelial tissue covers the body and lines internal organs. • Closely packed cells form a continuous surface. • Thin layers favour exchange. • Multiple layers favour protection. • Specialised cells carry out secretion, sensation and absorption.