2 Tissues of the Human Body
Learn how the four primary tissue types are structured and function, and how the body responds to tissue injury.
The four primary tissues
The body has four primary types: epithelial, connective, muscle, and nervous. They combine in different ways to form organs, with each type contributing a different function.
For example, the digestive tract includes an epithelial lining, for support, smooth muscle to move its contents, and that helps regulate activity.
, also called epithelium, covers body surfaces, lines internal cavities and passageways, and forms many glands. Its cells are closely packed into one or more layers, with little material between them. Epithelium has a free surface and is attached to underlying by a . It has no blood vessels, so nutrients diffuse to it from nearby tissues.
Naming and arrangement
is named according to cell shape and the number of layers. Squamous cells are flat, cuboidal cells are roughly cube-shaped, and columnar cells are taller than they are wide. A single layer is simple; multiple layers are stratified.
Functions and examples
Protection: Stratified squamous epithelium resists abrasion in the skin and the lining of the mouth.
Absorption and secretion: Simple columnar epithelium lines much of the digestive tract.
Exchange: Thin simple squamous epithelium allows gases to pass across lung air sacs and substances to move across capillary walls.
Movement of material: Ciliated epithelium in parts of the respiratory tract helps move mucus and trapped particles.
supports, binds, protects, and connects other tissues. Unlike epithelium, its cells are usually separated by an made of ground substance and protein fibers.
Collagen fibers provide strength, elastic fibers allow stretch and recoil, and reticular fibers form a fine supporting network. The amount and composition of the matrix vary by . Blood has a fluid matrix, while bone has a hardened, mineral-rich matrix.
Types and roles
Areolar surrounds and supports organs and blood vessels.
Adipose stores energy, cushions, and insulates.
Dense is rich in collagen. It forms tendons, which attach muscle to bone, and ligaments, which connect bone to bone.
Cartilage has a firm but flexible matrix that supports structures such as the nose and cushions joints.
Bone has a mineralized matrix that provides rigid support and protection.
Blood transports gases, nutrients, wastes, and other substances.
consists of elongated, excitable cells that contract to produce force and movement. Its three forms differ in structure, control, and function.
Skeletal muscle: Long, striated fibers are usually under voluntary control. Skeletal muscle moves the skeleton, such as when lifting an arm.
Cardiac muscle: Striated, branching cells are joined at specialized connections and contract involuntarily. Cardiac muscle contracts rhythmically to pump blood and is found in the heart.
Smooth muscle: Non-striated, spindle-shaped cells contract involuntarily. Smooth muscle moves materials through hollow organs and is found in the digestive tract and blood-vessel walls.
detects information, processes it, and communicates signals. It consists of neurons, which receive and transmit electrical and chemical signals, and supporting cells called glia, which help maintain the environment around neurons and provide support.
is found in the brain, spinal cord, and nerves. Signals from sensory neurons can alert the nervous system to a hot surface, while signals to muscle help produce a response.
injury and repair
After an injury, the body limits damage and begins repair. In a typical wound, the stages overlap rather than occurring as wholly separate events.
Inflammation: Local blood flow and immune activity increase. Redness, warmth, swelling, and pain may occur as the body responds to injury and clears damaged material.
Clotting and cleanup: If blood vessels are damaged, a clot helps reduce blood loss and temporarily seals the wound. Immune cells remove microbes and cellular debris.
rebuilding: New cells may replace lost cells through . Fibroblasts also produce collagen and other matrix components, and new small blood vessels can grow into the area.
Remodeling: The repaired area strengthens and reorganizes. If the original cannot be fully restored, collagen-rich scar , or , may remain.
The result depends on the , the size and depth of the injury, and whether enough healthy cells and supporting structures remain. Surface epithelium often replaces lost cells, while deep wounds are more likely to leave scars. Repair capacity also varies among connective, muscle, and nervous tissues, and some injuries do not restore the original structure completely.
Repair may restore the original cells through , replace some lost structure with and scar, or involve both processes.