A tissue is a group of similar cells organized to perform a shared function. Tissues are the level of organization between individual cells and organs — cells assemble into tissues, tissues assemble into organs, organs assemble into organ systems.
The human body has four fundamental tissue types:
Epithelial tissue
Lines surfaces and forms barriers. The skin’s outer layer (epidermis), the lining of the airways, the lining of the gut, the lining of blood vessels — all are epithelial tissue. Epithelial tissue controls what crosses from one compartment to another: nutrients from the gut lumen into the blood, oxygen from the alveoli into the capillaries, drugs from the intestinal wall into the portal circulation. The epithelial barrier of the intestinal wall is what determines oral bioavailability.
Connective tissue
Supports, connects, and separates other tissues and organs. Connective tissue includes:
- Bone — rigid support; stores calcium and phosphorus; contains marrow where blood cells are produced
- Cartilage — flexible support in joints, airways, ears, nose
- Tendons — connect muscle to bone; transmit the force of muscle contraction
- Ligaments — connect bone to bone; stabilize joints
- Fascia — sheets of connective tissue that envelop muscles, organs, and body compartments. Structural Integration (Rolfing) works primarily with fascia, treating it as a continuous tension network that distributes force throughout the body (tensegrity)
- Blood — technically a connective tissue; plasma (fluid) with suspended cells (red blood cells, white blood cells, platelets)
- Adipose (fat) tissue — energy storage, insulation, cushioning; also acts as an endocrine organ. Fat tissue is pharmacologically significant because lipophilic drugs accumulate in it, creating a reservoir that prolongs drug effect
Muscle tissue
Contracts to produce force and movement. Three types:
- Skeletal muscle — attached to bones; voluntary control; produces movement. Somatics works primarily with skeletal muscle — pandiculation restores voluntary cortical control over skeletal muscles locked in habitual contraction patterns
- Cardiac muscle — found only in the heart; involuntary; contracts rhythmically to pump blood
- Smooth muscle — found in the walls of blood vessels, airways, gut, bladder; involuntary; controls vessel diameter, airway caliber, peristalsis. Bronchospasm (smooth muscle contraction in the airways producing wheeze) and vasodilation (smooth muscle relaxation in blood vessels lowering blood pressure) are both smooth muscle phenomena
Nervous tissue
Generates and transmits electrical and chemical signals. Composed of neurons (which carry signals) and glial cells (which support neurons). Nervous tissue constitutes the brain, spinal cord, and peripheral nerves. It is the substrate of sensation, movement, thought, and autonomic regulation — and the substrate in which pain is produced.
Tissue damage and repair
When tissue is damaged (by trauma, infection, ischemia, or chemical injury), the body initiates inflammation — a process that clears damaged cells, fights infection, and begins repair. Acute pain is the sensory experience associated with tissue damage. Whether pain persists after tissue repair depends on whether the nervous tissue itself has undergone lasting changes (central sensitization).
Related terms
- Cell — the unit of which tissues are composed
- Organ — the structures that tissues compose
- Inflammation — the tissue response to damage