H3 BIO

From Cells to Organ Systems

Tissues: Groups of Cells with a Common Function

• There are four primary tissue types in humans:

  • Epithelial tissue

  • Connective tissue

  • Muscle tissue

  • Nervous tissue

Epithelial Tissues

Epithelial tissues form protective sheets that cover body surfaces and line internal cavities and organs. Their structure is closely related to their protective and absorptive functions.

• Classification by cell shape:

  • Squamous: Flattened, plate-like cells (e.g., lining of lungs, blood vessels).

  • Cuboidal: Cube-shaped cells (e.g., kidney tubules, glands).

  • Columnar: Tall, rectangular cells (e.g., lining of digestive tract).

• Classification by number of layers:

  • Simple epithelium: Single layer of cells (efficient for absorption and filtration).

  • Stratified epithelium: Multiple layers (provides protection, e.g., skin).

• Cell junctions:

  • Tight junctions: Create leak-proof seals between cells (e.g., intestinal lining).

  • Adhesion junctions: Anchor cells together, allowing flexibility and movement.

  • Gap junctions: Channels for direct transfer of water and ions between adjacent cells (important in cardiac muscle).

Example: The lining of the small intestine is made of simple columnar epithelium, which aids in nutrient absorption.

Connective Tissues

Connective tissues provide structural and metabolic support for other tissues and organs. They are characterized by an abundant extracellular matrix.

• Fibrous Connective Tissue:

  • Loose connective tissue: Surrounds internal organs, muscles, and blood vessels; provides cushioning and flexibility.

  • Dense connective tissue: Forms strong structures like tendons (connect muscle to bone) and ligaments (connect bone to bone).

  • Elastic connective tissue: Found in organs that change size regularly, such as the stomach and arteries.

  • Reticular connective tissue: Forms the internal framework for soft organs like the liver and lymph nodes.

• Specialized Connective Tissue:

  • Cartilage: Maintains shape of body parts and cushions joints (e.g., ear, nose, joints).

  • Bone: Provides rigid support and forms the skeleton.

  • Blood: Fluid matrix (plasma) that transports gases, nutrients, and wastes throughout the body.

  • Adipose tissue: Stores energy as fat, insulates, and cushions organs.

Example: Tendons are composed of dense fibrous connective tissue, allowing them to withstand tension during muscle contraction.

Muscle Tissues

Muscle tissues are specialized for contraction, enabling movement of body parts and substances within the body. They are classified based on structure and control mechanisms.

• Skeletal muscle: Attached to bones, responsible for voluntary movements (e.g., walking, writing).

• Cardiac muscle: Found only in the heart, contracts involuntarily to pump blood.

• Smooth muscle: Found in walls of hollow organs (e.g., blood vessels, intestines), controls involuntary movements such as peristalsis.

Example: Cardiac muscle cells are connected by gap junctions, allowing coordinated heart contractions.

Nervous Tissue

Nervous tissue forms the communication network of the body, transmitting electrical impulses to coordinate bodily functions.

• Neurons: Specialized cells that generate and conduct electrical impulses.

• Glial cells: Support, protect, and nourish neurons; maintain homeostasis in the nervous system.

Example: The brain and spinal cord are composed primarily of nervous tissue.

Organs and Organ Systems

Organs are structures composed of two or more tissue types working together for a specific function. Organ systems are groups of organs that collaborate to perform broad physiological tasks.

• Organ: Structure with at least two tissue types (e.g., the stomach contains muscle, epithelial, connective, and nervous tissues).

• Organ system: Group of organs with a common function (e.g., digestive system).

• Humans have 11 major organ systems (e.g., circulatory, respiratory, nervous, etc.).

• Body cavities:

  • Anterior cavity: Includes thoracic (chest), abdominal, and pelvic cavities.

  • Posterior cavity: Includes cranial (brain) and spinal cavities.

Example: The heart is an organ composed of muscle, connective, and nervous tissues, and is part of the circulatory system.

The Integumentary System (The Skin)

The integumentary system, primarily the skin, serves as the body's first line of defense against environmental hazards and helps regulate body temperature.

• Functions: Protection from injury, infection, and dehydration; temperature regulation; sensation.

• Layers of the skin:

  • Epidermis: Outermost layer, composed of stratified squamous epithelial cells; provides a barrier to pathogens and water loss.

  • Dermis: Inner layer of connective tissue containing blood vessels, nerves, hair follicles, and glands.

  • Hypodermis (subcutaneous layer): Layer of adipose tissue beneath the skin; insulates and cushions the body.

Example: The epidermis continuously sheds dead cells and is replaced by new cells from the basal layer, maintaining the skin's protective function.