Body Cavities

Overview of Body Cavities

Body cavities are enclosed internal spaces that house and protect vital organs. They are classified into dorsal and ventral cavities, each containing specific organs.

• Dorsal body cavity

  • Cranial cavity: Contains the brain.

  • Vertebral cavity: Contains the spinal cord.

• Ventral body cavity

  • Thoracic (chest) cavity

    • Pericardial cavity: Contains the heart.

    • Pleural cavities: Each contains a lung.

  • Abdominal cavity: Contains digestive organs.

  • Pelvic cavity: Contains urinary bladder, reproductive organs, and rectum.

Example: The thoracic cavity is separated from the abdominal cavity by the diaphragm.

Organs and Membranes

Organ Structure

Organs consist of two or more tissue types and most contain all four basic tissue types. Membranes are specialized structures lining or covering organs and body surfaces.

• Epithelial membranes: Simplest organs, lining inner and outer surfaces of the body and its organs.

• Cutaneous membranes: The skin, composed of stratified squamous epithelium (epidermis) and underlying connective tissue (dermis).

Epithelial Membranes

• Mucous membranes (mucosa)

  • Line cavities that open directly to the body exterior (e.g., digestive, respiratory, urinary, reproductive tracts).

  • Consist of:

    • Epithelial layer: Apical surface in contact with the luminal space; avascular; often contains goblet cells.

    • Lamina propria: Underlying vascular loose areolar connective tissue layer; nourishes epithelial cells.

• Serous membranes (serosa)

  • Double-layered membranes lining the abdominal and thoracic body cavities and their organs.

  • Each layer is composed of serous fluid-secreting simple squamous epithelium fused to thin areolar connective tissue.

  • Layers:

    • Visceral layer: Fused to organ wall.

    • Parietal layer: Fused to cavity wall.

• Synovial membranes

  • Located within some joints; formed of synovial fluid-secreting areolar connective tissue; no epithelium.

Serous Membrane Subtypes

• Pericardium: Surrounds the heart.

• Pleura: Surrounds the lungs.

• Peritoneum: Surrounds alimentary tract and abdominal organs.

Organ Systems

Definition and Example

An organ system is a group of organs that work together to accomplish a common purpose. The integumentary system is one such system, including the skin, hair, nails, glands, muscles, nervous system sense receptors, and hypodermis.

The Integumentary System

Skin Structure

The skin is composed of two main layers: the epidermis and the dermis. It serves as a protective barrier and is involved in sensation, temperature regulation, and synthesis of vitamin D.

• Epidermis: Superficial layer; stratified squamous epithelium; avascular; primarily composed of keratinocytes (cells producing keratin).

• Dermis: Connective tissue proper; contains blood vessels, nerves, glands, and hair follicles; divided into papillary and reticular layers.

• Hypodermis: Subcutaneous layer; composed of adipose and areolar connective tissue; loosely binds skin to underlying structures.

Epidermal Layers (Strata)

The epidermis is separated into five distinct strata (four in thin skin):

• Stratum basale: Basal layer attached to the dermis; single row of keratinocytes; stem cells divide by mitosis; contains melanocytes (produce melanin for UV protection).

• Stratum spinosum: Spiny layer; irregular shaped keratinocytes; limited mitosis; cells begin to move away from blood supply.

• Stratum granulosum: Granular layer; thin layer with granules that help form keratin and waterproofing glycolipids.

• Stratum lucidum: Clear layer; thin band of flat, dead keratinocytes (present only in thick skin).

• Stratum corneum: Surface layer; thick band of dead cells filled with keratin; glycolipids create waterproof layer; continually shed and replaced from below.

Example: Psoriasis is an autoimmune disorder causing accelerated mitosis of keratinocytes in the stratum basale, resulting in scaly plaques on the skin.

Thin vs. Thick Skin

• Thin skin: Has hair follicles, sebaceous glands, and arrector pili muscles; covers most of the body; no stratum lucidum apparent.

• Thick skin: Found only on the palm of the hand and sole of the foot; no hair follicles, sebaceous glands, or arrector pili muscles; prominent stratum lucidum.

Dermis Structure

• Papillary layer: Superficial areolar tissue; vascular; supplies epidermis with nutrients; has projections (dermal papillae and ridges) that extend into the epidermis to create fingerprints.

• Reticular layer: Forms most of the dermis; dense irregular connective tissue (lots of collagen).

Skin Colouration

• Melanin: Red, brown, and black pigment types; responsible for most variation in skin colour; produced by melanocytes and transferred to keratinocytes; freckles and moles are due to local accumulations; albinism is the inability to produce melanin.

• Hemoglobin: Blood pigment; most evident in fair-skinned people (e.g., blushing).

Epidermal Derivatives

Hair

• Hair: Dead epithelial cells containing hard keratin; has a root (within the skin) and a shaft (projects from skin).

• Hair follicles: Invagination of epidermis (root sheath) surrounded by connective tissue sheath; expanded region at base is the hair bulb.

• Hair matrix: Single layer of rapidly dividing cells; derived from stratum basale cells; site of hair growth; contains melanocytes.

• Associated structures:

  • Root hair plexus: Nerve endings.

  • Arrector pili muscle: Smooth muscle causing 'goosebumps'.

  • Hair papilla: Dermal tissue projecting into the matrix; provides blood supply and signals for regulation.

  • Sebaceous (oil) gland: Opens into follicle.

Nails

• Nails: Scale-like modification of the epidermis; highly keratinized stratum corneum cell arising from the nail matrix.

Skin Exocrine Glands

• Sebaceous (oil) glands: Found in thin skin; secrete sebum (oil, salt, protein mixture) into hair follicles; softens and lubricates hair and skin; antibacterial.

• Sudoriferous (sweat) glands: Secretion is primarily H2O; secretory portion located in dermis with a duct opening on the surface; helps regulate body temperature via evaporation; antibacterial; removes some body wastes (urea, ammonia).

• Ceruminous glands: Modified sweat glands in the auditory canals; secretion mixes with sebum to form ear wax, a substance with antimicrobial/antifungal properties.

• Mammary glands: Modified sweat glands; secrete milk, a nutritious secretion.

Table: Comparison of Membrane Types

Key Formulas and Scientific Terms

• Keratin: A fibrous protein that provides protective properties to skin, hair, and nails.

• Melanin: Pigment responsible for skin colour and UV protection.

• Serous fluid: Lubricating fluid found between serous membrane layers.

*Additional info: The notes have been expanded with academic context and definitions to ensure completeness and clarity for college-level Anatomy & Physiology students.*