Integumentary System

Structure and Layers of the Skin

The skin is the largest organ of the body and consists of multiple layers, each with specialized functions. The main layers are the epidermis, dermis, and hypodermis.

• Epidermis: The superficial layer composed of stratified squamous keratinized epithelium. It is avascular and relies on diffusion of oxygen and nutrients from the dermis.

• Dermis: The deeper layer made of loose and dense irregular collagenous connective tissue.

• Hypodermis: The deepest layer, primarily composed of adipose tissue (subcutaneous tissue).

Glands of the Skin

Skin contains several types of glands that contribute to thermoregulation and other functions.

• Eccrine Sweat Glands: Most numerous, produce watery sweat for thermoregulation, active in all ages.

• Apocrine Sweat Glands: Located in axillae, anal area, and areola; secrete thick sweat rich in proteins, become active at puberty, and can produce odor when bacteria act on secretions.

Functions of the Integumentary System

The integumentary system serves several vital roles in the body:

• Protection: Acts as a barrier against mechanical damage, pathogens, and UV radiation.

• Sensation: Contains sensory receptors for touch, pain, and temperature.

• Thermoregulation: Regulates body temperature via sweat and blood flow.

• Excretion: Eliminates wastes and toxins.

• Synthesis of Vitamin D: Required for calcium ion absorption.

Negative Feedback Loops in Thermoregulation

Body temperature is regulated by negative feedback mechanisms involving the skin and hypothalamus.

• If body temperature rises, thermoreceptors in the skin send signals to the hypothalamus, which stimulates sweating and vasodilation. When temperature normalizes, sweating and vasodilation decrease.

• If body temperature drops, thermoreceptors detect the decrease and signal the hypothalamus, which triggers vasoconstriction and redirects blood to deeper tissues.

Layers of the Epidermis

The epidermis is organized into distinct layers, each with specific cell types and functions.

• Stratum basale: Deepest layer; site of most mitosis.

• Stratum spinosum: Thickest layer; provides strength.

• Stratum granulosum: Cells fill with keratin.

• Stratum lucidum: Only in thick skin (palms, soles).

• Stratum corneum: Outermost layer; cells migrate here in 40-50 days.

Mnemonic: "Come Let's Get Sun Burned" (Corneum, Lucidum, Granulosum, Spinosum, Basale).

Cell Types in the Epidermis

• Keratinoctyes: Make up 95% of epidermal cells; produce keratin for protection and water resistance.

• Dendritic (Langerhans) cells: Phagocytes of the immune system.

• Merkel cells: Detect light touch and discriminate shapes/textures.

• Melanocytes: Produce melanin pigment for UV protection.

Specialized Structures and Pigments

• Callus: Area of additional stratum corneum due to repetitive pressure.

• Carotene: Orange pigment from diet; protects from UV.

• Hemoglobin: Iron-containing pigment in blood; binds oxygen.

• Cyanosis: Blue appearance due to lack of oxygen.

• Erythema: Redness from increased blood flow.

• Pallor: Paleness from decreased blood flow.

Hair and Nails

• Hair: Filamentous structures covering most of the body except thick skin, lips, and parts of external genitalia. Arrector pili muscles cause hair to stand on end.

• Lanugo: Thin, non-pigmented hair on fetuses.

• Terminal hair: Thick, pigmented hair (scalp, eyebrows).

• Vellus hair: Thin, non-pigmented hair; replaced by terminal hair at puberty.

• Nail plate: Visible portion of the nail.

• Hyponychium: Anchors distal end of nail plate.

• Lunula: Half-moon shaped region of proximal nail.

Skin Integrity and Burns

• Wound: Disruption in skin integrity.

• First-degree burn: Least severe; red and slightly painful, no permanent damage.

• Second-degree burn: Partial thickness; blisters, may require skin grafts.

• Rule of nines: Divides body into sections for estimating burn severity.

Skin Cancer

• Basal cell carcinoma: Most common, least likely to metastasize.

• Squamous cell carcinoma: Cancer of keratinocytes; more likely to metastasize than basal cell.

• Malignant melanoma: Cancer of melanocytes; most likely to metastasize.

• ABCDE Rule: Used to distinguish skin cancer from normal moles:

  • A - Asymmetry

  • B - Border

  • C - Color

  • D - Diameter

  • E - Evolving

Skeletal System

Functions of the Skeletal System

The skeletal system provides structure, protection, and support for the body, and is involved in movement, mineral storage, and blood cell formation.

• Protection: Shields vital organs.

• Mineral storage: Stores calcium and phosphate.

• Acid-base homeostasis: Buffers blood pH.

• Blood cell formation: Occurs in bone marrow (hematopoiesis).

• Fat storage: Yellow bone marrow stores fat.

• Movement: Provides attachment for muscles.

• Support: Maintains body shape.

Classification of Bones

• Long bones: Longer than wide; e.g., femur.

• Short bones: Cube-shaped; e.g., carpal bones.

• Flat bones: Two layers of compact bone with spongy bone in between; e.g., cranial bones.

• Irregular bones: Do not fit other categories; e.g., vertebrae.

• Sesamoid bones: Located within tendons; e.g., patella.

Structure of Long Bones

• Diaphysis: Shaft of a long bone.

• Epiphysis: Ends of a long bone.

• Periosteum: Covering of the diaphysis.

• Spongy bone: Honeycomb-like structure, resists force from many directions.

• Epiphyseal line: Remnant of the growth plate.

• Bone marrow: Red marrow produces blood cells; yellow marrow stores fat.

Bone Matrix and Cells

• Extracellular matrix: Contains inorganic (strength, resistance to compression) and organic (flexibility) components.

• Osteoblasts: Bone-forming cells.

• Osteocytes: Mature bone cells.

• Osteoclasts: Bone-resorbing cells; different structure from other bone cells.

Microscopic Structure of Bone

• Osteon: Compact bone unit; cross-section resembles tree rings (lamellae).

• Lacunae: Small cavities between lamellae for osteocytes.

• Canaliculi: Small passages for communication between osteocytes.

• Trabeculae: "Ribs" of spongy bone.

Bone Formation and Growth

• Intramembranous ossification: Forms flat bones (e.g., skull).

• Endochondral ossification: Forms bones below the head (except clavicles); occurs within hyaline cartilage.

• Longitudinal growth: Growth in length.

• Appositional growth: Growth in width.

• Epiphyseal plate: Growth plate with zones for reserve cartilage, proliferation, hypertrophy, calcification, and ossification.

Hormonal Regulation of Bone Growth

• Testosterone: Increases appositional growth.

• Estrogen: Increases longitudinal growth, inhibits osteoclast activity.

Bone Remodeling and Repair

• Compression: Stimulates bone deposition.

• Tension: Stretching force.

• Calcium ion homeostasis: Regulated by negative feedback involving parathyroid hormone and calcitonin.

Calcium Regulation Equations:

• When blood calcium is low: $\text{Parathyroid hormone} \uparrow \Rightarrow \text{Osteoclast activity} \uparrow \Rightarrow \text{Blood calcium} \uparrow$

• When blood calcium is high: $\text{Calcitonin} \uparrow \Rightarrow \text{Osteoblast activity} \uparrow \Rightarrow \text{Blood calcium} \downarrow$

Bone Fractures and Healing

• Simple fracture: Skin and tissue around fracture remain intact.

• Compound fracture: Skin and tissue around fracture are damaged.

• Healing process:

  1. Hematoma forms.

  2. Fibroblasts and chondroblasts form a soft callus.

  3. Osteoblasts build a bone callus.

  4. Bone is remodeled; primary bone replaced with secondary bone.

  Typical healing time: 6-8 weeks.

Axial and Appendicular Skeleton

• Axial skeleton: Skull, vertebral column, thoracic cage.

• Appendicular skeleton: Limbs and girdles.

Skull Bones and Sutures

• Cranial bones: 8 total (2 parietal, 2 temporal, 1 frontal, 1 occipital, 1 sphenoid, 1 ethmoid).

• Facial bones: 14 total (mandible, vomer, 2 nasal, 2 palatine, 2 lacrimal, 2 zygomatic, 2 maxilla, 2 inferior nasal conchae).

• Sutures: Coronal, lambdoid, squamous, sagittal.

• Keystone bone: Sphenoid; articulates with every other cranial bone.

• Ethmoid bone: Located deep in anterior cranium between eyes and behind nasal bones.

• Fontanels: Soft spots between cranial bones in infants.

• Hyoid bone: Only bone not articulating with other bones.

Vertebral Column

• Vertebrae: 7 cervical, 12 thoracic, 5 lumbar.

• Atlas (C1): Articulates with skull.

• Axis (C2): Contains dens for head rotation.

• Transverse foramen: Found in cervical vertebrae.

• Thoracic vertebrae: Articulate with ribs.

• Lumbar vertebrae: Largest body, bear most weight.

• Spinal curvatures: Primary (thoracic, sacral) and secondary (cervical, lumbar).

• Abnormal curvatures: Lordosis (swayback), kyphosis (hunchback), scoliosis (lateral curve).

Bone Markings

• Condyle: Rounded end that articulates with another bone.

• Epicondyle: Small projection proximal to condyle.

• Foramen: Hole for passage of vessels/nerves.

• Fossa: Indentation for articulation.

• Spine: Sharp process.

• Tubercle: Small rounded projection.

• Trochanter: Large projection (femur only).

• Crest: Ridge or projection.

Joints and Cartilage

• Intervertebral discs: Cartilage pads between vertebrae; absorb shock.

• Sternum: Breastbone; consists of manubrium, body, xiphoid process.

Pectoral and Pelvic Girdles

• Pectoral girdle: Clavicle (collar bone) and scapula (shoulder blade).

• Glenoid cavity: Articulates with head of humerus.

• Acromion: Forms point of shoulder.

Upper Limb Bones

• Humerus: Arm bone.

• Radius: Forearm bone (thumb side).

• Ulna: Forearm bone (pinky side).

• Carpals: Wrist bones (scaphoid, lunate, triquetrum, pisiform, trapezium, trapezoid, capitate, hamate).

• Metacarpals: Bones of the hand.

Pelvic Girdle and Lower Limb Bones

• Pelvis: Made of ilium, ischium, pubis.

• Sex differences: Female pelvis has wider opening, shorter sacrum, more movable coccyx.

• Acetabulum: Deep socket for femur head.

• Femur: Largest, strongest bone.

• Tibia: Shinbone.

• Tarsals: Ankle bones (calcaneus, talus, navicular, cuneiforms, cuboid).

• Metatarsals: Foot bones.

• Phalanges: Toe bones; big toe has two, others have three.

Additional info: Some explanations and examples have been expanded for clarity and completeness. All key terms and processes are described to provide a comprehensive study guide for Anatomy & Physiology students.