Summary lab

Organ system

A group of organs working together toward a particular function.

Integumentary system – organs & functions

Skin, hair, nails, glands; forms external covering, protects tissues, synthesizes vitamin D, houses sensory receptors, regulates temperature.

Skeletal system – organs & functions

Bones, cartilage, joints; supports and protects organs, provides muscle attachment framework, stores minerals, produces blood cells.

Muscular system – organs & functions

Skeletal muscles; produces locomotion, maintains posture, generates heat, allows facial expression.

Nervous system – organs & functions

Brain, spinal cord, nerves, sensory receptors; fast-acting control system that detects stimuli and activates muscles/glands.

Endocrine system – organs & functions

Pituitary, pineal, thyroid, parathyroid, thymus, adrenal glands, pancreas, ovaries, testes; secretes hormones to regulate growth, metabolism, and reproduction.

Cardiovascular system – organs & functions

Heart and blood vessels; heart pumps blood, vessels transport O2, nutrients, hormones, and wastes.

Lymphatic system – organs & functions

Lymph nodes, lymphatic vessels, spleen, thymus; recovers fluid leaked from vessels, houses immune cells, removes debris.

Respiratory system – organs & functions

Nasal cavity, larynx, trachea, bronchi, lungs; supplies blood with O2 and removes CO2 via gas exchange in lung air sacs.

Digestive system – organs & functions

Oral cavity, esophagus, stomach, small/large intestine, liver, gallbladder, pancreas; breaks food into absorbable nutrients, eliminates indigestible waste as feces.

Urinary system – organs & functions

Kidneys, ureters, bladder, urethra; filters blood, produces urine, regulates fluid/electrolyte/acid-base balance.

Reproductive system – function

Production of offspring; testes produce sperm and testosterone; ovaries produce eggs and female sex hormones.

Histology

The study of tissues.

Four tissue types

Epithelial, connective, muscle, and nervous tissue.

Connective tissue – general features

Most abundant tissue type; large ECM relative to cells; functions include support, transport, fat storage, tissue repair, and binding.

Extracellular matrix (ECM)

Non-living material secreted by cells consisting of ground substance and fibers; gives connective tissue its functional properties.

Ground substance – composition

Made of glycosaminoglycans (GAGs, like hyaluronic acid), interstitial fluid, and adhesion proteins; can be liquid, gel-like, semi-solid, or solid.

Hyaluronic acid

A GAG that binds 1000x its weight in water; key to ground substance's ability to resist compression.

Three ECM fiber types

Collagen (white, tensile strength), elastic (stretch/recoil), reticular (fine meshwork support).

Areolar tissue

Loose CT; gel-like matrix; all 3 fiber types; fibroblasts, macrophages, mast cells, adipocytes; wraps/cushions organs, holds fluid; found in lamina propria under epithelia.

Adipose tissue

Loose CT; adipocytes with large lipid vacuole (ring/gem appearance); minimal visible matrix; energy storage, insulation, organ protection, hormone secretion.

Adipocyte appearance

Looks like a gemstone ring — large triglyceride vacuole minimizes cytoplasm, pushing the nucleus to the edge like a "gem."

Reticular tissue

Loose CT; predominantly reticular fibers; forms soft framework of lymphoid organs (spleen, lymph nodes).

Dense regular CT

Collagen fibers run parallel; few fibroblasts; resists force in ONE direction; found in tendons (muscle→bone) and ligaments (bone→bone).

Dense irregular CT

Collagen fibers in random orientation; resists force in MULTIPLE directions; found in dermis, joint capsules, organ capsules.

Elastic CT

Predominantly elastic fibers; found in walls of large arteries and bronchial tubes.

Cartilage – shared features

Avascular and aneural; chondrocytes in lacunae; semi-solid matrix allows nutrient diffusion; resists compression, cushions, supports.

Hyaline cartilage

Diffuse collagen; glassy/smooth; covers long bone ends, tracheal C-rings, nose tip, larynx, ribs-to-sternum; most common type.

Elastic cartilage

Elastic fibers predominate; provides structure AND flexibility; found in external ear and epiglottis.

Fibrocartilage

Thick parallel collagen bundles; greatest compression resistance; found in intervertebral discs and pubic symphysis.

Bone – key features

Calcified (hydroxyapatite) solid matrix + collagen; osteocytes in lacunae; Haversian systems (osteons); resists compression and tension; supports and protects.

Haversian system (osteon)

Concentric rings (lamellae) of matrix and cells surrounding a central canal containing blood vessels and nerves; structural unit of compact bone.

Periosteum

Layer of dense irregular CT wrapping the outer surface of bone; anchors tendons/ligaments.

Blood – key features

Fluid CT; plasma matrix; erythrocytes (flat biconcave discs), leukocytes (immune), platelets (clotting fragments); NO structural fibers; transports gases, nutrients, wastes, hormones.

Blood – what distinguishes it

Only connective tissue with a liquid (fluid plasma) matrix and no collagen, elastic, or reticular fibers.

Skeletal muscle – key features

Striated; voluntary; multinucleated; long cylindrical bundled fibers; attached to bone; responsible for locomotion and posture.

Cardiac muscle – key features

Striated; involuntary; branching cardiomyocytes; intercalated discs coordinate contraction; found only in the heart.

Smooth muscle – key features

Non-striated; involuntary; spindle-shaped, uninucleate; arranged in sheets; stimulated by stretch; produces peristalsis; found in hollow organs, blood vessels, arrector pili.

Intercalated discs

Gap junctions unique to cardiac muscle that electrically couple cardiomyocytes for coordinated, rhythmic contraction.

Peristalsis

Wave-like smooth muscle contractions that propel substances through hollow organs.

Arrector pili

Smooth muscle attached to hair follicles; contracts in response to cold or fear to produce goosebumps.

Skeletal vs. cardiac muscle – similarities

Both striated (actin/myosin); both have ECM with collagen and elastin.

Skeletal vs. cardiac muscle – differences

Skeletal is voluntary/multinucleated/cylindrical; cardiac is involuntary/branching/has intercalated discs.

Cardiac vs. smooth muscle – similarities

Both involuntary.

Cardiac vs. smooth muscle – differences

Cardiac is striated/branching/only in heart; smooth is non-striated/spindle-shaped/in hollow organs.

Epithelial tissue – key features

Avascular; tightly packed cells with little ECM; covers surfaces, lines cavities, forms glands; functions in protection, secretion, and absorption.

Epithelial cell shapes

Squamous (flat), cuboidal (cube), columnar (tall/column).

Epithelial layering types

Simple (1 layer), stratified (multiple layers), pseudostratified (all on basement membrane but nuclei at different levels).

Mucous membrane

Epithelial membrane in areas open to exterior (GI, respiratory, urogenital tracts); secretes mucus to trap/remove foreign particles; stratified squamous where friction is high, simple columnar where absorption/secretion occurs.

Serous membrane

Double-walled epithelial membrane lining closed body cavities and organ surfaces; secretes watery serous fluid to reduce friction.

Cutaneous membrane

The skin; external body covering; made of stratified squamous epithelium; dry and protective.

Parietal vs. visceral peritoneum

Parietal peritoneum lines the cavity wall; visceral peritoneum wraps individual organs; the space between them is the peritoneal cavity.

Retroperitoneal organs

Organs behind the peritoneum: kidneys, pancreas, ureters, adrenal glands, aorta, esophagus, rectum, parts of stomach and colon.

Lamina propria

Areolar CT layer just beneath mucous membranes that supports the epithelium.

Thermoregulation – skin mechanism

Sweat glands secrete sweat onto the skin surface; as sweat evaporates, heat energy is removed, cooling the body.

Alcohol vs. water on skin

Alcohol evaporates faster (lower boiling point, 82°C vs. 100°C) → removes heat faster → skin feels cooler.

Fingerprints – basis

Dermal papillae genetically determine the pattern of epidermal friction ridges; sweat pores make the pattern visible as fingerprints.

Common fingerprint patterns

Arch, loop, and whorl.

Unusual fingerprint patterns

Accidental and other mixed/irregular patterns.

Capillary refill time (CRT)

Clinical test measuring seconds for skin to return to normal color after pressure is released from a fingertip; assesses blood circulation.

Normal CRT values

Newborns ≈ 2 sec; adults ≈ 3 sec; elderly slightly > 3 sec.

Temperature effect on CRT

Warm room → vasodilation → faster refill; cold room → vasoconstriction → slower refill.

Dehydration effect on CRT

Low blood volume reduces capillary pressure, slowing refill time.

Mechanoreceptors

Somatosensory receptors in the skin (part of nervous system) that detect pressure, stretch, touch, or vibration.

Meissner's corpuscles

In upper dermis projecting into epidermis; fingertips and eyelids; detect light touch and vibration.

Pacinian corpuscles

Deep in dermis; detect deep pressure and vibration.

Ruffini endings

Deep in dermis; detect skin stretch and warmth.

Merkel's disks

At base of epidermis (papillary layer); high density on fingertips and lips; detect sustained light touch.

Two-point discrimination test (2PD)

Measures minimum distance at which two touch points can be felt separately; indicates receptor density and tactile sensitivity.

Tactile agnosia

Inability to process tactile stimuli due to neurological damage; identified with 2PD testing.

Receptor density and sensitivity

Higher receptor density = smaller discrimination distance = greater sensitivity (e.g., fingertips and lips most sensitive).

Three skin layers (superficial to deep)

Epidermis, dermis, hypodermis (subcutaneous layer).

Epidermis – key features

Outermost layer; avascular; stratified squamous epithelium; renewed every ~30 days from stratum basale upward.

Five epidermal layers (superficial to deep)

Stratum corneum, stratum lucidum (thick skin only), stratum granulosum, stratum spinosum, stratum basale.

Stratum corneum

Outermost layer; dead, flat, keratinized cells; glycolipids waterproof skin; shed by friction/washing every ~2 weeks.

Stratum lucidum

Thin clear layer of dead cells; found only in thick skin (palms, soles); provides extra durability.

Stratum granulosum

Cells flatten, fill with granules, produce keratin and lipids; cells begin to die here.

Stratum spinosum

Spiky appearance from pre-keratin filaments; contains Langerhans cells; provides strength and flexibility.

Stratum basale

Deepest epidermal layer; stem cells continuously divide to renew epidermis; contains melanocytes and Merkel cells; attached to basement membrane.

Basement membrane

Junction between epidermis and dermis; nutrients, gases, and wastes diffuse across it from dermal blood supply into avascular epidermis.

Keratinocytes

Most abundant epidermal cell; produce keratin to form the skin's barrier.

Melanocytes

In stratum basale; produce melanin to absorb UV radiation and protect DNA.

Langerhans cells

Immune cells in stratum spinosum; present antigens to the immune system.

Merkel cells

Sensory cells in stratum basale; paired with nerve endings to detect light touch.

Fibroblasts

Cells that produce collagen and protein filaments; found in the dermis.

Dermis – two regions

Papillary region (superficial) and reticular region (deep).

Reticular region of dermis

Contains main blood supply network; deeper region; collagen and elastic fibers allow skin to stretch without damage.

Dermabrasion

Surgical skin planing using a motorized rotary tool to remove epidermal layers; used for acne scars, wrinkles, hyperpigmentation, and conditions like LPPig.

Why dermabrasion causes bleeding and pain

The epidermis is avascular and aneural; bleeding and pain occur when abrasion reaches the vascularized, innervated dermis.

Goal of dermabrasion

Remove all epidermal layers down to (but not including) stratum basale to allow regeneration from stem cells.

Skin regeneration backup

If stratum basale is destroyed, hair follicles and dermal glands extending into the dermis can still contribute to epidermal regeneration.

Lichen planus pigmentosus (LPPig)

Uncommon autoimmune variant of lichen planus; T cell-mediated attack on basal keratinocytes causes brownish pigmentation patches; common in darker-skinned ethnicities; treated with dermabrasion.

Punch biopsy

Diagnostic procedure where a circular tool removes a core of skin for microscopic examination.