Comprehensive Study Guide: Human Anatomy and Physiology

Anatomy and Physiology Defined

• Anatomy: The science of body structures and the relationships among them. First studied by dissection (dis- = apart; -section = act of cutting), the careful cutting apart of body structures to study their relationships.

• Physiology: The science of body functions—how the body parts work.

• Structure and function are intimately connected; the structure of a body part reflects its functions (e.g., tight skull bones protect the brain, thin lung air sac walls permit rapid gas exchange).

• Selected Branches of Anatomy:

  • Embryology: The first eight weeks of development after fertilization of a human egg.
  • Developmental biology: The complete development of an individual from fertilization to death.
  • Cell biology: Cellular structure and functions.
  • Histology: Microscopic structure of tissues.
  • Gross anatomy: Structures that can be examined without a microscope.
  • Systemic anatomy: Structure of specific systems of the body (e.g., nervous or respiratory systems).
  • Regional anatomy: Specific regions of the body (e.g., head or chest).
  • Surface anatomy: Surface markings of the body to understand internal anatomy through visualization and palpation (gentle touch).
  • Imaging anatomy: Internal body structures visualized with techniques such as X-rays, MRI, CT scans.
  • Pathological anatomy: Structural changes (gross to microscopic) associated with disease.

• Selected Branches of Physiology:

  • Molecular physiology: Functions of individual molecules such as proteins and DNA.
  • Neurophysiology: Functional properties of nerve cells.
  • Endocrinology: Hormones (chemical regulators in blood) and how they control body functions.
  • Cardiovascular physiology: Functions of the heart and blood vessels.
  • Immunology: Body's defenses against disease-causing agents.
  • Respiratory physiology: Functions of air passageways and lungs.
  • Renal physiology: Functions of the kidneys.
  • Exercise physiology: Changes in cell and organ functions due to muscular activity.
  • Pathophysiology: Functional changes associated with disease and aging.

Levels of Structural Organization and Body Systems

• The human body is organized into six structural levels:
  • Chemical Level: Includes atoms (smallest units of matter participating in chemical reactions, e.g., Carbon, Hydrogen, Oxygen, Nitrogen, Phosphorus, Calcium, Sulfur) and molecules (two or more atoms joined together, e.g., DNA, glucose).
  • Cellular Level: Molecules combine to form cells, the basic structural and functional living units of an organism. Examples include muscle cells, nerve cells, and epithelial cells.
  • Tissue Level: Groups of cells and surrounding materials working together to perform a particular function. Four basic tissue types:
  • Epithelial tissue: Covers body surfaces, lines hollow organs and cavities, forms glands.
  • Connective tissue: Connects, supports, and protects body organs while distributing blood vessels.
  • Muscular tissue: Contracts to make body parts move and generates heat.
  • Nervous tissue: Carries information via nerve impulses.
  • Organ Level: Structures composed of two or more different types of tissues; have specific functions and recognizable shapes (e.g., stomach, skin, heart, liver, lungs, brain).
  • System (Organ-System) Level: Consists of related organs with a common function (e.g., digestive system). Some organs belong to more than one system (e.g., pancreas in digestive and endocrine systems).
  • Organismal Level: Any living individual; all body systems functioning together.

• 11 Systems of the Human Body:
  • Integumentary System: Skin, hair, nails, sweat and oil glands. Protects body, regulates temperature, eliminates waste, synthesizes vitamin D, detects sensations.
  • Skeletal System: Bones, joints, cartilages. Supports and protects body, provides muscle attachment surface, aids body movements, stores blood-cell-producing cells and minerals/fats.
  • Muscular System: Skeletal muscle tissue. Produces body movements, stabilizes posture, generates heat.
  • Nervous System: Brain, spinal cord, nerves, special sense organs. Generates action potentials (nerve impulses) to regulate body activities, detects internal/external environmental changes, interprets changes, causes muscular contractions or glandular secretions.
  • Endocrine System: Hormone-producing glands (pituitary, thyroid, pancreas, adrenal, gonads) and hormone-producing cells. Regulates body activities by releasing hormones.
  • Cardiovascular System: Blood, heart, blood vessels. Heart pumps blood through blood vessels; blood carries oxygen/nutrients to cells and carbon dioxide/wastes away; regulates acid-base balance, temperature, water content; blood components defend against disease and repair damaged vessels.
  • Lymphatic System: Lymphatic fluid/vessels, spleen, thymus, lymph nodes, tonsils, B cells, T cells. Returns proteins/fluid to blood, carries lipids from GI tract to blood, contains sites of maturation/proliferation of B cells and T cells that protect against disease.
  • Respiratory System: Lungs, pharynx, larynx, trachea, bronchial tubes. Transfers oxygen from inhaled air to blood and carbon dioxide from blood to exhaled air; regulates acid-base balance of body fluids; produces sound.
  • Digestive System: Mouth, pharynx, esophagus, stomach, small and large intestines, anus, salivary glands, liver, gallbladder, pancreas. Achieves physical and chemical breakdown of food, absorbs nutrients, eliminates solid wastes.
  • Urinary System: Kidneys, ureters, urinary bladder, urethra. Produces, stores, eliminates urine; eliminates wastes; regulates volume and chemical composition of blood; maintains acid-base balance, mineral balance, and red blood cell production.
  • Reproductive System: Gonads (testes, ovaries) and associated organs (epididymis, ductus deferens, penis; uterine tubes, uterus, vagina), mammary glands. Gonads produce gametes (sperm, oocytes) and release hormones that regulate reproduction and other body processes.

Characteristics of the Living Human Organism

• Six Basic Life Processes:

  • Metabolism: Sum of all chemical processes in the body.
  • Catabolism: Breakdown of complex chemical substances into simpler components (e.g., splitting proteins into amino acids).
  • Anabolism: Building up of complex chemical substances from smaller, simpler components (e.g., building new structural proteins).
  • Responsiveness: Ability to detect and respond to changes in internal or external environments.
  • Movement: Motion of the whole body, individual organs, single cells, or structures within cells.
  • Growth: Increase in body size resulting from an increase in cell size, cell number, or amount of intercellular material.
  • Differentiation: Development of a cell from an unspecialized state to a specialized state. Precursor cells that divide and undergo differentiation are stem cells.
  • Reproduction: Formation of new cells for tissue growth, repair, or replacement, OR the production of a new individual.

• Clinical Noninvasive Diagnostic Techniques:

  • Inspection: Observing the body for any deviations from normal.
  • Palpation: Feeling body surfaces with hands (e.g., palpating abdomen for enlarged organs).
  • Auscultation: Listening to body sounds to evaluate organ function (e.g., using a stethoscope to listen to lungs or heart).
  • Percussion: Tapping body surfaces with fingertips and listening to resulting sounds (hollow vs. solid).

• Autopsy (Necropsy): Postmortem examination and dissection of internal organs to determine cause of death, uncover undetected diseases, or educate students.

Homeostasis

• Homeostasis: Maintenance of relatively stable conditions in the body's internal environment through dynamic regulatory processes.
• Body Fluids: Dilute, watery solutions containing dissolved chemicals inside and outside cells.
  • Intracellular Fluid (ICF): Fluid within cells.
  • Extracellular Fluid (ECF): Fluid outside cells.
  • Interstitial Fluid: ECF filling narrow spaces between tissue cells (serves as the body's internal environment).
  • Blood Plasma: ECF within blood vessels.
  • Lymph: ECF within lymphatic vessels.
  • Cerebrospinal Fluid (CSF): ECF in/around brain and spinal cord.
  • Synovial Fluid: ECF in joints.
  • Aqueous Humor & Vitreous Body: ECF in the eyes.

• Control of Homeostasis: Regulated primarily by the nervous system (rapid nerve impulses) and endocrine system (slower-acting hormones).
• Feedback Systems (Feedback Loops):
  • Controlled Condition: A monitored variable (e.g., body temperature, blood pressure, blood glucose).
  • Stimulus: Any disruption that alters a controlled condition.
  • Components of a Feedback System:
  • Receptor: Body structure monitoring changes; sends input via an afferent pathway (toward control center).
  • Control Center: Evaluates input, sets set point, generates output commands via an efferent pathway (away from control center).
  • Effector: Receives output, produces a response that alters controlled condition.

• Negative Feedback Systems: Reverses a change in a controlled condition (e.g., regulation of blood pressure).
  • Example: Increase in blood pressure $\rightarrow$ detected by baroreceptors in blood vessels $\rightarrow$ nerve impulses to brain $\rightarrow$ output to heart and blood vessels $\rightarrow$ heart rate decreases and blood vessels dilate $\rightarrow$ blood pressure drops back to normal.

• Positive Feedback Systems: Strengthens or reinforces a change in a controlled condition (requires an outside event to shut it off).
  • Example: Childbirth labor contractions $\rightarrow$ fetus pushed into cervix $\rightarrow$ stretch-sensitive nerve cells in cervix send impulses to brain $\rightarrow$ oxytocin released $\rightarrow$ uterine wall muscles contract more forcefully $\rightarrow$ further stretching $\rightarrow$ continues until baby is born.
  • Other examples: Blood loss escalation, blood clotting.

• Homeostatic Imbalances:
  • Disorder: Any abnormality of structure or function.
  • Disease: Specific illness characterized by a recognizable set of signs and symptoms.
  • Local disease: Affects one part or limited region.
  • Systemic disease: Affects entire body or several parts.
  • Symptoms: Subjective changes not apparent to an observer (e.g., headache, nausea, anxiety).
  • Signs: Objective changes that can be observed and measured (e.g., rash, swelling, fever, high blood pressure).
  • Epidemiology: Science of why, when, and where diseases occur and how they are transmitted.
  • Pharmacology: Science dealing with effects and uses of drugs in disease treatment.
  • Diagnosis: Science and skill of distinguishing one disorder/disease from another using medical history, physical examination, vital signs, and lab tests.

Basic Anatomical Terminology

• Anatomical Position: Standard reference position where the subject stands erect facing the observer, head level, eyes forward, lower limbs parallel, feet flat on floor forward, upper limbs at sides with palms facing forward.

  • Prone: Lying facedown.
  • Supine: Lying faceup.

• Major Body Regions:

  • Head (Cephalic): Skull (cranial) and face (facial).
  • Neck (Cervical): Supports head, attaches to trunk.
  • Trunk: Chest (thoracic), abdomen (abdominal), pelvis (pelvic).
  • Upper Limb: Shoulder, armpit (axillary), arm (brachial), front of elbow (antecubital), forearm (antebrachial), wrist (carpal), hand (manual: palmar/volar, dorsum, pollex/thumb, digital/phalangeal/fingers).
  • Lower Limb: Buttock (gluteal), thigh (femoral), anterior knee (patellar), hollow behind knee (popliteal), leg (crural), calf (sural), ankle (tarsal), foot (pedal: heel/calcaneal, sole/plantar, top/dorsum, hallux/great toe, digital/phalangeal/toes).
  • Groin (Inguinal): Front surface crease where trunk attaches to thighs.

• Directional Terms:
  • Superior (Cephalic/Cranial): Toward head or upper structure.
  • Inferior (Caudal): Away from head or lower structure.
  • Anterior (Ventral): Nearer to or at front of body.
  • Posterior (Dorsal): Nearer to or at back of body.
  • Medial: Nearer to midline.
  • Lateral: Farther from midline.
  • Intermediate: Between two structures.
  • Ipsilateral: On same side of body.
  • Contralateral: On opposite side of body.
  • Proximal: Nearer to attachment of limb to trunk or origination of structure.
  • Distal: Farther from attachment of limb to trunk or origination.
  • Superficial (External): Toward or on surface of body.
  • Deep (Internal): Away from surface of body.

• Planes and Sections:
  • Sagittal plane: Vertical plane dividing body/organ into right and left sides.
  • Midsagittal (Median) plane: Passes through midline, dividing into equal right and left sides.
  • Parasagittal plane: Divides into unequal right and left sides.
  • Frontal (Coronal) plane: Divides body/organ into anterior and posterior portions.
  • Transverse (Cross-sectional/Horizontal) plane: Divides body/organ into superior and inferior portions.
  • Oblique plane: Passes through body/organ at an angle other than $90^\circ$.
  • Section: A cut made along a plane.

• Body Cavities:
  • Cranial Cavity: Formed by cranial bones, contains brain.
  • Vertebral Canal: Formed by vertebral column, contains spinal cord. Lined by three protective tissue layers called meninges.
  • Thoracic Cavity (Chest Cavity): Enclosed by ribs, sternum, chest muscles, thoracic vertebrae.
  • Pericardial Cavity: Fluid-filled space surrounding heart.
  • Pleural Cavities: Two fluid-filled spaces surrounding each lung.
  • Mediastinum: Central portion of thoracic cavity between lungs, extending from sternum to vertebral column and first rib to diaphragm. Contains heart, esophagus, trachea, thymus, large blood vessels.
  • Diaphragm: Dome-shaped muscle separating thoracic and abdominopelvic cavities.
  • Abdominopelvic Cavity: Extends from diaphragm to groin.
  • Abdominal Cavity (superior): Contains stomach, spleen, liver, gallbladder, small intestine, most of large intestine.
  • Pelvic Cavity (inferior): Contains urinary bladder, portions of large intestine, internal reproductive organs.
  • Viscera: Organs within thoracic and abdominopelvic cavities.

• Thoracic and Abdominal Cavity Membranes:

  • Serous Membrane: Double-layered, slippery membrane lining cavity walls and covering viscera.
  • Parietal layer: Lines cavity walls.
  • Visceral layer: Covers viscera.
  • Serous fluid: Lubricating fluid between layers.
  • Pleura: Serous membrane of pleural cavities (Visceral & Parietal pleura).
  • Pericardium: Serous membrane of pericardial cavity (Visceral & Parietal pericardium).
  • Peritoneum: Serous membrane of abdominal cavity (Visceral & Parietal peritoneum).
  • Retroperitoneal Organs: Organs posterior to peritoneum (kidneys, adrenal glands, pancreas, duodenum, ascending/descending colons, abdominal aorta, inferior vena cava).

• Abdominopelvic Regions and Quadrants:

  • Nine Regions (Tic-tac-toe grid using subcostal, transtubercular, and left/right midclavicular lines): Right hypochondriac, Epigastric, Left hypochondriac, Right lumbar, Umbilical, Left lumbar, Right inguinal (iliac), Hypogastric (pubic), Left inguinal (iliac).
  • Four Quadrants (Midsagittal and transumbilical lines crossing at umbilicus): Right Upper Quadrant (RUQ), Left Upper Quadrant (LUQ), Right Lower Quadrant (RLQ), Left Lower Quadrant (LLQ).

The Chemical Level of Organization

• Matter, Mass, and Weight:

  • Matter: Anything occupying space and having mass (solids, liquids, gases).
  • Mass: Amount of matter in an object (constant).
  • Weight: Force of gravity acting on mass (variable).

• Chemical Elements:

  • 118 total known elements (92 occur naturally).
  • 26 elements in human body:
  • 4 Major Elements ($96\%$ body mass): Oxygen ($65.0\%$), Carbon ($18.5\%$), Hydrogen ($9.5\%$), Nitrogen ($3.2\%$).
  • 8 Lesser Elements ($3.6\%$ body mass): Calcium ($1.5\%$), Phosphorus ($1.0\%$), Potassium ($0.35\%$), Sulfur ($0.25\%$), Sodium ($0.2\%$), Chlorine ($0.2\%$), Magnesium ($0.1\%$), Iron ($0.005\%$).
  • 14 Trace Elements ($0.4\%$ body mass): Al, B, Cr, Co, Cu, F, I, Mn, Mo, Se, Si, Sn, V, Zn.

• Atomic Structure:

  • Atoms: Smallest units of matter retaining element properties.
  • Nucleus: Dense central core containing Protons ($p^+$) and Neutrons ($n^0$).
  • Electrons ($e^-$): Move in an electron cloud around nucleus within electron shells.
  • Shell capacities: 1st shell = max 2, 2nd shell = max 8, 3rd shell = max 18.
  • Atomic Number: Number of protons in nucleus.
  • Mass Number: Sum of protons and neutrons.
  • Isotopes: Atoms of an element with different numbers of neutrons (same atomic number, different mass number).
  • Radioisotopes: Unstable radioactive isotopes that decay. Half-life is time for half of radioisotope to decay.
  • Atomic Mass (Daltons / amu): Average mass of all naturally occurring isotopes of an element.

• Ions, Molecules, Compounds, and Free Radicals:

  • Ion: Atom with a positive charge (cation, lost electrons) or negative charge (anion, gained electrons). Ionization is electron loss/gain.
  • Molecule: Two or more atoms sharing electrons ($\text{O}_2$, $\text{H}_2\text{O}$).
  • Compound: Substance containing atoms of two or more different elements ($\text{H}_2\text{O}$, $\text{NaCl}$).
  • Free Radical: Atom or group of atoms with an unpaired electron in outermost shell (e.g., superoxide $\text{O}_2^{\bullet-}$). Highly reactive and destructive. Antioxidants inactivate free radicals (vitamins C, E, beta-carotene, selenium, zinc).

Chemical Bonds

• Valence Shell: Outermost electron shell determining bonding likelihood.
• Octet Rule: Atoms interact to achieve 8 valence electrons (chemically stable configuration).
• Types of Chemical Bonds:
  • Ionic Bonds: Force of attraction holding oppositely charged ions together. In solution, ionic compounds break into ions called electrolytes.
  • Covalent Bonds: Two or more atoms share pairs of valence electrons (single, double, or triple).
  • Nonpolar Covalent Bond: Equal sharing of electrons.
  • Polar Covalent Bond: Unequal sharing due to differences in electronegativity (e.g., $\text{H}_2\text{O}$ where oxygen is $\delta^-$ and hydrogen is $\delta^+$).
  • Hydrogen Bonds: Weak attraction between partial positive charge ($\delta^+$) of hydrogen and partial negative charge ($\delta^-$) of neighboring electronegative atoms (O or N). Provides cohesion and surface tension in water; stabilizes large 3D molecules (proteins, nucleic acids).

Chemical Reactions and Energy

• Chemical Reaction: Formation or breaking of chemical bonds between atoms.

  • Metabolism: All chemical reactions in the body.
  • Reactants: Starting substances ($2\text{H}_2 + \text{O}_2$).
  • Products: Ending substances ($2\text{H}_2\text{O}$).

• Forms of Energy:

  • Potential Energy: Stored energy due to position.
  • Kinetic Energy: Energy associated with matter in motion.
  • Chemical Energy: Potential energy stored in chemical bonds.
  • Law of Conservation of Energy: Energy is neither created nor destroyed, only converted.

• Energy Transfer:

  • Exergonic Reactions: Release more energy than they absorb (catabolism).
  • Endergonic Reactions: Absorb more energy than they release (anabolism).
  • ATP Coupling: Exergonic reactions drive endergonic ones via adenosine triphosphate ($\text{ATP}$).

• Activation Energy: Initial collision energy investment needed to break chemical bonds.

  • Influenced by concentration and temperature.
  • Catalysts / Enzymes: Speed up reactions by lowering activation energy without being altered.

Inorganic Compounds and Solutions

• Inorganic vs. Organic Compounds:

  • Inorganic: Usually lack carbon, structurally simple, ionic or covalent bonds ($\text{H}_2\text{O}$, salts, acids, bases, $\text{CO}_2$, $\text{HCO}_3^-$).
  • Organic: Always contain carbon, usually hydrogen, always covalent bonds.

• Water ($\text{H}_2\text{O}$): Most abundant inorganic compound ($55\text{--}60\%$ body mass).

  • Solvent: Polarity creates hydration spheres around ions. Hydrophilic (polar/charged) vs Hydrophobic (nonpolar).
  • Reactions: Medium for reactions; participates in hydrolysis (adding water to break bonds) and dehydration synthesis (removing water to form bonds).
  • Thermal Properties: High heat capacity (absorbs/releases large heat with small temp change), high heat of vaporization (cooling via evaporation).
  • Lubricant: Major component of mucus, synovial fluid, serous fluid.

• Mixtures:

  • Solution: Transparent, solute particles tiny and evenly dispersed, do not settle out.
  • Colloid: Translucent/opaque, larger solute particles scatter light, do not settle out (e.g., milk).
  • Suspension: Suspended material mixes temporarily, eventually settles out (e.g., blood).

• Concentration:

  • Mass per Volume %: Grams solute per $100\,\text{mL}$ solution.
  • Molarity (M): Moles per liter ($\text{mol/L}$). A mole is mass in grams equal to atomic/molecular mass. $1\,\text{mole} = 6.023 \times 10^{23}$ particles (Avogadro's number).

• Inorganic Acids, Bases, and Salts:

  • Acid: Dissociates into $\text{H}^+$ (proton donor) and anions ($\text{HCl} \rightarrow \text{H}^+ + \text{Cl}^-$).
  • Base: Removes $\text{H}^+$ from solution (proton acceptor); dissociates into $\text{OH}^-$ and cations ($\text{KOH} \rightarrow \text{K}^+ + \text{OH}^-$).
  • Salt: Dissociates into cations and anions, neither of which is $\text{H}^+$ or $\text{OH}^-$ ($\text{KCl} \rightarrow \text{K}^+ + \text{Cl}^-$).

• Acid-Base Balance and pH:

  • pH Scale: $0\text{--}14$ scale measuring $\left[\text{H}^+\right]$ concentration in $\text{mol/L}$. $\text{pH} = -\log_{10}\left[\text{H}^+\right]$.
  • Neutral: $\text{pH} = 7$ ($\left[\text{H}^+\right] = \left[\text{OH}^-\right] = 10^{-7}\,\text{mol/L}$).
  • Acidic: $\text{pH} < 7$ ($\left[\text{H}^+\right] > \left[\text{OH}^-\right]$).
  • Basic (Alkaline): $\text{pH} > 7$ ($\left[\text{H}^+\right] < \left[\text{OH}^-\right]$).
  • Blood pH: Normal range $7.35\text{--}7.45$. Acidosis ($<7.35$), Alkalosis ($>7.45$).
  • Buffers: Convert strong acids/bases into weak ones by binding/releasing $\text{H}^+$.
  • Carbonic acid-bicarbonate buffer system:
    • Excess $\text{H}^+$ (acidic): $\text{H}^+ + \text{HCO}_3^- \rightarrow \text{H}_2\text{CO}_3$
    • Shortage $\text{H}^+$ (alkaline): $\text{H}_2\text{CO}_3 \rightarrow \text{H}^+ + \text{HCO}_3^-$

Organic Compounds

• Carbon Properties: 4 valence electrons, forms rings/chains (carbon skeleton), yields hydrocarbons.
• Functional Groups: Hydroxyl ($\text{--OH}$), Sulfhydryl ($\text{--SH}$), Carbonyl ($\text{--C=O}$), Carboxyl ($\text{--COOH}$), Ester, Phosphate ($\text{--PO}_4^{2-}$), Amino ($\text{--NH}_2$).
• Macromolecules & Polymers: Built from monomers via dehydration synthesis. Broken down via hydrolysis.
• Isomers: Same molecular formula, different structural arrangement (e.g., glucose and fructose: $\text{C}_6\text{H}_{12}\text{O}_6$).

Carbohydrates

• Contains C, H, O in $1:2:1$ ratio ($\text{CH}_2\text{O}$). Functions as ATP source or structural units (deoxyribose).
• Monosaccharides: Simple sugars (3-7 carbons: trioses, tetroses, pentoses [ribose, deoxyribose], hexoses [glucose, fructose, galactose]).
• Disaccharides: Formed by combining 2 monosaccharides via dehydration synthesis (Sucrose = glucose + fructose, Lactose = glucose + galactose, Maltose = glucose + glucose).
• Polysaccharides: Tens to hundreds of monosaccharides. Insoluble in water.
  • Glycogen: Stored glucose in liver and skeletal muscles.
  • Starches: Polysaccharides formed by plants (pasta, potatoes).
  • Cellulose: Plant cell wall polysaccharide, indigestible fiber in humans.

Lipids

• Contain C, H, O (fewer polar covalent bonds, hydrophobic). Joined with proteins to form lipoproteins for blood transport.
• Fatty Acids: Carboxyl group + hydrocarbon chain.
  • Saturated Fatty Acid: Single covalent bonds only; saturated with H.
  • Unsaturated Fatty Acid: One or more double covalent bonds (kinks in chain). Monounsaturated vs Polyunsaturated.
• Triglycerides (Triacylglycerols): Glycerol backbone + 3 fatty acids linked by ester linkages.
  • Fats: Solid at room temp (mostly saturated).
  • Oils: Liquid at room temp (mostly unsaturated).
• Phospholipids: Amphipathic. Glycerol backbone + 2 fatty acid tails (nonpolar, hydrophobic) + 1 phosphate group head (polar, hydrophilic). Forms membrane bilayer.
• Steroids: 4 rings of carbon atoms. Sterols have an $\text{--OH}$ group (Cholesterol, Estrogens, Testosterone, Cortisol, Bile salts, Vitamin D).
• Eicosanoids: Derived from 20-carbon arachidonic acid. Includes Prostaglandins (inflammation, fever, pain, blood clotting, uterine contractions) and Leukotrienes (allergic/inflammatory responses).

Proteins

• Contain C, H, O, N, sometimes S. Built from 20 different Amino Acids.
• Amino Acid Structure: Central carbon + Hydrogen atom + Amino group ($\text{--NH}_2$) + Carboxyl group ($\text{--COOH}$) + Side chain / R group.
• Peptide Bond: Covalent bond joining carboxyl carbon of one amino acid to amino nitrogen of another via dehydration synthesis.
  • Dipeptide (2), Tripeptide (3), Peptide (4-9), Polypeptide (10-2000+).
• Levels of Protein Structure:
  • Primary: Unique sequence of amino acids held by peptide bonds.
  • Secondary: Repeated twisting/folding stabilized by hydrogen bonds (Alpha helixes, Beta pleated sheets).
  • Tertiary: 3D shape of a single polypeptide chain (disulfide bridges $\text{S--S}$, ionic/hydrogen/hydrophobic interactions; assisted by chaperones).
  • Quaternary: Arrangement of multiple polypeptide chains.
• Fibrous vs Globular:
  • Fibrous: Insoluble, parallel strands (collagen, elastin, keratin, dystrophin, fibrin, actin, myosin).
  • Globular: Soluble, spherical (enzymes, antibodies, hemoglobin, lipoproteins, albumins, hormones).
• Denaturation: Unraveling/loss of 3D shape due to altered environment (heat, pH); loses function.
• Enzymes: Protein catalysts consisting of apoenzyme (protein) + cofactor (metal ion or organic coenzyme).
  • Properties: Highly specific (active site, substrate, induced fit), extremely efficient ($10^8\text{--}10^{10}\times$ rate increase), subject to cellular controls.
  • Lower activation energy by properly orienting colliding particles.

Nucleic Acids and ATP

• Nucleic Acids: Huge organic molecules (C, H, O, N, P) composed of nucleotide chains.
  • Nucleotide Structure: Nitrogenous base + Pentose sugar + Phosphate group.
  • Deoxyribonucleic Acid (DNA): Double helix (Watson-Crick model). Sugar = deoxyribose. Bases = Adenine (A), Thymine (T), Cytosine (C), Guanine (G). Purines (A, G - double ring) vs Pyrimidines (T, C - single ring). Base pairing: A-T ($2$ H-bonds), G-C ($3$ H-bonds). Encodes genetic traits.
  • Ribonucleic Acid (RNA): Single-stranded. Sugar = ribose. Bases = A, Uracil (U), C, G. Base pairing: A-U, G-C. Types: mRNA, rRNA, tRNA.
• Adenosine Triphosphate (ATP): Principal energy currency. Adenine + Ribose + 3 Phosphate groups.
  • Hydrolysis by ATPase: $\text{ATP} + \text{H}_2\text{O} \xrightarrow{\text{ATPase}} \text{ADP} + \text{P} + \text{Energy}$
  • Synthesis by ATP synthase: $\text{ADP} + \text{P} + \text{Energy} \xrightarrow{\text{ATP synthase}} \text{ATP} + \text{H}_2\text{O}$
  • Cellular respiration phases: Anaerobic phase (glycolysis, 2 ATP) and Aerobic phase (mitochondria, 30-32 ATP).

The Cellular Level of Organization

• Three Main Parts of a Cell:
  • Plasma Membrane: Flexible outer barrier separating internal from external environment.
  • Cytoplasm: Cellular contents between plasma membrane and nucleus (Cytosol + Organelles).
  • Nucleus: Large organelle housing DNA/chromosomes/genes.

The Plasma Membrane

• Fluid Mosaic Model: Sea of fluid lipids containing a mosaic of floating/anchored proteins.
• Lipid Bilayer:
  • Phospholipids ($75\%$), Cholesterol ($20\%$), Glycolipids ($5\%$).
  • Amphipathic: Hydrophilic polar heads face outward; hydrophobic nonpolar fatty acid tails face interior.
• Membrane Proteins:
  • Integral / Transmembrane: Extend through bilayer.
  • Peripheral: Attached to inner/outer surfaces.
  • Glycoproteins & Glycocalyx: Sugary coat enabling cell recognition, adhesion, protection.
  • Protein Functions: Ion channels, Carriers/Transporters, Receptors (bind ligands), Enzymes, Linkers, Cell-identity markers (MHC proteins).
• Membrane Fluidity: Allows rotation, lateral movement, self-sealing. Dependent on lipid double bonds and cholesterol.
• Selective Permeability: Highly permeable to nonpolar molecules ($\text{O}_2$, $\text{CO}_2$, steroids); moderately permeable to small uncharged polar molecules ($\text{H}_2\text{O}$, urea); impermeable to ions and large uncharged polar molecules (glucose).
• Gradients Across Membrane:
  • Concentration gradient: Difference in chemical concentration.
  • Electrical gradient / Membrane potential: Difference in electrical charges across membrane.
  • Electrochemical gradient: Combined concentration and electrical gradients.

Transport Across the Plasma Membrane

• Passive Processes (No ATP, down concentration gradient):
  • Diffusion: Random mixing of particles down concentration gradient until equilibrium. Factors: gradient steepness, temperature, mass, surface area, diffusion distance.
  • Simple Diffusion: Passive movement directly through lipid bilayer ($\text{O}_2$, $\text{CO}_2$, fatty acids, steroids, fat-soluble vitamins, $\text{H}_2\text{O}$, urea).
  • Facilitated Diffusion: Aided by transmembrane proteins.
    • Channel-mediated: Ions pass through gated or ungated ion channels ($\text{K}^+$, $\text{Cl}^-$, $\text{Na}^+$, $\text{Ca}^{2+}$).
    • Carrier-mediated: Solute binds carrier protein, causing shape change (glucose, fructose, galactose). Exhibits transport maximum and saturation.
  • Osmosis: Net movement of solvent (water) across selectively permeable membrane toward higher solute concentration via lipid bilayer or aquaporins (AQPs).
    • Hydrostatic pressure vs Osmotic pressure.
    • Tonicity: Measure of solution's ability to change cell volume.
    • Isotonic: Same solute concentration; no volume change.
    • Hypotonic: Lower solute concentration outside; water enters cell $\rightarrow$ hemolysis / lysis.
    • Hypertonic: Higher solute concentration outside; water leaves cell $\rightarrow$ crenation.

• Active Processes (Requires ATP, against concentration gradient):
  • Active Transport:
  • Primary Active Transport: Energy from ATP hydrolysis directly pumps substance. Example: Sodium-Potassium Pump ($\text{Na}^+\text{--}\text{K}^+$ ATPase) pumps $3\,\text{Na}^+$ OUT and $2\,\text{K}^+$ IN per ATP.
  • Secondary Active Transport: Energy stored in $\text{Na}^+$ or $\text{H}^+$ concentration gradient drives other substances.
    • Symporters: Move two substances in same direction ($\text{Na}^+$-glucose, $\text{Na}^+$-amino acids).
    • Antiporters: Move two substances in opposite directions ($\text{Na}^+\text{--}\text{Ca}^{2+}$, $\text{Na}^+\text{--}\text{H}^+$).

• Transport in Vesicles:
  • Endocytosis: Inward vesicle formation.
    • Receptor-mediated endocytosis: Selective uptake of ligands (LDLs, transferrin) via clathrin-coated pits/vesicles, uncoating, fusion with endosomes, recycling receptors, degradation in lysosomes.
    • Phagocytosis: "Cell eating" by phagocytes (macrophages, neutrophils) extending pseudopods around particle to form a phagosome, which fuses with lysosome.
    • Bulk-phase endocytosis (Pinocytosis): "Cell drinking" of nonselective extracellular fluid droplets.
  • Exocytosis: Membrane-enclosed secretory vesicles fuse with plasma membrane to release contents (neurotransmitters, enzymes, hormones).
  • Transcytosis: Endocytosis on one side, movement across cell, exocytosis on opposite side (e.g., antibody transfer across placenta).

Cytoplasm and Organelles

• Cytosol: Fluid portion ($55\%$ cell volume, $75\text{--}90\%$ water), site of metabolic reactions (glycolysis). Includes inclusions like lipid droplets and glycogen granules.
• Cytoskeleton: Protein filament network.
  • Microfilaments: Thinnest, made of actin and myosin. Movement, support, form core of microvilli.
  • Intermediate Filaments: Exceptionally strong, stabilize organelle positions, resist mechanical stress (keratin).
  • Microtubules: Largest, hollow tubes made of tubulin. Determine cell shape, move organelles, chromosomes, cilia/flagella.
• Organelles:
  • Centrosome: Microtubule organizing center near nucleus. Pair of centrioles (9 triplets of microtubules) + pericentriolar matrix (tubulin rings for mitotic spindle).
  • Cilia and Flagella: Motile surface projections containing $9+2$ array of microtubules anchored by a basal body.
  • Cilia: Short, numerous; move fluids across cell surface (oarlike beat).
  • Flagella: Long, single; move entire cell (sperm tail).
  • Ribosomes: Sites of protein synthesis. Large and small subunits made in nucleolus, assembled in cytoplasm. Free vs ER-attached.
  • Endoplasmic Reticulum (ER): Membranous network.
  • Rough ER: Studded with ribosomes; synthesizes secretory, membrane, and organellar proteins, glycoproteins, phospholipids.
  • Smooth ER: Lacks ribosomes; synthesizes fatty acids and steroids, detoxifies drugs/alcohol, stores/releases $\text{Ca}^{2+}$ in muscle cells (sarcoplasmic reticulum), removes phosphate from glucose-6-phosphate.
  • Golgi Complex: 3-20 flattened sacs (cisterns).
  • Cis / Entry face: Convex, faces rough ER, receives transport vesicles.
  • Medial cisterns: Add carbohydrates/lipids to form glycoproteins/lipoproteins.
  • Trans / Exit face: Concave, faces plasma membrane; sorts and packages into secretory vesicles, membrane vesicles, or transport vesicles (to lysosomes).
  • Lysosomes: Membrane-enclosed vesicles from Golgi containing acidic hydrolytic enzymes ($\text{pH} \approx 5$). Perform autophagy (digesting old organelles in autophagosomes) and autolysis (destroying entire cell).
  • Peroxisomes (Microbodies): Contain oxidases to oxidize amino/fatty acids and toxic substances. Produces $\text{H}_2\text{O}_2$, which is broken down by catalase.
  • Proteasomes: Barrel-shaped structures containing proteases that degrade unneeded, damaged, or faulty cytosolic proteins cut into peptides.
  • Mitochondria: "Powerhouses" of cell. Outer and inner mitochondrial membranes, mitochondrial cristae (folds), mitochondrial matrix (fluid). Site of aerobic cellular respiration (ATP production). Triggers apoptosis via cytochrome c release. Contains self-replicating circular maternal mtDNA.

Nucleus

• Spherical/oval structure housing most DNA.
• Nuclear Envelope: Double membrane with nuclear pores (proteins controlling active/passive transport).
• Nucleoli: Spherical non-membrane bodies producing ribosomal subunits.
• Genes & Chromosomes: 46 chromosomes ($23$ pairs) in human somatic cells. Genome is total genetic information.
• Chromatin: Complex of DNA, proteins (histones), and RNA.
  • Nucleosomes: "Beads on a string" consisting of double-stranded DNA wrapped twice around a core of 8 histone proteins, connected by linker DNA.
  • Condenses into paired chromatids joined at a centromere.

Protein Synthesis

• Gene Expression: Using DNA template to synthesize proteins.
  • Proteome: All proteins produced by an organism.
  • Genetic Code: Set of rules relating DNA base triplets to RNA codons and amino acids.
• Transcription (In nucleus): DNA template copied into RNA.
  • Catalyzed by RNA Polymerase, starting at a promoter and ending at a terminator.
  • Complementary base pairing: DNA A, T, G, C $\rightarrow$ RNA U, A, C, G.
  • Pre-mRNA processing: Introns (non-coding) snipped out by snRNPs; Exons (coding) spliced together. Alternative splicing allows one gene to code for multiple proteins.
• Translation (In cytoplasm on ribosomes):
  • mRNA carries codons.
  • tRNA brings specific amino acid; carries an anticodon complementary to mRNA codon.
  • Ribosome binding sites: P site (peptidyl), A site (aminoacyl), E site (exit).
  • Initiation: Initiator tRNA binds start codon (AUG, coding for methionine).
  • Elongation: Peptide bond formation catalyzed by ribosome, shifting along mRNA (polyribosomes allow multiple simultaneous translations).
  • Termination: Reaches a stop codon.

Cell Division

• Somatic Cell Division: Replaces dead/injured cells, adds new ones. Involves mitosis (nuclear division) and cytokinesis (cytoplasmic division) yielding 2 identical diploid ($2n$) cells ($46$ chromosomes / $23$ homologous pairs).
• Cell Cycle:
  • Interphase: High metabolic activity, cell growth, component duplication.
  • $G_1$ Phase: Duplicates organelles/cytosolic components ($8\text{--}10\,\text{hours}$). Cells in $G_0$ do not divide.
  • $S$ Phase: DNA replication ($8\,\text{hours}$).
  • $G_2$ Phase: Cell growth, enzyme/protein synthesis, centrosome replication completed ($4\text{--}6\,\text{hours}$).
  • Mitotic (M) Phase:
  • Mitosis:
    • Prophase: Chromatin condenses into visible chromosomes (paired chromatids at centromere with kinetochores). Centrosomes build mitotic spindle and move to poles. Nucleolus and nuclear envelope disappear.
    • Metaphase: Chromatid pairs line up at metaphase plate.
    • Anaphase: Centromeres split, sister chromatids separate to opposite poles (now individual chromosomes).
    • Telophase: Chromosomes uncoil into chromatin, nuclear envelopes and nucleoli reappear, mitotic spindle breaks up.
  • Cytokinesis: Formation of a cleavage furrow via an actin contractile ring pinching cytoplasm in two.

• Control of Cell Destiny:
  • Regulated by Cyclin-dependent protein kinases (Cdk's) and Cyclins.
  • Apoptosis: Orderly, programmed cell death (shrinks, DNA fragments, phagocytized, no inflammation).
  • Necrosis: Pathological cell death from injury (cells swell, burst, cause inflammation).
• Reproductive Cell Division: Meiosis occurs in gonads producing haploid ($n$) gametes ($23$ chromosomes).
  • Meiosis I:
  • Prophase I: Synapsis forms tetrads (4 chromatids). Crossing-over (exchange of genetic material between nonsister chromatids) produces genetic recombination.
  • Metaphase I: Tetrads line up side by side.
  • Anaphase I: Homologous pairs separate (sister chromatids remain attached).
  • Telophase I: Yields 2 haploid cells.
  • Meiosis II: Similar to mitosis; centromeres split, sister chromatids separate. Yields 4 haploid gametes.

Cellular Diversity and Aging

• Sizes & Shapes: Sizes range from $8\,\mu\text{m}$ (RBC) to $140\,\mu\text{m}$ (oocyte). Shapes reflect function (sperm flagellum, disc RBC, spindle smooth muscle, branched nerve cell).
• Aging: Progressive alteration of homeostatic responses. Studies by Gerontology and Geriatrics. Linked to telomeres, free radicals, autoimmune responses.

Tissue Level of Organization & Cell Junctions

• Four Basic Tissue Types: Epithelial, Connective, Muscular, Nervous. (Studied in Histology; diseases examined by Pathologist via Biopsy).
• Five Main Cell Junctions:
  • Tight Junctions: Weblike strands of transmembrane proteins fusing adjacent plasma membranes to seal off passageways (stomach, intestines, urinary bladder).
  • Adherens Junctions: Contain plaque attached to microfilaments, joined by transmembrane glycoproteins called cadherins forming adhesion belts.
  • Desmosomes: Plaque attached to intermediate filaments (keratin), joined by cadherins (epidermis, cardiac muscle).
  • Hemidesmosomes: Do not link adjacent cells; use integrins to attach intermediate filaments to laminin in the basement membrane.
  • Gap Junctions: Membrane proteins called connexins form fluid-filled tunnels called connexons connecting adjacent cells for ion/signal exchange (avascular tissues, nerve/cardiac impulses).

Epithelial Tissue

• General Features: Closely packed cells, continuous sheets, little extracellular matrix, avascular (relies on connective tissue diffusion), high rate of cell division.
• Surfaces: Apical (free) surface (microvilli/cilia), Lateral surfaces (junctions), Basal surface (anchored to basement membrane).
• Basement Membrane: Basal lamina (laminin, collagen, secreted by epithelium) + Reticular lamina (collagen, secreted by connective tissue fibroblasts).
• Classification:
  • Arrangement: Simple (single layer), Pseudostratified (appears layered, all touch basement membrane), Stratified (two or more layers).
  • Shapes: Squamous (flat), Cuboidal (cube/hexagon), Columnar (tall), Transitional (variable).
• Types of Covering & Lining Epithelium:
  • Simple Squamous: Single layer flat cells. Filtration, diffusion, secretion in serous membranes.
  • Endothelium: Lines heart, blood/lymphatic vessels.
  • Mesothelium: Forms epithelial layer of serous membranes (pleura, pericardium, peritoneum).
  • Simple Cuboidal: Single layer cube-shaped. Secretion and absorption (kidney tubules, gland ducts).
  • Nonciliated Simple Columnar: Single layer tall cells with microvilli and goblet cells (secrete mucus). Secretion and absorption (gastrointestinal tract).
  • Ciliated Simple Columnar: Single layer tall ciliated cells with goblet cells. Moves mucus/particles (uterine tubes, respiratory tract).
  • Nonciliated Pseudostratified Columnar: Appears layered; lacks cilia/goblet cells (epididymis, male urethra).
  • Ciliated Pseudostratified Columnar: Appears layered; contains cilia and goblet cells (upper respiratory tract).
  • Stratified Squamous: Multiple layers; apical cells flat. Protection against abrasion.
  • Keratinized: Apical layers contain tough keratin (epidermis of skin).
  • Nonkeratinized: Lacks keratin, moistened by mucus (mouth, esophagus, vagina).
  • Stratified Cuboidal: Rare; 2+ layers cube-shaped (sweat gland ducts).
  • Stratified Columnar: Rare; apical layer columnar (pharynx, male urethra).
  • Transitional Epithelium (Urothelium): Variable shape (cuboidal to flat when stretched). Lines urinary bladder, ureters, urethra.
• Glandular Epithelium:
  • Endocrine Glands: Ductless; secrete hormones directly into interstitial fluid and blood (pituitary, thyroid, adrenal).
  • Exocrine Glands: Secrete products into ducts opening onto surface/lumen (sweat, oil, salivary glands).
  • Unicellular: Single-celled (goblet cells).
  • Multicellular:
    • Duct branching: Simple (unbranched) vs Compound (branched).
    • Secretory shape: Tubular, Acinar (alveolar), or Tubuloacinar.
  • Functional Classes:
    • Merocrine: Released via exocytosis in secretory vesicles (salivary glands, pancreas).
    • Apocrine: Secretory product accumulates at apical surface, which pinches off (milk fat in mammary glands).
    • Holocrine: Cell accumulates product, ruptures, and dies (sebaceous glands).

Connective Tissue

• General Features: Consists of cells and extracellular matrix (ground substance + fibers). Highly vascular (except cartilage) and innervated.
• Connective Tissue Cells:
  • Fibroblasts: Large flat cells secreting fibers and ground substance.
  • Macrophages: Phagocytes from monocytes (fixed vs wandering).
  • Plasma Cells: Secrete antibodies.
  • Mast Cells: Produce histamine during inflammation, kill bacteria.
  • Adipocytes: Fat cells storing triglycerides.
  • Leukocytes: White blood cells migrating to sites of infection/allergy.

• Extracellular Matrix:
  • Ground Substance: Water + Glycosaminoglycans (GAGs) (hyaluronic acid, chondroitin sulfate, dermatan sulfate, keratan sulfate) linked to proteins as proteoglycans. Adhesion proteins include fibronectin.
  • Fibers:
  • Collagen Fibers: Very strong, flexible, parallel bundles of collagen protein.
  • Elastic Fibers: Stretchable (up to 150%), elasticity due to elastin and fibrillin.
  • Reticular Fibers: Fine collagen bundles coated with glycoprotein; form stroma of soft organs.
• Types of Connective Tissue:
  • Embryonic:
  • Mesenchyme: Gives rise to all connective tissues.
  • Mucous (Mucoid): Umbilical cord (Wharton's jelly).
  • Mature Connective Tissue:
  • Connective Tissue Proper:
    • Loose Connective Tissue:
    • Areolar: Fibers randomly arranged + all cell types. Subcutaneous layer, packing material.
    • Adipose: Adipocytes storing fat. White adipose vs Brown adipose tissue (BAT - heat generation).
    • Reticular: Reticular fibers and cells; forms stroma of lymph nodes, spleen, liver.
    • Dense Connective Tissue:
    • Dense Regular: Parallel collagen bundles + fibroblasts in rows (tendons, ligaments).
    • Dense Irregular: Irregularly arranged collagen fibers (dermis, periosteum, heart valves).
    • Elastic: Branching elastic fibers + fibroblasts (lung tissue, elastic arteries).
  • Supporting Connective Tissue:
    • Cartilage: Chondrocytes in lacunae, surrounded by perichondrium (except fibrocartilage/articular). Avascular (antiangiogenesis factor).
    • Hyaline Cartilage: Resilient gel, fine collagen (ends of bones, nose, trachea, fetal skeleton).
    • Fibrocartilage: Thick visible collagen bundles, lacks perichondrium (intervertebral discs, pubic symphysis).
    • Elastic Cartilage: Threadlike elastic fiber network (epiglottis, external ear).
    • Growth: Interstitial (from within) vs Appositional (outer surface).
    • Bone (Osseous) Tissue: Compact bone (organized into osteons / haversian systems: lamellae, lacunae, osteocytes, canaliculi, central canal) vs Spongy bone (trabeculae).
  • Liquid Connective Tissue:
    • Blood: Blood plasma + formed elements (RBCs, WBCs, platelets).
    • Lymph: Extracellular fluid in lymphatic vessels.

Membranes

• Epithelial Membranes:
  • Mucous Membrane (Mucosa): Lines cavities opening to exterior (digestive, respiratory, reproductive, urinary). Epithelium + lamina propria (areolar connective tissue).
  • Serous Membrane (Serosa): Lines closed cavities and covers organs. Mesothelium + areolar connective tissue. Parietal and visceral layers separated by serous fluid (Pleura, Pericardium, Peritoneum).
  • Cutaneous Membrane (Skin): Epidermis + Dermis.
• Synovial Membranes: Line joint cavities; lack epithelium. Composed of synoviocytes and connective tissue secreting synovial fluid.

Muscular and Nervous Tissue

• Muscular Tissue: Elongated cells (muscle fibers / myocytes) using ATP to generate force.
  • Skeletal Muscle: Long, cylindrical, striated, multinucleated, voluntary.
  • Cardiac Muscle: Branched, striated, single central nucleus, intercalated discs (desmosomes + gap junctions), involuntary.
  • Smooth Muscle: Spindle-shaped, nonstriated, single central nucleus, gap junctions, involuntary (walls of hollow internal organs).
• Nervous Tissue:
  • Neurons: Sensitive to stimuli, convert to action potentials. Parts: Cell body, Dendrites (input), Axon (output).
  • Neuroglia: Supportive cells (do not generate impulses).
• Excitable Cells: Neurons and muscle fibers respond to stimuli via action potentials.

Tissue Repair and Aging

• Tissue Repair: Replacement of worn-out/damaged cells from stroma (connective tissue) or parenchyma (functional cells).
  • Stem cells / division of mature cells.
  • Fibrosis: Formation of scar tissue (granulation tissue).
• Aging: Tissues heal slower, collagen/elastic fibers change.

The Integumentary System - Structure of the Skin

• Components: Skin, hair, oil/sweat glands, nails, sensory receptors.
• Skin (Cutaneous Membrane): Largest organ by weight ($2\,\text{m}^2$, $4.5\text{--}5\,\text{kg}$, $7\%$ body weight, thickness $0.5\text{--}4.0\,\text{mm}$).
• Structure:
  • Epidermis: Superficial, avascular, keratinized stratified squamous epithelium.
  • Dermis: Deeper, vascular, dense irregular connective tissue.
  • Subcutaneous Layer (Hypodermis): Deep to dermis (areolar + adipose tissue); contains lamellated (pacinian) corpuscles.

• Epidermis Cells:
  • Keratinocytes ($90\%$): Produce keratin and lamellar granules (water-repellent sealant).
  • Melanocytes ($8\%$): Produce melanin (absorbs UV light, shields nuclear DNA).
  • Intraepidermal Macrophages (Langerhans cells): Immune response against invading microbes.
  • Tactile Epithelial Cells (Merkel cells): Contact tactile (Merkel) discs of sensory neurons for touch sensation.
• Epidermal Strata (Deep to Superficial):
  • Stratum Basale (Germinativum): Single row of cuboidal/columnar keratinocytes with stem cells; contains keratin intermediate filaments, melanocytes, tactile cells.
  • Stratum Spinosum: 8-10 layers of keratinocytes with coarse keratin intermediate filaments; thornlike spines on slides; contains Langerhans cells and melanocyte projections.
  • Stratum Granulosum: 3-5 layers of flattened keratinocytes undergoing apoptosis; contains keratohyalin granules and lamellar granules; boundary between living and dead cells.
  • Stratum Lucidum: Present ONLY in thick skin (fingertips, palms, soles); 4-6 layers of clear, dead, flattened keratinocytes.
  • Stratum Corneum: 25-30+ layers of dead, flat, scale-like keratinocytes filled with keratin. Forms calluses upon friction.
• Keratinization & Growth: Cells pushed upward, accumulate keratin, die ($4\text{--}6\,\text{weeks}$ process). Driven by epidermal growth factor (EGF).
• Dermis:
  • Papillary Region ($1/5\text{th}$): Areolar connective tissue, fine elastic fibers. Features dermal papillae, capillary loops, corpuscles of touch (Meissner corpuscles), free nerve endings (warmth, cool, pain, tickle, itch), and epidermal ridges (fingerprints/footprints, dermatoglyphics).
  • Reticular Region ($4/5\text{ths}$): Dense irregular connective tissue, thick collagen bundles, coarse elastic fibers. Provides strength, extensibility, elasticity. Contains hair follicles, glands, nerves. Tension lines (lines of cleavage) indicate collagen direction. Striae (stretch marks) are internal tears in dermis.
• Skin Color Pigments:
  • Melanin: Synthesized from tyrosine by tyrosinase in melanosomes. Two forms: Pheomelanin (yellow to red) and Eumelanin (brown to black). Accumulations form freckles, age spots, and nevi (moles).
  • Hemoglobin: Oxygen-carrying red pigment in dermal capillaries (gives pink/red skin tone).
  • Carotene: Yellow-orange precursor of vitamin A stored in stratum corneum and subcutaneous fat.
  • Clinical signs: Albinism (inability to produce melanin), Vitiligo (loss of melanocytes in patches), Cyanosis (bluish skin due to hypoxia), Jaundice (yellowish due to bilirubin buildup), Erythema (redness), Pallor (paleness).

Accessory Structures of the Skin

• Hair (Pili):
  • Anatomy: Hair shaft (above surface), Hair root (deep to shaft).
  • Concentric layers: Medulla (inner), Cortex (middle), Cuticle of the hair (outermost single layer of overlapping flat cells).
  • Hair Follicle: Epithelial root sheath (External & Internal root sheaths) surrounded by Dermal root sheath.
  • Hair Bulb: Base of follicle; contains papilla of the hair (blood vessels) and hair matrix (germinal stem cells for growth).
  • Associated structures: Sebaceous glands, Arrector pili muscle (smooth muscle causing goose bumps), Hair root plexus (touch sensitive dendrites).
  • Growth Cycle: Growth stage (matrix divides, $2\text{--}6\,\text{years}$ on scalp), Regression stage (matrix stops dividing, follicle shrinks, $2\text{--}3\,\text{weeks}$), Resting stage ($3\,\text{months}$). Normal loss: $70\text{--}100\,\text{hairs/day}$. Alopecia = hair loss.
  • Types of Hair: Lanugo (fetal downy hair), Terminal hair (coarse, pigmented; scalp, eyebrows, adult pubic/axillary/facial), Vellus hair ("peach fuzz").

• Skin Glands:

  • Sebaceous (Oil) Glands: Simple branched acinar glands connected to hair follicles. Secrete sebum (triglycerides, cholesterol, proteins, inorganic salts) to prevent drying and inhibit bacteria. Inflammation leads to acne (cystic acne).
  • Sudoriferous (Sweat) Glands:
  • Eccrine Sweat Glands: Simple coiled tubular glands throughout body (forehead, palms, soles). Secretory portion in deep dermis; duct opens at pore on epidermis. Function in thermoregulation (thermoregulatory sweating, insensible/sensible perspiration) and emotional sweating ("cold sweat").
  • Apocrine Sweat Glands: Simple coiled tubular glands in axillae, groin, areolae, bearded face. Secretory portion in lower dermis/subcutaneous; duct opens into hair follicle. Secretes milky/yellowish sweat (lipid/protein rich) that produces body odor when metabolized by bacteria. Active during emotional sweating/sexual excitement (functional at puberty).
  • Ceruminous Glands: Modified sweat glands in ear canal secreting waxy material that combines with sebum to form cerumen (earwax).

• Nails: Plates of tightly packed, hard, dead, keratinized epidermal cells.

  • Anatomy: Nail body (plate), Free edge, Nail root, Lunula (white crescent), Hyponychium (thickened stratum corneum beneath free edge), Nail bed (skin beneath nail plate), Eponychium (cuticle) (stratum corneum band at proximal border), Nail matrix (proximal epithelium containing dividing stem cells, growth $\approx 1\,\text{mm/week}$).

Functions of the Skin and Wound Healing

• Skin Functions:
  • Thermoregulation: Sweat evaporation + dermal blood vessel dilation/constriction.
  • Blood Reservoir: Houses $8\text{--}10\%$ of resting adult blood flow.
  • Protection: Keratin, lamellar lipids, sebum, acidic sweat pH, melanin, Langerhans cells, macrophages.
  • Cutaneous Sensations: Tactile (touch, pressure, vibration, tickle), thermal (warmth, cool), pain.
  • Excretion & Absorption: Excretes water, heat, salts, carbon dioxide, ammonia, urea. Absorbs fat-soluble vitamins (A, D, E, K), gases ($\text{O}_2$, $\text{CO}_2$), organic solvents, heavy metal salts, topical steroids.
  • Synthesis of Vitamin D: UV sunlight activates precursor in skin $\rightarrow$ liver & kidneys convert to calcitriol (active vitamin D hormone) for calcium absorption.
• Skin Wound Healing:
  • Epidermal Wound Healing: Abrasions/minor burns affecting only epidermis. Basal cells enlarge, detach from basement membrane, migrate across wound until meeting (contact inhibition). Epidermal growth factor (EGF) stimulates basal stem cells to divide and rebuild strata. No scar.
  • Deep Wound Healing: Extends to dermis/subcutaneous layer. Four phases:
  1. Inflammatory Phase: Blood clot forms, inflammation delivers neutrophils/macrophages/fibroblasts.
  2. Migratory Phase: Clot becomes scab, epithelial cells bridge wound beneath scab, fibroblasts synthesize scar tissue (granulation tissue).
  3. Proliferative Phase: Extensive epithelial cell growth, random collagen deposition, blood vessel growth.
  4. Maturation Phase: Scab sloughs off, collagen reorganizes, blood vessels normalize.
  • Fibrosis: Process of scar tissue formation. Hypertrophic scar (raised within wound boundary) vs Keloid scar (extends beyond original boundary).