Essential Concepts in Human Anatomy and Physiology

Anatomy and Physiology Fundamentals

Anatomical Terminology

• Anatomical Descriptors
  • Anterior — at or near the front of the body
  • Posterior — at or near the back of the body
  • Midline — imaginary vertical line dividing the body equally
  • Lateral — farther from the midline
  • Medial — nearer to the midline
  • Superior — toward the head or upper part of a structure
  • Inferior — away from the head or lower part of a structure
  • Superficial — close to the surface of the body
  • Deep — away from the surface of the body
  • Proximal — near the origination of a structure
  • Distal — farther from the origination of a structure
  • Contralateral — on the opposite side
• Anatomical Planes
  • Sagittal — body divided into left and right halves by a vertical plane passing through the midline
  • Coronal (Frontal) — body divided into anterior and posterior sections
  • Transverse (Axial) — body divided into upper and lower sections
• Tissue Descriptors
  • Lumen — hollow inner cavity of a tubular structure (e.g., blood vessel or intestine); allows fluids to pass through
  • Central — relating to or situated at the center; located near the center or middle of the body
  • Peripheral — relating to or situated on/near the edge or periphery; structures situated away from the center
  • Medullary — relating to/resembling marrow or the medulla oblongata; innermost part/core of an organ
  • Cortical — relating to/consisting of cortex or external layer; outer layer of an organ
  • Basal — relating to/situated at the base; lowest part/base of an organ
  • Apical — relating to/situated at the apex or uppermost point; uppermost part/apex of an organ

Regional Anatomy

• Abdominal Quadrants
  • Right Upper Quadrant (RUQ) — contains liver, gallbladder, pancreas, duodenum, and right kidney
  • Left Upper Quadrant (LUQ) — contains parts of the stomach, pancreas, left kidney, and adrenal gland
  • Right Lower Quadrant (RLQ) — contains parts of the appendix, right ureter, right ovary/testis, and part of the large and small intestine
  • Left Lower Quadrant (LLQ) — contains parts of the left ureter, left ovary/testis, and parts of the large intestine
• Abdominopelvic Regions

  Right hypochondriac region	Epigastric region	Left hypochondriac region
  Right lumbar region	Umbilical region	Left lumbar region
  Right inguinal region	Hypogastric region	Left inguinal region

Body Cavities

• Major Body Cavities
  • Cranial cavity — houses and protects the brain in the skull
  • Spinal cavity — encloses the spinal cord in the spinal column
  • Thoracic cavity — contains the lungs, heart, and major blood vessels
• Other Body Cavities
  • Dorsal body cavity
  • Vertebral cavity
  • Abdominal cavity
  • Pelvic cavity
  • Abdominopelvic cavity
  • Ventral body cavity (includes both thoracic and abdominopelvic cavities)
• Minor Body Cavities
  • Superior mediastinum
  • Pleural cavity
  • Pericardial cavity (within the mediastinum)
  • Diaphragm

Organization of Life

• What is a System? A group of interacting, interrelated, or interdependent elements that act according to rules to form a unified whole.
  • Circulatory System — transports oxygen and nutrients to cells, removes waste, and equalizes temperature in the body
  • Respiratory System — brings oxygen into the body and removes carbon dioxide
  • Digestive System — breaks down food and absorbs nutrients
  • Integumentary System — barrier between external and internal environments; contains sensory receptors
  • Skeletal System — supports the body and enables movement
  • Muscular System — enables movement and helps maintain body temperature
  • Nervous System — detects and processes sensory information and activates bodily responses
  • Endocrine System — secretes hormones and regulates bodily processes
  • Lymphatic System — returns fluid to the blood and defends against pathogens
  • Urinary System — controls water balance in the body and removes wastes from the blood
  • Female Reproductive System — produces gametes and sex hormones, and maintains fertilized eggs
  • Male Reproductive System — produces androgens (testosterone), maintains male reproductive function, promotes spermatogenesis, and transports sperm for fertilization
• What is a Function? The special, normal, or proper activity of an organ or part of an organism.
• What is an Organ? Structures made up of two or more tissues that carry out a specific function or group of functions within an organism.

Homeostasis and Feedback Loops

• Homeostasis refers to the ability to maintain a stable internal environment.
  • *Example:* Regulation of body temperature, blood glucose, and water balance. The hypothalamus and endocrine system play key roles in detecting changes and triggering corrective mechanisms.
• Feedback loops are important mechanisms for maintaining homeostasis and driving change in complex systems.
  • Positive Feedback — amplifies deviations, leading to explosive growth (e.g., childbirth)
  • Negative Feedback — reduces deviations, leading to stability (e.g., temperature regulation)

Body Chemistry and Biochemistry

Basic Chemical Components

• What is an Atom? The basic building block of every kind of matter; it can join or bind with other atoms and cannot be broken into smaller parts using ordinary chemical processes.
• What is a Molecule? A group of two or more atoms joined together by attractive forces called chemical bonds. Thousands of molecules exist in the body, and they can contain atoms from the same or different species.
• What is an Element? A pure substance with specific properties.
  • Major elements in the body = Carbon, Hydrogen, Oxygen, and Nitrogen (CHON)
• What is a Compound? Formed when atoms from different elements join.
• What is an Ion? Any atom or group of atoms that bears one or more electric charges, positive or negative.
  • Opposites attract, similar charges repel.
    • Cation — positive charge (e.g., Na⁺)
    • Anion — negative charge (e.g., Cl⁻)

Major Macromolecules and Contribution to Physiology

• Carbohydrates — contain carbon, hydrogen, and oxygen (e.g., sugars, starches, cellulose, chitin). They provide most of the energy needed for life and are important components in cells.
  • Monosaccharides — simple carbohydrates, containing 3-7 carbon atoms (e.g., glucose, the main energy-supplying compound)
  • Disaccharides — two monosaccharides joined together; can be split back into them
  • Polysaccharides — the largest carbohydrates; may contain tens or hundreds of monosaccharides
• Lipids — insoluble in water (hydrophobic), fatty, waxy, or oily compounds. They are soluble in organic solvents but insoluble in polar solvents (like water).
  • Triglycerides — the most plentiful lipids in the body; provide protection, insulation, and energy
  • Phospholipids — lipids containing phosphorus; important in cell membranes
  • Steroids — lipids containing rings of carbon atoms
  • Fat-soluble vitamins A, D, E, and K
• Proteins — structural and regulatory, made up of amino acids. A polypeptide chain is a chain of amino acids.
  • Functions include contractile, transport, and catalyst roles.
• Nucleic Acids — huge organic molecules made of carbon, hydrogen, oxygen, nitrogen, and phosphorus. The basic unit is the nucleotide.
  • DNA (Deoxyribonucleic Acid) — storage of hereditary information
  • RNA (Ribonucleic Acid) — carries the DNA blueprint to the ribosomes to synthesize proteins

Water Solubility

• Hydrophilic Substances
  • Dissolve in polar solvents (e.g., water)
  • Will be ionic or possess polar bonds
  • E.g., NaCl, ethanol, glucose, and some proteins
• Hydrophobic Substances
  • Dissolved in non-polar solvents (e.g., petrol, kerosene)
  • Will consist of non-polar bonds
  • E.g., grease, petroleum jelly, and lipids

Acids, Bases, and pH

• Acids — release hydrogen ions (H⁺) in a solution.
  • Increase the concentration of hydrogen ions in a water solution.
  • Considered electrolytes as they release ions in solution.
• Bases — increase the number of hydroxide ions (OH⁻) in a solution.
  • More OH⁻ dissolved in solution means it is more alkaline/basic.
  • Decrease the concentration of hydrogen ions in a water solution.
• Strong Acid — most common is HCl (found in the stomach).
• Weak Acid — carbonic acid and organic acids (reversible arrows in compound equation are used).
• pH Scale — concentration of acid/base in biological systems is often very small.
  • Blood H⁺ concentration is 3.98 x 10⁻⁸ mol/L.
  • Water [H⁺] = 1 x 10⁻⁷ mol/L.
  • The pH scale shows the concentration of hydrogen ions in solution.
    • Pure water pH = 7.0, blood pH = 7.4.
  • A lower number means more acidic, and a higher number means more alkaline.

Buffers and Electrolytes

• Buffers (chemicals) help maintain physiological pH — they resist rapid and drastic changes in pH when moderate amounts of acid or base are added.
• Electrolytes — maintain electrical neutrality in cells and produce action potentials in nerves and muscles; essential for life.
  • Bodily chemical reactions occur in aqueous solutions.
  • The dissolved substances in the solution are solutes.
  • A mineral dissociated from a salt that carries an electrical charge (an ion) is called an electrolyte.
    • Cation electrolytes in the body = sodium (Na⁺), potassium (K⁺), hydrogen (H⁺), calcium (Ca²⁺), and magnesium (Mg²⁺).
    • Anion electrolytes in the body = chloride (Cl⁻), hydroxide (OH⁻), bicarbonate (HCO₃⁻), sulfate (SO₄²⁻), and phosphate (PO₄³⁻).

Membrane Transport Principles

• Diffusion — a form of passive transport (substances move in/out of cells without energy). It is the movement of the solute from an area of high solute concentration to an area of low solute concentration.
• Concentration Gradients — forms if a solute can diffuse from an area of higher concentration to an area of lower concentration. The gradient is an interim phase.
• Osmosis — a specific type of diffusion where the solvent moves through a semi-permeable membrane from a low concentration of the solute to a high concentration of the solute.
• Tonicity — affects water movement into and out of cells in a solution.
  • Hypertonic Solutions — have a higher concentration of dissolved solutes than blood. This results in the osmotic movement of fluid *out* of cells into the intravascular space.
    • Used to increase intravascular fluid volume and to treat hyponatremia (deficit of Na⁺).
  • Isotonic Solutions — have a similar concentration of solutes as blood. Fluid *stays* in the intravascular space (no imbalance).
    • Used for extracellular fluid volume deficit.
  • Hypotonic Solutions — have a lower concentration of dissolved solutes than blood. This causes osmosis of water from the intravascular compartment *into* the intracellular space.
    • Used for cellular dehydration.

Cell Structure, DNA, and Heredity

Cellular Organelles

• Nucleus — the control center in most living things; contains genetic material.
• Ribosomes — made of RNA and protein; produces proteins from genetic material.
  • Free ribosomes = floating in the cytoplasm.
  • Attached ribosomes = bound to the endoplasmic reticulum.
• Endoplasmic Reticulum (ER) — a membrane network attached to the nuclear membrane; site of protein synthesis (smooth vs. rough ER).
• Golgi Apparatus — a network of flattened and smooth membranes; packages proteins and lipids from the endoplasmic reticulum into secretory vesicles and transports them. Vesicles transport proteins and lipids to other organelles or release them from the cell.
• Mitochondria — the powerhouse of the cell. Surrounded by a double lipid bilayer membrane (the inner layer forms cristae); involved in ATP production.
• Cytoskeleton — maintains cell shape and internal organization; provides a pathway for the transport of substances within the cell.
• Cytoplasm — fills the cytoplasmic matrix (intracellular fluid); consists of water and dissolved proteins, sugars, and wastes; suspends organelles.
  • Intracellular Fluid = cytoplasm.
  • Extracellular Fluid = interstitial (fills space between cells within tissues) and intravascular (fluid inside the blood vessels).

Cell Membrane Structure and Function

• The Cell Membrane contains the cells and is composed of proteins and lipids; controls the composition of the intracellular space.
• Involved in cell-cell recognition, maintenance of cell shape, and cell mobility.
• Composed of a phospholipid bilayer with polar-hydrophilic heads and hydrophobic non-polar tails.
• Proteins — integral/transmembrane and peripheral — act as receptors, transporters, enzymes, surface markers/cell recognition, and cell adhesion molecules (CAMs).
• Controls the passage of substances into/out of the cell, allowing it to regulate the internal environment.
  • Cholesterol in the membrane decreases its fluidity.
  • Membrane proteins have many functions (e.g., channels and cell identity markers).

Membrane Proteins and Receptors

• Membrane Proteins
  • Integral (or Transmembrane)
    • Extend from one side of the membrane to the other.
    • Act as channels (e.g., for water, electrolytes, non-lipid soluble substances).
    • Act as pumps to transport substances across.
    • Involved in identification (glycoproteins).
    • Form connections between cells.
    • Act as enzymes.
    • Translate hormone signals into chemical signals (cell signal transduction).
  • Peripheral (or Surface)
• Cellular Receptors
  • Allow cells to monitor their environment.
  • They are protein molecules.
  • Locations: cell membrane, cytoplasm, and nucleus.
  • Receptors can bind with many substances and become activated or inactivated.

Cellular Metabolism and ATP

• Cellular Metabolism — all chemical tasks of maintaining essential cellular functions.
  • Anabolism — energy-using processes.
  • Catabolism — energy-releasing processes.
• Cellular respiration — aerobic and anaerobic.
• ATP (Adenosine Triphosphate) — the energy-carrying molecule used in the synthesis of organic molecules, muscle contraction, and transport.

Genetics and Chromosomes

• DNA is packaged with proteins to form chromosomes. Humans have 46 individual chromosomes; each parent provides 23 chromosomes (gametes). Genes occur along the length of the DNA molecule.
  • DNA is folded around proteins called histones.
    • In non-dividing cells, DNA and histones appear as a tangle of long threads known as chromatin.
    • In dividing cells, chromatin condenses just before cell division and appears as individual, tightly coiled chromosomes.
  • When a chromosome copies itself, it consists of two chromatids (sister chromatids) connected by a centromere.
• Chromosomes — Karyotypes
  • A visual representation of the number and appearance of an individual's complete set of chromosomes.
• Genes — small, discrete sections of a chromosome; usually codes for a single protein.
  • Gene expression, alleles (alternative forms of a gene), traits, and mutations.
  • Genotype — genetic makeup of an organism.
  • Phenotype — observable, detectable, or outward appearance of the genetics of an organism.
• Dominance and Recessiveness — homozygous (two copies of the same allele) and heterozygous (two different alleles).
  • The observable allele is dominant, and the one whose effects are hidden is recessive. Dominant alleles are expressed by upper case letters; recessive by lower case.
  • Alleles can be codominant.
    • Each allele produces a slightly different protein, and heterozygous individuals express both forms (e.g., A/B/O blood groups).
• Autosomal and X-linked Inheritance — autosomal (caused by genes on autosomes) and X-linked traits (caused by genes on the X chromosome, often showing gender imbalance).

Cell Proliferation and Division

• Cell Proliferation — production of gametes (meiosis) and reproduction of other body cells (mitosis and cytokinesis).
• Cell Division — the process by which a cell reproduces itself.
  • Mitosis — results in the formation of two identical cells (diploid somatic cells); necessary for growth and repair.
    • Prophase
    • Metaphase
    • Anaphase
    • Telophase
  • Meiosis — results in the formation of four non-identical cells (haploid gametes); necessary for sexual reproduction.
  • DNA replication must occur *before* any cell division takes place.
• The Cell Cycle — damaged and worn-out cells need replacing. Apoptosis is the “self-destruct” genetically controlled cell death. The cycle is in two phases:
  • Interphase (growth phase) — newly formed cells produce a wide variety of molecules for growth and produce organelles.
  • M phase (division phase) — includes mitosis (PMAT).
• Control of Cell Division — checkpoints — influenced by hormones. Failures at checkpoints mean abnormalities are repaired or the cell undergoes apoptosis.
• DNA and DNA Replication — stores genetic information in the nucleus. Deoxyribose sugar-phosphate backbone with 4 nitrogenous bases (cytosine, thymine, adenine, guanine) forms a nucleotide. The double helix model involves strands uncoupling to replicate.
  • Replication occurs in the S phase. Hydrogen bonds break to form a single strand, and unpaired bases attract free nucleotides to form a new complementary strand (complementary base pairing).

Transport Across the Plasma Membrane

• Transport across the plasma membrane includes passive transport (diffusion, facilitated diffusion, and osmosis) and active transport (pumps, vesicular — exocytosis and endocytosis).
• The plasma membrane is selectively permeable — it controls the passage of substances across it to an extent, allowing the internal environment to be different from the interstitial fluid.
  • Particle movement is regulated by:
    • Particle size — small particles may more easily pass through.
    • Pores/specific channels — allow some substances but not others.
    • Pumps and carriers — these are specific for certain substances.
• Mechanisms for Transport Across the Membrane
  

  	Passive Transport	Active Transport
  Energy Requirements	Energy not required	Energy required (ATP)
  Movement	From higher to lower concentrations (“downhill”)	From lower to higher concentration (“uphill”)
  Examples	Simple diffusion, osmosis (diffusion of water)	Protein pumps, vesicles (phagocytosis)

  
  
• Passive Transport Details
  • Filtration — movement of water and solutes through a membrane due to a greater pushing pressure (i.e., force) on one side of the membrane than the other.
    • Hydrostatic pressure is the mechanical force that water exerts against cell membranes; generated in vessels when the heart contracts; important for the urinary system.
    • Movement is from high concentration to low concentration (particles move down the concentration gradient).
  • Simple Diffusion — nonpolar molecules easily dissolved in and diffuse through the lipid bilayer. Some small hydrophilic particles can pass through some channels.
    • Movement is from high concentration to low concentration (particles move down the concentration gradient).
  • Facilitated Diffusion — uses integral proteins to assist with diffusion (e.g., glucose and amino acids transporters).
    • Integral proteins form channels.
    • Carrier molecules attract the substance to be transferred.
      • As binding occurs, the carrier molecule changes shape, depositing the particle to the other side of the membrane.
      • Carrier sites are specific and limited in number.
  • Osmosis — net movement of water across a semi-permeable membrane. Water molecules move from a high to a low water concentration gradient.
    • Water moves to where the solutes are higher in concentration.
    • Only occurs when the membrane is permeable to water and impermeable to at least one solute.

• Tonicity Revisited — a measure of an osmotic (e.g., water) gradient across a membrane.

  • Isotonic Solution — same solute concentration on either side of the cell membrane (i.e., same concentration in extracellular fluid and cytosol). The cell maintains shape and volume since there is no net osmosis.
  • Hypotonic Solution — higher solute concentration inside the cell than outside (i.e., more solutes in the cytosol than in the extracellular fluid). The cell swells as water enters and may undergo lysis (known as hemolysis).
  • Hypertonic Solution — higher solute concentration outside the cell than inside (i.e., more solute in the extracellular fluid than in the cytosol). The cell shrinks and may undergo crenation.
• Active Transport Details — requires the use of energy (ATP) to transport solutes across plasma membranes from an area of low concentration to an area of high concentration (against the concentration gradient).
  • Specialized protein carrier molecules are specific for a particular particle (e.g., sodium-potassium pump).
  • Calcium Pump = found in muscle cells; maintains unequal concentrations of Ca²⁺ on either side of the membrane. Allows the cell to force intracellular Ca²⁺ into special compartments or force Ca²⁺ out of the cell (two calcium ions are pumped out).
  • Sodium-Potassium Pump = found in all cells; maintains unequal concentrations of Na⁺ and K⁺ on either side of the membrane. Na⁺ has a high concentration outside of the cell, so Na⁺ tends to diffuse into the cell (vice versa for K⁺). (3 Na⁺ must be pumped out as 2 K⁺ are pumped in).
  • Vesicular Transport — moves particles across the membrane.
    • Endocytosis — particles moved by vesicles *into* the cell (engulfed by extensions of the cytoplasm).
      • Pinocytosis: brings fluid in.
      • Phagocytosis: allows cells to ‘eat’ large particles.
    • Exocytosis: particles are expelled from the cell (waste material).

Cellular Environment and Homeostasis

• The cellular environment — the cell is enclosed by a membrane that separates it from the external environment. Intracellular fluid exchanges nutrients and waste with extracellular fluid.

• Fluid exchange between compartments:
  

  
  

• Homeostasis at the cellular level — intracellular and extracellular fluid exchange equals cellular homeostasis (fluid, electrolytes, oxygen, and carbon dioxide balance).

• Cellular receptors allow cells to monitor their environment; they are protein molecules that are located in the cell membrane, cytoplasm, and nucleus.
  • Can bind with many substances and become activated/inactivated.

• Agonists and antagonists.
  

  
  

Pathogens and Infection

• Pathogen = any disease-causing microorganism. Pathogens cause disease by direct damage to tissues and through the production of toxins that may be disseminated through the body.
  • Opportunistic Pathogen = may be a part of the normal microbiota that causes a disease if it gains access to other tissues (e.g., skin bacteria entering underlying connective tissue).
• Infection = when the pathogen is able to multiply in the tissues of its host.
  • Infectious Disease = occurs when the pathogen has overwhelmed local host defense mechanisms.
  • Classes of Infectious Agents: bacteria, viruses (not cellular), parasites (uncommon), protozoa (uncommon), and algae (uncommon).
    • Classification schemes:
      • Bacteria (typically prokaryotes)
      • Archaea (different but similar to bacteria in appearance; thrive in extreme environments)
      • Eukarya (eukaryotic cells; protists, fungi, and all plants/animals)
• Fixation = heat (by passing a slide through a flame) or methanol (kills the organism, anchors it to a slide, and preserves its shape).
• Staining: helps to view organisms better.
  • Simple Stain, e.g., methylene blue, is sufficient to observe bacterial shape, flagellum, or spores.
  • Gram Stain — works by staining the peptidoglycan in the bacteria (thick layer vs. thin layer in their cell walls).
    • Crystal violet gets trapped in the thick layer, but *not* the thin layer.
• Chain of Infection: agent, virulence of pathogen, dose of pathogen, means of exposure to pathogen, and susceptibility of the host to the pathogen.
• Pathogenicity = ability of a pathogen to cause disease (ability to gain entry, attach to tissue and multiply, evade host defenses, and damage host tissues and cause symptoms).

Tissues and the Integumentary System

Tissue Structure

• Tissue — a group of similar cells that perform a common function (structures and organs are made up of tissue).
• Matrix — nonliving intercellular (between cells) material.
• Extracellular Matrix (ECM) — composed of water, proteins (structural proteins such as collagen and elastin), and glycoproteins (proteins with carbohydrates attached). It holds tissue in a single mass (skeletal muscle) and provides intercellular junctions that hold groups of cells together (plasma).

Main Types of Tissues

• Epithelial Tissue/Epithelium — covers body surfaces, lines cavities, and forms glands (lines heart, lungs, and digestive tract).
  • Supplied oxygen and nutrients from the capillaries in the connective tissue.
  • Functions = protection, absorption, secretion, and diffusion.

• Connective Tissue — binds organs, stores energy, and participates in body defenses.
  • Defined by exclusion (not epithelial, muscle, or nervous).
  • Loose connective (adipose/fat and reticular tissue).
  • Dense fibrous (regular - tendons and ligaments | irregular — dermis, capsules around liver, kidneys, spleen).
  • Functions: protection, fat storage, support, transport, connecting, and building tissues and organs.
• Muscle Tissue — produces physical force for body movement.
  • Skeletal, cardiac, and smooth.
• Nervous Tissue — detects and responds to changes in the internal and external environment.

Skeletal System

Bone Structure and Function

• Structure of Bone: Bone is a rigid connective tissue composed of cells, collagen fibers, ground substance, and mineral deposits.
  • Compact (Cortical) Bone: Dense, forms the outer layer; organized into Haversian systems.
  • Spongy (Trabecular) Bone: Found in the epiphyses; contains trabeculae, no Haversian systems.
• Function of Bone:
  • Provides form, support, and protection for vital organs.
  • Allows movement by serving as attachment points for muscles.
  • Is the site of blood cell formation (hematopoiesis).
  • Stores minerals, especially calcium and phosphate.

Bone Cell Types

• Osteoblasts: Bone-forming cells that produce osteoid (non-mineralized bone matrix); mature into osteocytes.
• Osteocytes: Mature bone cells that maintain bone tissue and regulate mineral content.
• Osteoclasts: Large, multinucleated cells that break down bone by releasing enzymes (resorption).

Skeletal Divisions

Axial Skeleton (80 bones):

• Skull: parietal, occipital, frontal, maxilla, mandible, nasal bones, orbits.
• Spine: vertebrae, sacrum, coccyx.
• Thorax: sternum (manubrium and xiphoid process), ribs.

Appendicular Skeleton (126 bones):

• Shoulder girdle: clavicle, scapula (acromion).
• Pelvis: ilium, pubis, ischium.
• Arms: humerus, radius, ulna.
• Hands: carpals, metacarpals, phalanges.
• Legs: femur (greater trochanter), patella, tibia, fibula.
• Feet: tarsals, metatarsals, phalanges.

Calcium Regulation

Regulation involves a balance between:

• Bone deposition (osteoblasts) – stores calcium.
• Bone resorption (osteoclasts) – releases calcium into blood.

Key Hormones:

• Parathyroid Hormone (PTH):
  • Secreted when blood calcium is low.
  • Stimulates osteoclasts to break down bone.
  • Enhances calcium reabsorption in kidneys.
  • Stimulates vitamin D activation → increases intestinal calcium absorption.
• Calcitonin (from the thyroid gland):
  • Secreted when blood calcium is high.
  • Inhibits osteoclasts.
  • Stimulates calcium deposition into bone via osteoblasts.

Organs Involved:

• Bones (storage and release),
• Kidneys (reabsorption or excretion),
• Intestines (absorption with vitamin D).

Joint Classification

Fibrous Joints:

Bones are joined by dense connective tissue.

• Types:
  • Suture: immovable (e.g., skull in children).
  • Syndesmosis: slightly movable (e.g., between radius and ulna).
  • Gomphosis: peg-in-socket (e.g., teeth in mandible/maxilla).
• Function: provide stability with little to no movement.

Cartilaginous Joints:

Bones joined by cartilage (either fibrocartilage or hyaline).

• Types:
  • Symphysis: fibrocartilage pad (e.g., intervertebral discs).
  • Synchondrosis: hyaline cartilage (e.g., between ribs and sternum).
• Function: allow more movement than fibrous joints, but less than synovial.

Synovial Joints (Diarthroses):

The most movable type of joint.

• Structure includes:
  • Joint capsule (fibrous tissue enclosing the joint).
  • Synovial membrane (secretes lubricating synovial fluid).
  • Joint cavity (space filled with synovial fluid).
  • Articular cartilage (covers bone ends, reduces friction).
  • Ligaments/tendons: reinforce and stabilize the joint.
  • Intraarticular menisci: shock absorbers (e.g., in the knee).
• Function:
  • Allow smooth movement in various directions (uniaxial, biaxial, or multiaxial).
  • Enable complex actions like walking, throwing, and grasping.

Muscular System

Muscle Structure and Function

• Structure:
  • Skeletal muscle is made of bundles of muscle fibers, which contain myofibrils made up of myofilaments (actin and myosin).
  • Connective Tissue Layers:
    • Epimysium surrounds the entire muscle.
    • Perimysium surrounds fascicles (bundles of muscle fibers).
    • Endomysium surrounds individual muscle fibers.
  • Muscle fibers are striated and under voluntary control.
  • Each muscle fiber has a sarcolemma (cell membrane), sarcoplasm (cytoplasm), and sarcoplasmic reticulum (stores calcium).
• Function:
  • Enable movement by contracting.
  • Maintain posture and stabilize joints.
  • Generate heat during activity.
  • Support body structures and protect organs.

Muscle Contraction Mechanism

• The neuromuscular junction (NMJ) is the synapse between a motor neuron and a muscle fiber.
• Steps of Transmission:
  • An action potential travels down the motor neuron.
  • At the NMJ, the neuron releases acetylcholine (ACh) into the synaptic cleft.
  • ACh binds to cholinergic receptors on the sarcolemma.
  • This triggers sodium influx and potassium efflux, causing depolarization.
  • The signal travels along T-tubules, leading to calcium release from the sarcoplasmic reticulum.
  • ACh is then broken down and recycled.
• Result: The muscle fiber initiates contraction in response to the nerve signal.

Role of Calcium and ATP

• Calcium (Ca²⁺):
  • Released from the sarcoplasmic reticulum in response to excitation.
  • Binds to troponin, causing tropomyosin to shift and expose myosin-binding sites on actin.
  • Enables cross-bridge formation between actin and myosin.
• ATP:
  • Energizes myosin heads to attach to actin.
  • Needed for cross-bridge cycling (attach, pull, release, reset).
  • Required for active transport of calcium back into the sarcoplasmic reticulum during relaxation.
  • Both contraction and relaxation are active processes requiring ATP.

Key Muscle Groups

Some key muscle groups and their clinical relevance include:

• Quadriceps Femoris (front thigh):
  • Includes rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius.
  • Important for knee extension; often injured in sports.
• Hamstrings (back thigh):
  • Biceps femoris, semitendinosus, semimembranosus.
  • Crucial for knee flexion and hip extension; commonly strained.
• Pectoralis Major (“pecs”):
  • Responsible for arm adduction and flexion.
  • Important in lifting and pushing movements.
• Rectus Abdominis (“abs”):
  • Stabilizes the core and spine.
  • Key in posture and injury prevention.
• Pelvic Floor Muscles:
  • Support bladder, bowel, and reproductive organs.
  • Vital in continence, childbirth, and pelvic health.
• Biceps Brachii (front upper arm):
  • Flexes the elbow and supinates the forearm.
• Triceps Brachii (back upper arm):
  • Extends the elbow; essential for pushing movements.

Nervous System

Neurons (Functional Unit)

• Cell body (soma): contains the nucleus.
• Dendrites: receive signals.
• Axon: transmits impulses.
• Myelin sheath: speeds up transmission.
• Types:
  • Sensory neurons: carry impulses to the CNS.
  • Motor neurons: carry impulses to effectors.
  • Interneurons: integrate information between sensory and motor neurons.

Action Potential

Definition:

An electrical signal generated by neurons to transmit information.

Key Ions:

• Na⁺ (sodium): rushes into the neuron during depolarization.
• K⁺ (potassium): exits during repolarization.
• Na⁺/K⁺ pump: restores the resting state (3 Na⁺ out, 2 K⁺ in).

Stages:

1. Resting membrane potential: -70 mV.
2. Depolarization: Na⁺ channels open → inside becomes positive.
3. Repolarization: K⁺ channels open → K⁺ exits the cell.
4. Hyperpolarization: membrane becomes more negative than the resting potential.
5. Refractory period:
   • Absolute: no new AP possible.
   • Relative: AP possible if the stimulus is strong.

Synapse and Transmission

Synapse:

A junction between two neurons (or a neuron and an effector), where communication occurs via neurotransmitters.

Key Components:

• Presynaptic neuron
• Synaptic cleft
• Postsynaptic neuron

Events:

1. Action potential reaches the axon terminal.
2. Neurotransmitter (e.g., ACh) is released into the synaptic cleft.
3. Binds to receptors on the postsynaptic membrane.
4. Ion channels open → change in membrane potential.
5. If the threshold is reached → action potential generated in the postsynaptic neuron.

Myelin's Role:

• Insulates axons and speeds up transmission.
• Allows the action potential to jump between nodes (saltatory conduction).

Haematology (Blood)

Composition and Functions of Blood

Composition of Blood:

• Plasma (50–55% of blood volume):
  • 90% water, 10% solutes.
  • Contains plasma proteins (albumins, globulins, fibrinogen), nutrients, hormones, and electrolytes.
• Formed Elements (45%):
  • Erythrocytes (RBCs) – oxygen transport.
  • Leukocytes (WBCs) – immune defense.
  • Platelets (thrombocytes) – clotting.

Functions of Blood:

• Transport of oxygen, nutrients, and hormones.
• Removal of waste products.
• Defense against infection (via WBCs).
• Clotting to prevent blood loss.
• Maintaining acid-base balance and homeostasis.

Erythrocytes and Gas Transport

Production (Erythropoiesis):

• Erythrocytes are produced in the bone marrow from erythroblasts.
• Stimulated by erythropoietin (EPO), secreted by the kidneys in response to hypoxia.
• Reticulocytes are released into the bloodstream and mature into erythrocytes.

Role in Gas Transport:

• Shape: Biconcave and deformable – increases surface area for gas exchange.
• Content: Filled with hemoglobin, which binds oxygen (O₂) and carbon dioxide (CO₂).
• No nucleus or mitochondria, allowing more room for hemoglobin.
• Hemoglobin contains iron, which binds O₂ in the lungs and releases it in tissues.

Haemostasis (Clotting)

The process that stops bleeding from damaged vessels, involving:

1. Vasoconstriction – immediate narrowing of blood vessels.
2. Platelet plug formation – platelets adhere to exposed vessel walls and to each other.
3. Coagulation cascade – clotting factors activate:
   • Converts fibrinogen (soluble) into fibrin (insoluble) to form a stable clot.
4. Clot retraction – platelets contract to tighten the clot and close the wound.
5. Fibrinolysis:
   • Plasminogen is activated to plasmin, which breaks down fibrin, dissolving the clot after healing.

Blood Group Systems

ABO Blood Group System:

• Based on the presence/absence of A and B antigens on RBCs.
  • Type A: A antigens, anti-B antibodies.
  • Type B: B antigens, anti-A antibodies.
  • Type AB: Both A and B antigens, no antibodies – universal recipient.
  • Type O: No A/B antigens, both antibodies – universal donor.

Rhesus (Rh) Factor:

• Rh-positive: Has D antigen.
• Rh-negative: Lacks D antigen and may develop anti-D antibodies if exposed to Rh-positive blood.

Safe Transfusion:

• Must match ABO and Rh type to prevent agglutination (clumping) and hemolytic reactions.
• O negative is the universal donor; AB positive is the universal recipient.

Immunology and Lymphatic System

Lymphatic System Components

Lymph:

• Structure/Content: Clear fluid derived from blood plasma, containing electrolytes, lipids, proteins, antigens, bacteria, lymphocytes, and macrophages.
• Function:
  • Maintains fluid balance in tissues.
  • Transports immune cells and waste.
  • Facilitates removal of pathogens and debris.

Lymphatic Vessels:

• Transport lymph through afferent vessels into lymph nodes and out via efferent vessels.
• Includes the right lymphatic duct and thoracic duct, which return filtered lymph to the bloodstream.

Lymph Nodes:

• Structure: Bean-shaped filters located throughout the body.
• Function:
  • Filter lymph to remove pathogens.
  • Facilitate lymphocyte maturation.
  • Act as hubs for immune cell activation.

Lymphoid Organs:

• Primary organs: Bone marrow (B cell maturation) & thymus (T cell maturation).
• Secondary organs: Lymph nodes, spleen, tonsils, Peyer’s patches.
  • Spleen:
    • White pulp: Contains lymphocytes and macrophages – immune surveillance.
    • Red pulp: Recycles old erythrocytes.

Immune Defenses

Innate Immunity (Non-specific):

• First Line of Defense:
  • Physical barriers – skin, mucous membranes.
  • Chemical barriers – enzymes (lysozyme), antimicrobial peptides, secretions like saliva and mucus.
• Second Line of Defense:
  • Phagocytes (neutrophils, macrophages) engulf pathogens.
  • Natural Killer (NK) cells destroy infected cells.
  • Chemical mediators like cytokines and interferons promote inflammation and recruit immune cells.

Adaptive Immunity (Specific):

• Third Line of Defense:
  • B lymphocytes (humoral immunity): Produce antibodies via plasma cells.
  • T lymphocytes (cell-mediated immunity): Include helper T cells (activate B and cytotoxic T cells) and cytotoxic T cells (kill infected cells).
• Key Features:
  • Specificity: Targets particular antigens.
  • Memory: Faster and stronger response upon re-exposure to the same antigen.

Immunological Memory and Vaccination

Immunological Memory:

• Developed through adaptive immunity.
• Involves the creation of memory B and T cells after exposure to an antigen.
• Leads to a faster and stronger secondary response upon re-exposure.
  • Primary response: Slower, involves IgM initially.
  • Secondary response: Rapid, high IgG production.

Vaccinations:

• Mimic infection using attenuated or inactivated antigens.
• Stimulate active immunity without causing disease.
• Promote long-term protection by forming memory cells.