Human Anatomy and Physiology: Body Systems, Tissues & Cells
Major Body Systems
Cardiovascular System
Heart: Pumps blood. Arteries/Veins: Transport blood. Blood: Carries oxygen, CO₂, nutrients, waste, and immune components.
Respiratory System
Nasal cavity: Filters air. Larynx: Produces voice and protects the airway. Lungs: Gas exchange (O₂, CO₂).
Digestive System
Mouth: Begins breakdown of food. Stomach: Acid and enzymes. Small intestine: Absorbs nutrients. Large intestine: Removes water and stores waste.
Urinary System
Kidneys: Form urine, maintain pH balance, regulate blood pressure. Bladder: Stores urine. Urethra: Excretes urine.
Reproductive System
Male: Testes (sperm), prostate, seminal fluid. Female: Ovaries (eggs, hormones), uterus (development), mammary glands (nutrition).
Tissues
Epithelial: Covers and protects. Connective: Provides support and transport. Muscle: Produces movement. Nervous: Conducts nerve impulses.
Supportive and Protective Systems
Integumentary System
Epidermis: Protects and produces vitamin D. Dermis: Contains hair, glands, nails, receptors; provides sensation, lubrication, and cooling. Hypodermis: Stores fat and attaches skin to underlying structures.
Muscular System
Axial: Supports and positions the skeleton. Appendicular: Moves the limbs. Tendons: Connect muscle to bone (rope-like). Aponeuroses: Connect muscle to muscle (sheet-like).
Skeletal System
Axial: Skull, spine, ribs (protect organs, support weight). Appendicular: Limb bones (support movement). Cartilage: Hyaline (ribs, joints), fibrocartilage (discs).
Bone marrow: Red: Blood cell production. Yellow: Fat storage.
Nervous System
CNS: Brain (control) and spinal cord (relays information). PNS: Links the CNS to the body. Special senses: Sight, hearing, smell, taste.
Endocrine System
Pineal: Regulates sleep (melatonin). Pituitary: Controls growth and fluid balance. Thyroid: Regulates metabolism (thyroxine). Parathyroid: Regulates calcium.
Adrenal: Responds to stress (cortisol, adrenaline). Pancreas: Controls glucose. Gonads: Support reproduction.
Lymphatic System
Vessels: Carry lymph. Nodes: Monitor fluid and initiate immune responses. Spleen: Monitors blood and recycles red blood cells. Thymus: Supports T-cell development.
Epithelial Tissue: Structure and Functions
Key Functions of Epithelium
Selective barriers – Regulate transfer of substances.
Secretory function – Release substances onto free surfaces.
Protective role – Shield against physical and chemical damage.
Cell Arrangements and Shapes
Cell arrangements:
- Simple epithelium – Single cell layer (for absorption, secretion, filtration).
- Stratified epithelium – Multiple layers (for protection).
- Pseudostratified epithelium – Appears layered but is actually a single layer.
Cell shapes:
- Squamous – Flat, allows diffusion.
- Cuboidal – Cube-shaped, involved in secretion and absorption.
- Columnar – Taller than wide, facilitates absorption and secretion.
- Transitional – Changes shape; found in stretchable organs (e.g., bladder).
Cell Junctions
Tight junctions – Prevent leakage (e.g., stomach, intestines, bladder).
Adherens junctions – Provide strong mechanical attachments.
Desmosomes – Resist mechanical stress (e.g., skin, heart muscle).
Hemidesmosomes – Attach cells to the basement membrane.
Gap junctions – Allow communication between cells.
Basement Membrane
Epithelial cells rest on a basement membrane, composed of:
- Basal lamina – Secreted by epithelial cells (contains collagen, laminin, etc.).
- Reticular lamina – Secreted by connective tissue cells (contains fibronectin, collagen, etc.).
Functions of the basement membrane: Provides structural support for epithelial cells, a surface for cell migration during wound healing, a physical barrier for filtration (e.g., kidneys), and it facilitates exchange of nutrients and waste via diffusion.
Covering and Lining Epithelium
Simple epithelium:
Simple squamous: Thin, flat cells for diffusion and filtration (e.g., lung alveoli, blood vessels).
Simple cuboidal: Cube-shaped, involved in secretion and absorption (e.g., kidney tubules, glands).
Simple columnar:
Non-ciliated: With microvilli for absorption (e.g., intestines).
Ciliated: Moves substances (e.g., fallopian tubes, respiratory tract).
Stratified and Transitional Epithelium
Stratified squamous:
Keratinized: Found in skin for protection.
Non-keratinized: Found on wet surfaces (e.g., mouth, esophagus).
Stratified cuboidal: Found in some glands (e.g., sweat glands).
Stratified columnar: Rare; found in the male urethra.
Transitional epithelium: Found in the bladder; allows stretching.
Pseudostratified Epithelium
Pseudostratified columnar epithelium:
Ciliated: Found in the upper respiratory tract; moves mucus.
Non-ciliated: Found in larger ducts of glands; involved in absorption and protection.
Types of Glands
Exocrine glands – Secrete onto surfaces or into ducts (e.g., sweat glands, salivary glands).
Endocrine glands – Release hormones directly into the bloodstream (e.g., thyroid, adrenal glands).
Connective Tissue: Composition and Functions
Functions of Connective Tissue
Connective tissue (CT) binds, supports, and strengthens other tissues. It functions as a major transport system (e.g., blood) and as major energy storage (e.g., adipose tissue).
Features of CT:
Unlike epithelia: CT is not found on surfaces and can be vascular (except cartilage and tendons).
Like epithelia: CT is supplied by nerves (except cartilage).
CT Composition
CT = ECM + cells. ECM = ground substance + fibers.
Ground substance: Water, proteins, and polysaccharides (GAGs).
GAGs form proteoglycans and trap water (jelly-like consistency).
Hyaluronic acid: Lubrication, joint movement, eye shape.
Chondroitin sulfate: Found in cartilage, bone, skin, and vessels.
Keratan sulfate: Present in bone, cartilage, and cornea.
Dermatan sulfate: Found in skin, tendons, vessels, and heart valves.
Connective Tissue Fibers and Disorders
Collagen fibers: Strong, flexible, most abundant; found in bone, cartilage, tendons, and ligaments.
Reticular fibers: Thin, branching support networks; found in basement membranes, adipose tissue, and nerve fibers.
Elastic fibers: Thin; can stretch ~150% without breaking; found in skin and blood vessels.
Disorders:
Marfan syndrome: Defect in elastic fibers; mutation in the fibrillin gene; affects growth, heart, and skeletal system.
Exophthalmos (thyroid disease): Excess GAGs cause water retention and swelling in the eye area.
Bone Cells and Liquid Connective Tissue
Bone cells:
Osteogenic: Stem cells.
Osteoblasts: Bone formation.
Osteocytes: Maintain bone.
Osteoclasts: Bone resorption.
Liquid Connective Tissue: Blood
Blood = Plasma (ECM) + Cells
Red blood cells (RBCs): Transport oxygen.
White blood cells (WBCs): Provide immunity.
Platelets: Support clotting.
Embryonic and Mature Connective Tissue
Embryonic Connective Tissue
Mesenchyme: Gives rise to all connective tissues.
Mucous connective tissue: Supports the umbilical cord.
Connective Tissue Cell Types
Fibroblasts: Secrete ECM components.
Adipocytes: Store fat.
Macrophages: Phagocytic (fixed or wandering).
Plasma cells: Produce antibodies.
Mast cells: Produce histamine.
Leukocytes: White blood cells for immune function.
Mature Connective Tissue Types
Loose CT (more cells, fewer fibers):
Areolar: Packing material; provides strength, elasticity, and support.
Adipose: Energy storage, insulation, and temperature control.
Reticular: Structural support network.
Dense CT (more fibers, fewer cells):
Regular: Tendons, ligaments, aponeuroses; slow healing.
Irregular & elastic: Structural support and elasticity.
Supporting CT: Cartilage
Hyaline: Most abundant; provides flexibility and movement.
Elastic & fibrocartilage: Specialized types; provide resilience and strength.
Supporting CT: Bone
Compact bone: Organized into osteons; stores calcium and phosphorus.
Spongy bone: Lacks osteons; stores fat and produces blood cells.
Bone Healing and Structure
Bone healing:
Chondroblasts lay down a cartilage callus.
Osteoclasts remove dead bone.
Osteoblasts form new bone.
Osteoclasts remodel bone structure.
Osteon (Haversian system):
Lamellae: Mineralized rings composed of calcium and collagen.
Lacunae: Spaces that house osteocytes.
Canaliculi: Channels for nutrient and waste exchange.
Haversian canal: Contains blood vessels, lymphatics, and nerves.
Muscle Tissue and Organization
Muscle Tissue: General Features
General features: Elongated myocytes use ATP to generate force → movement, posture, and heat.
Types of muscle (3):
Skeletal: Striated, voluntary, cylindrical, multinucleate, attached to bones via tendons (approximately 650 muscles).
Cardiac: Striated, branched, single nucleus, involuntary, contains intercalated discs (desmosomes + gap junctions).
Smooth: Non-striated, involuntary, spindle-shaped, single nucleus; found in hollow organs and blood vessels.
Skeletal Muscle Structure
Myofibrils: Contain actin (thin) and myosin (thick) filaments, organized into sarcomeres.
Sarcomere components:
- A band: Region containing thick filaments.
- I band: Region containing thin filaments only.
- H zone: Region containing thick filaments only.
- M line: Middle line that holds thick filaments.
- Z disc: Separates sarcomeres and anchors thin filaments.
Connective tissue layers:
Epimysium: Surrounds the whole muscle.
Perimysium: Surrounds fascicles.
Endomysium: Surrounds individual muscle fibers.
Cardiac and Smooth Muscle Features
Cardiac muscle special features:
Intercalated discs: Contain desmosomes (adhesion) and gap junctions (communication).
Purkinje fibers: Specialized for electrical conduction; have fewer myofibrils and more gap junctions.
Smooth muscle special features:
No striations, but contains actin and myosin. Dense bodies act as Z-disc equivalents and anchor actin filaments. Intermediate filaments (non-contractile) stabilize the cell. Contraction often causes cell twisting.
Nervous System: Function and Cells
Functions of the Nervous System
Sensory: Detects stimuli and sends information to the CNS.
Integrative: Processes, stores, and analyzes information.
Motor: Sends signals to effectors (muscles and glands).
Divisions of the Nervous System
CNS: Brain and spinal cord.
PNS: Sensory (afferent): Sends information to the CNS. Motor (efferent): Sends information from the CNS to effectors.
Neurons
General properties: Limited cell division, high metabolic rate; neurons die quickly without oxygen.
Structure: Dendrites: Receive signals. Axon: Transmits signals away. Soma (cell body): Processes information.
Types of Neurons
Multipolar: Many dendrites and one axon; most common in the CNS and motor neurons.
Bipolar: One dendrite and one axon; found in special senses (eye, ear, olfaction).
Unipolar: Dendrite and axon are continuous; found in sensory neurons.
Anaxonic: No clear distinction between axons and dendrites; found in the brain with unclear functions.
Glial Cells
CNS glial cells:
Astrocytes: Maintain the blood-brain barrier, regulate ions, and provide support.
Oligodendrocytes: Myelinate CNS axons and increase signal speed.
Microglia: Macrophages of the CNS; provide immune defense.
Ependymal cells: Produce cerebrospinal fluid (CSF); have cilia and microvilli.
PNS glial cells:
Schwann cells: Myelinate PNS axons (one cell per internode) or support multiple unmyelinated axons.
Satellite cells: Support neuron cell bodies and regulate the local environment (analogous to astrocytes in the CNS).
Cell Structure, Organelles and Theory
Main Parts of the Cell
Three main parts:
- Plasma membrane
- Nucleus
- Cytoplasm
Plasma membrane: A selectively permeable barrier composed of a phospholipid bilayer with hydrophilic heads and hydrophobic tails. Membrane proteins include integral (e.g., transmembrane) and peripheral proteins. Functions include transport, cell signaling, intercellular communication, and cell identification.
Cell Organelles
Nucleus: Stores DNA, synthesizes RNA, assembles ribosomes; contains the nuclear envelope, nucleolus, chromatin, and chromosomes.
Rough & Smooth ER: Rough ER synthesizes secretory and membrane proteins; smooth ER synthesizes lipids and stores ions.
Golgi apparatus: Modifies, sorts, and ships proteins received from the ER.
Lysosomes: Contain digestive enzymes for breaking down substances, autophagy, and autolysis.
Mitochondria: Produce ATP and contain their own genome.
Cytoskeleton: Microfilaments (actin), intermediate filaments (e.g., keratin), and microtubules (tubulin). Functions: structural support, intracellular transport, cell movement, and division.
Cell Theory and Organization
Cell theory: All living organisms are made of cells, and all cells arise from pre-existing cells.
Eukaryotic vs. prokaryotic cells: Eukaryotes have membrane-bound organelles and are generally larger. Prokaryotes lack a membrane-bound nucleus.
DNA, RNA, proteins: Central dogma – DNA → RNA → Protein.
DNA structure: Chromatin, nucleosomes, histones, and chromosomes.
Membrane proteins: Include transporters, channels, enzymes, and signaling molecules.
Endomembrane system: Works with the plasma membrane for intracellular trafficking of molecules.
ATP, Cellular Respiration and Energy Metabolism
ATP and Energy
Key concepts for ATP and cellular respiration:
ATP breakdown and energy release: ATP hydrolysis releases energy; ATP is the cell’s energy currency. The ATP cycle transfers energy between complex and simple molecules in the body.
Major fuel sources for ATP production:
- Carbohydrates → broken down into simple sugars.
- Proteins → broken down into amino acids.
- Fats → broken down into fatty acids and glycerol.
Overview of Cellular Respiration
Glycolysis: Occurs in the cytosol; splits glucose into two pyruvate molecules, generating a net of 2 ATP and 2 NADH.
Pyruvate oxidation: Converts pyruvate into acetyl-CoA in the mitochondrial matrix, producing NADH and CO₂.
Citric acid cycle (Krebs cycle): Occurs in the mitochondrial matrix; produces ATP, NADH, FADH₂, and CO₂.
Oxidative phosphorylation: Occurs in the inner mitochondrial membrane; involves the electron transport chain and chemiosmosis, producing most ATP (approximately 26–28 ATP).
ATP Generation Mechanisms
Substrate-level phosphorylation: ATP generated by direct transfer of a phosphate from a substrate to ADP.
Oxidative phosphorylation: ATP generated through the electron transport chain and chemiosmosis.
Electron transport chain and chemiosmosis: NADH and FADH₂ donate electrons, creating a proton gradient used by ATP synthase to produce ATP. Oxygen is the final electron acceptor, forming water. The electron transport chain can be blocked by agents such as cyanide, causing cell death.
Regulation of Cellular Respiration and Blood Glucose
Phosphofructokinase: The gatekeeper of glycolysis; inhibited by citrate and ATP and stimulated by AMP (when ATP usage is high).
Blood glucose regulation:
Insulin: Secreted by beta cells of the pancreas; promotes glucose uptake for ATP production and storage as glycogen.
Glucagon: Secreted by alpha cells; stimulates glycogen breakdown to release glucose into the bloodstream.
Diabetes Mellitus
Type 1 diabetes: Autoimmune destruction of beta cells leading to lack of insulin production; requires insulin replacement.
Type 2 diabetes: Insulin resistance; often associated with older age, obesity, and other pathologies.
Homeostasis of blood glucose: Normal fasting blood glucose is around 4–6 mmol/L. Hyperglycemia is >7 mmol/L and hypoglycemia is <4 mmol/L.
Symptom paradox in diabetes: Increased hunger and weight loss – despite eating more, the body cannot utilize glucose effectively, leading to persistent hunger and weight loss.
Cell Communication and Signaling
Why Cells Communicate
Cell communication types:
- Autocrine: Signals act on the signaling cell itself.
- Paracrine: Signals act on nearby cells (e.g., fibroblast growth factor).
- Synaptic: Neurotransmitters like acetylcholine act on neighboring cells.
- Endocrine: Hormones travel through the circulatory system to distant cells (e.g., insulin).
Cell signaling process:
Reception: A signaling molecule binds to a receptor protein; the receptor undergoes a conformational and/or chemical change.
Transduction: The activated receptor triggers a chain of molecular events, including enzyme activation (e.g., G-proteins, adenylyl cyclase).
Phosphorylation cascade: A sequence of protein kinases adds phosphates to subsequent proteins, activating them.
Response: Final proteins lead to cellular responses such as gene expression changes, ion channel regulation, or metabolic adjustments.
Types of Receptors and Secondary Messengers
G protein-coupled receptors (GPCRs): Transmembrane proteins that traverse the membrane seven times. They are activated when ligands bind, which activates the associated G-protein.
Signaling mechanism: GDP is exchanged for GTP on the G-protein, activating it and triggering downstream cellular changes. GTP is later hydrolyzed to GDP, turning off the signal.
Ligand-gated ion channels: Ion channels that open or close upon ligand binding (e.g., neurotransmitters). Examples include channels for Na⁺, K⁺, and Ca²⁺; they mediate rapid cellular responses such as action potentials in nerve cells.
Secondary messengers:
cAMP (cyclic AMP): Produced by adenylyl cyclase after receptor activation; activates protein kinase A (PKA), which phosphorylates target proteins to elicit a response. Example: cholera toxin disrupts this pathway.
Calcium ions (Ca²⁺): Normally low inside the cell and high outside. Ca²⁺ acts as a second messenger by binding to proteins and activating pathways. Example: Calcium release from the ER activates proteins such as phospholipase C.
Signal Amplification and Cellular Responses
Signal amplification: A single signaling molecule can trigger a cascade that amplifies the response, activating a large number of downstream molecules. Example: Adrenaline stimulates glycogen breakdown to glucose-1-phosphate, producing a rapid energy boost.
Cellular responses to signaling (examples):
- Changes in gene expression (activation or inhibition of transcription factors).
- Alteration of protein function (enzyme activation).
- Opening or closing of ion channels.
- Metabolic changes.
- Cytoskeletal rearrangements.
Turning off the signal: Phosphodiesterase (PDE) breaks down cAMP, ensuring signals are terminated after the response. Some drugs (e.g., sildenafil/Viagra) inhibit specific PDEs to prolong signaling.
Example of signaling: adrenaline/epinephrine signaling:
In response to stress, adrenaline binds to GPCRs, leading to production of cAMP, which activates protein kinases that promote glycogen breakdown, rapidly increasing available energy.