Anatomy, Physiology, and the Science of Tissues

Foundations of Anatomy and Physiology

  • Definition of Disciplines:
    • Anatomy: The study of the structure and relationships between body parts. It focuses on what the body is.
    • Physiology: The science of how body parts come together to function and keep the body alive. It focuses on what the body does.
  • The Science of Us: These disciplines draw heavily from other sciences, including chemistry and physics, and utilize a specialized vocabulary consisting of Latin and Greek terms.
  • Applications of Study: Understanding anatomy and physiology explains why we are alive, how we came to be, how disease harms the body, and how recovery occurs from illness or injury.
  • Historical Context of Anatomy:
    • Early Taboos: For centuries, dissecting human bodies was taboo, leading to a slow and often clandestine history for the field.
    • Galen (2nd Century): A Greek physician who deduced information about the human form by performing vivisections on pigs.
    • Leonardo da Vinci: Sketched detailed anatomical drawings based on human dissections until the Pope ordered him to stop.
    • 17th and 18th Centuries: Certified anatomists were permitted to perform regulated public human dissections, sometimes involving admission fees and attended by figures like Michelangelo and Rembrandt.
    • The Anatomy Act of 1832 (Britain): Passed to combat the lucrative and illegal occupation of grave robbing by providing students with the corpses of executed murderers.
    • Modern Education: Today, cadavers for study are provided by volunteers who donate their bodies to science.

Fundamental Principles of Organization and Function

  • Complementarity of Structure and Function: A unifying theme stating that the function of a cell, organ, or organism always reflects its physical form.
    • Example: Heart valves allow blood to flow in only one direction because their shape prevents backward flow.
    • Example: Bones are strong and hard, allowing them to support and protect soft tissues.
  • Hierarchy of Organization:
    1. Chemical Level: The body is composed of approximately 7×10277 \times 10^{27} atoms (seven octillion).
    2. Cellular Level: The smallest units of living things. They range in size from red blood cells (5×10−6 m5 \times 10^{-6}\,m) to motor neurons that can be one meter long.
    3. Tissue Level: Groups of similar cells that work together (e.g., muscles, nervous, connective tissues).
    4. Organ Level: Two or more tissue types combining to perform specific functions (e.g., heart, liver, stomach).
    5. Organ System Level: Groups of organs working together (e.g., the digestive system combining the liver, stomach, and intestines).
    6. Organism Level: The highest level, representing the complete human body.
  • Homeostasis: The ability of all living systems to maintain stable internal conditions (balance of materials and energy) regardless of changes in the external environment.
    • Ultimate Cause of Death: The extreme and irreversible loss of homeostasis (e.g., organ failure, hypothermia, suffocation).
    • Example of Blood Pressure: A loss of blood leads to a drop in blood pressure, preventing oxygen delivery to cells, leading to a loss of homeostasis and eventual death.

Anatomical Language and Directional Terms

  • Standard Anatomical Position: The body is erect, facing straight ahead, arms at the sides, with palms facing forward.
  • Body Planes:
    • Sagittal Plane: Vertically divides the body into left and right parts.
    • Parasagittal Plane: Parallel to the sagittal plane but off to one side.
    • Coronal (Frontal) Plane: Vertically divides the body into front and back.
    • Transverse (Horizontal) Plane: Divides the body into top and bottom (superior and inferior).
  • Body Divisions:
    • Axial Part: Includes the head, neck, and trunk.
    • Appendicular Part: Includes the limbs (appendages) attached to the body's axis.
  • Directional Pairs:
    • Anterior (Ventral): Toward the front of the body (e.g., eyes are anterior).
    • Posterior (Dorsal): Toward the back of the body (e.g., the butt is posterior).
    • Superior (Cranial): Toward the top of the body (e.g., the jaw is superior to the lungs).
    • Inferior (Caudal): Toward the bottom of the body (e.g., the pelvis is inferior to the stomach).
    • Medial: Toward the midline of the body.
    • Lateral: Farther away from the midline.
    • Proximal: Closer to the center of the trunk (used for limbs, e.g., the knee is proximal to the ankle).
    • Distal: Farther away from the center of the trunk (e.g., the wrist is distal to the elbow).

Histology: The Study of Tissues

  • Definition: Tissues are groups of similar cells that perform a common function. The term "tissue" literally means "woven."
  • History of Histology:
    • Hans and Zacharias Janssen (1590s): Invented the first microscope.
    • Anton von Leeuwenhoek (Late 1600s): Created high-power microscopes (270×270\times magnification) and was the first to observe microorganisms, bacteria, and muscle fibers.
    • Staining: Necessary to see individual cells. Joseph von Gerlach (1850s) discovered a neural stain using a diluted version of carmine (red dye from crushed insects), which revealed the nuclei of brain cells and their connecting fibers.
  • The Four Primary Tissue Types:
    1. Nervous Tissue: Provides control and communication.
    2. Muscle Tissue: Provides movement.
    3. Epithelial Tissue: Lines body cavities, covers and protects the body; creates order out of chaos.
    4. Connective Tissue: Provides support.

Nervous and Muscle Tissues

  • Nervous Tissue:
    • Functions: Sensing stimuli and sending electrical impulses.
    • Neurons: Specialized building blocks that generate and conduct electrochemical nerve impulses. Structure includes the cell body (soma), dendrites (listening end), and axon (transmission cable).
    • Glial Cells: The "pit crew" that provides support, insulation, and protection, tethering neurons to blood vessels.
  • Muscle Tissue: Highly vascularized (rich blood supply).
    • Skeletal Muscle: Long, cylindrical, striated (pinstriped), and voluntary. Attaches to bones to facilitate movement.
    • Cardiac Muscle: Found only in the heart. Striated but usually uninucleate (one nucleus). Cells divide and converge; connected by intercalated discs to pass signals. Involuntary.
    • Smooth Muscle: Non-striated, uninucleate, and involuntary. Lines blood vessels and hollow organs (digestive, urinary tracts, uterus).

Epithelial Tissue

  • Proper Epithelium: Covers and lines outer and inner body surfaces.
  • Glandular Epithelium: Forms glands and secretes hormones.
  • Characteristics:
    • Polarity: Cells have an apical (upper) side exposed to the surface/cavity and a basal (inner/bottom) side attached to the basement membrane (mostly collagen fibers).
    • Avascular: No direct blood supply; relies on diffusion from connective tissue.
    • Selectively Permeable: Allows for regulated absorption, filtration, and excretion.
  • Classification by Shape:
    • Squamous: Flat, scale-like; nucleus is also flat. Found where fast absorption/transport is needed (e.g., lung air sacs).
    • Cuboidal: Cube-shaped; circular nucleus; used for secretion and absorption (e.g., sweat glands).
    • Columnar: Tall and thick; ellipse-shaped nucleus; cushions tissues and secretes (e.g., stomach lining).
  • Classification by Layering:
    • Simple: One layer of cells.
    • Stratified: Multiple layers set on top of each other (for protection, e.g., skin).
    • Pseudostratified: A single layer of varying shapes and heights that gives the appearance of multiple layers.
  • Glands:
    • Endocrine: Secrete hormones directly into the bloodstream (e.g., thyroid secreting thyroxine).
    • Exocrine: Secrete substances into ducts that lead to an epithelial surface (e.g., sweat, saliva, mucus, stomach acid, milk).

Connective Tissue and Marfan Syndrome

  • Case Study: Flo Hyman: An Olympic volleyball player who died in 1986 from an undiagnosed case of Marfan Syndrome. Marfan is a genetic disorder that weakens connective tissue over time, often resulting in tall, thin stature and potentially fatal aortic tears.
  • Common Origin: All connective tissues develop from mesenchyme, a fluid embryonic tissue.
  • Extracellular Matrix (ECM): Connective tissues are mostly nonliving material. The ECM consists of:
    • Ground Substance: A watery, rubbery material made of proteoglycans and glycosaminoglycans (GAGs).
    • Fibers:
      • Collagen: The strongest and most abundant; tough and flexible.
      • Elastic Fibers: Made of elastin; allow tissues to stretch and coil.
      • Reticular Fibers: Short, fine collagen fibers that form sponge-like networks.
  • Cell Types:
    • Blast Cells: Immature, forming state (e.g., chondroblasts for cartilage, osteoblasts for bone); secrete ground substance and fibers.
    • Site Cells: Mature, less active phase (e.g., chondrocytes, osteocytes); maintain matrix health.
    • Immune Cells: Macrophages (eat bacteria/dead cells) and Leukocytes (white blood cells).

Subtypes of Connective Tissue

  • Connective Tissue Proper:
    • Loose:
      • Areolar: Found under epithelium; stores ground substance.
      • Adipose: Fat tissue; stores lipids; average body weight is 18%18\% adipose.
      • Reticular: Provides internal framework (stroma) for the spleen and lymph nodes.
    • Dense:
      • Regular: Parallel collagen bundles; found in tendons (muscle to bone) and ligaments (bone to bone).
      • Irregular: Erratic fiber arrangement; found in the leathery dermis.
      • Elastic: Found in large artery walls and connecting vertebrae.
  • Cartilage: Avascular; stands up to tension and compression.
    • Hyaline: Glassy; connects ribs to sternum and forms the tip of the nose.
    • Elastic: High stretchability (e.g., the ear).
    • Fibrocartilage: Shock absorber (e.g., intervertebral discs, knee joints).
  • Bone (Osseous Tissue): Calcified connective tissue.
    • Spongy: Found in the heads of long bones; stores marrow.
    • Compact: External layer; stores calcium.
  • Blood: Matrix is liquid plasma. Cells include erythrocytes (red blood cells), leukocytes (white blood cells), and platelets (cell fragments for clotting).

The Integumentary System (Skin)

  • Overview: Composed of skin, hair, nails, and glands. Measures roughly 2 m22\,m^2 and weighs 3–5 kg3\text{--}5\,kg.
  • Layers of Skin:
    1. Epidermis: Outer visible layer (stratified squamous epithelium).
    2. Dermis: Deep middle layer; site of most "skin jobs."
    3. Hypodermis (Subcutis): Deepest layer; mostly adipose tissue for insulation and energy storage.
  • Epidermal Cells:
    • Keratinocytes: Produce keratin (tough, waterproof protein). Replaced every 4 to 6 weeks.
    • Melanocytes: Synthesize melanin (pigment). Skin color depends on melanin breadth, not the number of cells.
    • Dendritic (Langerhans) Cells: Star-shaped immune cells.
    • Merkel (Tactile) Cells: Sensory receptors for touch.
  • Layers of the Epidermis (Mnemonic: "Come Let's Get Sunburned"):
    1. Stratum Corneum: Toughest, outermost layer (20–3020\text{--}30 sheets of dead cells).
    2. Stratum Lucidum: Clear layer found only in "thick skin" (palms and soles).
    3. Stratum Granulosum: Contains living keratinocytes producing keratin.
    4. Stratum Spinosum: Spiny layer where filaments help cells hold together.
    5. Stratum Basale: Deepest level; single layer of columnar cells; site of rapid cell production (mitosis).
  • Dermal Structure:
    • Papillary Layer: Thin areolar tissue; contains papillae (peglike projections) that form fingerprints.
    • Reticular Layer: 80%80\% of the dermis; dense irregular connective tissue.
  • Clinical Skin Signs:
    • Cyanosis: Blue skin indicating heart failure or respiratory issues.
    • Jaundice: Yellowing indicating liver disorder (bile accumulation).
    • Erythema: Redness indicating fever, inflammation, or allergy.

Skin Appendages and Glands

  • Hair (Pili): Strands of dead keratin protein cells. Parts include the shaft (fully keratinized) and root (follicle).
  • Sweat Glands (Sudoriferous Glands):
    • Eccrine: Most abundant; palms, forehead, soles. Simple tubes to the surface.
    • Apocrine: Appear at puberty; armpits and groin. Secrete viscous sweat with fats/proteins. Breakdown by bacteria causes body odor.
    • Modified Apocrine Glands: Mammary glands (milk) and Ceruminous glands (earwax).
  • Oil Glands (Sebaceous): Secrete sebum into hair follicles to soften skin and prevent water loss.
  • Sunlight and Vitamin D: Skin converts a molecule into Vitamin D using UV light. It is activated in the liver and kidneys to become calcitriol to support bones.

The Nervous System Fundamentals

  • Principal Functions:
    1. Sensory Input: Receptors detect stimuli (e.g., a spider on a knee).
    2. Integration: Processing input and deciding action.
    3. Motor Output: Response (activating muscles or glands).
  • Organizational Levels:
    • Central Nervous System (CNS): Brain and spinal cord (control center).
    • Peripheral Nervous System (PNS): Nerves branching from the CNS.
      • Sensory (Afferent) Division: Carries information to the brain.
      • Motor (Efferent) Division: Carries information from the brain to muscles.
        • Somatic Nervous System: Voluntary movement.
        • Autonomic Nervous System: Involuntary (heart, lungs).
          • Sympathetic: Mobilizes body (Fired up).
          • Parasympathetic: Relaxes body (Talks it down).
  • Nervous Tissue Cells:
    • Neuroglia (Glial Cells) in CNS:
      • Astrocytes: Anchors neurons to blood supply; govern exchange.
      • Microglial Cells: Immune defense.
      • Ependymal Cells: Circulate cerebrospinal fluid.
      • Oligodendrocytes: Produce the myelin sheath (insulation).
    • Neuroglia in PNS:
      • Satellite Cells: Support neuron cell bodies.
      • Schwann Cells: Produce the myelin sheath.
  • Neuron Structural Classification:
    • Multipolar: 99%99\% of neurons; three or more processes (one axon, many dendrites).
    • Bipolar: Two processes (one axon, one dendrite); rare (retina).
    • Unipolar: One process; found in sensory receptors.

Electrochemical Signaling

  • Basics of Electricity:
    • Voltage: Measure of potential energy (VV).
    • Membrane Potential: Difference in charge across the cell membrane.
    • Resting Membrane Potential: Fixed at −70 mV-70\,mV for a resting neuron (polarized).
    • Ion Gradients: Outside is rich in Sodium (Na+Na^+); inside is rich in Potassium (K+K^+) and negatively charged proteins.
  • Sodium-Potassium Pump: Straddles the membrane; pumps out 3 Na+3\,Na^+ for every 2 K+2\,K^+ pumped in to maintain the gradient.
  • Ion Channels:
    • Voltage-gated: Open at specific membrane potentials (e.g., Na+Na^+ channels open at −55 mV-55\,mV).
    • Ligand-gated: Open when a neurotransmitter or hormone binds to them.
    • Mechanically-gated: Open in response to physical stretching of the membrane.
  • Action Potential: The electrical impulse frequency used by neurons to communicate. It is a uniform signal strength; only frequency and location vary to communicate intensity.