Week 2 - 3 Review: Active/Passive Transport, Organic/Inorganic, Chemical Reactions, Integumentary System, and Skeletal System

Week 2 Review: Foundations of Anatomy and Physiology

Cellular Transport Mechanisms

• Simple Diffusion
  • Molecules Moved: Small, nonpolar molecules.
  • Uses Energy? No.
  • Example/Disease: Pulmonary edema (related to fluid balance affecting diffusion).
• Facilitated Diffusion
  • Molecules Moved: Polar molecules, larger ions.
  • Uses Energy? No.
  • Example/Disease: GLUT4 transporter (involved in glucose uptake) / Diabetes Mellitus Type II.
• Primary Active Transport
  • Molecules Moved: Molecules moving against their concentration gradient, coupled to the hydrolysis of ATP.
  • Uses Energy? Yes.
  • Example/Disease: Sodium-potassium pump, proton pump / atrial fibrillation, acid reflux.
• Secondary Active Transport
  • Molecules Moved: One molecule going with its gradient, coupled to another molecule going against its gradient.
  • Uses Energy? Yes.
  • Example/Disease: Sodium-calcium exchanger, SGLT2 (Sodium-Glucose Co-transporter 2).

Organic vs. Inorganic Compounds

• Organic Compounds
  • Contain carbon, usually bonded to hydrogen.
  • Examples: DNA, Sugar, Methane ($ext{CH}<em>4$), Ethanol ($ext{CH}</em>3 ext{CH}_2 ext{OH}$).
• Inorganic Compounds
  • Usually do not contain carbon.
  • Examples: Table Salt ($ext{NaCl}$), Hydrochloric Acid ($ext{HCl}$), Quartz ($ext{SiO}_2$).
  • Exceptions: Some inorganic carbon compounds do exist, including carbon dioxide ($ext{CO}<em>2$), some carbonates ($ext{CO}</em>3^{2-}$), cyanides ($ext{CN}^-$), and carbides.

Types of Chemical Reactions

Four main categories describe how substances interact:

• Synthesis Reaction
  • Description: Two or more reactants combine to form a larger, more complex product.
  • Format: $A + B \rightarrow AB$
• Decomposition Reaction
  • Description: A larger molecule is broken down into two or more smaller products.
  • Format: $AB \rightarrow A + B$
• Single Replacement Reaction
  • Description: One element replaces another element in a compound.
  • Format: $AB + C \rightarrow B + AC$
• Double Replacement Reaction
  • Description: The ions of two compounds exchange places with each other to form two new compounds.
  • Format: $AB + CD \rightarrow AC + BD$

Anabolic (Genesis) vs. Catabolic (Lysis) Pathways

These terms relate to metabolic processes occurring in the body, often connected to the absorptive (fed) and post-absorptive (fasting) states.

• Anabolic Pathways ("Genesis" = Formation)
  • Definition: Building up complex molecules from simpler ones, usually requiring energy.
  • Glycogenesis (glyco-genesis): The formation of glycogen (storage form of glucose).
  • Lipogenesis (lipo-genesis): The formation of lipids (fats).
  • Gluconeogenesis (gluco-neo-genesis): The formation of glucose from non-carbohydrate sources.
  • Associated State: Often occurs during the absorptive state when nutrients are plentiful.
• Catabolic Pathways ("Lysis" = Breakdown)
  • Definition: Breaking down complex molecules into simpler ones, usually releasing energy.
  • Glycogenolysis (glycogen-o-lysis): The breakdown of glycogen into glucose.
  • Lipolysis (lipo-lysis): The breakdown of lipids (fats).
  • Associated State: Often occurs during the post-absorptive state when the body needs to mobilize stored energy.

The Generalized Cell and Organelle Functions

A typical body cell is comprised of a plasma membrane, cytoplasm (containing organelles), and a nucleus.

The Plasma Membrane

• Function: Forms the cell's outer boundary, separating the internal environment from the outside.
• Key Characteristics:
  • It is a selectively permeable barrier, controlling what enters and exits the cell.
  • Plays a crucial role in cellular communication.
  • Covers and protects the cell.
  • Links to other cells.
  • Expresses surface structures that identify the cell.

Organelle Functions (City Analogy)

• Nucleus: City hall (stores instructions & directs activities).
• Ribosomes: Factories (make goods/proteins).
• Endoplasmic Reticulum (ER): Roads/highways (transport system).
• Golgi Apparatus: Post office (packages & ships goods).
• Mitochondria: Power plant (makes energy/ATP).
• Lysosomes: Trash/recycling center (breaks down waste).
• Cell Membrane: City limits/walls (controls who enters/exits).
• Cytoplasm: Open land/parks (space where things happen).

The Integumentary System

Introduction and Overview

• Definition: The integumentary system consists of the skin and its accessory structures including hair, nails, and glands, as well as blood vessels, muscles, and nerves.
  • Tissue Representation: All four basic tissue types are well-represented:
    • Epithelium: Found in hair, nails, and the epidermis of the skin.
    • Connective Tissue (C.T.): Forms the dermis.
    • Muscle: Attached to hair follicles and found in the walls of arteries and veins.
    • Nerves: Provide abundant sensation throughout the system.
• Dermatology: The medical specialty focused on the diagnosis and treatment of disorders of the integumentary system.

Properties of the Integument (Cutaneous Membrane)

• Largest Organ: The integument (skin) is the largest organ of the body by both surface area and weight.
  • Area: Approximately $2 ext{ square meters}$, or $22 ext{ square feet}$.
  • Weight: Ranging from $4.5-5 ext{ kg}$ ($10-11 ext{ lb}$), which is about $16\%$ of total body weight.
  • Thickness: Varies from $0.5-4 ext{ mm}$, being thinnest on the eyelids and thickest on the heels.
• Skin Epithelium Loss: We lose almost $1 ext{ kg}$ of skin epithelium per year, which becomes a major component of household dust.

Functions of the Skin

Besides protection, the skin performs several vital roles:

• Regulation of Body Temperature: Through sweating and modulation of blood flow.
• Sensory Perceptions: Detects touch, pressure, pain, temperature, and vibration.
• Synthesis of Vitamin D: Ultraviolet (UV) radiation is necessary for the skin to synthesize Vitamin D.
• Emotional Expression: Facial expressions largely depend on skin and underlying muscles.
• Blood Reservoir: Serves as an important reservoir of blood.

Memory Trick: ABCDE for Skin Functions

• A: Acts as a barrier (physical, chemical, immune).
• B: Balances fluids (prevents dehydration, maintains electrolytes).
• C: Controls temperature (sweating, vasodilation/vasoconstriction).
• D: Detects sensations (touch, pain, temperature, pressure).
• E: Enables vitamin D synthesis.

Layers of the Skin and Subcutaneous Layer

The skin has two major layers, the epidermis and the dermis, and is underlain by the subcutaneous layer (hypodermis).

1. Epidermis – The Protective Roof

• Description: The outer, thinner layer, composed of epithelial tissue (keratinized stratified squamous epithelium).
• Avascular: Lacks its own blood supply.
• Layers (Superficial $\rightarrow$ Deep):
  • Stratum Corneum: The most superficial layer, consists of "shingles" of dead, keratinized cells, providing a strong protective barrier.
  • Stratum Lucidum: A thin, clear layer found only in the thick skin of the palms and soles.
  • Stratum Granulosum: Cells flatten, nuclei and organelles disintegrate, and the cells accumulate keratin granules.
  • Stratum Spinosum: Cells are joined by desmosomes, giving a "spiny" appearance; contains Langerhans cells.
  • Stratum Basale (Stratum Germinativum): The deepest layer, attached to the dermis. It contains newly formed cells and the main site of continuous cell division, producing all other layers above it. It also forms epidermal ridges that extend into the deeper dermis.
• Functions: Primary barrier against pathogens and chemicals.

Cells of the Epidermis

Four major types of cells are present:

• Keratinocytes: Make up $90\%$ of epidermal cells. They produce keratin, a tough fibrous protein that provides protection and contributes to the barrier function.
• Melanocytes: Produce the pigment melanin, which protects against damage from ultraviolet (UV) radiation. Located in the stratum basale.
• Langerhans Cells (Dendritic Cells): Macrophages that originate in the red bone marrow. They are involved in immune responses, patrolling for pathogens.
• Merkel Cells (Tactile Epithelial Cells): Function in the sensation of touch along with adjacent tactile discs (receptors).

Skin Pigments & Color Contribution

Skin color is determined by three interacting pigments:

• Melanin:
  • Produced by melanocytes in the stratum basale, which project superficially.
  • Secreted in granules and taken up by more superficial keratinocytes, where the pigment protects the nuclei from UV damage.
  • Contributes to various skin colors (brown, black, yellow, red).
• Carotene:
  • A yellow-orange pigment.
  • Accumulates in the stratum corneum, dermis, and subcutaneous layer.
  • Can be particularly noticeable in areas with thick stratum corneum, like palms and soles.
• Hemoglobin:
  • Produces "red" tones in the skin.
  • Located within erythrocytes (red blood cells) flowing through dermal capillaries.
  • The degree of redness depends on the amount of blood flow and oxygenation.

2. Dermis – The Foundation & Utilities

• Description: The inner, thicker layer of the skin, composed primarily of connective tissue (containing collagen and elastic fibers).
• Two Regions:
  • Papillary Layer: Lies just below the epidermis.
    • Consists of areolar connective tissue with thin collagen and elastic fibers.
    • Contains dermal papillae (finger-like projections that extend into the epidermis), which house capillary loops (for nutrient supply), corpuscles of touch (Meissner corpuscles for light touch), and free nerve endings (for pain, temperature).
    • Epidermal ridges (fingerprints) reflect contours of underlying dermal papillae and increase firmness of grip by increasing friction.
  • Reticular Layer: The deeper, thicker part of the dermis, providing strength and elasticity.
    • Consists of dense irregular connective tissue with collagen and elastic fibers.
    • Contains adipose cells, hair follicles, nerves, sebaceous (oil) glands, and sudoriferous (sweat) glands.
    • Stretch marks (striae): Tears or excessive stretching in this region.
• Functions: Provides strength and elasticity, houses blood supply and nerves, contributes to sensation and thermoregulation.
• Lines of Cleavage (Tension Lines): Indicate the predominant direction of underlying collagen fibers in the reticular layer. Surgical incisions made parallel to these lines minimize scarring.

3. Hypodermis – The Basement Insulation (Subcutaneous Layer)

• Description: Located underneath the dermis; not technically considered a layer of the skin itself.
• Composition: Primarily loose areolar connective tissue and ample adipose tissue.
• Functions:
  • Attaches the skin to underlying tissues and organs (muscles and bone).
  • Cushions and protects underlying organs.
  • Insulates the body (temperature regulation) due to the presence of fat reserves.
  • Stores energy in the form of fat.
  • Contains blood vessels and nerves that are in transit to more superficial layers.
  • Houses lamellated (Pacinian) corpuscles, which detect deep external pressure applied to the skin.

Sensory Receptors of the Skin

The skin contains various sensory receptors to detect different tactile sensations:

• Light Touch, Pressure, Vibration, Itch, and Tickle.
• Superficial Receptors:
  • Merkel Discs: Detect sustained touch and pressure.
  • Free Nerve Endings: Detect many stimuli including pain, temperature, itch, and some touch.
  • Meissner Corpuscles (Corpuscles of Touch): Detect light touch and low-frequency vibration.
  • Hair Root Plexuses: Detect movements of hair.
• Deep Receptors:
  • Pacinian Corpuscles (Lamellated Corpuscles): Detect deep pressure and high-frequency vibration.

Accessory Structures of the Skin

1. Hair ("Pili")

• Location: Present on most surfaces of the body, except the palms, anterior surfaces of fingers, and soles of the feet.
• Composition: Composed of dead, keratinized epidermal cells.
• Characteristics: Genetics determines hair thickness and distribution.
• Functions:
  • Aids in touch sensations.
  • Protects the body against the harmful effects of the sun.
  • Protects against heat loss.
• Associated Structures:
  • Hair Shaft: The superficial part of the hair.
  • Hair Root: The part embedded in the skin.
  • Hair Follicle: Surrounds the root, extending down into the dermis or even hypodermis.
  • Hair Root Plexus: Sensory nerve endings wrapped around the hair follicle, sensitive to hair movement.
  • Arrector Pili Muscle: Smooth muscle attached to the hair follicle; contracts to make hair stand on end (e.g., in cold or fear, causing "goosebumps").
  • Hair Bulb: The base of the hair follicle.
  • Papilla of the Hair: Indentation at the base of the bulb, containing blood vessels for hair nourishment.

2. Skin Glands (Glands are epithelial cells that secrete a substance)

• Sebaceous (Oil) Glands:
  • Connection: Usually connected to hair follicles (holocrine glands).
  • Secretion: Secrete an oily substance called sebum into the hair follicle.
  • Functions of Sebum:
    • Prevents dehydration of hair and skin.
    • Inhibits the growth of certain bacteria.
• Sudoriferous (Sweat) Glands: Simple, coiled tubular glands.
  • Eccrine Sweat Glands:
    • Location: Located virtually all over the body.
    • Secretion: Secrete a watery solution directly onto the skin surface.
    • Functions: Helps to cool the body (evaporative cooling), eliminates small amounts of waste, and responds to emotional stress.
  • Apocrine Sweat Glands:
    • Location: Mainly found in the skin of the axilla (armpit), groin, areolae (around nipples), and bearded facial regions of adult males.
    • Secretion: Secrete a slightly viscous sweat that contains lipids and proteins.
    • Function: Becomes active during sexual excitement and emotional stress. Bacterial decomposition of these secretions contributes to body odor.

3. Nails

• Composition: Composed of hard, keratinized epidermal cells.
• Location: Located over the dorsal surfaces of the ends of fingers and toes.
• Structures:
  • Free Edge: The part of the nail that extends past the end of the finger or toe.
  • Transparent Nail Body (Plate): The main visible part of the nail.
  • Lunula: A whitish, crescent-shaped area at the base of the nail body; appears white due to a thickened underlying stratum basale obscuring capillaries.
  • Nail Root: The portion of the nail embedded in a fold of skin.

Age-Related Changes to the Integumentary System

• Wrinkles: Develop due to decreased elasticity and collagen production.
• Dehydration and Cracking: Skin becomes drier and more prone to cracking.
• Decreased Sweat Production: Reduced activity of sudoriferous glands affects thermoregulation.
• Pigmentation Changes: A decrease in functional melanocytes results in gray hair and potential for atypical skin pigmentation (e.g., age spots).
• Skin Thinning: Loss of subcutaneous fat and a general decrease in skin thickness.
• Nails: May become more brittle.
• Increased Susceptibility to Pathological Conditions: Such as decubitus ulcers (pressure ulcers or "bed sores"), which are common in elderly bedridden patients.

Practical Implications (Discussion Questions):

• Why do elderly patients bruise easily? Thinner skin, decreased collagen and elastin, and more fragile blood vessels make them more susceptible to bruising.
• Why are babies prone to dehydration? Babies have a higher surface area to volume ratio, thinner skin, and immature kidney function, leading to greater insensible fluid loss and difficulty regulating fluid balance.
• Why do burn patients need fluid resuscitation? Damage to the skin barrier results in massive fluid and electrolyte loss from the burned surface, as well as plasma loss, requiring aggressive fluid replacement to prevent shock and maintain homeostasis.

The Skeletal System

Functions of Bone and the Skeletal System

Bone is a dynamic tissue, constantly undergoing remodeling.

• Support: Forms the structural framework of the body, supports soft tissues, and provides attachment points for skeletal muscle tendons.
• Protection: Guards important internal organs (e.g., cranium protects the brain, vertebrae protect the spinal cord, ribs protect the lungs and heart, pelvis partially protects reproductive and urinary organs).
• Assistance in Movement: Serves as levers for muscles, enabling movement.
• Mineral Homeostasis: Stores several minerals (especially calcium and phosphorus). It can release minerals into the blood to maintain proper levels.
• Blood Cell Production (Hemopoiesis): Red bone marrow produces red blood cells, white blood cells, and platelets.
• Triglyceride Storage: Yellow bone marrow stores triglycerides (fats) in adipose cells, serving as a potential chemical energy reserve.

Tissues of the Skeletal System

The skeletal system is composed of several different tissues, primarily bone (osseous tissue) and cartilage.

Bone (Osseous Tissue)

• Description: A highly vascularized connective tissue with a hard, mineralized extracellular matrix.
• Arrangements:
  • Compact Bone:
    • Function: Good at providing protection and support.
    • Location: Forms the diaphysis (shaft) of long bones and the external layer of all bones.
    • Histology: Components are arranged in repeating structural units called osteons (Haversian systems). The organization of osteons changes in response to physical demands (remodeling).
  • Spongy Bone (Trabecular Bone):
    • Function: Lightweight and provides tissue support.
    • Location: Forms much of the epiphysis (ends) and the internal cavity of long bones.
    • Histology: Lacks osteons. Instead, lamellae are arranged in a lattice of thin columns called trabeculae. Trabeculae support and protect red bone marrow and are oriented along lines of stress to help bones resist stresses without breaking.
    • Hematopoiesis: Occurs in the red marrow within spongy bone.

Cartilage

• Articular Cartilage:
  • Description: A thin layer of hyaline cartilage.
  • Location: Covers the epiphysis of long bones where the bone forms an articular (joint) surface, allowing one bone to move smoothly against another.

Periosteum

• Description: A tough sheath of dense, irregular connective tissue that covers the outer surface of bone where there is no articular cartilage.
• Components: Contains osteoblasts.
• Functions:
  • Helps the bone grow in thickness (appositional growth) but not in length.
  • Assists with fracture repair.
  • Serves as an attachment point for tendons and ligaments.

Cells Present in Bone Tissue

Four principal types of cells are involved in bone formation and maintenance:

• Osteogenic Cells (Osteoprogenitor Cells):
  • Unspecialized stem cells that undergo cell division.
  • The resulting cells develop into osteoblasts.
• Osteoblasts:
  • Bone-building cells.
  • Synthesize and secrete collagen fibers and other organic components that form the extracellular matrix of bone tissue.
  • Initiate calcification (mineralization), depositing mineral salts (primarily calcium phosphate) that crystallize in the framework formed by collagen fibers.
• Osteocytes:
  • Mature bone cells.
  • Derived from osteoblasts that become trapped within the calcified matrix.
  • Maintain the bone matrix and exchange nutrients and wastes with the blood.
• Osteoclasts:
  • Large, multinucleated cells derived from monocytes.
  • Release powerful lysosomal enzymes and acids that digest the mineral components and collagen fibers of the bone matrix, a process called resorption.
  • Play a critical role in the maintenance, repair, and remodeling of bone by breaking down old bone tissue.
  • Help regulate blood calcium levels.

Extracellular Matrix of Bone Tissue

• Composition: Surrounds widely separated cells.
  • $25\%$ water
  • $25\%$ collagen fibers (provide bone's flexibility and tensile strength)
  • $50\%$ crystallized mineral salts, primarily calcium phosphate ($\text{Ca}<em>3( ext{PO}</em>4)_2$), which is the most abundant chemical compound.

Other Tissues in the Skeletal System

• Epithelium (Endothelium): Forms the capillary walls within bones.
• Nerves: Highly innervated; the periosteum is especially rich in sensory nerves, making it sensitive to tearing or tension.
• Red Marrow: Site of hematopoiesis (blood cell production).
• Yellow Marrow: Primarily composed of adipose tissue for fat storage.

Structure of a Long Bone

• Diaphysis: The main, cylindrical shaft of a long bone.
• Epiphysis: The proximal and distal ends of a long bone.
• Metaphysis: The region between the diaphysis and epiphysis. In a growing bone, it contains the epiphyseal (growth) plate, a layer of hyaline cartilage that allows the diaphysis to grow in length.
• Articular Cartilage: A thin layer of hyaline cartilage covering the epiphysis where it forms a joint.
• Periosteum: A tough connective tissue sheath covering the external surface of the bone.
• Medullary Cavity: The hollow, cylindrical space within the diaphysis that contains fatty yellow bone marrow in adults.
• Endosteum: A membrane lining the medullary cavity and the trabeculae of spongy bone. It contains osteoclasts, osteoblasts, and connective tissue.

Blood and Nerve Supply of Bone

• Bones are richly supplied with blood vessels.
• Periosteal Arteries and Veins: Supply the periosteum and compact bone.
• Nerves typically accompany blood vessels.
• The periosteum is rich in sensory nerves, making it exquisitely sensitive to tearing or tension (e.g., in a fracture).

Bone Formation (Ossification or Osteogenesis)

Ossification is the process of forming new bone, occurring in four main situations:

1. Formation of bone in an embryo.
2. Growth of bones until adulthood.
3. Remodeling of bone throughout life.
4. Repair of fractures.

1. Formation of Bone in an Embryo

Begins around the 6th week of embryonic development by two different methods:

• Intra-membranous Ossification:
  • Produces spongy bone directly from mesenchymal connective tissue.
  • This spongy bone may subsequently be remodeled to form compact bone.
  • Forms flat bones of the skull, most of the facial bones, mandible, and the medial part of the clavicle.
• Endochondral Ossification:
  • The method for the formation of most bones in the body, especially long bones.
  • Involves the replacement of a hyaline cartilage model by bone tissue.
  • Forms both compact and spongy bone.
  • Involves one primary ossification center (in the diaphysis) and two secondary ossification centers (in the epiphyses).
  • Sequence of Events:
    1. Development of Cartilage Model: Mesenchymal cells develop into chondroblasts, which form a hyaline cartilage model.
    2. Growth of Cartilage Model: Growth occurs by cell division of chondrocytes, increasing length and width.
    3. Development of Primary Ossification Center: In the diaphysis, bone tissue replaces most of the cartilage.
    4. Development of the Medullary (Marrow) Cavity: Bone breakdown by osteoclasts forms the medullary cavity.
    5. Development of Secondary Ossification Centers: These occur in the epiphyses of the bone, with bone replacing cartilage.
    6. Formation of Articular Cartilage and Epiphyseal Plate: Both structures consist of hyaline cartilage; articular cartilage remains on joint surfaces, and the epiphyseal plate allows for longitudinal growth.

2. Bone Growth During Infancy, Childhood, and Adolescence

• Growth in Length (Longitudinal Growth): Primarily occurs at the epiphyseal plate (growth plate) of long bones.
  • Involves two major events:
    1. Growth of Cartilage: Cartilage cells (chondrocytes) within the epiphyseal plate divide and enlarge, pushing the epiphysis away from the diaphysis.
    2. Replacement of Cartilage by Bone: Osteoclasts dissolve the calcified cartilage on the diaphysial side of the plate, and osteoblasts invade the area, laying down new bone matrix.
  • Continuous remodeling occurs concurrently to maintain the proper shape of the growing bone.
  • Closure of Epiphyseal Plates: At adulthood, around $18-21$ years of age, the epiphyseal plates close as bone replaces all the cartilage, leaving a bony structure called the epiphyseal line. Growth in length then ceases.
• Growth in Width (Appositional Growth):
  • Occurs as osteoblasts in the periosteum lay down new bone tissue on the outer surface of the bone.
  • Simultaneously, osteoclasts in the lining of the medullary cavity (endosteum) resorb bone from the inside.
  • This process allows the bone to grow wider while maintaining a relatively constant wall thickness, preventing the bones from becoming too heavy.

3. Bone Growth and Remodeling

Bone is a dynamic tissue constantly being broken down and built up. A delicate balance must exist between the actions of osteoclasts (resorption) and osteoblasts (deposition).

• Imbalances:
  • Excessive new tissue formation: Leads to abnormally thick and heavy bones (e.g., acromegaly – excess growth hormone after epiphyseal plate closure).
  • Excessive loss of calcium (resorption > deposition): Weakens bones (e.g., osteoporosis).
  • Bones becoming too "soft": Due to inadequate mineralization (e.g., rickets in children, osteomalacia in adults), often caused by vitamin D deficiency.

Factors Influencing Normal Bone Metabolism

• Minerals: Essential components for bone growth and remodeling.
  • Large amounts of calcium ($ext{Ca}$) and phosphorus ($ext{P}$) are required.
  • Smaller amounts of magnesium ($ext{Mg}$), fluoride ($ext{F}$), and manganese ($ext{Mn}$) are also necessary.
• Vitamins: Crucial for various aspects of bone health.
  • Vitamin A: Stimulates the activity of osteoblasts.
  • Vitamin C: Essential for the synthesis of collagen fibers, which provide bone's tensile strength.
  • Vitamin D: Critically important for healthy bones as it promotes the absorption of calcium from foods in the gastrointestinal tract into the blood.
• Hormones: Play a significant role in bone homeostasis.
  • Sex Hormones (Estrogen and Testosterone): Cause a dramatic effect on bone growth, such as the sudden "growth spurt" during teenage years. They also promote the widening of the pelvis in the female skeleton and are responsible for closing the epiphyseal plates at the end of puberty.
  • Parathyroid Hormone (PTH) and Calcitonin: Are critical for balancing the levels of calcium ($ext{Ca}^{2+}$) and phosphorus ($ext{PO}_4^{3-}$) between blood and bone.
    • Maintaining a normal serum $ext{Ca}^{2+}$ level takes precedence over mineralizing bone.

Bone's Role in Calcium Homeostasis

Bone acts as the body's major calcium reservoir.

• Levels of calcium in the blood ($ext{Ca}^{2+}$) are tightly maintained by controlling the rates of calcium resorption from bone into blood and of calcium deposition from blood into bone.
• Importance of Calcium Ions:
  • Both nerve and muscle cells depend on $ext{Ca}^{2+}$ to function properly (e.g., neurotransmitter release, muscle contraction).
  • Blood clotting also requires $ext{Ca}^{2+}$.
  • Many enzymes require $ext{Ca}^{2+}$ as a cofactor for their activity.
• **Day-to-day Control of Calcium Regulation (Hormonal):
  • Parathyroid Hormone (PTH): Released when blood $ext{Ca}^{2+}$ levels are low.
    • Stimulates osteoclastic activity, increasing bone resorption and releasing $ext{Ca}^{2+}$ into the blood.
    • Increases reabsorption of $ext{Ca}^{2+}$ by the kidneys and promotes production of calcitriol (active Vitamin D).
    • Effect: Raises serum calcium level.
  • Calcitonin: Released when blood $ext{Ca}^{2+}$ levels are high.
    • Stimulates osteoblastic activity, promoting bone building and encouraging the deposition of $ext{Ca}^{2+}$ into bone matrix.
    • Effect: Lowers serum calcium level.
  • Vitamin D (Calcitriol): Essential for the absorption of $ext{Ca}^{2+}$ and $ext{PO}_4^{3-}$ ions from the small intestine.
  • The regulation of serum $ext{Ca}^{2+}$ levels and bone mineralization is carefully balanced under hormonal control to maintain blood $ext{Ca}^{2+}$ levels within a narrow physiological range.

4. Repair of Fractures

(Not explicitly detailed in the provided transcript, but listed as one of the four situations for bone formation, implying its importance in bone dynamics).

Exercise and Bone Tissue

• Mechanical Stress & Strength: Under mechanical stress (e.g., from the pull of skeletal muscles and the pull of gravity during weight-bearing activities), bone tissue becomes stronger.
  • This strengthening occurs through the deposition of mineral salts and increased production of collagen fibers by osteoblasts.
  • This is an example of the principle that function dictates structure (Wolff's Law).
• Unstressed Bones: Conversely, bones that are not subjected to mechanical stress (e.g., during prolonged bed rest or in paralysis) become weaker due to decreased bone deposition and increased resorption.

Aging and Bone Tissue

• Decrease in Bone Mass: Occurs as the level of sex hormones (estrogen and testosterone) diminishes during middle age, especially in women after menopause.
  • Bone resorption by osteoclasts begins to outpace bone deposition by osteoblasts.
  • Since female bones are generally smaller and less massive than males to begin with, old age often has a greater adverse effect on bone density in females.
• Brittleness: Collagen fibers contribute to bone's tensile strength. With age, protein synthesis decreases, leading to a loss of tensile strength. This causes bones to become very brittle and more susceptible to fractures.
• Osteoporosis: A common condition in aging where bone resorption significantly outpaces bone deposition, making bones porous and fragile.
  • Often due to depletion of calcium from the body or inadequate calcium/Vitamin D intake over a lifetime.

Study Guide Questions

Anatomical Planes and Orientation

• How do the three major anatomical planes (sagittal, transverse, frontal) help us describe movements or medical imaging?
• Explain the difference between "deep" and "superficial" using an example from the human body.

Chemical Bonds and Water

• Compare covalent, ionic, and hydrogen bonds in terms of strength and importance in biology. Why does water's polarity make it essential for life?
• How do hydrogen bonds give water its unique properties? Why are those properties critical for cells?

Macromolecules

• Compare the structure and function of triglycerides vs. phospholipids. How does their structure explain their role in the body?
• Proteins have many roles in the cell. Why aren't they primarily used for energy?
• How does RNA connect the information in DNA to the structure and function of proteins?

Cell Structure & Transport

• Why do red blood cells rely on glycolysis instead of aerobic respiration? What structural feature explains this?
• Compare simple diffusion, facilitated diffusion, and active transport. How do structure and energy use influence each type?
• Why is the sodium-potassium pump critical for maintaining cell function? How does its structure explain its role?

Major Tissue Types

• How do the four major tissue types differ in both structure and function? Give an example of each.
• Compare simple squamous, simple columnar, and stratified squamous epithelium in terms of structure and primary function.
• Why is fibrocartilage the right tissue for intervertebral discs, while dense regular connective tissue is found in tendons?
• Compare smooth, cardiac, and skeletal muscle tissue. How does their structure explain their specific roles in the body?

Epidermis vs. Dermis + Accessories

• How does the epidermis differ from the dermis in both structure and function?
• Why are sebaceous and sweat glands located in the dermis and not in the epidermis?

Bone Formation vs. Growth vs. Remodeling

• Compare bone growth, bone remodeling, and bone repair. How does the role of osteoblasts and osteoclasts differ in each?
• How does physical activity influence bone strength and remodeling? Connect this to the concept that function dictates structure.

Aging & Tissues

• How does aging affect connective tissue, bone, and skin? How do these changes affect function?
• Why might tissue repair slow down with age?