Seeley\'s Essentials of Anatomy & Physiology Chapter 6 PDF

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Ch. 6: Skeletal System ● 206 bones in skeletal system ● Functions of Skeletal System: 1. Support a. Strong bone and firm, flexible cartilage provide support 2. Protection a. Bone is hard and protects organs 3. Movement a. Allow movement 4. Storage a. Calcium and phosphorous are stored in bone b. Adipose tissue is stored within bone cavities 5. Blood Cell Production a. Bone cavities are filled with red bone marrow i. Red bone marrow produces blood cells and platelets ● Components of Skeletal System: ○ Bone (self repair → metabolically active, lots of blood/nutrients) ○ Cartilage (can be easily destroyed by obesity → lots of pressure on knee) ■ Reduce friction → cushion ■ Model for bone formation ■ Cartilage CAN’T repair itself well ○ Tendons ■ Attach muscle to bone ○ Ligaments ■ Attach bone to bone ■ CANNOT repair itself ● Extracellular Matrix (differentiates components of skeletal system) ■ Bone: ● Collagen & minerals ● Flexible, able to bear weight ■ Cartilage: ● Collagen & proteoglycans ● Good shock absorber ● ■ Tendons & Ligaments ● Extracellular matrix is collagen ● Very strong and tough ● General Features of Bone ○ Long bones → longer than they are wide ■ Bones of upper and lower limbs (femur, tibia, fibula) ○ Short bones → as wide as they are long ■ Wrist and ankle bones (carpals, tarsals, phalanges)

○ Flat bones → thin, flat shape ■ Certain skull bones, ribs, scapulae, and sternum ■ Ribs, sternum, skull ○ Irregular bones → shapes that do not fit readily into the other 3 categories ■ Vertebrae, facial bone, Sphenoid bone (butterfly shaped) ● Structure of Long Bone ○ Diaphysis: central shaft of a long bone ■ Open space in middle → Medullary Cavity ○ Epiphysis: end of a bone; separated from the remainder of the bone by the epiphyseal plate/line ○ Each long bone consists of a central shaft and two ends ○ Thin layer of articular cartilage covers ends of epiphysis where bone joints with other bones ○ Epiphyseal Plate (growth plate): site of growth between diaphysis and epiphysis ■ Epiphyseal plate → indicates juvenile (young) ■ Epiphyseal line → indicates older bone (plate is fused) ■ Puberty → steroid hormones are responsible for fusing the epiphyseal plate → fusing the plate stops growth!! ■ When bone growth stops, epiphyseal plate is replaced by bone and becomes an epiphyseal line! ○ Diaphysis has a large Medullary Cavity: large, marrow-filled cavity in the diaphysis of long bone ○ Cavities in bone are filled with marrow ○ Yellow marrow → consists of adipose tissue (fat) ○ Red marrow → consists of blood-forming cells and is the only site of blood formation in adults ■ Children have more red marrow in their bones than adult bones ● As one ages, red marrow is replaced by yellow marrow ■ In adults, red marrow is confined to bones of central axis of body and most proximal epiphyses of limbs ○ Periosteum (outside): membrane around bone’s outer surface ■ Contains blood vessels and nerves → vascular! ● Has 2 layers: ○ Inner layer of Periosteum:

■ Attach bone ○ Outer layer of Periosteum: ■ Blood vessels ○ Endosteum (inside): lines medullary cavity! Periosteum and endosteum both contain osteoblasts ■ Osteoblasts: bone-making cells; can divide ● Turn into osteocytes ■ Osteocytes: mature bone cell surrounded by bone matrix; maintain bone matrix ■ Osteoclasts: remodel bone ○ Bone is formed in thin sheets of extracellular matrix known as Lamellae (thin sheet/layer of bone) ○ Osteocytes are located between the lamellae within spaces known as Lacunae (small space, cavity, or depression; space in bone matrix where osteocyte is located) ○ Cell processes extend from the osteocytes across the extracellular matrix of the lamellae within tiny canals called Canaliculi (tiny canal in bone)

● Compact Bone ○ Located in outer part of diaphysis (long bones) and thinner surfaces of other bones ○ Osteons: structural unit of compact bone; single central canal with its contents and the associated lamellae and osteocytes surrounding it ■ Each osteon has concentric rings of lamellae (rings of bone matrix) surrounding a central canal (Haversian canal) ■ Nutrients leave the blood vessels of central canal and diffuse to osteocytes via the canaliculi ● Waste products diffuse in the opposite direction! ○ Lamella: rings of bone matrix ○ Lacunae: spaces between lamella, filled w/ Osteocytes ○ Canaliculus: tiny canals built by Osteocytes, transport nutrients and waste into central canal ○ Central Canal: center of osteon ● Spongy Bone (Cancellous/Trabecular Bone) ○ NO OSTEONS!! ○ Located in epiphyses of long bones and center of other bones ○ Trabeculae: interconnecting rods, spaces contain marrow! ■ No central canal!

■ Add strength to a bone without adding weight since it’s not forming a solid, mineralized matrix ■ Spaces in between trabeculae are filled with marrow ○ Each trabecula consists of several lamellae w/ osteocytes between them ■ No blood vessels penetrate trabeculae ○ Nutrients exit vessels in marrow and pass by diffusion through canaliculi to osteocytes of trabeculae

● Bone Ossification → process ○ Ossification: formation of bone by osteoblasts ■ After osteoblast is fully surrounded by bone matrix, it becomes osteocyte ○ Ossification Center: where bone formation begins ○ Primary Ossification Center: where bone first begins to appear ■ Forms diaphyses ○ Secondary Ossification Center: forms epiphyses ■ Intramembranous Ossification → bone formation inside connective tissue membranes! ● Osteoblasts build bone ○ Occurs in skull bones ■ Endochondral Ossification → bone formation inside cartilage! ● Occurs in all bones (except skull) ● Cartilage models are replaced by bone ● Steps in Endochondral Ossification: 1. Chondroblasts build a cartilage model, the chondroblasts become chondrocytes 2. Cartilage model calcifies (hardens) 3. Osteoblasts invade calcified cartilage, primary ossification center forms diaphysis 4. Secondary ossification centers form epiphysis 5. Original cartilage model is almost completely ossified and remaining cartilage is articular cartilage

● Bone Growth ○ Infancy and youth: ■ Long bones lengthen at epiphyseal plate ■ Long bones widen by adding more lamella → Appositional Growth ○ End of bone growth (in length): ■ Epiphyseal plate is replaced by epiphyseal line ● Bone Remodeling ○ Removal of existing bone by osteoclasts (break down), deposition of new bone by osteoblasts (make) ○ Occurs in all bone, normal process ○ Remodeling is responsible for: ■ Change in bone shape ■ Adjustment of bone to stress ● i.e. Astronauts lose strength in bone → Osteoporosis ○ No force on bones ■ Bone repair ■ Calcium ion regulation ● Calcium is removed from bones when blood calcium levels decrease ● Calcium is deposited when dietary calcium is adequate ● Both removal and deposition are under hormonal control ○ If too much is deposited, bones become thick or have abnormal lumps that can interfere w/ normal function ○ If too little is deposited, bones weaken and become susceptible to fracture → Osteoporosis

● Bone Repair 1. Broken bone causes bleeding and a blood clot (hematoma) forms 2. Callus forms, which is fibrous network between 2 fragments 3. Cartilage model forms first a. Then, osteoblasts enter the callus and form cancellous bone (spongy bone) for 4-6 weeks after injury 4. Cancellous bone (spongy bone) is slowly remodeled ● Hematopoietic Tissue ○ Makes blood cells ■ Red blood cells ■ White blood cells ○ Red marrow → location of blood-forming cells ○ Yellow marrow → mostly fat ○ Location of Hematopoietic tissue in newborns: ■ Most bones (red marrow) ○ Location of Hematopoietic tissue in adults: ■ Red is replaced w/ yellow marrow ■ Red marrow is mainly in epiphyses of femur and humerus now

● Blood & Calcium Homeostasis ○ Bone is major storage site for calcium

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■ Movement of calcium into/out of bone helps determine blood calcium levels ● Necessary for normal muscle and nervous system function ■ Calcium moves into bone as osteoblasts build new bone ■ Calcium moves out of bone as osteoclasts break down bone When blood calcium levels are too low, osteoclast activity increases ○ Osteoclasts release calcium from bone into blood to increase blood calcium levels When blood calcium levels are too high, osteoclast activity decreases ○ Osteoblasts remove calcium from blood to produce new bone to decrease blood calcium levels Calcium homeostasis is maintained by 3 hormones: ○ Parathyroid Hormone (PTH) from parathyroid glands ○ Vitamin D from skin/diet ■ Both PTH and vitamin D are secreted when blood calcium levels are too low ■ Decreasing blood calcium levels will stimulate PTH secretion ○ Calcitonin from thyroid gland ■ Secreted when blood calcium levels are too high Parathyroid Hormone (PTH): ○ Indirectly stimulates osteoclasts to break down bone, which releases stored calcium into blood ○ Stimulates kidney to take up calcium from urine and return it to blood ○ Stimulates formation of active vitamin D, which promotes increases calcium absorption from small intestine Calcitonin: ○ Works to decrease blood calcium levels by inhibiting osteoclast activity ■ In the absence of osteoclast activity, osteoblast activity still continues! (osteoblasts continue to remove calcium of blood and deposit into bone)

● General Considerations of Bone Anatomy ○ 206 bones in the body ● Axial Skeleton Consists of: skull, vertebra, thoracic cage ● Skull ○ Frontal bone of skull forms majority of anterior cranial fossa ○ Occipital bone of skull forms majority of posterior cranial fossa ○ Styloid process: attachment site for tongue

○ Mandibular fossa: depression where lower jaw and skull meet ○ Glenoid fossa: where humerus meets scapula ○ Sphenoid bone → location of many foramen for cranial nerves ○ Foramen Ovale ○ Anosmia → shearing of axon with head blow → loss of smell ○ Axons can regenerate :) ○ Hard palate: roof of mouth ○ Palatine bones are in back of hard palate ○ Mandible: lower jaw, moves ○ Maxilla: upper jaw, doesn’t move ● Vertebral column ○ 7 cervical ○ 12 thoracic ○ 5 lumbar ○ 1 sacrum ■ (5 fused sacral vertebrae) ○ 1 coccyx ○ Atlas → 1st vertebra, holds the head ○ Axis → 2nd vertebra, rotates head ○ Functions of Vertebral column: ■ Support ■ Protect spinal cord ● Thoracic cage ○ Protects vital organs ○ 12 pairs of ribs ○ Sternum: breastbone ○ True ribs: attach directly to sternum by cartilage (1-7) ○ False ribs: attach indirectly to sternum by cartilage (8-12) ○ Floating ribs: not attached to sternum ● Appendicular Skeleton: ○ Consists of bones of upper limb and lower limb, pectoral girdle, pelvic girdle ■ Pectoral Girdle ● Scapula ● Clavicle ■ Upper Limb bones ● Humerus ● Ulna ● Radius (outer bone of forearm) ● Carpals (wrist) ● Metacarpals (hand) ● Phalanges (fingers) ■ Pelvic Girdle ● Pelvis: pelvic girdle + coccyx

○ Shaped differently for men and women (adaptation for childbirth) ● Ischium: inferior and posterior region ● Ilium: most superior region ● Acetabulum: hip socket (joint) ■ Lower limb bones ● Femur → largest bone in body (strongest, too) ● Patella ● Tibia ● Fibula ● Tarsals (ankle) → 7 tarsal bones ● Metatarsals (foot) ● Phalanges (toes and fingers)

● Joints ○ Synarthrosis: non-movable joint ■ e.g. Skull ○ Amphiarthrosis: slightly movable joint ■ e.g. between vertebrae ○ Diarthrosis: freely movable joint ■ e.g. Knee, Elbow, Wrist ➢ Fibrous joints ○ 2 bones united by fibrous tissue → little to no movement ■ Sutures ● Fibrous joint between flat bones of skull ● In newborns, some parts of sutures are wide and are called fontanels (soft spots) → allow flexibility during birth process and growth of head after birth ■ Syndesmoses ● Fibrous joint in which bones are separated by some distance and held together by ligaments ○ e.g. fibrous membrane connecting radius and ulna ■ Gomphoses

● Pegs fitted into sockets, held in place by ligaments ○ e.g. joint between tooth and its socket

➢ Cartilaginous joints ○ Unites 2 bones via cartilage → slight movement ■ e.g. cartilage at epiphyseal plates of growing long bones ■ e.g. cartilage between ribs and sternum ○ Cartilage can be reinforced by additional collagen fibers in cartilaginous joints where lots of stress is placed on the joint ■ Forms a type of cartilage: Fibrocartilage ➢ Synovial joints ○ Freely moveable joints ■ Contain fluid in a cavity surrounding ends of articulating bones ○ Joints that unite bones of appendicular skeleton are mostly synovial joints ■ (Most in axial skeleton are not → less mobile than appendicular skeleton)

○ Features of a synovial joints: ■ Articular cartilage → covers articular surfaces of bones within synovial joints ● Provides smooth surface where bones meet ■ Joint cavity → filled with fluid, enclosed by joint capsule ■ Joint capsule → hold bones together, allows movement ■ Synovial membrane → lines joint cavity everywhere, except over articular cartilage ● (May extend as a pocket/sac known as a bursa between structures exposed to rubbing together) ● (May extend as a tendon sheath along some tendons associated w/ joints) ● Synovial membrane produces synovial fluid (mixture of polysaccharides, proteins, lipids, and cells)

● Types of Movement: ○ Flexion: bending ○ Extension: straightening ○ Abduction: movement AWAY from the midline ○ Adduction: movement TOWARD midline ○ Pronation: rotation of forearm with palms down ○ Supination: rotation of forearm with palms up ○ Rotation: movement of structure about long axis...