How bones are built, remodeled, and joined
TEAS 7 outline: Skeletal system: anatomy and physiology
Also covers HESI A2 Anatomy & Physiology (Skeletal system)
- The skeleton supports, protects, aids movement, stores calcium and makes blood cells.
- Adults have 206 bones: 80 axial and 126 appendicular.
- Osteoblasts build bone, osteocytes maintain it, osteoclasts break it down.
- PTH raises blood calcium; calcitonin from the thyroid lowers it.
- Ligaments join bone to bone; tendons attach muscle to bone.
Key facts
- The adult skeleton has 206 bones: 80 in the axial skeleton (skull, vertebral column including sacrum and coccyx, ribs, sternum) and 126 in the appendicular skeleton (limbs plus the girdles that attach them).
- Bones are classified by shape as long, short, flat, irregular, or sesamoid; long describes shape, not size, so finger bones are long bones.
- Compact bone is dense and built from osteons (Haversian systems); spongy bone is a lighter lattice of trabeculae whose spaces can hold red marrow.
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- Osteoblasts form new bone, osteocytes (trapped former osteoblasts) maintain the matrix, and osteoclasts resorb (break down) bone.
- When blood calcium falls, PTH increases osteoclast activity, kidney reabsorption of calcium, and vitamin D synthesis; when it rises, calcitonin from the thyroid inhibits osteoclasts.
- Red marrow, found in the spaces of spongy bone, is where blood cells are made (hematopoiesis); yellow marrow fills the medullary cavity of long bones and stores fat.
- Functionally, joints are synarthroses (immobile, such as skull sutures), amphiarthroses (slightly movable, such as intervertebral discs), or diarthroses (freely movable, which includes all synovial joints, such as hinge, pivot, and ball-and-socket joints).
Quick check1 question · 2 more at the end
Which of the following bones is part of the appendicular skeleton?
Correct answer: C.
The appendicular skeleton contains the bones of the upper and lower limbs plus the bones that attach each limb to the axial skeleton. The scapula, together with the clavicle, forms the pectoral (shoulder) girdle that attaches the upper limb to the trunk, so it is appendicular. The axial skeleton is the central axis: skull and associated bones such as the hyoid, the vertebral column including the sacrum and coccyx, and the thoracic cage of ribs and sternum.
A The sacrum is part of the vertebral column and therefore axial, even though it sits in the pelvis.
B The sternum is part of the thoracic cage, which belongs to the axial skeleton.
C Correct: the scapula is part of the pectoral girdle, which attaches the upper limb to the axial skeleton, so it is appendicular.
D The hyoid bone is one of the bones associated with the head and is counted in the axial skeleton.
Full lesson
Each part starts with its main idea. Open a part when you want the details.
The short answer: The skeleton supports and protects the body, gives muscles levers to move, stores calcium and phosphorus, and makes blood cells in red marrow. Its 206 adult bones split into the axial skeleton (skull, vertebral column, rib cage) and the appendicular skeleton (limbs and girdles). Osteoblasts build bone, osteocytes maintain it, and osteoclasts break it down.
1 What does the skeletal system do?Your skeleton is made of bones, cartilage, and ligaments, and it does far more than hold you up. 1 min
Your skeleton is made of bones, cartilage, and ligaments, and it does far more than hold you up. It supports the body against gravity, gives muscles a rigid frame to pull on so you can move, and protects the brain, spinal cord, heart, and lungs.
Bone is the body's main storage site for calcium and phosphorus. The red marrow inside some bones is where blood cells are made (hematopoiesis), and yellow marrow stores fat.
2 Which bones are axial and which are appendicular?An adult has 206 bones, split into two divisions. 1 min
An adult has 206 bones, split into two divisions.
The axial skeleton (80 bones) forms the body's central vertical axis. It includes the skull, the vertebral column (24 vertebrae plus the sacrum and coccyx), and the thoracic cage (12 pairs of ribs and the sternum). The hyoid bone and the tiny ear ossicles are counted here too.
The appendicular skeleton (126 bones) includes all the bones of the upper and lower limbs plus the girdles that attach them to the axial skeleton. The pectoral (shoulder) girdle is the clavicle and scapula, which connect each upper limb to the sternum.
3 What shapes can bones have?Bones are grouped by shape, and each shape matches a job. 1 min
Bones are grouped by shape, and each shape matches a job. "Long" describes shape, not size: a tiny finger bone is still a long bone.
| Shape | What it looks like | Main job | Examples |
|---|---|---|---|
| Long | Longer than it is wide | Act as levers | Femur, humerus, phalanges |
| Short | Cube-like | Stability with a little motion | Carpals, tarsals |
| Flat | Thin, often curved | Protect organs, anchor muscles | Skull bones, sternum, ribs, scapulae |
| Irregular | Complex shape | Protect internal organs | Vertebrae, some facial bones |
| Sesamoid | Small and round, inside a tendon | Protect tendons from pressure | Patella (kneecap) |
4 What is inside a long bone?A long bone has a shaft, the diaphysis, and a wider end at each side, the epiphysis. 1 min
A long bone has a shaft, the diaphysis, and a wider end at each side, the epiphysis. The walls of the shaft are compact bone, and the hollow center, the medullary cavity, holds yellow marrow. The epiphyses are filled with spongy bone, and red marrow fills its spaces.
In a growing bone, a layer of hyaline cartilage called the epiphyseal (growth) plate sits between shaft and end. When growth stops in early adulthood, it is replaced by bone and becomes the epiphyseal line. A tough outer membrane, the periosteum, carries vessels and nerves, and tendons and ligaments attach to it. Where bones meet at joints, the ends are covered by smooth articular cartilage instead.
Compact bone is dense and strong. Its building block is the osteon (Haversian system): rings of hard matrix around a central canal carrying blood vessels and nerves. Spongy (cancellous) bone is a lattice of thin struts called trabeculae that line up along lines of stress. It makes bones lighter, and its spaces can hold red marrow.
5 Which cells remodel bone, and how does that control blood calcium?Bone is living tissue that is constantly rebuilt. 1 min
Bone is living tissue that is constantly rebuilt. Its matrix is collagen fibers (for flexibility) hardened by calcium-based mineral crystals called hydroxyapatite (for strength).
- Osteogenic cells are stem cells and the only bone cells that divide. They become osteoblasts.
- Osteoblasts build new bone by secreting collagen matrix and calcium salts.
- Osteocytes are osteoblasts that got trapped in the matrix they made. They are the most common bone cell and maintain the matrix.
- Osteoclasts are large, multinucleated cells that resorb (break down) bone. They come from white blood cells (monocytes and macrophages), not from osteogenic cells.
This build-and-break balance lets bone act as a calcium bank. When blood calcium drops, the parathyroid glands release parathyroid hormone (PTH). PTH increases osteoclast activity so bone releases calcium. It also makes the kidneys reclaim calcium from urine and stimulates vitamin D synthesis, so the small intestine absorbs more calcium from food.
When blood calcium is too high, the thyroid releases calcitonin, which inhibits osteoclasts and promotes calcium uptake into bone.
6 How are joints classified?A joint (articulation) is where bones meet. 1 min
A joint (articulation) is where bones meet. Joints are sorted two ways.
By structure: in a fibrous joint, bones are united by fibrous connective tissue. In a cartilaginous joint, they are united by cartilage. In a synovial joint, bones meet inside a fluid-filled joint cavity. Synovial joints are the most common type.
By function (how much they move):
- Synarthrosis: immobile, such as the sutures between skull bones.
- Amphiarthrosis: slightly movable, such as the intervertebral discs and the pubic symphysis.
- Diarthrosis: freely movable. Every synovial joint is a diarthrosis. The elbow moves in one plane, while the shoulder and hip move in all three.
Inside a synovial joint, smooth articular cartilage caps each bone end and a fibrous articular capsule encloses the cavity. The capsule's inner synovial membrane makes synovial fluid, which lubricates the joint and feeds the cartilage (cartilage has no blood vessels of its own). Ligaments, strong bands of fibrous connective tissue, hold bone to bone and block abnormal movement; tendons attach muscle to bone.
Synovial joints are also named by the shape of their surfaces:
- Hinge (one plane, bend and straighten): elbow, knee.
- Pivot (rotation around one axis): the joint between the first two neck vertebrae that turns the head side to side.
- Condyloid and saddle (two planes): knuckles and wrist; the base of the thumb.
- Plane (small gliding motions): between the carpal bones of the wrist.
- Ball-and-socket (all three planes, the widest range): shoulder and hip.
Common mistakes
The traps students fall into most. Read these before you practice.
Watch out: OsteoBlasts Build bone and osteoClasts Chew it up, not the other way around.
Watch out: The sacrum and coccyx are axial (part of the vertebral column) even though they sit in the pelvis, while the scapula and clavicle are appendicular because they attach the upper limb to the trunk.
Watch out: PTH raises blood calcium and calcitonin lowers it; calcitonin comes from the thyroid gland, not the parathyroid glands.
Watch out: Compact bone forms the walls of the shaft and spongy bone fills the epiphyses, not the reverse; osteons belong to compact bone, trabeculae to spongy bone.
Watch out: Ligaments join bone to bone; tendons attach muscle to bone, not the other way around.
How to remember it
Memory hook: osteoBlasts Build, osteoClasts Chew, osteocytes sit (they are the settled cells). For calcium, calciTONin TONes down blood calcium, and PTH does the opposite.
Self-check:
- Sort these as axial or appendicular: sacrum, scapula, hyoid, femur, sternum.
- Explain why the epiphysis is filled with spongy bone instead of compact bone.
- Predict what happens to blood calcium if osteoclasts become much less active.