Step by Step
Sup
Support — the rigid framework
Bones provide the rigid scaffold that holds the body upright and gives muscles something to pull against.
Prot
Protection — shielding vital structures
The skull protects the brain, vertebrae protect the spinal cord, ribs protect the heart and lungs, and the pelvis protects the reproductive organs.
Move
Movement — bones as levers
Bones act as levers, with muscles attaching via tendons and pulling on them to create movement at joints.
Store
Storage and blood cell production
Bones store 99% of the body's calcium (along with phosphate), with mineral release regulated by PTH and calcitonin. Bones also carry out hematopoiesis (blood cell production) in red bone marrow, producing red blood cells, white blood cells, and platelets — in adults, red marrow is concentrated in the axial skeleton, pelvis, and the proximal ends of long bones.
In an adult, red bone marrow — responsible for actually producing blood cells — is largely confined to the axial skeleton, pelvis, and proximal long bone epiphyses, unlike in children, where red marrow is far more widespread throughout the skeleton.
Applied Walkthrough
1
A physician needs to perform a bone marrow biopsy on an adult patient and selects the posterior iliac crest (part of the pelvis) as the site, rather than, say, a bone in the hand or foot.
2
Ask: why choose the pelvis specifically? In adults, red bone marrow (the tissue actually responsible for hematopoiesis) is concentrated in the axial skeleton, pelvis, and the proximal ends of long bones — not spread evenly throughout the entire skeleton the way it is in children.
3
A bone in the hand or foot in an adult would likely contain mostly yellow marrow (fatty tissue with little to no active blood cell production), making it a poor choice for sampling actual hematopoietic activity.
4
This is exactly why understanding where red marrow is concentrated in adults — rather than assuming it's uniform throughout the skeleton — has direct, practical clinical relevance for procedures like bone marrow biopsies.
Exam Application
Exams test all five skeletal functions (support, protection, movement, storage, blood cell production) and specific supporting details — which organs each protective bone shields, the 99% calcium storage figure, and the specific adult locations of red marrow (axial skeleton, pelvis, proximal long bones).
⚠ Common Trap
The most common trap is assuming red marrow (and therefore active blood cell production) is distributed evenly throughout the adult skeleton. In adults, it's concentrated specifically in the axial skeleton, pelvis, and proximal long bone epiphyses — most other bones contain yellow (fatty) marrow instead.
✓ Quick Self-Check
1. What percentage of the body's calcium is stored in bone?
99%.
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2. What two hormones regulate calcium release from and deposition into bone?
PTH (releases calcium from bone) and calcitonin (deposits calcium into bone).
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3. What is hematopoiesis, and where does it occur?
Blood cell production; it occurs in red bone marrow.
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4. In adults, where is red bone marrow (and therefore active hematopoiesis) concentrated?
The axial skeleton, pelvis, and the proximal ends of long bones.
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5. Give an example of the skeletal system's protective function.
The skull protects the brain (also: vertebrae protect the spinal cord, ribs protect the heart and lungs, pelvis protects reproductive organs).
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