01 / 22

Skeletal System
Mineral Homeostasis

Master bone structure, calcium and phosphate regulation, and the hormonal axes tested on the MCAT: Section 3B.

MCAT Biology

Why the Skeletal System Matters

Bones are not inert scaffolding. They are dynamic, living organs that serve mechanical, metabolic, and endocrine roles: protection, movement, mineral storage, blood cell production, and acid-base buffering.

206 Bones
Adult human skeleton: axial (80) + appendicular (126)
99% of \(\text{Ca}^{2+}\)
Stored in bone as hydroxyapatite mineral matrix
85% of \(\text{PO}_4^{3-}\)
Stored in bone; rest in soft tissues and blood

Bone Composition: Organic + Inorganic

ORGANIC (~35%) Type I Collagen Provides tensile strength Osteoid (unmineralized) Secreted by osteoblasts Proteoglycans + Glycoproteins Ground substance Cells: Blasts, Cytes, Clasts INORGANIC (~65%) Hydroxyapatite Provides compressive strength
\(\text{Ca}_{10}(\text{PO}_4)_6(\text{OH})_2\)
Calcium + Phosphate Main mineral reservoir
Also: \(\text{Mg}^{2+}, \text{Na}^{+}, \text{CO}_3^{2-}\)

The Three Key Bone Cells

Osteoblasts
Build bone. Secrete osteoid (Type I collagen + ground substance), which is then mineralized. Derived from mesenchymal stem cells.
Osteocytes
Maintain bone. Mature osteoblasts trapped in lacunae. Sense mechanical stress and coordinate remodeling via canaliculi network.
Osteoclasts
Break down bone. Multinucleated cells from monocyte/macrophage lineage. Secrete \(\text{H}^+\) and acid hydrolases into resorption pits (Howship's lacunae).

Compact vs. Spongy Bone

Compact (Cortical) HC Lamellae (concentric layers) Haversian Canal (blood vessels + nerves) Osteocytes in lacunae Dense, organized osteons 80% of skeletal mass Spongy (Trabecular) Red marrow (hematopoiesis) Porous trabeculae lattice High surface area for remodeling

Calcium Homeostasis: The Big Picture

Plasma \(\text{Ca}^{2+}\) is tightly regulated near 10 mg/dL (2.5 mM). Deviations cause neuromuscular dysfunction: hypocalcemia causes tetany and seizures; hypercalcemia causes lethargy and cardiac arrhythmias.

PTH
Raises blood \(\text{Ca}^{2+}\). Released by parathyroid glands when \(\text{Ca}^{2+}\) drops.
Calcitonin
Lowers blood \(\text{Ca}^{2+}\). Released by thyroid C-cells when \(\text{Ca}^{2+}\) rises.
Vitamin D
Raises blood \(\text{Ca}^{2+}\). Active form: \(1,25\text{-(OH)}_2\text{D}_3\) (calcitriol). Enhances gut absorption.

PTH: The Master Calcium Regulator

PTH Released
(low \(\text{Ca}^{2+}\) detected)
 BONE Stimulates osteoclasts
Releases \(\text{Ca}^{2+}\) and \(\text{PO}_4^{3-}\)
 KIDNEY
Reabsorbs \(\text{Ca}^{2+}\)
Excretes \(\text{PO}_4^{3-}\)
 GUT (indirect) PTH activates Vitamin D in kidney (1-alpha-hydroxylase)
Blood \(\text{Ca}^{2+}\) RISES
 Negative Feedback

The Vitamin D Activation Pathway

1
Skin: UV-B converts 7-dehydrocholesterol to cholecalciferol (\(\text{Vitamin D}_3\)).
2
Liver: 25-hydroxylase converts \(\text{D}_3\) to \(25\text{-(OH)D}_3\) (calcidiol). This is the storage form measured clinically.
3
Kidney: \(1\alpha\)-hydroxylase (stimulated by PTH) converts \(25\text{-(OH)D}_3\) to \(1,25\text{-(OH)}_2\text{D}_3\) (calcitriol): the active form.
4
Gut: Calcitriol increases intestinal absorption of \(\text{Ca}^{2+}\) and \(\text{PO}_4^{3-}\) (both rise).

Calcitonin: The Counter-Regulator

Calcitonin is secreted by parafollicular (C) cells of the thyroid gland in response to high blood \(\text{Ca}^{2+}\). It opposes PTH:

1
Inhibits osteoclasts: reduces bone resorption, keeping \(\text{Ca}^{2+}\) locked in bone.
2
Promotes renal excretion: increases \(\text{Ca}^{2+}\) and \(\text{PO}_4^{3-}\) loss in urine.
3
Net effect: blood \(\text{Ca}^{2+}\) decreases.

PTH vs. Calcitonin vs. Vitamin D

PTH Calcitonin Vitamin D
Source Parathyroid glands Thyroid C-cells Skin/Liver/Kidney
Stimulus Low \(\text{Ca}^{2+}\) High \(\text{Ca}^{2+}\) PTH (activates)
Blood \(\text{Ca}^{2+}\) Raises Lowers Raises
Blood \(\text{PO}_4^{3-}\) Lowers Lowers Raises
Bone Effect Stimulates resorption Inhibits resorption Supports mineralization
Kidney Effect \(\text{Ca}^{2+}\) reabsorption \(\text{Ca}^{2+}\) excretion \(\text{Ca}^{2+}\) reabsorption

The Bone Remodeling Cycle

Activation Osteoclasts recruited Resorption Bone broken down Reversal Transition phase Formation Osteoblasts build Mineralization Hydroxyapatite deposited in osteoid Cycle Repeats Complete cycle: ~4-6 months

How to Solve Mineral Homeostasis Questions

1
Identify the disturbance: Is \(\text{Ca}^{2+}\) high or low? Is the problem primary (gland) or secondary (target organ)?
2
Determine the hormone response: Low \(\text{Ca}^{2+}\) triggers PTH. High \(\text{Ca}^{2+}\) triggers calcitonin and suppresses PTH.
3
Trace the target organs: PTH acts on bone (resorption), kidney (\(\text{Ca}^{2+}\) reabsorption, \(\text{PO}_4^{3-}\) excretion), and gut (via Vitamin D).
4
Check phosphate separately: PTH lowers \(\text{PO}_4^{3-}\); Vitamin D raises \(\text{PO}_4^{3-}\). This distinguishes clinical scenarios.
5
Apply negative feedback: Once \(\text{Ca}^{2+}\) normalizes, PTH secretion decreases. Look for feedback loops in passage-based questions.

Worked Example: Hypoparathyroidism

Scenario: A patient has their parathyroid glands accidentally removed during thyroidectomy. Predict the changes in blood \(\text{Ca}^{2+}\) and \(\text{PO}_4^{3-}\).

1
No PTH means no signal to raise \(\text{Ca}^{2+}\).
2
Bone: Osteoclast activity decreases: less \(\text{Ca}^{2+}\) released from bone.
3
Kidney: Less \(\text{Ca}^{2+}\) reabsorbed, and less \(\text{PO}_4^{3-}\) excreted (phosphate retention).
4
Vitamin D: \(1\alpha\)-hydroxylase not stimulated: less calcitriol: reduced gut \(\text{Ca}^{2+}\) absorption.

Worked Example: Primary Hyperparathyroidism

Scenario: A patient has a parathyroid adenoma secreting excess PTH. What labs would you expect?

\(\text{Ca}^{2+}\)
Elevated. Excess PTH causes increased bone resorption, kidney reabsorption, and Vitamin D activation.
\(\text{PO}_4^{3-}\)
Low. PTH is phosphaturic: increased renal excretion of phosphate.

Worked Example: Vitamin D Deficiency

Scenario: A child with poor sunlight exposure and low dietary Vitamin D presents with bowed legs. Trace the hormonal cascade.

1
Low Vitamin D means decreased intestinal \(\text{Ca}^{2+}\) and \(\text{PO}_4^{3-}\) absorption.
2
Blood \(\text{Ca}^{2+}\) drops, triggering secondary hyperparathyroidism (compensatory PTH rise).
3
PTH acts on bone: increased resorption to maintain blood \(\text{Ca}^{2+}\), weakening the skeleton.
4
Inadequate mineralization of osteoid leads to rickets (children) or osteomalacia (adults).

Common Mistakes on MCAT

✕ Confusing PTH + Vitamin D on Phosphate
PTH lowers blood \(\text{PO}_4^{3-}\) (renal excretion). Vitamin D raises it (gut absorption). They act together on \(\text{Ca}^{2+}\) but oppositely on phosphate.
✕ Calcitonin Source
Calcitonin comes from thyroid C-cells, not the parathyroid. It lowers \(\text{Ca}^{2+}\) (opposite of PTH). Its clinical role is minor but heavily tested.
✕ Osteoclast Lineage
Osteoclasts derive from monocyte/macrophage lineage (hematopoietic). Osteoblasts come from mesenchymal stem cells. Don't swap their origins.

Clinical Summary: Key Disease States

Condition \(\text{Ca}^{2+}\) \(\text{PO}_4^{3-}\) PTH
Primary Hyperparathyroidism High Low High
Hypoparathyroidism Low High Low
Vitamin D Deficiency Low Low High (2\(^{\circ}\))
Chronic Kidney Disease Low High High (2\(^{\circ}\))
Osteoporosis Normal Normal Normal

Question 1 of 5

A patient presents with muscle tetany and labs showing low blood \(\text{Ca}^{2+}\) and high blood \(\text{PO}_4^{3-}\). PTH levels are undetectable. Which condition is most likely?

Question 2 of 5

Which hormone raises both blood \(\text{Ca}^{2+}\) and blood \(\text{PO}_4^{3-}\)?

Question 3 of 5

Osteoclasts are multinucleated cells that resorb bone. From which cell lineage do they differentiate?

Question 4 of 5

In a patient with chronic kidney disease, the kidneys cannot convert \(25\text{-(OH)D}_3\) to active calcitriol. What is the expected change in PTH?

Question 5 of 5

According to Wolff's Law, what happens to the bones of an astronaut after extended time in microgravity?