Anatomy & Physiology · Chapter 6

Bone Tissue & the Skeletal System

It is easy to picture the skeleton as a dry rack of poles, the part of us that lasts after everything else is gone. That picture is wrong in the most important way. Bone is living tissue. It is laced with blood vessels and nerves. It is rebuilt every day of your life. And it quietly does jobs that have nothing to do with holding you up.

In this chapter you will meet the skeleton as an organ system, a group of organs that work together. You will see how it is built and what its cells do. You will learn how it keeps the calcium in your blood in a very narrow range. You will also see how it heals itself when it breaks. Once you see bone as alive, the rest of the skeleton makes sense.

By the end of this chapter you can

List the major functions of the skeletal system. Split the skeleton into its axial and appendicular parts.
Sort bones by shape: long, short, flat, irregular, and sesamoid. Give an example of each.
Name the main parts of a long bone that you can see without a microscope. Describe how compact and spongy bone differ.
Name the four bone cells. Explain how remodeling balances building bone up against breaking it down.
Describe the organic and inorganic parts of bone matrix and what each one adds.
Draw how parathyroid hormone and calcitonin keep blood calcium near its set point.
Recognize common fracture patterns. Put the four stages of bone repair in order.

Section 1What the skeleton is for

The human skeleton, with the axial and appendicular parts shown in different colors — **Reference.** The human skeleton, axial and appendicular divisions — OpenStax, CC BY 3.0 (via Wikimedia Commons)

A frame that is also a factory

Every bone is an organ. It is made of several tissue types working together. These include bone tissue, cartilage, dense connective tissue, blood, and nerve tissue. And a bone is very much alive. Snap a fresh bone and it bleeds. Leave a broken one alone and it heals.

All your bones, cartilages, and joints together make up the skeletal system. This system does far more than give you a shape. It works as a support beam, a suit of armor, and a set of levers. It is also a mineral bank and a factory for blood cells. Often it does all of these at the same time.

The jobs bones do

It is worth learning the skeleton's main jobs as a short list. Almost everything else in this chapter is one of these jobs seen up close. Bone stores fat and minerals, makes blood, moves you, protects you, and holds you up.

Function	How the skeleton does it
Support	A stiff frame that holds the body up. It gives soft organs and muscles a place to attach.
Protection	Hard cases shield key organs. The skull guards the brain. The vertebrae guard the spinal cord. The rib cage guards the heart and lungs.
Movement	Bones act as levers that muscles pull on. Where two bones meet at a joint, motion happens.
Mineral & fat storage	Bone is a storehouse for calcium and phosphate. It stores them or releases them as needed. Yellow marrow stores energy as fat.
Blood-cell formation	Hematopoiesis, the making of blood cells. Red marrow inside some bones makes red cells, white cells, and platelets.

Axial and appendicular: two halves of one set

An adult skeleton has 206 bones. They split into two named groups. The axial skeleton has about 80 bones. It runs down the center line, or axis, of the body. It includes the skull, the backbone (vertebral column), and the rib cage (thoracic cage). Its main jobs are protection and support.

The appendicular skeleton has about 126 bones. It is everything that hangs off the axis. That means the arms and legs. It also includes the two girdles, the shoulder (pectoral) and hip (pelvic) girdles, which attach the limbs to the trunk. Its main job is movement. In the reference figure, the axial bones are one color and the appendicular bones are another. This is a handy way to keep the two straight.

Bone is the body's calcium bank. About 99% of the body's calcium is stored in the skeleton. That fact drives one of this chapter's biggest ideas. When blood calcium runs low, the body can take calcium out of bone. It will do this even if the skeleton suffers. A steady blood level matters more from moment to moment than any single bone.

Tap each of the skeleton's jobs in the interactive to see what carries it out.

Interactive Bones are not a dead frame. They are living organs doing five jobs at once, from holding you up to making your blood. Open full ↑

Key points

A bone is a living organ. The skeletal system is bones, cartilages, and joints together.
Main jobs: support, protection, movement, mineral/fat storage, and blood-cell formation (hematopoiesis).
The axial skeleton (about 80 bones) protects and supports. The appendicular skeleton (about 126 bones) lets you move. An adult has 206 bones in all.
About 99% of the body's calcium is stored in bone.

Section 2Classifying bones by shape

Examples of bones of each shape — **Reference.** Classification of bones by shape — BruceBlaus, CC BY 3.0 (via Wikimedia Commons)

Shape follows the job

Bones come in a few shapes you can learn to spot. As with the rest of the body, the shape is a clue to the job. A bone built to act as a lever looks nothing like a bone built to shield a soft organ. Sorting bones into five shape classes gives you quick words to describe any bone you meet.

The five shapes

Class	Shape & role	Examples
Long bone	Longer than it is wide, with a shaft and two ends. Acts as a lever for movement.	Femur (thigh), humerus (upper arm), phalanges (fingers & toes)
Short bone	Roughly cube-shaped. Gives steady support with only a little motion.	Carpals (wrist), tarsals (ankle)
Flat bone	Thin plates that are often curved. They protect organs and give muscles a wide place to attach.	Skull bones, sternum (breastbone), ribs, scapula (shoulder blade)
Irregular bone	Odd shapes that fit no other class. They do many kinds of protection and support.	Vertebrae, hip bones, many face bones
Sesamoid bone	Small bones set inside a tendon. They protect the tendon and give it a better pull.	Patella (kneecap)

Notice that the phalanges of your fingers count as long bones, even though they are tiny. The label is about proportion, not size. A long bone is longer than it is wide and has a shaft. It also works as a lever. The reference figure also shows a sutural (Wormian) bone. This is a small extra bone that can show up in the seams of the skull. Think of it as a special kind of flat bone, not a sixth class.

The patella is the body's largest sesamoid. A sesamoid bone sits inside a tendon. It lifts the tendon away from the joint. This gives the muscle more leverage, the way a longer handle makes a wrench stronger. That is why the kneecap makes the quadriceps better at straightening the knee. Small sesamoids also hide in the tendons of the hands and feet.

Tap each shape in the interactive to see the bone and where it lives in the body.

Interactive Bones come in five shapes, and the shape follows the job. Tap each one to see it and where you will find it. Open full ↑

Key points

There are five shape classes: long, short, flat, irregular, and sesamoid.
Long bones are levers. Short bones give steady support. Flat bones protect. Irregular bones do special jobs. Sesamoids sit inside tendons.
The patella is the largest sesamoid. The phalanges are small long bones.

Section 3Inside a long bone

A long bone cut open to show its parts — **Reference.** Anatomy of a long bone (femur) — OpenStax, CC BY 3.0 (via Wikimedia Commons)

The shaft and the ends

A long bone, like the femur, has a neat layout once you learn its parts. The long middle tube is the diaphysis, or shaft. Its wall is a tube of dense compact bone. Its hollow center is the medullary cavity. In adults it is filled with fatty yellow marrow.

Each rounded end is an epiphysis. It is filled with light spongy (trabecular) bone. The spaces in spongy bone hold red marrow, where blood cells are made. Where the end meets another bone at a joint, it is capped with slick articular cartilage. This cartilage cushions the joint and lets it glide.

Between the shaft and each end is the metaphysis. In a growing child, it holds the epiphyseal (growth) plate. This is a disc of cartilage where the bone gets longer. When growth ends, the plate turns to bone. It leaves a thin epiphyseal line.

The living wrappers

Two thin layers keep bone alive and connected. The periosteum is a tough sheath on the outside of the bone. It covers every surface except the joint caps. It carries blood vessels and nerves. It anchors tendons and ligaments. It also holds bone-making cells for growth and repair.

The inner surfaces are lined by the thin endosteum. It lines the medullary cavity and the spaces in spongy bone. It also holds bone cells. A nutrient artery enters the shaft through a small hole. It feeds the bone from the inside.

Zoom in: the osteon

Under a microscope, compact bone is not a solid block. It is a bundle of tiny pillars that bear weight, called osteons (Haversian systems). Each osteon is a set of rings of bone matrix, like the rings in a tree trunk. These rings are the lamellae. They wrap around a central canal that carries a blood vessel and a nerve.

Small pockets between the rings are called lacunae. Mature bone cells, the osteocytes, are trapped inside them. The cells stay linked to each other and to the central vessel by hair-thin channels called canaliculi. This system of canals and channels works like plumbing. It keeps the buried cells in hard bone fed and alive.

Part	What it is or holds
Diaphysis	The shaft. A tube of compact bone around the medullary cavity.
Epiphysis	A wide end filled with spongy bone and red marrow
Metaphysis	The area between the shaft and the end. Home of the growth plate or growth line.
Medullary cavity	The hollow center of the shaft. Holds yellow (fatty) marrow in adults.
Articular cartilage	Smooth cartilage that caps the joint surface of each epiphysis
Periosteum	Outer layer. Holds vessels, nerves, attachment points, and bone-making cells.
Endosteum	Inner lining of the marrow cavity and the spongy bone spaces

The growth plate is a child's weak point. The epiphyseal plate is made of cartilage. So it is softer than the bone around it. In children, a hard blow can crack right through it. These growth-plate (Salter–Harris) fractures need careful care. Damage there can change how the limb grows. In adults, the plate has sealed into the epiphyseal line. Then this weak point is gone.

Tap each labeled part in the interactive to find it on the bone and learn its role.

Interactive A long bone is more organized than it looks. It has a hollow shaft, spongy ends, and living wrappers. Tap each part to find it and learn its role. Open full ↑

Key points

The diaphysis (shaft) is compact bone around a marrow-filled medullary cavity. The epiphyses (ends) are spongy bone with red marrow.
The metaphysis holds the epiphyseal plate (growth plate). It becomes the epiphyseal line when growth stops.
The periosteum wraps the outside. The endosteum lines the inside.
Compact bone is built of osteons. Each is rings of lamellae around a central canal. Osteocytes sit in lacunae, linked by canaliculi.

Section 4Bone cells & remodeling

Bone cells breaking down old bone and building new bone — **Reference.** Bone remodeling: resorption and deposition by bone cells — Laboratoires Servier, CC BY-SA 3.0 (via Wikimedia Commons)

What bone is made of

Bone tissue is a wonder of design. It mixes two opposite qualities. Its organic part is mostly collagen fibers. Collagen gives bone flexibility and tensile strength, the strength to resist being pulled. So bone bends a little under a load instead of snapping.

Its inorganic part is made of mineral salts of calcium and phosphate. These form hard crystals called hydroxyapatite. They give bone its hardness and compressive strength, the strength to resist being squeezed. Take the mineral away, and bone turns rubbery. Take the collagen away, and it turns as brittle as chalk. Healthy bone needs both, in balance.

Four cells, one crew

Four cell types build and care for this matrix. The easiest way to remember them is by their jobs. Osteogenic (osteoprogenitor) cells are like stem cells. They divide to make the builders. Osteoblasts lay down new bone matrix. They build.

An osteoblast can wall itself into the matrix it made. Then it matures into an osteocyte. This is the caretaker cell. It lives in a lacuna and keeps the tissue healthy. The wrecking crew is the osteoclasts. These are large cells with many nuclei. They dissolve old bone and release its minerals back into the blood.

Cell	Job	Memory hook
Osteogenic cell	Stem cell that divides to make osteoblasts	The source
Osteoblast	Builds new bone matrix	“b” for build
Osteocyte	Mature cell. Keeps bone healthy and senses stress on it.	A retired builder, walled in
Osteoclast	Breaks down bone. Frees calcium.	“c” for consume/carve

The tug-of-war called remodeling

Bone is never finished. All through life, osteoclasts dissolve old bone. Osteoblasts lay down fresh bone right behind them. This nonstop cycle is called remodeling. It replaces about 10% of the skeleton every year. When the two crews stay balanced, the amount of bone stays steady.

Two things tune the balance. The first is mechanical stress, the load placed on a bone. Bone gets thicker where it is loaded and thinner where it is not. This rule is called Wolff's law. It is why exercise that puts weight on your bones makes them stronger. It is also why bones weaken during bed rest or in space. The second is hormones, especially the two calcium hormones you will meet next.

Sometimes breakdown beats building for years. This often happens after menopause. The result is osteoporosis, bone full of holes that breaks easily.

Blasts build, clasts consume. The whole system is a balance between two cells. Osteoblasts build bone up. Osteoclasts chew it down. Keep them even, and the skeleton stays strong. Tip the balance toward the clasts, and bone thins toward osteoporosis.

Watch the crews trade places in the interactive, and meet the four cells one at a time.

Interactive One crew lays down new bone while another dissolves old bone. This constant tug-of-war is remodeling. Watch it happen, and meet the four cells. Open full ↑

Key points

Bone matrix is collagen (for bending) plus hydroxyapatite mineral (for hardness). It needs both.
There are four cells: osteogenic (source), osteoblast (build), osteocyte (maintain), and osteoclast (break down).
Remodeling balances breakdown and building. It replaces about 10% of bone each year. It responds to load (Wolff's law) and to hormones.
If breakdown wins for a long time, the result is osteoporosis.

Section 5Calcium homeostasis

How PTH and calcitonin control blood calcium — **Reference.** Calcium homeostasis: parathyroid hormone and calcitonin — OpenStax, CC BY 3.0 (via Wikimedia Commons)

Why calcium is worth guarding

Blood calcium (Ca²⁺) must stay in a tight range, about 9–10 mg/dL. So much depends on it. Calcium makes muscles contract. It lets nerves fire. It is needed for blood clotting. It also carries messages inside cells.

If calcium drops too low, nerves get jumpy. This causes muscle spasms and cramps, called tetany. If it climbs too high, nerves and muscles get sluggish. This is a classic example of homeostasis, the idea from Chapter 1. The body holds a value near a set point using feedback, much like a thermostat. The skeleton is the store the body draws on to protect that value.

PTH raises calcium

When blood calcium falls, the parathyroid glands respond. These are four small glands on the back of the thyroid. They release parathyroid hormone (PTH). PTH raises calcium in three ways at once.

In bone, it spurs osteoclasts to dissolve matrix. This releases calcium into the blood. In the kidneys, it tells the tubes to take back calcium that would be lost in urine. It also has the kidneys turn vitamin D into its active form, calcitriol. Calcitriol then makes the intestines absorb more calcium from food. Bone, kidneys, and gut all pour calcium into the blood. The level climbs back toward normal.

Calcitonin lowers calcium

When blood calcium runs high, the thyroid gland responds. Its parafollicular (C) cells release calcitonin. Calcitonin does the opposite of PTH. It holds back osteoclasts and favors osteoblasts. So calcium is pulled out of the blood and stored back in bone. The level settles down.

The two hormones work as a push-pull pair. PTH is the gas pedal, and calcitonin is the brake. Together they keep blood calcium very steady.

Feature	Parathyroid hormone (PTH)	Calcitonin
Released when	Blood calcium is low	Blood calcium is high
Source	Parathyroid glands	Thyroid C (parafollicular) cells
Effect on calcium	Raises it	Lowers it
Bone	Spurs osteoclasts (breakdown)	Holds back osteoclasts (favors building)
Kidney / gut	Kidneys take back Ca. Activates vitamin D → gut absorbs more.	Minor role

The body protects blood calcium before the skeleton. Say your diet is short on calcium. PTH will keep pulling calcium out of bone to protect the blood level. The heart needs normal blood calcium to beat, and nerves need it to fire, right now. Over years, this steady loss is one road to weak, brittle bone. That is why calcium and vitamin D in your diet matter so much.

Knock blood calcium off its set point in the interactive and watch the right hormone respond.

Interactive Blood calcium must stay near 9–10 mg/dL. Two hormones guard it. PTH raises it, and calcitonin lowers it. Knock it off balance and watch the right one respond. Open full ↑

Key points

Blood calcium is kept near 9–10 mg/dL because nerves, muscles, and clotting depend on it.
PTH (from the parathyroids) raises calcium. It acts on bone, kidneys, and gut (through calcitriol, the active vitamin D).
Calcitonin (from thyroid C cells) lowers calcium by holding back osteoclasts.
The body will take calcium from bone to protect the blood level.

Section 6Fractures & repair

Drawings of different kinds of broken bones — **Reference.** Types of bone fractures — OpenStax, CC BY 4.0 (via Wikimedia Commons)

How bones break

A fracture is simply a broken bone. Doctors name the pattern of the break. The pattern hints at what caused it and how to treat it. The first and most important question is whether the skin stayed whole.

A closed (simple) fracture leaves the skin unbroken. An open (compound) fracture breaks through the skin. An open fracture is far more serious. It opens a door for infection. Breaks are also named for the line they follow and how bad they are.

Type	What it looks like
Closed vs open	Skin whole (closed) vs bone poking through the skin (open). Open fractures risk infection.
Transverse	The break runs straight across the shaft.
Oblique	The break runs at an angle across the bone.
Spiral	A twisting break that winds around the shaft. It points to a twisting force.
Comminuted	The bone shatters into three or more pieces.
Impacted	One broken end is driven into the other.
Greenstick	A partial break. The bone bends and cracks on one side. Seen mostly in children.

How a bone heals: four stages

A broken bone repairs itself in a set order. The full process can take months. First, torn blood vessels bleed and clot. This forms a fracture hematoma, a blood clot that seals the gap and sets the stage.

Next, cells build a fibrocartilaginous (soft) callus out of collagen and cartilage. It splints the broken ends together. Then osteoblasts replace this soft bridge with a bony (hard) callus of spongy bone. This happens about six weeks after the injury. Last, over months, the bone remodels. Osteoclasts and osteoblasts reshape the callus back toward the bone's first shape. The same stresses that shape every bone guide this step.

Stage	What happens
1. Fracture hematoma	Bleeding forms a clot that fills and seals the break.
2. Soft (fibrocartilaginous) callus	Collagen and cartilage bridge the gap and splint the ends.
3. Hard (bony) callus	Osteoblasts replace the soft callus with spongy bone (about 6 weeks).
4. Bone remodeling	Over months, the callus is reshaped toward the bone's first shape.

Doctors help this along in two steps. If the broken ends have moved apart, the fracture is first reduced. This means the ends are lined back up. It can be done by moving them from the outside (closed reduction) or by surgery (open reduction). Then the bone is immobilized, or held still, in a cast or with metal hardware. This keeps the fragile callus safe while it matures.

Children bend before they break. A child's bones have more collagen and less mineral than an adult's. So they are springier. That is why children get greenstick fractures. The bone bends and cracks partway through, like a fresh green twig, instead of snapping clean. The same bendiness helps children heal faster than adults.

Explore each fracture pattern in the interactive, then step through the four stages of repair.

Interactive Bones break in typical patterns. Then they heal themselves in four stages. Explore the types, then step through how a bone repairs. Open full ↑

Key points

Closed means the skin is whole. Open (compound) means the bone breaks the skin and risks infection.
Patterns include transverse, oblique, spiral, comminuted, impacted, and greenstick.
Healing runs in four stages: hematoma → soft callus → hard callus → remodeling.
Treatment is reduction (lining the ends up) and then immobilization (holding still) to protect the callus.

The chapter in one breath

Bone is living tissue. The skeletal system does five jobs: support, protection, movement, mineral/fat storage, and blood-cell formation.
The skeleton splits into the axial (about 80 bones) and appendicular (about 126 bones) parts, 206 in all.
Bones are sorted by shape as long, short, flat, irregular, or sesamoid. The shape follows the job.
A long bone has a diaphysis of compact bone around a marrow cavity. Its epiphyses are spongy. Compact bone is built of osteons.
Four cells drive remodeling, the lifelong balance of building and dissolving bone. They are the osteogenic cell, osteoblast, osteocyte, and osteoclast.
The matrix mixes collagen (for bending) with hydroxyapatite mineral (for hardness).
PTH raises blood calcium and calcitonin lowers it. Together they keep it near 9–10 mg/dL. The skeleton is the calcium bank.
Fractures come in named patterns. They heal in four stages: hematoma → soft callus → hard callus → remodeling.

ReferenceWords worth knowing

Here is a quick list of the terms in this chapter. Skim it now, or come back whenever a word trips you up.

Appendicular skeleton: The bones of the arms and legs, plus the shoulder and hip girdles that attach them to the axis (about 126 bones). Built for movement.
Axial skeleton: The bones along the body's center line: skull, backbone, and rib cage (about 80 bones). Built for support and protection.
Calcitonin: A hormone from the thyroid's parafollicular (C) cells. It lowers blood calcium by holding back osteoclasts.
Calcitriol: The active form of vitamin D. It helps the gut absorb calcium, and PTH helps switch it on.
Callus: The repair bridge that forms across a break. First it is a soft (fibrocartilaginous) callus, then a hard (bony) callus.
Compact bone: Dense, strong bone tissue built of osteons. It forms the outer wall of bones and the shaft of long bones.
Diaphysis: The tube-shaped shaft of a long bone. It is compact bone around the medullary cavity.
Endosteum: The thin layer that lines the inside surfaces of bone: the medullary cavity and the spaces in spongy bone.
Epiphyseal plate: The cartilage growth plate in the metaphysis, where a long bone gets longer. It becomes the epiphyseal line when growth ends.
Epiphysis: The wide end of a long bone. It is spongy bone with red marrow, capped by articular cartilage.
Hematopoiesis: The making of blood cells (red cells, white cells, and platelets) in the red marrow of bone.
Hydroxyapatite: The crystal mineral of calcium and phosphate in bone matrix. It makes bone hard.
Medullary cavity: The hollow center of a long bone's shaft. In adults it stores fatty yellow marrow.
Osteoblast: The bone-building cell that makes new matrix. It becomes an osteocyte once bone surrounds it.
Osteoclast: A large cell with many nuclei. It dissolves bone matrix and releases its minerals into the blood.
Osteocyte: A mature bone cell that lives in a lacuna. It keeps the matrix healthy and senses stress on the bone.
Osteon: The basic building block of compact bone. It is rings of matrix (lamellae) around a central (Haversian) canal.
Parathyroid hormone (PTH): The hormone from the parathyroid glands that raises blood calcium. It acts on bone, kidneys, and intestine.
Periosteum: The tough outer layer of a bone. It carries vessels and nerves, anchors tendons, and holds bone-making cells.
Remodeling: The lifelong cycle in which osteoclasts break down bone and osteoblasts build it back, in balance.
Sesamoid bone: A small bone set inside a tendon, such as the patella. It protects the tendon and gives it more leverage.
Spongy bone: Light bone tissue full of holes, made of thin struts (trabeculae). It is found in the epiphyses, and its spaces hold red marrow.

Check yourself

1. A friend says the skeleton is just “dead scaffolding.” Give two pieces of evidence that bone is living tissue.

Bone has many blood vessels and nerves. Living cells remodel it all the time. And it heals itself after a fracture. Any of these shows that bone is a living organ, not dead scaffolding. A broken bone even bleeds and repairs itself.

2. List the major functions of the skeletal system. Which one involves red bone marrow?

Support, protection, movement, mineral and fat storage, and blood-cell formation. The last one, hematopoiesis, happens in red marrow. Red marrow in spongy bone makes red cells, white cells, and platelets.

3. Sort these into the axial or appendicular skeleton: femur, sternum, a vertebra, the scapula, the skull.

Axial: sternum, vertebra, skull. Appendicular: femur, scapula. The axial skeleton is the center line (skull, backbone, rib cage). The appendicular skeleton is the limbs and their girdles. The scapula is part of the shoulder (pectoral) girdle.

4. Sort each bone by shape: the femur, a carpal, a rib, a vertebra, the patella.

Femur = long. Carpal = short. Rib = flat. Vertebra = irregular. Patella = sesamoid. Shape follows the job. Long bones are levers, short bones steady, flat bones protect, irregular bones do special jobs, and sesamoids sit inside tendons.

5. Name the shaft of a long bone. What fills its hollow center in an adult? What tissue is found in the wide ends?

The shaft is the diaphysis. The medullary cavity holds yellow (fatty) marrow. The epiphyses hold spongy bone with red marrow. Compact bone forms the wall of the shaft. Spongy bone fills the ends.

6. How is the epiphyseal plate different from the epiphyseal line? Why does this matter for children's injuries?

The epiphyseal plate is the active cartilage growth plate in a growing bone. The epiphyseal line is the bony mark left after growth stops. The plate is softer cartilage. So a break through it in a child can change how the limb grows later.

7. Match each cell to its job: osteoblast, osteoclast, osteocyte. Which one raises blood calcium when it is active?

The osteoblast builds new bone. The osteocyte keeps bone healthy. The osteoclast breaks bone down. The osteoclast raises blood calcium by dissolving matrix. Remember: blasts build, clasts consume.

8. Bone matrix has an organic part and an inorganic part. Name each and what it gives bone.

Organic collagen gives flexibility and tensile strength. Inorganic hydroxyapatite (calcium phosphate) gives hardness and compressive strength. Bone needs both. Without mineral it is rubbery. Without collagen it is brittle.

9. What is bone remodeling? How does Wolff's law explain why weight-bearing exercise makes bone stronger?

Remodeling is the ongoing balance of breakdown by osteoclasts and building by osteoblasts. Wolff's law says bone remodels in response to the stress put on it. Loading the skeleton signals osteoblasts to add bone where it is needed. So weight-bearing exercise builds denser bone, while lack of use thins it.

10. Blood calcium drops below normal. Which hormone responds? Where is it made? What are its three targets?

Parathyroid hormone (PTH), made by the parathyroid glands. It acts on bone (spurs osteoclasts), the kidneys (take back calcium and activate vitamin D), and the intestine (absorbs more calcium through calcitriol). All three raise blood calcium back toward its set point of 9–10 mg/dL.

11. Compare an open fracture with a closed fracture. Why is one more dangerous?

In a closed (simple) fracture, the skin stays whole. In an open (compound) fracture, the broken bone pokes through the skin. The open fracture is more dangerous. The wound exposes bone to the outside, so the risk of infection is high.

12. Put the four stages of bone repair in order, starting right after the break.

Fracture hematoma → soft (fibrocartilaginous) callus → hard (bony) callus → bone remodeling. A clot fills the gap. Cartilage and collagen splint it. Spongy bone replaces that bridge. Last, the callus is reshaped toward the bone's first shape.

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