Bone Anatomy and Physiology

Parts of a bone, cellular level of the skeletal system, feedback loop of PTH and calcitonin

Where is fat stored within the bone?

Medullary cavity

Through the action of osteoclasts,

bony matrix is dissolved

________ bone reduces the weight of the skeleton and reduces the load on muscles.

Spongy

Red bone marrow functions to __________, whereas yellow bone marrow functions to ________.

Produce blood cells, store fat

What happens when blood levels of calcium drop below normal?

parathyroid hormone secretion increases and osteoclast activity increases

What is the function of the hormone calcitonin?

increase calcium absorption to bone and decrease calcium excretion to blood.

Elevated levels of calcium ions in the blood stimulate the secretion of which hormone?

Calcitonin

A lack of exercise could lead to which of the following to happen?

Results in porous and weak bones.

While on a school skiing trip in Colorado, 18 year old Heidi falls and breaks her tibia and fibula in a comminuted fracture.  Blood tests reveal elevated levels of PTH and calcium. What disease would be suspected?

Osteoporosis

What does the presence of an epiphyseal plate indicate?

Bone length is increasing

Identify the bones that belong to the appendicular skeletal system.

There are 4 stages in the healing of a bone fracture.  Which of the following is the correct sequence?

1. Bone remodeling

2. Bony callus formation

3. Hematoma formation

4. Fibrocartilage callus formation

3,4,2,1: Hematoma formation, Fibrocartilage callus formation, bony callus formation, bone remodeling

________ fractures are completely internal; they do not break through the skin.

Closed

Which of the following types of fractures causes the bone to shattered into many pieces?

Comminuted

Below is an x-ray of a forearm. Identify the two bones present in the image.

radius, ulna

This is an x-ray from a 10 year old child. Identify the type of fracture present in the x-ray below. 

greenstick

Identify the type of fracture present in the x-ray of this tibia.

transverse

Explain how hyperparathyroidism leads to osteoporosis in young adults. Your explanation should include the following terms: osteocytes, osteoclast, osteoblast, parathyroid hormone, calcitonin, and calcium.

Osteocytes can be broken into two different groups, osteoclasts which break down bone, and osteoblasts which build up bone. When there is high blood calcium levels, calcitonin is released which promotes osteoblasts to use the calcium to build up bone. When there is low blood calcium levels, parathyroid hormone is released and promotes osteoclasts to break down bone for it's calcium. Hyperparathyroidism is the overproduction of the parathyriod hormone even when blood calcium levels are high, and over time more and more bone is broken down for calcium. This leads the bones to become weak and porus as the osseos tissue is siphoned away due to the overproduction of PTH, leading the condition osteoporosis. 

Diaphysis

Shaft of bone: Compact bone

Epiphysis (head)

• Proximal and distal

  • Spongy bone

  • Red bone marrow = where blood cells form

Articular Cartilage

Cushions ends of bones so joints can function.

Periosteum

Membrane on outside of bone

Medullary Cavity

• Yellow bone marrow stored here.

  • adipose tissue (commonly known as body fat) stored.

Osteocyte

Bone cell

Lacuna

where bone cells sit.

Canaliculus:

Canals that connect lacunae. Deliver nutrients to bone cells.

Lamella

Rings of bone tissue

Central Haversian Canal

Openings for blood vessel to move through bone.

Bone growth Formation: Ossification

1. In an embryo: bone is cartilage

2. In a fetus: bone slowly converts from hyaline cartilage to bone

3. In a child: hyaline cartilage converts to bone tissue

Ossification:

process of turning cartilage into bone by depositing phosphorus and calcium.

Bone Growth

• Grow in length and width

• Length: under articular cartilage and under epiphyseal plate as cartilage forms.

• Width: osteocytes grow under periosteum.

  

The epiphyseal plate

The epiphyseal plate, which is the growth plate in long bones, gradually turns into bone throughout childhood and adolescence

• The presence of an epiphyseal plate (also known as a growth plate) indicates that a bone is still in a period of growth and lengthening

• Increases bone length

Epiphyseal line

Once the cartilage growth stops and is fully replaced by bone, marking the end of bone growth in length

What is bone growth determined by

• Controlled by Ca²⁺ levels in the blood

  • High blood Ca²⁺ leads to bone formation by addition of osteoblasts

  • Low blood Ca²⁺ leads to bone breaking down by osteoclasts

Bone Remodeling

• Parathyroid Hormone (PTH): determines WHEN bone is broken down or formed in response to Ca levels.

• Stress/gravity acting on bones determines WHERE bone matrix is broken down or formed.

Parathyroid Hormone (PTH):

• Secreted by the parathyroid glands (small glands behind the thyroid). when low calcium levels are detected

• Stimulates breakdown of bone (osteoclasts) to release calcium

• Increases blood calcium levels.

Calcitonin

• When high calcium levels are detected

• Secreted by the thyroid gland (specifically by C cells).

• Inhibits bone breakdown; promotes calcium storage in bone (Osteoblasts)

• Decreases blood calcium levels.

Osteoclasts

Break down bone, PTH (stimulates activity)

Osteoblasts

Build new bone, Calcitonin (stimulates activity)

Explain the relationship of PTH and levels of calcium in the blood.

When Blood Calcium is Low,

1. The parathyroid glands detect the drop.

2. They release PTH into the bloodstream.

3. PTH works to increase blood calcium levels by stimulating osteoclasts (bone-breaking cells) to break down bone and releases calcium into the blood.

How does a disruption of the above relationship lead to osteoporosis?

Chronic overproduction of PTH leads to Hyperparathyroidism where too much PTH is released over time (even when calcium levels are normal), bone is constantly broken down by increased osteoclast activity

• Excessive loss of bone mass and minerals.

• Bones become porous, fragile, and prone to fractures

• Calcium levels in the blood stay high

Leading to osteoporosis

Explain the relationship of calcitonin and levels of calcium in the blood.

• The thyroid gland detects the elevated calcium.

• It releases calcitonin into the blood.

• Inhibits osteoclasts (cells that break down bone).

• Reduces the release of calcium from bones into the blood.

Fractures: Closed/Simple:

Bone breaks cleanly without penetrating the skin.

Fractures: Open/Compound

Bone breaks through skin.

Transverse

Fracture occurs at exactly 90^o horizontal angle.

Oblique

Fracture occurs sloped up/down at an angle.

Fracture: Comminuted

Bone breaks into many fragments.

Avulsion

A ligament or tendon pulls away from its attachment on the bone and a fragment of the bone breaks with it.

Compression and Depression

Compression: Bone is crushed

Depression: Bone is pressed inward.

Impacted

Broken bone ends are forced into each other.

Spiral

Ragged break due to excessive twisting.

Fissure/Hairline

An incomplete bone fracture; multiple small lines are often visible, but do not pass through the entire body.

Greenstick

Bone breaks incompletely, like a green twig.

• Most common in children with growing bones.

Treatment for Fractures

Reduction: realignment of broken bone ends.

• Closed Reduction: physician’s hands put ends back together.

Open Reduction: surgery is performed and bone ends are tied together by pins/wires.

Healing Process for Fractures

• Takes 6-8 weeks

1. Hematoma forms

2. Fibrocartilage callus forms

3. Bony callus forms

4. Bone remodeling occurs

• External callus forms here (scar tissue)

Axial skeletal system

Central core of body, that supports and protects brain and spinal cord

• Skull

• Spine

• Rib Cage

Appendicular skeletal system

Bones of limbs that connect them to the axial skeletal system, responsible for movement.

• Upper limbs

• Lower limbs

• Shoulders

• Hip Bones

Regulation of low blood calcium

Stimulus: When low blood calcium is detected

Sensor/Integrator: Parathyroids

Effector: Parathyroid hormone is released to encourage breakdown of bone by osteoclasts

Blood level calcium increases to return it back to homeostasis

Regulation of high blood calcium

Stimulus: high blood calcium is detected

Sensor/Integrator: Thyroids

Effector: Calcitonin is released to encourage build up of bone by osteoblasts

Blood level calcium decreases to return it back to homeostasis

Osteoporosis

A condition where bones have decreased bone density because they have become more porous (larger pores) and weak due to lack of bone tissue

• Can be controlled by diet, exercise, and smoking