Fractures and Homeostasis Summary Questions

Fractures: Bone Repair

By the end of this section, you will be able to:

• Differentiate among the different types of fractures
• Describe the steps involved in bone repair

 

A fracture is a broken bone.

• It will heal whether or not a physician resets it in its anatomical position.
  • If the bone is not reset correctly, the healing process will keep the bone in its deformed position.
  • When a broken bone is manipulated and set into its natural position without surgery, the procedure is called a closed reduction.
  • Open reduction requires surgery to expose the fracture and reset the bone. While some fractures can be minor, others are quite severe and result in grave complications.
    • For example, a fractured diaphysis of the femur has the potential to release fat globules into the bloodstream.
    • These can become lodged in the capillary beds of the lungs, leading to respiratory distress and if not treated quickly, death.

 

Types of Fractures

Fractures are classified by their complexity, location, and other features

• Some fractures may be described using more than one term because it may have the features of more than one type (e.g., an open transverse fracture).

 

Compare healthy bone with different types of fractures: (a) closed fracture, (b) open fracture, (c) transverse fracture, (d) spiral fracture, (e) comminuted fracture, (f) impacted fracture, (g) greenstick fracture, and (h) oblique fracture.

 

 Types of Fractures

Type of Fracture	Description
Transverse	Occurs straight across the long axis of the bone
Oblique	Occurs at an angle that is not 90 degrees
Spiral	Bone segments are pulled apart as a result of a twisting motion
Comminuted	Several breaks result in many small pieces between two large segments
Impacted	One fragment is driven into the other, usually as a result of compression
Greenstick	A partial fracture in which only one side of the bone is broken
Open (or compound)	A fracture in which at least one end of the broken bone tears through the skin; carries a high risk of infection
Closed (or simple)	A fracture in which the skin remains intact

 

Bone Repair

When a bone breaks, blood flows from any vessel torn by the fracture.

• These vessels could be in the periosteum, osteons, and/or medullary cavity.
• The blood begins to clot, and about six to eight hours after the fracture, the clotting blood has formed a fracture hematoma.
  • The disruption of blood flow to the bone results in the death of bone cells around the fracture.
• Within about 48 hours after the fracture, chondrocytes from the endosteum have created an internal callus (plural = calli) by secreting a fibrocartilaginous matrix between the two ends of the broken bone, while the periosteal chondrocytes and osteoblasts create an external callus of hyaline cartilage and bone, respectively, around the outside of the break.
  • This stabilizes the fracture.
• Over the next several weeks, osteoclasts resorb the dead bone; osteogenic cells become active, divide, and differentiate into osteoblasts.
  • The cartilage in the calli is replaced by trabecular bone via endochondral ossification.
• Eventually, the internal and external calli unite, compact bone replaces spongy bone at the outer margins of the fracture, and healing is complete.
• A slight swelling may remain on the outer surface of the bone, but quite often, that region undergoes remodeling, and no external evidence of the fracture remains.

 

 

The healing of a bone fracture follows a series of progressive steps:

• (a) A fracture hematoma forms.
• (b) Internal and external calli form.
• (c) Cartilage of the calli is replaced by trabecular bone.
• (d) Remodeling occurs.

 

Exercise, Nutrition, Hormones, and Bone Tissue

By the end of this section, you will be able to:

• Describe the effect exercise has on bone tissue
• List the nutrients that affect bone health
• Discuss the role those nutrients play in bone health
• Describe the effects of hormones on bone tissue

 

All of the organ systems of your body are interdependent, and the skeletal system is no exception.

• The food you take in via your digestive system and the hormones secreted by your endocrine system affect your bones.
• Even using your muscles to engage in exercise has an impact on your bones.

 

Exercise and Bone Tissue

During long space missions, astronauts can lose approximately 1 to 2 percent of their bone mass per month.

• This loss of bone mass is thought to be caused by the lack of mechanical stress on astronauts’ bones due to the low gravitational forces in space.
• Lack of mechanical stress causes bones to lose mineral salts and collagen fibers, and thus strength.
• Similarly, mechanical stress stimulates the deposition of mineral salts and collagen fibers.
  • The internal and external structure of a bone will change as stress increases or decreases so that the bone is an ideal size and weight for the amount of activity it endures.
  • That is why people who exercise regularly have thicker bones than people who are more sedentary.
  • It is also why a broken bone in a cast atrophies while its contralateral mate maintains its concentration of mineral salts and collagen fibers.
• The bones undergo remodeling as a result of forces (or lack of forces) placed on them.

 

Numerous, controlled studies have demonstrated that people who exercise regularly have greater bone density than those who are more sedentary.

• Any type of exercise will stimulate the deposition of more bone tissue, but resistance training has a greater effect than cardiovascular activities.
  • Resistance training is especially important to slow down the eventual bone loss due to aging and for preventing osteoporosis.

 

Nutrition and Bone Tissue

The vitamins and minerals contained in all of the food we consume are important for all of our organ systems.

• However, there are certain nutrients that affect bone health.

 

Calcium and Vitamin D

You already know that calcium is a critical component of bone, especially in the form of calcium phosphate and calcium carbonate.

• Since the body cannot make calcium, it must be obtained from the diet.
• However, calcium cannot be absorbed from the small intestine without vitamin D.
  • Therefore, intake of vitamin D is also critical to bone health.
  • In addition to vitamin D’s role in calcium absorption, it also plays a role, though not as clearly understood, in bone remodeling.
• Milk and other dairy foods are not the only sources of calcium.
  • This important nutrient is also found in green leafy vegetables, broccoli, and intact salmon and canned sardines with their soft bones.
  • Nuts, beans, seeds, and shellfish provide calcium in smaller quantities.

 

Sunlight is one source of vitamin D.

• Except for fatty fish like salmon and tuna, or fortified milk or cereal, vitamin D is not found naturally in many foods.
  • The action of sunlight on the skin triggers the body to produce its own vitamin D, but many people, especially those of darker complexion and those living in northern latitudes where the sun’s rays are not as strong, are deficient in vitamin D.
  • In cases of deficiency, a doctor can prescribe a vitamin D supplement.

 

Other Nutrients

Vitamin K also supports bone mineralization and may have a synergistic role with vitamin D in the regulation of bone growth.

• Green leafy vegetables are a good source of vitamin K.

 

The minerals magnesium and fluoride may also play a role in supporting bone health.

• While magnesium is only found in trace amounts in the human body, more than 60 percent of it is in the skeleton, suggesting it plays a role in the structure of bone.

 

Fluoride can displace the hydroxyl group in bone’s hydroxyapatite crystals and form fluorapatite.

• Similar to its effect on dental enamel, fluorapatite helps stabilize and strengthen bone mineral.
• Fluoride can also enter spaces within hydroxyapatite crystals, thus increasing their density.

 

Omega-3 fatty acids have long been known to reduce inflammation in various parts of the body.

• Inflammation can interfere with the function of osteoblasts, so consuming omega-3 fatty acids, in the diet or in supplements, may also help enhance production of new osseous tissue.

 

Nutrients and Bone Health

Nutrient	Role in bone health
Calcium	Needed to make calcium phosphate and calcium carbonate, which form the hydroxyapatite crystals that give bone its hardness
Vitamin D	Needed for calcium absorption
Vitamin K	Supports bone mineralization; may have synergistic effect with vitamin D
Magnesium	Structural component of bone
Fluoride	Structural component of bone
Omega-3 fatty acids	Reduces inflammation that may interfere with osteoblast function

 

Hormones and Bone Tissue

The endocrine system produces and secretes hormones, many of which interact with the skeletal system.

• These hormones are involved in controlling bone growth, maintaining bone once it is formed, and remodeling it.

 

Hormones That Influence Osteoblasts and/or Maintain the Matrix

Several hormones are necessary for controlling bone growth and maintaining the bone matrix.

• The pituitary gland secretes growth hormone (GH), which, as its name implies, controls bone growth in several ways.
  • It triggers chondrocyte proliferation in epiphyseal plates, resulting in the increasing length of long bones.
  • GH also increases calcium retention, which enhances mineralization, and stimulates osteoblastic activity, which improves bone density.

 

GH is not alone in stimulating bone growth and maintaining osseous tissue.

• Thyroxine, a hormone secreted by the thyroid gland promotes osteoblastic activity and the synthesis of bone matrix.
• During puberty, the sex hormones (estrogen in girls, testosterone in boys) also come into play.
  • They too promote osteoblastic activity and production of bone matrix, and in addition, are responsible for the growth spurt that often occurs during adolescence.
  • They also promote the conversion of the epiphyseal plate to the epiphyseal line (i.e., cartilage to its bony remnant), thus bringing an end to the longitudinal growth of bones.
• Additionally, calcitriol, the active form of vitamin D, is produced by the kidneys and stimulates the absorption of calcium and phosphate from the digestive tract.

 

Aging and the Skeletal System

Osteoporosis is a disease characterized by a decrease in bone mass that occurs when the rate of bone resorption exceeds the rate of bone formation, a common occurrence as the body ages.

• Notice how this is different from Paget’s disease.
  • In Paget’s disease, new bone is formed in an attempt to keep up with the resorption by the overactive osteoclasts, but that new bone is produced haphazardly.
  • In fact, when a physician is evaluating a patient with thinning bone, he or she will test for osteoporosis and Paget’s disease (as well as other diseases).
  • Osteoporosis does not have the elevated blood levels of alkaline phosphatase found in Paget’s disease.

 

Bone density peaks at about 30 years of age.

• Women lose bone mass more rapidly than men.

 

While osteoporosis can involve any bone, it most commonly affects the proximal ends of the femur, vertebrae, and wrist.

• As a result of the loss of bone density, the osseous tissue may not provide adequate support for everyday functions, and something as simple as a sneeze can cause a vertebral fracture.
• When an elderly person falls and breaks a hip (really, the femur), it is very likely the femur that broke first, which resulted in the fall.

 

Histologically, osteoporosis is characterized by a reduction in the thickness of compact bone and the number and size of trabeculae in cancellous

bone.

• Women lose bone mass more quickly than men starting at about 50 years of age.
  • This occurs because 50 is the approximate age at which women go through menopause.
  • Not only do their menstrual periods lessen and eventually cease, but their ovaries reduce in size and then cease the production of estrogen, a hormone that promotes osteoblastic activity and production of bone matrix.
  • Thus, osteoporosis is more common in women than in men, but men can develop it, too.
• Anyone with a family history of osteoporosis has a greater risk of developing the disease, so the best treatment is prevention, which should start with a childhood diet that includes adequate intake of calcium and vitamin D and a lifestyle that includes weight-bearing exercise.
  • These actions, as discussed above, are important in building bone mass.
  • Promoting proper nutrition and weight-bearing exercise early in life can maximize bone mass before the age of 30, thus reducing the risk of osteoporosis.

 

For many elderly people, a hip fracture can be life threatening.

• The fracture itself may not be serious, but the immobility that comes during the healing process can lead to the formation of blood clots that can lodge in the capillaries of the lungs, resulting in respiratory failure; pneumonia due to the lack of poor air exchange that accompanies immobility; pressure sores (bed sores) that allow pathogens to enter the body and cause infections; and urinary tract infections from catheterization.

 

Current treatments for managing osteoporosis include bisphosphonates (the same medications often used in Paget’s disease), calcitonin, and estrogen (for women only).

• Minimizing the risk of falls, for example, by removing tripping hazards, is also an important step in managing the potential outcomes from the disease.

 

Hormones That Influence Osteoclasts

Bone modeling and remodeling require osteoclasts to resorb unneeded, damaged, or old bone, and osteoblasts to lay down new bone.

• Two hormones that affect the osteoclasts are parathyroid hormone (PTH) and calcitonin.
  • PTH stimulates osteoclast proliferation and activity.
    • As a result, calcium is released from the bones into the circulation, thus increasing the calcium ion concentration in the blood.
    • PTH also promotes the reabsorption of calcium by the kidney tubules, which can affect calcium homeostasis (see below).
    • The small intestine is also affected by PTH, albeit indirectly.
    • Because another function of PTH is to stimulate the synthesis of vitamin D, and because vitamin D promotes intestinal absorption of calcium, PTH indirectly increases calcium uptake by the small intestine.
  • Calcitonin, a hormone secreted by the thyroid gland, has some effects that counteract those of PTH.
    • Calcitonin inhibits osteoclast activity and stimulates calcium uptake by the bones, thus reducing the concentration of calcium ions in the blood.
    • As evidenced by their opposing functions in maintaining calcium homeostasis, PTH and calcitonin are generally not secreted at the same time.

 

Hormones That Affect the Skeletal System

Hormone	Role
Growth hormone	Increases length of long bones, enhances mineralization, and improves bone density
Thyroxine	Stimulates bone growth and promotes synthesis of bone matrix
Sex hormones	Promote osteoblastic activity and production of bone matrix; responsible for adolescent growth spurt; promote conversion of epiphyseal plate to epiphyseal line
Calcitriol	Stimulates absorption of calcium and phosphate from digestive tract
Parathyroid hormone	Stimulates osteoclast proliferation and resorption of bone by osteoclasts; promotes reabsorption of calcium by kidney tubules; indirectly increases calcium absorption by small intestine
Calcitonin	Calcitonin Inhibits osteoclast activity and stimulates calcium uptake by bones

 

Calcium Homeostasis: Interactions of the Skeletal

System and Other Organ Systems

By the end of this section, you will be able to:

• Describe the effect of too much or too little calcium on the body
• Explain the process of calcium homeostasis

 

Calcium is not only the most abundant mineral in bone, it is also the most abundant mineral in the human body.

• Calcium ions are needed not only for bone mineralization but for tooth health, regulation of the heart rate and strength of contraction, blood coagulation, contraction of smooth and skeletal muscle cells, and regulation of nerve impulse conduction.
• The normal level of calcium in the blood is about 10 mg/dL. When the body cannot maintain this level, a person will experience hypo- or hypercalcemia.

 

Hypocalcemia, a condition characterized by abnormally low levels of calcium, can have an adverse effect on a number of different body systems including circulation, muscles, nerves, and bone.

• Without adequate calcium, blood has difficulty coagulating, the heart may skip beats or stop beating altogether, muscles may have difficulty contracting, nerves may have difficulty functioning, and bones may become brittle.
• The causes of hypocalcemia can range from hormonal imbalances to an improper diet.
• Treatments vary according to the cause, but prognoses are generally good.

 

Conversely, in hypercalcemia, a condition characterized by abnormally high levels of calcium, the nervous system is underactive, which results in lethargy, sluggish reflexes, constipation and loss of appetite, confusion, and in severe cases, coma.

 

Obviously, calcium homeostasis is critical.

• The skeletal, endocrine, and digestive systems play a role in this, but the kidneys do, too.
• These body systems work together to maintain a normal calcium level in the blood.

 

The body regulates calcium homeostasis with two pathways; one is signaled to turn on when blood calcium levels drop below normal and one is the pathway that is signaled to turn on when blood calcium levels are elevated.

 

Calcium is a chemical element that cannot be produced by any biological processes.

• The only way it can enter the body is through the diet.
• The bones act as a storage site for calcium: The body deposits calcium in the bones when blood levels get too high, and it releases calcium when blood levels drop too low.
  • This process is regulated by PTH, vitamin D, and calcitonin.

 

Cells of the parathyroid gland have plasma membrane receptors for calcium.

• When calcium is not binding to these receptors, the cells release PTH, which stimulates osteoclast proliferation and resorption of bone by osteoclasts.
• This demineralization process releases calcium into the blood.
• PTH promotes reabsorption of calcium from the urine by the kidneys, so that the calcium returns to the blood.
• Finally, PTH stimulates the synthesis of vitamin D, which in turn, stimulates calcium absorption from any digested food in the small intestine.

When all these processes return blood calcium levels to normal, there is enough calcium to bind with the receptors on the surface of the cells of the parathyroid glands, and this cycle of events is turned off.

 

When blood levels of calcium get too high, the thyroid gland is stimulated to release calcitonin, which inhibits osteoclast activity and stimulates calcium uptake by the bones, but also decreases reabsorption of calcium by the kidneys.

• All of these actions lower blood levels of calcium.
• When blood calcium levels return to normal, the thyroid gland stops secreting calcitonin.