BONE HOMEOSTASIS

What does bone remodeling accomplish?

1. Changes the way bone matrix resists stress 2 .Removes worn or injured bone 3. Provides a reservoir for body calcium

PTH has three targets (effectors)

1.increase bone resorption 2.increase calcium reabsorption by the kidneys 3.increased absorption of calcium by the gut (in conjunction with vitamin D -thru release of calcitriol)) The response to these effects is an increase in blood calcium concentration. The final result is a return to homeostasis as blood calcium levels are brought back into the 9.5 - 10.5 mg% range and the feedback system turns off. Calcitonin has just the opposite effects.

Consider the role of bone in calcium homeostasis. How is it hormonally controlled to either store or release calcium dependent upon the body's needs at the moment.

Blood calcium levels are very tightly controlled between 9.5-10.5 mg%. The hormones parathyroid hormone (PTH) and calcitonin (CT), as well as Vitamin D (Calcitriol), are the principal regulators of blood calcium concentrations.

Osteoporosis

Breakdown of spongy bone matrix (trabeculae destroyed)

What is the process of bone remodeling?

Remodeling is the ongoing replacement of old bone tissue by new bone tissue. It occurs as a delicate balance between bone resorption by osteoclasts and bone formation by osteoblasts.

This control is regulated by negative feedback mechanisms that are related to the amount of calcium in the blood. What happens if blood calcium drops too low?

The controlled condition is blood calcium concentration. In this case it has dropped below 9.5 mg%. Parathyroid gland cells detect the lowered calcium concentration. This serves as input into the control center for the feedback system. The parathyroid gland cells respond to the input from the receptors by secreting parathyroid hormone into the blood. This is the output of the system.