Calcium Regulation in the Body

Homeostatic Setpoint

• Normal calcium levels: 9 to 11 mg/100 mL of blood.

• Regulation occurs through two primary hormones: calcitonin and parathyroid hormone (PTH).

Calcitonin

• Released by the thyroid gland's C cells in response to rising blood calcium levels.

• Function:

  • Binds to osteoblasts, activating them to take calcium from the blood and deposit it into bones.

  • Stimulates the kidneys to excrete excess calcium in urine.

  • Reduces calcium absorption from the digestive tract to lower blood calcium levels.

• Outcome: Helps lower blood calcium levels back to homeostatic setpoint.

Parathyroid Hormone (PTH)

• Released when blood calcium levels fall, from the parathyroid glands.

• Function:

  • Inhibits osteoblasts and activates osteoclasts, which break down bone matrix to release calcium into the bloodstream.

  • Increases calcium absorption from the digestive tract.

• Outcome: Restores calcium levels, ensuring muscle and heart function.

Effects of Calcium Imbalance

• Long-term calcium deficiency can lead to:

  • Osteopenia: Reduced bone density.

  • Osteoporosis: Increased fragility and risk of fractures due to loss of bone density.

Ossification Processes

Overview of Ossification

• Ossification: Formation of bone, also known as osteogenesis.

• Two main types: endochondral ossification and intramembranous ossification.

Endochondral Ossification

• Involves converting a cartilage model into bone tissue.

• Essential for:

  • Formation of the fetal skeleton into bone.

  • Growth in length of long bones through epiphyseal plates (growth plates).

• Process:

  • Osteoblasts replace cartilage cells, forming ossification centers.

  • Blood vessels bring nutrients necessary for further mineralization.

  • Longitudinal bone growth occurs as osteoblasts continue to deposit the mineral matrix in the diaphysis.

• Epiphyseal plate remains until growth stops (occurs earlier in females than males, up to age 21).

Intramembranous Ossification

• Occurs between the periosteum and endosteum without a cartilage model.

• Function:

  • Ossification centers form between membranes, laying down a mineralized matrix directly.

  • Common in the development of flat bones such as those in the skull.

• Video demonstration can enhance understanding of this process.

Conclusion

• Understanding calcium regulation and ossification processes is crucial in physiology and can have significant implications for overall health, particularly regarding bone density and strength.