Regulation of Calcium in the Body

Hormonal Regulation

• Two key hormones regulate calcium levels: calcitonin and parathyroid hormone (PTH).

Homeostatic Setpoint

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

• Feedback loops help maintain calcium levels within this range.

Role of Calcitonin

• Trigger: An increase in blood calcium levels.

• Source: Released from C cells of the thyroid.

• Function:

  • Binds to receptors on osteoblasts (bone-forming cells).

  • Triggers osteoblasts to absorb calcium from the blood and deposit it into bones, lowering blood calcium levels.

  • Increases calcium excretion in urine.

  • Decreases calcium absorption in the digestive tract to prevent further elevation of calcium levels.

Role of Parathyroid Hormone (PTH)

• Trigger: A decrease in blood calcium levels.

• Source: Released from the parathyroid glands.

• Function:

  • Suppresses osteoblast activity and activates osteoclasts (bone-resorbing cells).

  • Osteoclasts secrete hydrochloric acid to dissolve bone minerals, releasing calcium into the bloodstream.

  • Increases calcium uptake from the digestive tract to restore calcium levels.

Consequences of Imbalance

• Prolonged low calcium intake can lead to osteopenia and osteoporosis due to chronic bone degradation, increasing the risk of fractures.

Ossification (Bone Formation)

Overview

• Ossification (or osteogenesis) is the process of bone formation by osteoblasts using calcium, phosphate, and collagen.

Types of Ossification

1. Endochondral Ossification

• Definition: Formation of bone from a cartilage model.

• Process:

  • Primary ossification occurs in long bones by replacing hyaline cartilage with osseous tissue.

  • Involves growth plates (epiphyseal plates) where cartilage grows and is replaced by bone, allowing for bone growth.

  • Osteoblasts migrate to the cartilage, create ossification centers, and deposit minerals.

• Examples: Growth in long bones during childhood, leading to elongation of the diaphysis.

2. Intramembranous Ossification

• Definition: Formation of bone directly within a membrane without a cartilage model.

• Process:

  • Occurs within the periosteum (outer layer) and endosteum (inner layer) of flat bones.

  • Ossification centers develop between these membranes, leading to direct mineral matrix formation.

Growth Plate Dynamics

• In children, the growth plate is crucial for height increase.

• Damage or disruption to growth plates can result in abnormal bone lengths or deformities.

• Growth plate ossification ceases after puberty, leading to the formation of an epiphyseal line.