Calcium Homeostasis:

Discussion opened about calcium movement in the body:
- Approximately 900 mg of calcium in extracellular fluid, while intracellular fluid contains around 9,000 mg.
- Calcium ions are significantly stored in bone and sarcoplasmic reticulum in muscles.
- Bone is not a static structure and is involved in constant remodeling, releasing calcium into the bloodstream when necessary.

PTH is critical for regulating calcium levels, influencing both the kidneys and bone to maintain homeostasis:
- Kidneys: Increases calcium reabsorption.
- Bone: Mobilizes calcium through the process of demineralization.
Vitamin D is essential for calcium absorption in the intestines and is synthesized from cholesterol with the help of sunlight:
- Production Pathway:
- UV light converts cholesterol to pre-Vitamin D3, which is then converted by heat and liver into active vitamin D forms (25 and 1,25 dihydroxyvitamin D3).
Regulatory Mechanisms of PTH:
- Released from parathyroid glands in response to low calcium levels.
- In high calcium conditions, it is inactivated.

Bone Dynamics:
- Osteoblasts build new bone, while osteoclasts break down bone.
- The role of cytokines is discussed in recruiting osteoclasts for remodeling.
Relationship between dietary calcium, sunlight exposure, and potential seasonal deficiencies of Vitamin D are highlighted.
Potential effects on mood related to Vitamin D levels, particularly during winter months, and the impact of social isolation were also discussed.

Review and discussions on the mechanisms of calcium and vitamin D, as well as PTH functioning, will take place in upcoming class sessions.
Students encouraged to prepare and engage with both the textbook materials and journal articles in anticipation of the final exam and completion of assignments.