Lecture Notes: Physiological Importance of Calcium

Introduction

• Calcium is crucial for several physiological processes:
  • Bone strength (with phosphorus)
  • Nerve impulse transmission
  • Muscle contraction
  • Blood clotting/coagulation
  • Cellular signaling
• Over 99% of calcium is stored as hydroxyapatite in bones.

Intracellular and Blood Calcium

• Intracellular calcium accounts for less than 1% of total calcium.
  • Mostly stored in modified organelles like the smooth endoplasmic reticulum in skeletal muscles.
• Blood calcium represents less than a tenth of a percent, regulated by hormones.

Calcium Intake and Absorption

• Calcium intake from diet requires intestinal absorption, highly dependent on Vitamin D.
  • Milk is fortified with Vitamin D for enhanced calcium absorption.

Calcium Homeostasis

Low Blood Calcium

• Detected by the parathyroid gland.
• Parathyroid gland secretes PTH (Parathyroid Hormone):
  • Bone: Targets osteoclasts to promote bone resorption for calcium release.
  • Kidneys: Promotes calcium reabsorption.
  • Small intestine: Enhances calcium absorption with Vitamin D as a coenzyme.

Elevated Blood Calcium

• Detected by parafollicular cells in the thyroid gland.
• Secretion of Calcitonin:
  • Decreases PTH activity.
  • Promotes calcium excretion.
  • Decreases osteoclast activity, leading to increased calcium deposition into bones.

Phosphate Homeostasis

• Linked closely with calcium homeostasis.

Effects of Calcium Imbalance

Hypercalcemia (Too Much Calcium)

• Muscles become non-responsive.
• Can lead to cardiac muscle inefficiency and potential heart failure.
• Calcium crystals can form in blood vessels and kidneys.

Hypocalcemia (Too Little Calcium)

• Muscles become hyper-excitable.
• Can lead to neuromuscular issues and cardiac muscle tetany.

Conclusion

• Calcium homeostasis is vital for maintaining various physiological functions and preventing severe health complications.