Overview
This lecture explains how bone serves as a storage site for calcium, the hormonal control of calcium homeostasis, and how different hormones affect bone cells and calcium absorption. It also covers the physiological importance of maintaining proper calcium levels in the blood.
Calcium Homeostasis and Bone
- Bone is a major reservoir for calcium in the body.
- Calcium homeostasis refers to the regulation of calcium movement between the blood and bone, which is controlled by hormones.
- Osteoclasts are cells that break down bone (“crash” bone), releasing calcium and phosphate into the bloodstream.
- Osteoblasts are cells that build bone, taking calcium and phosphate from the blood and depositing them into bone.
- The balance between osteoclast and osteoblast activity determines whether calcium and phosphate are released from bone or stored in bone.
Hormonal Regulation of Calcium
- Three main hormones regulate calcium homeostasis: parathyroid hormone (PTH), calcitonin, and calcitriol (the active form of vitamin D).
- PTH and calcitriol both increase calcium and phosphate levels in the blood by:
- Stimulating osteoclast activity, which breaks down bone and releases calcium and phosphate.
- Increasing calcium absorption from the intestines and reabsorption from the kidneys.
- Calcitonin has the opposite effect:
- It decreases calcium and phosphate levels in the blood by promoting osteoblast activity, which builds bone and stores calcium and phosphate.
- It also reduces calcium absorption in the intestines and kidneys.
- When calcium and phosphate levels rise in the blood, their levels in bone decrease, and vice versa. Calcium and phosphate move together between blood and bone.
Hormonal Effects on Bone Cells and Absorption
- PTH and calcitriol decrease osteoblast (bone-building) activity and increase osteoclast (bone-breaking) activity, leading to more calcium and phosphate released into the blood.
- Calcitonin increases osteoblast activity and decreases osteoclast activity, resulting in more calcium and phosphate being stored in bone.
- PTH and calcitriol both enhance calcium absorption from the intestines and reabsorption from the kidneys, further increasing blood calcium levels.
- Calcitonin reduces calcium absorption in the intestines and kidneys, lowering blood calcium levels.
- The effects of these hormones can be summarized:
- PTH and calcitriol: Increase blood calcium and phosphate by breaking down bone and increasing absorption.
- Calcitonin: Decreases blood calcium and phosphate by building bone and reducing absorption.
Physiological Importance of Calcium Levels
- The concentration of free calcium ions in the blood is crucial for normal body function.
- High blood calcium (hypercalcemia) makes cell membranes less excitable, causing symptoms like lethargy, fatigue, and memory loss.
- Low blood calcium (hypocalcemia) makes cell membranes more excitable, leading to muscle cramps and convulsions.
- The endocrine system’s regulation of calcium homeostasis is essential for preventing these problems and maintaining overall health.
Key Terms & Definitions
- Calcium homeostasis: The hormonal regulation of calcium levels in the blood and bone.
- Osteoclast: A cell that breaks down bone, releasing calcium and phosphate into the bloodstream.
- Osteoblast: A cell that builds bone by depositing calcium and phosphate from the blood.
- Parathyroid hormone (PTH): A hormone that increases blood calcium by stimulating bone breakdown and increasing absorption from the gut and kidneys.
- Calcitonin: A hormone that decreases blood calcium by promoting bone formation and reducing absorption.
- Calcitriol: The active form of vitamin D, which increases blood calcium in a manner similar to PTH.
Action Items / Next Steps
- Review and memorize how PTH, calcitonin, and calcitriol affect osteoblast and osteoclast activity, as well as calcium absorption in the intestines and kidneys.
- Understand the physiological consequences of abnormal blood calcium levels, including symptoms of both high and low calcium.
- Practice summarizing the roles of each hormone and their effects on calcium and phosphate movement between bone and blood.