Calcium Homeostasis

Calcium Homeostasis

Overview of Total Body Calcium Distribution

  • Total Body Calcium: Approximately 1,000 grams, primarily stored in the bones (99%).

  • Non-Bone Calcium Distribution:

    • Extracellular: 1%

    • Intracellular and Other Forms:

    • Bone Fluid

    • Bloodstream

    • Interstitial Fluid

Hormonal Control Systems of Calcium Concentration

Increase Blood Calcium Concentration

  • Parathyroid Hormone (PTH): Secreted by the/., parathyroid gland and plays a crucial role in increasing blood calcium levels.

  • Vitamin D: Enhances calcium absorption in the intestines and assists in maintaining adequate levels of calcium in the blood.

Decrease Blood Calcium Concentration

  • Calcitonin: Hormone produced by the thyroid gland that allegedly reduces blood calcium levels, though its significance in healthy adults is uncertain.

Parathyroid Gland Anatomy

Structure of the Thyroid and Parathyroid Glands

  • Thyroid:

    • Composed of thyroid follicles.

    • Anterior and posterior views exist, indicating different anatomical orientations.

  • Parathyroid Gland:

    • Found adjacent to the thyroid gland, consisting of various parts including the right inferior parathyroid.

    • Contains the recurrent laryngeal nerve, which is significant for vocal cord function.

Regulation of Calcium Ion Concentrations

Homeostatic Regulation Mechanism

Calcium Levels and Hormonal Response

  • Normal Calcium Levels: Ranges from 8.5 to 11 mg/dL.

  • When Calcium Levels Rise:

    • Homeostasis disturbed leading to increased secretion of calcitonin from the thyroid, promoting bone deposition and inhibiting osteoclasts.

  • When Calcium Levels Fall:

    • Homeostasis disturbed, leading to the secretion of PTH from the parathyroid gland, stimulating the release of calcium from bones, enhancing reabsorption in the kidneys, and promoting intestinal absorption through calcitriol (active Vitamin D).

Diagram of Homeostatic Regulation

  • RESTORATION of Homeostasis:

    • Rising or falling calcium levels trigger hormonal secretions that normalize blood calcium levels.

    • Essential processes include increased renal calcium reabsorption, stimulation of bone turnover, and absorption in the digestive tract.

Calcium Hormone Overview

Detail on Parathyroid Hormone (PTH)

  • Secretion Regulation: PTH secretion is influenced by extracellular calcium concentration, mediated via calcium-sensing receptors (CaSR).

  • Biological Actions of PTH:

    • Leads to calcium and magnesium resorption from the kidneys.

    • Increases calcium reabsorption and decreases phosphate reabsorption.

    • Stimulates osteoclastic activity leading to bone resorption and formation.

Calcium Levels and Pharmacological Interventions

  • Calcimimetics and Calcilytics: Substances that modify calcium levels through their effects on PTH secretion with varying influence on serum calcium concentrations.

Physiological Effects of Calcium Regulation

Calcium Deprivation and Loading

  • Calcium Deprivation: Results in increased secretion of PTH which stimulates the production of vitamin D, enhancing calcium and phosphate release from bones.

  • Calcium Loading: Decreased PTH secretion occurs when calcium levels are sufficient, leading to reduced vitamin D activation.

Factors Affecting Osteoclast and Osteoblast Activity

  • Osteoclasts: Mediate bone resorption from the monocyte-macrophage lineage and respond to PTH signaling.

  • Osteoblasts: Responsible for bone formation and express PTH receptors to mediate their response during calcium regulation.

  • Osteocytes: Terminally differentiated forms of osteoblasts that detect mechanical and hormonal signals to regulate bone metabolism.

Vitamin D Metabolism

Sources of Vitamin D

  • Sunlight Exposure: Converts 7-dehydrocholesterol in the skin into cholecalciferol (Vitamin D3).

  • Dietary Sources: Ergocalciferol (Vitamin D2) is derived from food and also contributes to vitamin D levels.

  • Conversion Pathways: Inactive forms are converted to active forms in the liver and kidneys through enzymatic processes (hydroxylation).

Role of Vitamin D in Calcium Homeostasis

  • Vitamin D functions to increase calcium absorption, foster bone turnover, and enhance plasma calcium concentrations through various mechanisms including upregulation of osteocalcin and osteopontin.

Disorders of Calcium Homeostasis

Hypercalcemia

Definition and Causes

  • Hypercalcemia: Total plasma calcium concentration greater than 10.4 mg/dL.

  • Principal Causes:

    • Hyperparathyroidism

    • Vitamin D toxicity

    • Malignancies such as cancers.

Symptoms of Hypercalcemia

  • "Stones": Renal stones, nephrocalcinosis, polyuria, polydipsia.

  • "Bones": Osteitis fibrosa with subperiosteal resorption, osteoclastomas, radiologic osteoporosis.

  • "Abdominal Groans": Constipation, indigestion, nausea, vomiting, peptic ulcer, pancreatitis.

  • "Psychic Moans": Lethargy, confusion, memory loss, psychoses, and personality changes.

Diagnosis of Parathyroid Disorders

  • Serum Calcium and PTH Measurements: Levels assessed to determine the presence of disorders.

  • Graphical Analysis: Presents relationships between Intact PTH levels and serum calcium concentrations, aiding diagnostic processes.

Treatment of Hypercalcemia

Options Available

  • Decreasing Intestinal Calcium Absorption: To reduce dietary intake's contribution to elevated calcium levels.

  • Increasing Urinary Calcium Excretion: Promoting renal clearance of excess calcium.

  • Decreasing Bone Resorption: Utilizing medications that inhibit osteoclastic activity.

  • Dialysis: In severe cases, removing calcium directly from the bloodstream.

Hypocalcemia

Definition and Causes

  • Hypocalcemia: Total plasma calcium concentration drops below 8.8 mg/dL under normal protein circumstances.

  • Causes Include:

    • Hypoparathyroidism

    • Vitamin D deficiency

    • Pseudohypoparathyroidism

Specific Causes of Hypoparathyroidism

  • Surgical Injury: Damage to glands during neck surgeries.

  • DiGeorge Syndrome: A congenital absence of parathyroid glands leading to hormone deficiencies.

Overview of Calcium Supplementation and Management

  • Natural Sources: Sunshine and vitamin D supplementation as primary means for improving calcium absorption.

Rickets and Osteomalacia

Clinical Presentation

  • Illustrated examples showing the impact of calcium metabolism disorders such as rickets and osteomalacia including pseudofractures in patients.

Type II Vitamin D Dependent Rickets

  • Documented cases illustrate hereditary resistance to 1,25(OH)2D3 with specific clinical manifestations.

Summary of Structural Analysis

  • Comparison of modeled structures of active vitamin D compounds, elucidating variants relevant in calcium metabolism and diseases linked to dysregulation.