6.7 Calcium Homeostasis: Interactions of the Skeletal System and Other Organ Systems

Calcium homeostasis is crucial for our bodies to function properly. From muscle contractions to nerve impulses, blood clotting to bone strength, calcium plays a vital role. When levels get out of whack, it can lead to serious issues like muscle spasms, confusion, or even kidney stones.

Our bodies have a complex system to keep calcium levels just right. Hormones like parathyroid hormone and vitamin D work to increase calcium levels, while calcitonin helps lower them. The skeleton, digestive system, and kidneys all play important parts in this delicate balancing act.

Calcium Homeostasis and Regulation

Effects of calcium imbalances

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• Calcium essential for various body functions
  • Muscle contraction involves calcium binding to troponin enabling myosin attachment to actin filaments
  • Nerve impulse transmission requires calcium triggering neurotransmitter release at synapses
  • Blood clotting utilizes calcium as a cofactor for clotting factors in the coagulation cascade
  • Bone mineralization occurs through calcium phosphate crystals (hydroxyapatite) providing strength and rigidity to bones
• Hypocalcemia (low blood calcium levels) consequences include
  • Tetany characterized by involuntary muscle contractions and spasms
  • Paresthesia involving tingling or numbness in fingers, toes, and around the mouth
  • Seizures and mental confusion can occur in severe cases
• Hypercalcemia (high blood calcium levels) consequences include
  • Fatigue, lethargy, and confusion
  • Constipation and abdominal pain
  • Polyuria (excessive urination) and dehydration
  • Kidney stones and renal damage can occur in severe cases

Hormones in calcium regulation

• Parathyroid hormone (PTH)
  • Secreted by parathyroid glands in response to low blood calcium levels
  • Increases blood calcium levels through three mechanisms
    1. Stimulates osteoclasts to break down bone and release calcium into the bloodstream
    2. Enhances renal reabsorption of calcium in the kidneys
    3. Activates vitamin D synthesis in the kidneys
  • Regulates phosphate levels by increasing renal excretion
• Vitamin D (calcitriol)
  • Synthesized in the skin upon exposure to UV light or obtained from the diet (fatty fish, fortified dairy products)
  • Activated by hydroxylation in the liver and kidneys stimulated by PTH
  • Increases blood calcium levels through two mechanisms
    1. Enhances intestinal absorption of calcium from the diet
    2. Stimulates osteoclast activity and bone resorption in conjunction with PTH
• Calcitonin
  • Secreted by parafollicular cells (C cells) in the thyroid gland in response to high blood calcium levels
  • Decreases blood calcium levels through two mechanisms
    1. Inhibits osteoclast activity and bone resorption
    2. Enhances renal excretion of calcium in the kidneys

Systemic calcium homeostasis

• Skeletal system
  • Serves as a reservoir for calcium, storing 99% of the body's calcium
  • Osteoclasts break down bone and release calcium into the bloodstream when blood calcium levels are low
  • Osteoblasts incorporate calcium into bone when blood calcium levels are high (bone mineralization)
  • Bone remodeling process continuously balances bone formation and resorption
• Endocrine system
  • Parathyroid glands secrete PTH in response to low blood calcium levels
  • Thyroid gland (C cells) secretes calcitonin in response to high blood calcium levels
  • Vitamin D synthesis is stimulated by PTH and UV light exposure
• Digestive system
  • Calcium is absorbed from the diet in the small intestine (duodenum and jejunum)
  • Vitamin D enhances intestinal absorption of calcium by increasing expression of calcium-binding proteins
• Renal system
  • Kidneys reabsorb calcium from the filtrate when blood calcium levels are low stimulated by PTH
  • Kidneys excrete excess calcium in the urine when blood calcium levels are high stimulated by calcitonin
  • Kidneys activate vitamin D through hydroxylation stimulated by PTH

Cellular mechanisms of calcium regulation

• Calcium-sensing receptors on parathyroid glands detect changes in blood calcium levels
• Calcium channels in cell membranes regulate calcium influx and efflux
• Calcium-binding proteins in various tissues facilitate calcium transport and storage