Calcium and Phosphate Metabolism and Physiology of Bone

Introduction

Calcium, phosphate, and magnesium are vital minerals that build and strengthen bones. Intestinal calcium absorption adjusts according to dietary intake to maintain balance. Healthy bone mass depends on continuous coordination between osteoblast-driven formation and osteoclast-mediated resorption.

• Plasma calcium and phosphate are mainly regulated by parathyroid hormone from the parathyroid glands, calcitonin from thyroid parafollicular cells, and vitamin D.
• Additional influences include glucocorticoids, growth factors, and insulin, which modify mineral metabolism indirectly. These regulators act chiefly on three organs: the intestine, kidneys, and bones.
• The intestine controls mineral absorption from food.
• The kidneys adjust excretion and reabsorption of calcium and phosphate.
• Bones serve as the major storage site and continuously undergo remodeling.
• Calcium and phosphate concentrations are closely linked and must remain balanced. Disturbance of this balance can impair neuromuscular, skeletal, and metabolic functions.

Calcium And Phosphate Metabolism

Calcium Metabolism

• Calcium metabolism is essential for normal cellular, muscular, neural, and skeletal function.
• Normal total plasma calcium concentration is about 9–11 milligrams per deciliter, with an average near 10 milligrams per deciliter.
• Plasma calcium is maintained within a very narrow range, reflecting its critical physiological importance.
• Even small changes in calcium concentration can disturb several body systems.

Functions of Calcium

• Calcium helps generate and maintain action potentials, especially in cardiac and smooth muscle cells. It contributes to formation of pacemaker potentials in tissues that produce rhythmic electrical activity.
• Calcium is required for excitation-contraction coupling, allowing muscle fibers to contract effectively. It regulates nerve and muscle excitability by influencing sodium channel activity.
• Reduced calcium levels increase neuromuscular irritability and may cause tetany with painful muscle spasms.
• Calcium is necessary for normal cell division and cellular growth processes. It is a major mineral component of bone and is required for bone mineralization and strength.
• Bones also serve as the principal storage reservoir for body calcium.
• Calcium triggers exocytosis, enabling endocrine and exocrine glands to release hormones and enzymes. It is essential for neurotransmitter release at synapses and neuromuscular junctions.

• Calcium functions as coagulation factor IV and is required for normal blood clotting.
• Several anticoagulants prevent clotting by binding free calcium ions.
• Many enzymes require calcium as a cofactor for optimal activity.
• Calcium also acts as an intracellular second messenger in multiple hormone signaling pathways. It supports gastrointestinal smooth muscle activity and normal intestinal motility.

Distribution of Calcium in the Body

• Calcium distribution in the body is carefully regulated between bones, extracellular fluid, and cells.
• In cells, calcium is mainly stored in the mitochondria and endoplasmic reticulum. These intracellular stores provide an immediate source of calcium for signaling and other cellular functions.
• Free intracellular calcium is extremely low, while most cellular calcium remains bound or sequestered.
• Temporary changes in plasma calcium are buffered by movement of calcium between body compartments.
• A healthy adult usually contains about 1 to 1.5 kilograms of total body calcium.
• Approximately 99 percent of body calcium is located in bones and teeth.
• The remaining fraction is present in soft tissues, cells, and extracellular fluid.
• The extracellular calcium pool is about 1 to 1.5 grams.
• The intracellular calcium pool is about 10 to 15 grams, mostly in bound form.

• About 50 percent of plasma calcium is ionized and biologically active.
• Around 40 percent is bound mainly to albumin.
• Nearly 10 percent is complexed with anions such as bicarbonate.
• Blood pH affects calcium ionization.
• In alkalosis, ionized calcium decreases.
• In acidosis, ionized calcium increases.

Metabolism

• Calcium metabolism maintains balance between dietary intake, absorption, storage, and excretion.
• Daily calcium intake commonly ranges from 200 milligrams to 2 grams.
• The approximate recommended adult intake is 800 milligrams per day.
• Intestinal calcium absorption is inversely related to calcium intake.

• When intake is low, absorption efficiency rises to preserve calcium balance.
• When intake is high, absorption efficiency falls to reduce overload.
• With a daily intake of 1 gram, about 30 percent or 300 milligrams is usually absorbed.
• Around 700 milligrams of unabsorbed calcium is lost in feces.
• Approximately 150 milligrams of endogenous calcium is secreted into the intestine each day. Therefore, total fecal calcium loss becomes about 850 milligrams daily.
• About 150 milligrams of calcium is excreted in urine each day.
• Total daily calcium loss roughly matches intake in a healthy balanced state.
• Calcium absorption declines with advancing age.
• Reduced absorption may contribute to osteoporosis and decreased bone mineral density.
• Absorbed calcium enters the extracellular fluid pool, which contains about 1,000 milligrams. This pool remains in equilibrium with a rapidly exchangeable body reservoir of about 4,000 milligrams.
• During normal bone remodeling, about 500 milligrams move into bone daily.
• A similar amount returns from bone to plasma, preserving mineral homeostasis.

Table 61.1: Distribution of calcium in the body.

Body Calcium Distribution	Approximate Amount
Total body calcium	1200 g
Bones and teeth	99% (~1188 g)
Intracellular fluid	0.9% (~11 g)
Extracellular fluid	0.1% (~1 g)

Phosphate Metabolism

• Phosphate metabolism is tightly regulated because phosphate is essential for many cellular functions.
• Normal plasma phosphate concentration is about 2.5 to 4.5 milligrams per deciliter.

Functions of Phosphorus

• Phosphate is a key component of adenosine triphosphate and creatine phosphate, which store and transfer energy.
• It is present in important coenzymes such as nicotinamide adenine dinucleotide and thiamine pyrophosphate.
• Phosphate forms part of intracellular second messengers, including cyclic adenosine monophosphate and inositol triphosphate.
• It is an essential structural component of deoxyribonucleic acid and ribonucleic acid.
• Phosphate is required for phosphorylation of proteins that regulate metabolism.
• It modifies the activity of many enzymes.
• Along with calcium, phosphate is a major mineral of bones and teeth.
• It also contributes to intracellular acid-base buffering.

Distribution of Phosphate in the Body

• Total body phosphate is approximately half of total body calcium content.
• An adult contains about 600 grams of phosphate.
• Nearly 86 percent of body phosphate is stored in bones as mineral salts.
• About 14 percent is present within cells, while only a very small fraction is in extracellular fluid.
• Muscle tissue contains a substantial share of intracellular phosphate.

Metabolism

Phosphate Metabolism

• Average daily phosphate intake is about 1400 milligrams.
• Roughly 75 percent is absorbed from the intestine.
• The remaining portion is excreted in feces.
• The extracellular phosphate pool is about 700 milligrams.
• About 200 milligrams is exchanged daily between extracellular fluid and bone during remodeling.
• Approximately 1000 milligrams of phosphate is normally excreted in urine.
• Renal excretion is the major factor controlling plasma phosphate concentration.
• Urinary and fecal losses usually balance daily intake in healthy individuals.

Effects of Phosphate Deficiency

• Low phosphate may cause muscle weakness and reduced exercise capacity.
• Severe deficiency can impair cardiac and respiratory muscle function.
• It may also lead to defective bone mineralization.
• Red blood cell membrane stability may be reduced.

Magnesium and Mineral Metabolism

• Magnesium is closely linked with calcium and phosphate metabolism. It is essential for neuromuscular transmission and normal muscle function.
• Magnesium acts as a cofactor for many enzymes, especially those using adenosine triphosphate.
• Normal plasma magnesium concentration is about 1.5 to 2.5 milligrams per deciliter.
• About 35 percent of plasma magnesium is protein bound.
• Total body magnesium is approximately 25 grams.
• Around half is stored in bones, and most of the remainder is intracellular.

• Magnesium depletion may cause neuromuscular dysfunction and ventricular arrhythmias.

Table 61.2: Distribution of phosphate in the body.

Body Phosphate Distribution	Approximate Amount
Total body phosphate	600 g
Bones and teeth	86% (~516 g)
Intracellular fluid	14% (~84 g)
Extracellular fluid	0.08% (~1.2 g)

Regulation of Calcium and Phosphate

Homeostasis

• Calcium and phosphate homeostasis depends mainly on exchange of these minerals between plasma and bone.
• Intestinal absorption and renal handling also play major roles in maintaining balance.
• Calcium is excreted chiefly in feces, about 850 milligrams daily, and less in urine, about 150 milligrams.
• Phosphate is excreted mainly in urine, about 1000 milligrams daily, and less in stool.

Intestinal Control of Calcium and Phosphate

• Intestinal absorption significantly influences plasma calcium and phosphate concentrations.
• The intestine helps maintain mineral balance by regulating uptake from food.

Absorption of Calcium

• About one third of ingested calcium is usually absorbed, while the remainder is excreted.
• Calcium absorption is higher in growing children, pregnancy, and lactation because body demand increases.
• Approximately 150 milligrams of endogenous calcium enters the intestine daily through digestive secretions and shed mucosal cells.
• Calcium absorption occurs by both active and passive mechanisms.
• Active transport mainly occurs in the duodenum and jejunum.
• Passive diffusion occurs predominantly in the ileum.
• Active calcium uptake is stimulated by 1,25-dihydroxyvitamin D, the active form of vitamin D. This hormone increases synthesis of calcium transport proteins in intestinal epithelial cells.

• Calcium enters brush border cells through TRPV6 channels.
• Inside the cell, calcium binds to calbindin, which facilitates intracellular transport and limits toxic free calcium rise.
• Calcium then exits the basolateral membrane into blood through the sodium-calcium exchanger and calcium adenosine triphosphatase pump.
• Some calcium absorption can still occur even when TRPV6 or calbindin activity is reduced.
• Calcitonin and vitamin D may also modify intestinal calcium handling.

Absorption of Phosphate

• Most dietary phosphorus, about 75 to 85 percent, is absorbed as inorganic phosphate.
• Phosphate absorption occurs through both active transport and passive diffusion.
• Active transport is the dominant mechanism under normal conditions.
• A sodium-dependent transporter called NaPi-IIb mediates much of intestinal phosphate uptake.
• Regulation of phosphate absorption is less precise than calcium regulation.
• To some extent, phosphate absorption parallels calcium absorption and vitamin D activity.

Renal Control of Calcium and Phosphate

• Renal regulation of calcium and phosphate is essential for maintaining normal plasma concentrations.
• The kidneys adjust mineral balance through filtration, reabsorption, and urinary excretion.

Reabsorption of Calcium

• About 60 percent of plasma calcium is filterable at the glomerulus, while the remainder is protein bound.
• Only 1 to 5 percent of filtered calcium is normally excreted in urine.
• Nearly 95 percent is reabsorbed back into the blood.
• About 65 percent of filtered calcium is reabsorbed in the proximal tubule.

• Around 25 percent is reabsorbed in the loop of Henle.
• Approximately 8 percent is reabsorbed in the distal tubule.
• Parathyroid hormone increases calcium reabsorption, especially in the distal nephron.

Reabsorption of Phosphate

• Phosphate is excreted mainly through urine, so the kidneys are central to phosphate homeostasis.
• Usually 75 to 85 percent of filtered phosphate is reabsorbed.
• The proximal tubule is the major site, reabsorbing about 70 percent of the filtered load.
• Sodium-phosphate cotransporters mediate phosphate uptake in tubular cells.
• Parathyroid hormone reduces phosphate reabsorption in the proximal tubule, increasing urinary phosphate loss.

Bone Physiology

• Bone is a living, vascular connective tissue with structural and metabolic functions. It plays a major role in calcium, phosphate, and magnesium homeostasis.
• Bones form the skeletal framework that supports body shape and posture. They provide a stable base that enables efficient movement with muscles and joints.
• Bones protect vital organs within the skull, thoracic cavity, and pelvic cavity.
• Bone tissue serves as a reservoir for important minerals.
• Bone marrow is the principal site of hematopoiesis. It produces red blood cells, white blood cells, and platelets essential for circulation and immunity.

Table 61.3: Constituent of bone.

Bone Constituents	Major Components
Inorganic phase	Calcium, phosphate, carbonate, magnesium, sodium, water
Organic matrix	Type I collagen (~95% of matrix)
Ground substance	Proteoglycans and other high-molecular-weight proteins (~5%)

Composition of Bone

 Bone composition consists of an organic collagen framework reinforced by deposited minerals. This combination gives bone both strength and flexibility.

Inorganic Components

• The inorganic portion contains calcium, phosphate, carbonate, magnesium, and sodium.
• Minerals make up about 25 percent of bone volume.
• Because of high density, they contribute nearly half of bone weight.
• Bone is the principal storage reservoir for several body minerals.
• Approximately 99 percent of body calcium is stored in bone.
• About 86 percent of body phosphate is present in bone.
• A large proportion of carbonate and magnesium is also stored in the skeleton.

Organic Matrix

• The organic matrix of bone is called osteoid. It is composed mainly of Type I collagen, which forms about 95 percent of the matrix.
• The remaining portion is ground substance containing proteoglycans and other high-molecular-weight compounds. These substances help bind fibers and support mineral deposition.
• In compact bone, collagen fibers are arranged in concentric layers around vascular channels. These cylindrical structural units are called osteons or Haversian systems.
• Calcium and phosphate form hydroxyapatite crystals within bone. These microscopic crystals align parallel to collagen fibers. This arrangement provides hardness, compressive strength, and durability.

• Loss of minerals causes demineralization and weakens bone strength.
• Loss of organic matrix reduces flexibility and makes bone brittle.

Structure of Bone

• Bone structure has two main components: outer cortical bone and inner trabecular bone. These regions differ in density, architecture, and function.
• Cortical bone, also called compact bone, forms about 80 percent of total bone mass. It creates the dense outer shell of all bones.
• In long bones, cortical bone forms thick concentric outer layers.
• In flat bones, it forms a thinner external layer.
• Trabecular bone, also called spongy or cancellous bone, contributes about 20 percent of bone mass. It consists of interconnected bony spicules or trabeculae. It is prominent in the axial skeleton and at the ends of long bones.
• Trabecular bone provides strength with lower weight and contains marrow spaces.

Parts of Bone

• Long bones have a central shaft called the diaphysis. Their expanded ends are called epiphyses.
• The epiphysis and diaphysis are separated by epiphyseal growth plates during development.
• Linear bone growth occurs at the epiphyseal plates through replacement of cartilage by bone.
• Closure of these plates usually occurs near the end of puberty.
• After closure, further increase in height ceases.
• Bone width can still increase by appositional growth beneath the periosteum.
• Nutrients diffuse through canaliculi to nourish trabecular bone cells.
• In compact bone, blood vessels within Haversian canals supply nutrients and oxygen.

Remodeling of Bone

• Bone remodeling is a lifelong process involving coordinated bone formation and bone resorption. This continuous turnover renews skeletal tissue and repairs microdamage.
• During growth, bone formation exceeds resorption, causing an increase in bone mass.
• In adulthood, formation and resorption are usually balanced, maintaining stable bone mass.
• After 40 to 50 years, resorption often becomes greater than formation. This leads to gradual loss of bone mass with aging.
• Remodeling is an important mechanism for calcium homeostasis.
• In adults, about 10 percent of total bone mass is renewed each year.

Cell Types in Bone

• Bone cells are specialized for formation, maintenance, and resorption of skeletal tissue.
• The three principal cell types are osteoblasts, osteocytes, and osteoclasts.
• Osteoblasts and osteocytes arise from mesenchymal osteoprogenitor precursor cells.
• Osteoclasts develop from hematopoietic precursors related to monocytes and macrophages.

Osteoblasts and Osteocytes

• Osteoblasts are the primary bone-forming cells. They are derived from cells similar to fibroblasts.
• Osteoblasts synthesize osteoid, the unmineralized organic matrix of bone.
• Osteoid contains mainly Type I collagen and ground substance proteins. These cells contain abundant rough endoplasmic reticulum and Golgi apparatus, reflecting active protein synthesis.
• Osteoblasts are usually located on bone surfaces where new bone is being formed. They secrete matrix toward the interior surface of developing bone.
• Osteoblasts also promote mineralization by facilitating deposition of calcium and phosphate salts.
• When osteoblasts become enclosed within mineralized matrix, they differentiate into osteocytes.
• Osteocytes are mature bone cells located in small spaces called lacunae. They have reduced bone-forming activity compared with osteoblasts.
• Osteocytes help maintain the surrounding bone matrix. They also function as mechanosensors that detect mechanical stress and strain.

• Cytoplasmic processes of osteocytes extend through tiny channels called canaliculi.
• Canaliculi connect osteocytes with other osteocytes and surface cells. These channels allow transfer of nutrients, signaling molecules, and waste products.

Osteoclasts

• Osteoclasts are large multinucleated cells responsible for bone resorption. They are commonly found on bone surfaces undergoing remodeling.
• Osteoclasts attach tightly to bone and create a sealed resorption zone. They secrete hydrogen ions, producing an acidic environment that dissolves mineral components.
• They also release proteolytic enzymes that digest the organic matrix. This process liberates calcium and phosphate into extracellular fluid.

Mechanism of Bone Formation

• Bone formation is primarily carried out by osteoblasts, which are specialized bone-forming cells present on bone surfaces.
• Osteoblasts synthesize type I collagen and release it into the extracellular space.
• The collagen fibers and ground substance together form the unmineralized organic matrix called osteoid.
• After osteoid formation, calcium and phosphate ions are deposited within the matrix. This process is known as mineralization. These minerals combine to form hydroxyapatite crystals, mainly composed of calcium phosphate, which provide hardness and strength to bone.
• As mineral deposition progresses, osteoid is gradually converted into organized lamellae of mature bone.

• During continued matrix deposition, some osteoblasts become trapped within the surrounding mineralized tissue. These trapped cells lose much of their secretory activity and differentiate into osteocytes.
• Osteocytes occupy small spaces called lacunae and remain connected through microscopic channels known as canaliculi.
• Canaliculi allow movement of nutrients, waste products, and ions between osteocytes and surrounding extracellular fluid.
• Osteocytes help regulate local calcium exchange between bone and body fluids. This rapid transfer of calcium from recently formed bone mineral to extracellular fluid is termed osteocytic osteolysis.
• Under normal conditions, osteocytic osteolysis does not significantly reduce total bone mass. It mainly mobilizes surface calcium salts when required.
• Vitamin D is essential for proper mineralization because it increases intestinal absorption and availability of calcium and phosphate.

• Adequate calcium intake, phosphate balance, hormones, and mechanical stress are also necessary for healthy bone formation.

Role of Chondrocytes and Endochondral Ossification

• Before epiphyseal closure, the growth plate contains actively dividing chondrocytes.
• Chondrocytes produce cartilage matrix, which enlarges the epiphyseal plate and supports longitudinal bone growth.
• Newly formed cartilage gradually surrounds older chondrocytes within lacunae.
• Older chondrocytes undergo degeneration and are replaced by newly formed cells in deeper zones.
• The cartilage matrix then becomes calcified, creating a scaffold for bone deposition.
• Blood vessels and osteogenic cells enter the calcified cartilage region.

• Osteoblasts deposit osteoid over this scaffold, which later mineralizes to form bone tissue. This process lengthens the shaft of long bones and is called endochondral ossification.

• Insulin-like growth factor one, insulin, and thyroid hormones stimulate chondrocyte proliferation and activity.
• During late puberty, growth plate chondrocytes gradually lose responsiveness to growth signals.
• The epiphysis finally fuses with the shaft, producing epiphyseal closure. After closure, further linear growth of bone stops.

Bone Resorption

• Bone resorption is the breakdown of mineralized bone matrix, resulting in reduced bone mass. It releases stored calcium and phosphate into the extracellular fluid, helping maintain mineral balance. This process is performed by osteoclasts, which are large multinucleated cells specialized for bone degradation.
• Osteoclasts contain numerous mitochondria and lysosomes to support active resorptive function. They secrete acids and proteolytic enzymes, including collagen-degrading enzymes, that dissolve mineral and organic matrix.
• During resorption, calcium, phosphate, hydroxyproline, and hydroxylysine are released.

• Increased urinary hydroxyproline may indicate accelerated bone resorption.

Phases of Bone Resorption

Bone resorption occurs in two coordinated phases and is mediated by osteoclasts.

First Phase

• Activated osteoclasts insert proton pumps into the cell membrane facing bone. These pumps secrete hydrogen ions, lowering local pH to about 4.
• The acidic environment dissolves hydroxyapatite crystals and releases mineral salts.
• Low pH also enhances activity of acid-dependent enzymes.

Second Phase

• Osteoclasts secrete proteolytic enzymes, especially cathepsin K, which degrade collagen and other organic matrix proteins.
• Collagen breakdown releases pyridinoline compounds and hydroxyproline.
• Increased urinary hydroxyproline may indicate accelerated bone resorption.

Balance Between Bone Formation and Resorption

• Normal bone mass is maintained by a balance between bone formation and bone resorption within remodeling units.
• Old or damaged bone is removed by osteoclasts, and the resorption cavity is later filled by osteoblasts.
• Local signaling molecules released from bone cells coordinate this sequential process.
• Factors released during resorption can recruit osteoblasts to the affected site.
• Osteoblasts then synthesize osteoid, which mineralizes to restore bone strength.
• Continuous calcium movement into and out of bone reflects ongoing remodeling and mineral exchange.

Factors that Control Bone Formation and Resorption

• Factors promoting bone formation include growth hormone, insulin, estrogen, testosterone, vitamin D, platelet-derived growth factor, and transforming growth factor beta.
• Cortisol reduces bone formation by suppressing osteoblast activity.
• Factors increasing resorption include parathyroid hormone, cortisol, excess thyroxine, prostaglandins, interleukin-1, interleukin-6, and tumor necrosis factor.

• Estrogen, androgens, calcitonin, interferon gamma, and transforming growth factor beta inhibit resorption.

Applied Physiology

Osteoporosis

• Osteoporosis is a systemic skeletal disorder characterized by reduced bone mass and deterioration of bone microarchitecture, leading to increased fracture risk.
• Peak bone mass is usually achieved between 25 and 35 years of age. After about 40 years, gradual age-related bone loss may occur. This is often called involutional osteoporosis.
• In women, bone loss accelerates after menopause because reduced estrogen increases osteoclastic bone resorption.
• With aging, bone remodeling continues, but resorption often exceeds new bone formation.
• Osteoblastic repair becomes insufficient, and mineralization may decline, lowering bone density.

Causes

• Hyperparathyroidism increases bone resorption.
• Hyperthyroidism accelerates bone turnover and bone loss.
• Cushing syndrome and prolonged glucocorticoid excess suppress bone formation.
• Ovarian disorders causing low estrogen levels increase risk.
• Cigarette smoking and excessive alcohol intake adversely affect bone health.
• Poor calcium intake reduces mineral availability.

• Vitamin D deficiency impairs calcium absorption and mineralization.
• Vitamin C deficiency may reduce collagen synthesis and weaken matrix quality.

Features

• Osteoporosis often remains silent until fracture occurs.
• Common fracture sites include vertebrae, hip, and distal forearm, especially in older adults.
• Trabecular bone is affected earlier because it has higher metabolic activity.
• Vertebral fractures may cause height loss, kyphosis, and chronic back pain.

Treatment

Adequate calcium, vitamin D, weight-bearing exercise, fall prevention, and antiresorptive or anabolic therapy may be required.

Osteopetrosis

• Osteopetrosis is a disorder caused by defective osteoclast-mediated bone resorption.
• Unopposed osteoblastic activity increases bone density, but bones become brittle and deformed.
• Narrowed foramina may compress nerves, causing neurologic deficits.

• Reduced marrow cavities can lead to anemia, thrombocytopenia, and recurrent infections.