Mineral Notes (ACBS 400A/500A)

Calcium ($\mathrm{Ca^{2+}}$)

• Main uses and roles

  • Formation of skeletal tissues

  • Transmission of neural impulses

  • Excitation-contraction coupling in skeletal and cardiac muscle

  • Blood clotting formation

  • Component in milk

  • $\mathrm{Ca^{2+}}$ is distributed between the extracellular fluid (ECF) and intracellular compartments with most in the skeleton

  • 98% of $\mathrm{Ca^{2+}}$ resides in the skeletal system; ~2% in the ECF

• Concentration and distribution

  • Total plasma $\mathrm{Ca^{2+}}$ in adults: $[\mathrm{Ca^{2+}}]_{\text{total}} = 2.2 \text{--} 2.5\; \mathrm{mmol/L}$ (9–10 mg/dL; 4.4–5 mEq)

  • Distribution in plasma:

  • 40–45% protein-bound (albumin)

  • 5% bound to organic parts of blood

  • 45–50% ionized (free/unbound)

  • Ionized $\mathrm{Ca^{2+}}$ is the physiologically active pool and is higher in low blood pH and lower in high pH

  • Normal functional range deviation tolerance: about $1-1.25\, \mathrm{mmol/L}$ from baseline to maintain normal functions

• Extracellular $\mathrm{Ca^{2+}}$

  • Skeletal mineralization occurs when plasma $\mathrm{Ca^{2+}}$ and phosphate ($\mathrm{P^{3-}}$) concentrations are normal; $\mathrm{Ca^{2+}}$ is a component of hydroxyapatite

  • Nerve membrane potential: positively charged $\mathrm{Ca^{2+}}$ increases potential difference across membranes

  • In hypocalcemia, reduced difference between ECF and ICF can bring neurons closer to action potential initiation

  • Essential for the clotting factor pathway

  • Muscle membrane potential: $\mathrm{Ca^{2+}}$ influx triggers acetylcholine release from nerves to muscle

  • Cardiac muscle uses a similar $\mathrm{Ca^{2+}}$-coupled excitation mechanism (slightly different action potential dynamics)

• Intracellular $\mathrm{Ca^{2+}}$

  • Initiates muscle contraction in skeletal, smooth, and cardiac muscle via release from the sarcoplasmic reticulum

  • Acts as a second messenger for outside-to-inside signaling; $\mathrm{Ca^{2+}}$ entry alters membrane potential

  • Calmodulin binds $\mathrm{Ca^{2+}}$ to stimulate ion channels, enzyme activity, or DNA transcription

• $\mathrm{Ca^{2+}}$ homeostasis (balance)

  • Delicate balance between $\mathrm{Ca^{2+}}$ loss pathways and dietary/bone resorption

  • Loss from ECF occurs via bone formation, digestive secretions, sweat, urine, milk production (lactation/eggs in birds/reptiles)

  • Restored by diet, bone resorption, and renal $\mathrm{Ca^{2+}}$ resorption

• Hypocalcemia: hormonal regulation and bone turnover

  • Parathyroid hormone (PTH) senses ECF $\mathrm{Ca^{2+}}$ levels (parathyroid gland near thyroid/trachea)

  • Fall in ECF $\mathrm{Ca^{2+}}$ triggers PTH secretion; PTH increases renal $\mathrm{Ca^{2+}}$ reabsorption

  • Large $\mathrm{Ca^{2+}}$ losses stimulate intestinal $\mathrm{Ca^{2+}}$ absorption and bone resorption to maintain $\mathrm{Ca^{2+}}$ levels

  • PTH is the primary regulator of renal production of 1,25-(OH)~2~D (calcitriol)

  • PTH prevents osteoclasts from entering the reversal phase and prevents osteoblasts from laying new matrix during active resorption

• Bone’s role in $\mathrm{Ca^{2+}}$ homeostasis

  • PTH stimulates osteocytic osteolysis with persistent hypocalcemia; activates osteoclasts by shrinking osteoblasts to expose bone matrix

  • Osteoblasts release paracrine factors (prostaglandin E2, IL-1, IL-6) that stimulate osteoclasts

  • Vitamin D hormone (calcitriol) indirectly affects bone $\mathrm{Ca^{2+}}$ by increasing intestinal absorption of $\mathrm{Ca^{2+}}$ and phosphorus, balancing blood $\mathrm{Ca^{2+}}$ and enabling bone mineralization

  • Calcitonin (secreted by C-cells in the thyroid when $\mathrm{Ca^{2+}}$ is high) inhibits bone resorption by acting on osteoclasts

• Quick reference equations

  • Distribution of $\mathrm{Ca^{2+}}$ in plasma:$\ [\mathrm{Ca^{2+}}]<em>{\text{total}} = [\mathrm{Ca^{2+}}]</em>{\text{ionized}} + [\mathrm{Ca^{2+}}]<em>{\text{protein-bound}} + [\mathrm{Ca^{2+}}]</em>{\text{organic-bound}}$

  • Ionized $\mathrm{Ca^{2+}}$ fractions (approximate):$\ f<em>{\text{ionized}} \approx 0.45-0.50, \quad f</em>{\text{protein-bound}} \approx 0.40-0.45, \quad f_{\text{organic-bound}} \approx 0.05-0.10$

  • PTH action cascade (simplified):

    Renin-angiotensin-aldosterone system (indirectly linked to $\mathrm{Ca^{2+}}$ via volume and gut absorption) and active vitamin D production; note: primary renal regulator of 1,25-(OH)~2~D synthesis

Phosphorus ($\mathrm{PO_4^{3-}}$)

• Function

  • Combine with oxygen to form phosphate anions; major bone mineral

  • Used in phospholipids, phosphoproteins, nucleic acids, and energy currency ATP

  • Important component of the body's acid-base buffer system

• Concentration and distribution

  • Plasma $\mathrm{PO<em>4^{3-}}$ concentration: $[\mathrm{PO</em>4^{3-}}]_{\text{plasma}} \approx 1.3-2.6\,\mathrm{mmol/L}$ (4–8 mg/dL)

  • Intracellular $\mathrm{PO<em>4^{3-}}$ concentration: $[\mathrm{PO</em>4^{3-}}]_{\text{ICF}} \approx 25\,\mathrm{mmol/L}$ (78 mg/dL)

  • About 30% of plasma phosphorus is present as inorganic phosphate; the remainder is in organic molecules (proteins, phospholipids)

  • The measured blood phosphate is $\mathrm{PO_4^{3-}}$

• Homeostasis and regulation

  • Absorption: primarily in the small intestine via active transport; stimulated by 1,25-(OH)~2~D

  • Deficiency adaptation: deficiency can upregulate intestinal absorption when renal 1,25-(OH)~2~D increases

  • Renal handling: excess absorbed phosphate is excreted in urine; saliva also plays a role

  • PTH impact: increases renal and salivary excretion of phosphate when $\mathrm{Ca^{2+}}$ homeostasis is being managed; hypocalcemia often accompanies hypophosphatemia but correcting $\mathrm{Ca^{2+}}$ does not automatically correct phosphate

  • TL;DR: phosphate is mobilized from bone with $\mathrm{Ca^{2+}}$ exchange, but kidneys and saliva increase excretion to compensate when necessary

• Notes on calcium/phosphorus interdependence

  • Phosphorus and $\mathrm{Ca^{2+}}$ homeostasis are tightly linked; strategies that alter $\mathrm{Ca^{2+}}$ can impact phosphate handling and vice versa

Sodium ($\mathrm{Na^{+}}$)

• Function

  • Major cation of the extracellular fluid (ECF)

  • Maintains osmotic pressure and water content of circulation

  • Key role in acid-base balance

  • Determines electrical potential of nervous tissue and transmission of nerve impulses

  • $\mathrm{Na^{+}}$-coupled transport is essential for absorption of carbohydrates and amino acids

• Concentration and distribution

  • ECF $\mathrm{Na^{+}}$ concentration: $[\mathrm{Na^{+}}]_{\text{ECF}} \approx 135-155\,\mathrm{mmol/L}$

  • ICF $\mathrm{Na^{+}}$ concentration is about one-tenth of the ECF level (roughly 10–15 mmol/L)

• Homeostasis and regulation (renal-centric)

  • Primary control via renal handling; other mechanisms exist (to be discussed later)

  • Low ECF $\mathrm{Na^{+}}$ (hyponatremia) triggers the renin-angiotensin system (RAS):

  • Juxtaglomerular cells release renin $\rightarrow$ angiotensinogen to Angiotensin I $\rightarrow$ ACE converts Ang I to Ang II

  • Ang II stimulates aldosterone secretion from the adrenal cortex

  • Aldosterone increases renal $\mathrm{Na^{+}}$ reabsorption in renal tubules $\rightarrow$ raises plasma $\mathrm{Na^{+}}$

  • Hypernatremia stimulates atrial natriuretic peptide (ANP) release from atrial cells

  • ANP inhibits renal $\mathrm{Na^{+}}$ reabsorption and decreases angiotensin II and aldosterone production, promoting natriuresis and diuresis

• Diagrammatic pathway (conceptual)

  • Low BP/volume triggers: liver produces angiotensinogen $\rightarrow$ renin (kidney) $\rightarrow$ Angiotensin I $\rightarrow$ Angiotensin II (via ACE) $\rightarrow$ adrenal cortex releases aldosterone $\rightarrow$ renal $\mathrm{NaCl}$ and water reabsorption $\uparrow$ $\rightarrow$ plasma $\mathrm{Na^{+}}$ and volume $\uparrow$

  • Volume expansion triggers ANP $\rightarrow$ natriuresis and diuresis $\rightarrow$ restoration toward normal

Chloride ($\mathrm{Cl^{-}}$)

• Function

  • Major extracellular anion; maintains osmotic pressure and water distribution in circulation

  • Crucial in acid-base balance

  • Participates in HCl formation in the stomach for digestion

  • In RBCs, participates in the chloride shift to aid $\mathrm{O2}$ transport: exchange of $\mathrm{Cl^{-}}$ for $\mathrm{HCO</em>3^{-}}$

• Concentration and distribution

  • ECF $\mathrm{Cl^{-}}$ concentration: $[\mathrm{Cl^{-}}]_{\text{ECF}} \approx 100-113\,\mathrm{mmol/L}$

  • ICF $\mathrm{Cl^{-}}$ concentration is roughly one-tenth of the ECF level

• Homeostasis

  • Maintained largely by the electrical potential of cells driving $\mathrm{Cl^{-}}$ movement

  • Renal excretion of $\mathrm{Cl^{-}}$ helps balance acid-base status and other losses (stomach, intestine, sweat, etc.)

Potassium ($\mathrm{K^{+}}$)

• Function

  • Most critical for establishing and maintaining resting membrane potential

  • Higher intracellular $\mathrm{K^{+}}$ concentration relative to extracellular space

  • $\mathrm{Na^{+}}$/$\mathrm{K^{+}}$-ATPase pump: maintains intracellular $\mathrm{K^{+}}$ and extracellular $\mathrm{Na^{+}}$ by moving 2 $\mathrm{K^{+}}$ into cells and 3 $\mathrm{Na^{+}}$ out of cells per cycle; this helps preserve electrical potential and cell volume

  • ECF $\mathrm{K^{+}}$ concentration: $[\mathrm{K^{+}}]_{\text{ECF}} \approx 3-6\,\mathrm{mmol/L}$

  • ICF $\mathrm{K^{+}}$ concentration: $[\mathrm{K^{+}}]_{\text{ICF}} \approx 150\,\mathrm{mmol/L}$

• Roles in physiology

  • Essential for growth and protein synthesis (amino acids added to proteins require normal intracellular $\mathrm{K^{+}}$)

  • Necessary for insulin secretion (glucose and $\mathrm{K^{+}}$ enter cells together)

  • Participates in acid-base balance via exchange with $\mathrm{H^{+}}$ to help buffer pH

• Metabolism and regulation (homeostatic responses)

  • All dietary $\mathrm{K^{+}}$ is absorbed; kidneys excrete excess to maintain balance

  • Hyperkalemia can stimulate aldosterone secretion $\rightarrow$ increased renal $\mathrm{K^{+}}$ excretion

  • GI secretion and renal control work together to prevent hyperkalemia; intracellular uptake after meals (buffering) is mediated by insulin which increases $\mathrm{Na^{+}}$/$\mathrm{K^{+}}$-ATPase activity

  • Hypokalemia is generally corrected by reduced aldosterone secretion unless dietary intake is inadequate

Magnesium ($\mathrm{Mg^{2+}}$)

• Function

  • Major intracellular cation; essential cofactor for many enzymatic reactions in metabolic pathways

  • Forms Mg-ATP, used by many kinases (e.g., adenylate cyclase, acyl-CoA synthetase, succinyl-CoA synthetase)

  • Glycolysis requires $\mathrm{Mg^{2+}}$ (often with ATP or AMP)

  • Intracellular concentration: $[\mathrm{Mg^{2+}}]_{\text{i}} \approx 13\,\mathrm{mmol/L}$

  • Extracellular concentration: $[\mathrm{Mg^{2+}}]_{\text{ECF}} \approx 0.75-1.0\,\mathrm{mmol/L}$

  • Bone formation also requires $\mathrm{Mg^{2+}}$

• Extracellular and nerve conduction roles

  • $\mathrm{Mg^{2+}}$ is necessary for proper nerve conduction; hypomagnesemia reduces membrane potential toward threshold for action potential

• Homeostasis

  • No dedicated hormonal mechanism for $\mathrm{Mg^{2+}}$ homeostasis

  • Kidneys excrete excess $\mathrm{Mg^{2+}}$ when plasma levels exceed renal reabsorption threshold

  • Renal threshold for $\mathrm{Mg^{2+}}$: $[\mathrm{Mg^{2+}}]_{\text{th}} \approx 0.75-0.9\,\mathrm{mmol/L}$

  • Lower levels indicate insufficient dietary absorption (little $\mathrm{Mg^{2+}}$ detected in urine)

  • PTH can raise renal threshold for $\mathrm{Mg^{2+}}$ and increase $\mathrm{Mg^{2+}}$ concentrations if $\mathrm{Ca^{2+}}$ absorption is good

Iodine ($\mathrm{I^{-}}$)

• Function

  • Essential for synthesis of thyroid hormones thyroxine (T4) and triiodothyronine (T3)

  • Thyroid hormones regulate energy metabolism; hormone production increases in cold weather to boost basal metabolic rate

• Homeostasis and distribution

  • Approximately 80–90% of dietary iodine is absorbed; unutilized iodine is excreted in urine and milk

  • Milk iodine content rises with higher dietary iodine intake

  • When diet iodine content is high, less than ~20% is incorporated into the thyroid; mild deficiency can result in ~30% incorporation and severe deficiency up to ~65%

Iron ($\mathrm{Fe}$)

• Function

  • Element of heme in hemoglobin and myoglobin; $\mathrm{Fe^{2+}}$ (ferrous) state enables $\mathrm{O_2}$ binding

  • Cofactor for enzymes in the electron transport chain (cytochrome oxidase, ferredoxin, myeloperoxidase, catalase, cytochrome P450, etc.)

• Homeostasis and transport

  • Iron intake depends on diet type (e.g., carnivore vs. herbivore)

  • Heme iron (from animal sources) is absorbed via heme transport proteins on enterocytes; heme is endocytosed; $\mathrm{Fe^{2+}}$ is freed in the cytoplasm; $\mathrm{Fe^{2+}}$ is oxidized to $\mathrm{Fe^{3+}}$ as it exits the cell and binds to transferrin for transport to tissues

  • Non-heme iron ($\mathrm{Fe^{3+}}$) is harder to absorb; reduced to $\mathrm{Fe^{2+}}$ by stomach acid and facilitated by chelators (amino acids, fructose) to increase absorption; enters cells via specific transporters; once inside, $\mathrm{Fe^{2+}}$ is oxidized to $\mathrm{Fe^{3+}}$ by ferroportin during export; $\mathrm{Fe^{3+}}$ binds to transferrin for tissue delivery

• Iron homeostasis and storage

  • Adequate iron status leads to $\mathrm{Fe^{2+}}$ within enterocytes not transferred to blood; it binds ferritin for storage in enterocytes until cell turnover

  • Absorbed iron regulated by ferritin content in enterocytes; ferritin can bind zinc and copper; high dietary iron can reduce copper and zinc absorption

  • Hepcidin (liver-derived hormone) downregulates ferroportin to reduce iron absorption when needed

  • Old RBCs are destroyed and their iron is recycled onto transferrin for reuse

General notes and cross-links

• All minerals interact with foundational principles of physiology, including cell membranes, enzyme function, and hormonal regulation

• Key feedback loops to remember:

  • $\mathrm{Ca^{2+}}$: PTH, calcitonin, and vitamin D (1,25-(OH)~2~D) regulate bone resorption, intestinal absorption, and renal reabsorption

  • Phosphorus: closely linked to $\mathrm{Ca^{2+}}$ homeostasis; PTH modulates renal/phosphorus excretion and calcitriol levels adjust intestinal absorption

  • $\mathrm{Na^{+}}$ and water balance: RAAS and ANP coordinate renal reabsorption/excretion to maintain plasma $\mathrm{Na^{+}}$ and volume

  • $\mathrm{Mg^{2+}}$ and $\mathrm{Ca^{2+}}$: PTH can influence $\mathrm{Mg^{2+}}$ handling to some degree; $\mathrm{Mg^{2+}}$ is required for many enzymatic processes including those that process $\mathrm{Ca^{2+}}$

  • Iron: Hepcidin–ferroportin axis controls absorption and release of iron; ferritin stores iron within enterocytes; transferrin ferries iron in blood

• Practical implications (clinical relevance)

  • Hypocalcemia can trigger PTH-mediated bone resorption and renal vitamin D activation to restore $\mathrm{Ca^{2+}}$

  • Hyperkalemia or hypokalemia have significant effects on cardiac and neuromuscular function and are tightly regulated via renal and hormonal processes

  • Iron deficiency or overload has wide-ranging effects on oxygen transport and metabolism; regulation occurs at intestinal absorption and systemic distribution through transferrin and ferritin

• Connections to foundational principles

  • Homeostasis relies on controlled exchange between compartments (ECF/ICF, bone, gut, kidney, liver)

  • Hormonal regulation links organ systems (parathyroid, thyroid, adrenal, liver, kidneys) to maintain mineral balance

• Notation and formulas used in these notes

  • $\mathrm{Ca^{2+}}$ distribution in plasma:$\ [\mathrm{Ca^{2+}}]<em>{\text{total}} = [\mathrm{Ca^{2+}}]</em>{\text{ionized}} + [\mathrm{Ca^{2+}}]<em>{\text{protein-bound}} + [\mathrm{Ca^{2+}}]</em>{\text{organic-bound}}$

  • Fractional distribution (approximate):$\ f<em>{\text{ionized}} \approx 0.45-0.50, f</em>{\text{protein-bound}} \approx 0.40-0.45,\ f_{\text{organic-bound}} \approx 0.05-0.10$

  • $\mathrm{Na^{+}}$ homeostasis cascade (conceptual):

    $\text{Renin} \rightarrow \text{Angiotensin I} \rightarrow \text{Angiotensin II} \rightarrow \text{Aldosterone} \rightarrow \uparrow \text{Na}^+ \text{reabsorption}$

  • Kidney hormone interactions include ANP and aldosterone as regulators of $\mathrm{Na^{+}}$ and water balance

  • For $\mathrm{Mg^{2+}}$, $\mathrm{Mg^{2+}}$ homeostasis is largely governed by renal thresholds and intake, with PTH modulation in some contexts

• If you want, I can tailor these notes to a specific exam format (e.g., flashcards, brief summaries, or problem-based questions) or add example clinical scenarios for each mineral.