Calcium

Calcium is one of the most abundant and essential minerals in the human body, accounting for approximately 1.5% of total adult body weight. Despite its relatively small proportion, it plays a central role in maintaining structural integrity and regulating vital physiological processes.

Under normal conditions, the serum (plasma) calcium concentration is tightly controlled within the range of 8.5–10.5 mg/dL. Interestingly, red blood cells contain very little calcium, emphasizing that most physiologically relevant calcium resides in extracellular fluids and skeletal tissues.

1. Distribution of Calcium in the Body

More than 99% of total body calcium is stored in the bones and teeth. Here, it exists primarily as hydroxyapatite crystals:

Ca₁₀(PO₄)₆(OH)₂

These hydroxyapatite crystals are responsible for providing strength, rigidity, and structural support to the skeletal system. However, bone is not a static storage site. It is a dynamic and metabolically active tissue that undergoes continuous remodeling throughout life. During this process, calcium is constantly exchanged between bone and the extracellular fluid (ECF).

This ongoing exchange plays a critical role in maintaining stable blood calcium concentrations, which are essential for normal physiological functions such as muscle contraction, nerve transmission, and blood clotting.

The regulation of calcium balance is primarily controlled by parathyroid hormone (PTH), which acts as a key hormonal regulator by influencing calcium release from bone, reabsorption in the kidneys, and absorption in the intestines.

PTH regulates calcium homeostasis by:

Increasing bone resorption (indirectly via osteoclast activation)
Increasing renal calcium reabsorption
Promoting activation of vitamin D (calcitriol)

2. Calcium Homeostasis Regulation

Calcium homeostasis is tightly regulated to maintain plasma calcium within a narrow physiological range, as even minor fluctuations can significantly affect neuromuscular and cellular function. This regulation is primarily controlled by three key hormones: parathyroid hormone (PTH), vitamin D (calcitriol), and calcitonin.

(a) Parathyroid Hormone (PTH)

Parathyroid hormone is the most important regulator of serum calcium levels and acts to increase serum Ca²⁺ concentration. It exerts its effects through multiple target organs:

Bone: PTH indirectly stimulates osteoclast activity (via osteoblast-mediated RANKL signaling), leading to increased bone resorption and release of calcium into the bloodstream.

Kidney: PTH increases renal reabsorption of calcium, particularly in the distal convoluted tubule, thereby reducing urinary calcium loss. It also promotes phosphate excretion (phosphaturic effect), preventing calcium–phosphate precipitation.

(b) Vitamin D (Calcitriol, 1,25-(OH)₂-D₃)

Calcitriol is the biologically active form of vitamin D and plays a crucial role in increasing intestinal absorption of calcium.

Intestine: It enhances calcium uptake in the small intestine (especially duodenum) by increasing expression of calcium transport proteins (e.g., TRPV6), significantly improving dietary calcium absorption.

Bone and Kidney: Calcitriol supports normal bone mineralization and works synergistically with PTH to maintain calcium balance.

(c) Calcitonin

Calcitonin is a hormone secreted by the parafollicular (C) cells of the thyroid gland. It plays a minor role in adult calcium homeostasis compared to PTH and vitamin D.

Bone: Calcitonin inhibits osteoclast activity, thereby reducing bone resorption and lowering serum calcium levels.
Its role is relatively limited in long-term calcium regulation in humans.

2. Target Organs in Calcium Homeostasis

Calcium balance is maintained through coordinated hormonal effects on three primary target organs:

Bone: Bone acts as a dynamic calcium reservoir:

Resorption: release of Ca²⁺ into blood (mainly stimulated by PTH)
Deposition: incorporation of Ca²⁺ into hydroxyapatite during bone formation

Kidney: The kidney plays a central role in conserving calcium:

~98–99% of filtered Ca²⁺ is reabsorbed along different nephron segments
PTH increases Ca²⁺ reabsorption in the distal convoluted tubule, reducing urinary loss
Also regulates phosphate excretion, indirectly influencing calcium levels
Loop diuretics increase calcium excretion
Thiazide diuretics decrease calcium excretion

Intestine: The intestine is the main site of calcium entry into the body:

Calcium absorption is vitamin D–dependent
Active transport occurs in the duodenum via TRPV6 channels (dominant in low intake states)
Passive paracellular absorption occurs in jejunum and ileum (important in high intake states)
Dietary calcium availability directly influences systemic calcium balance

Together, these three hormones and target organs maintain a tightly regulated calcium equilibrium, ensuring stability of serum calcium levels essential for neuromuscular, skeletal, and metabolic function.

3. Plasma (Serum) Calcium

In the bloodstream, calcium exists in three distinct and physiologically important forms. Understanding these forms is essential for interpreting clinical conditions and laboratory results.

1. Protein-Bound (Non-Diffusible) Calcium

Approximately 40% of plasma calcium is bound to proteins, mainly albumin.
Because of this binding:

It cannot freely cross cell membranes
It is not readily filtered by the kidneys
It is considered biologically inactive

Clinical Significance

In conditions such as hypoproteinemia (low albumin levels), total serum calcium may appear reduced. However, the biologically active fraction (ionized calcium) may remain normal. This phenomenon is known as pseudohypocalcemia, where laboratory values can be misleading if albumin levels are not considered.

A more accurate assessment can be made using corrected calcium:
Corrected Ca²⁺ = Measured Ca²⁺ + 0.8 × (4 − Albumin)

2. Ionized Calcium (Physiologically Active Form)

This is the most important and functionally active form of calcium.

Accounts for 45–50% of total plasma calcium
Normal level: 4.5–5.2 mg/dL (≈ 1.1–1.3 mmol/L)

Functions:

Nerve conduction
Muscle contraction
Blood coagulation
Hormone secretion
Intracellular signaling

Clinical Importance:

Increased in hyperparathyroidism
Decreased in hypoparathyroidism

A critical drop in ionized calcium leads to tetany, a condition characterized by:

Muscle spasms
Increased neuromuscular excitability

3. Complexed Calcium

The remaining ~10% of plasma calcium exists in a loosely bound form with small anions such as:

Citrate
Phosphate
Bicarbonate

Although this fraction is ultrafilterable, it is not biologically active. It serves as a minor transport form rather than a functional one.

4. Dietary Sources of Calcium

Calcium is obtained exclusively through the diet, and its availability depends not only on intake but also on intestinal absorption and food composition.

Rich dietary sources include:

Milk and dairy products (highest bioavailability and primary source)
Egg yolk
Beans and legumes
Leafy green vegetables (bioavailability varies)
Hard drinking water (minor contribution in some regions)

Milk is widely considered the best dietary source due to its high content and good bioavailability. Although plant sources may contain calcium, absorption is often limited due to binding compounds that reduce bioavailability.

5. Dietary Requirement

(a) Recommended Daily Intake (RDA)

Calcium requirements vary with age and physiological state:

Adults: ~1000 mg/day
Adolescents, pregnancy, and lactation: ~1200–1300 mg/day

Higher requirements during growth and pregnancy reflect increased skeletal demand and fetal needs.

6. Factors Affecting Calcium Absorption

Absorption enhancers:

Vitamin D (major regulator of intestinal calcium uptake)

Lactose (improves solubility and absorption in milk-based diets)

Absorption inhibitors:

Oxalates (e.g., spinach) – form insoluble calcium oxalate
Phytates (grains and legumes) – bind calcium and reduce absorption
Excess dietary fat – forms insoluble calcium soaps in the intestine

Key Summary

The majority of calcium is stored in bones as hydroxyapatite crystals.
Ionized calcium is the only biologically active form.
Protein-bound calcium depends on albumin levels and is inactive.
Complexed calcium forms a small, inactive fraction.
Calcium homeostasis is regulated primarily by PTH and vitamin D.
Dietary intake is essential, but absorption depends on bioavailability, not just content.
Acid–base balance can significantly affect ionized calcium levels