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.

   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:

These crystals provide strength, rigidity, and structural support to the skeleton.

However, bone is not a static reservoir. Instead, it is a dynamic tissue, constantly undergoing remodeling. Calcium is continuously exchanged between bone and the extracellular fluid (ECF). This exchange is crucial for maintaining stable blood calcium levels and is largely regulated by the parathyroid hormone (PTH), which acts as a key hormonal controller of calcium balance.

   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 easily 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.

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: 4.5–5.6 mg/dL (≈ 1.1–1.3 mmol/L)

Unlike protein-bound calcium, ionized calcium is free and readily available to participate in biological processes.

Functions

Ionized calcium is essential for:

• Nerve conduction
• Muscle contraction
• Blood coagulation
• Hormone secretion

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

This highlights how even small fluctuations in ionized calcium can have profound physiological consequences.

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.

   Dietary Sources of Calcium

Calcium must be obtained through the diet to maintain normal body stores and physiological functions.

Rich Sources

• Milk and dairy products (approximately 120 mg per 100 mL)
• Egg yolk
• Beans and legumes
• Leafy green vegetables
• Hard drinking water

Milk is widely considered the best dietary source due to its high content and good bioavailability.

   Bioavailability of Calcium

An important concept in nutrition is that calcium content does not always equal calcium absorption.

For example:

• Spinach contains a relatively high amount of calcium
• However, it is a poor source of absorbable calcium

This is because it contains oxalates, which bind calcium to form insoluble calcium oxalate, preventing its absorption in the intestine.

Therefore, the chemical form of calcium and accompanying compounds significantly influence how much calcium the body can actually utilize.

   Additional Sources

In some populations, small amounts of calcium may also be consumed as calcium hydroxide (Ca(OH)₂), particularly in traditional practices such as betel leaf preparations.

   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
• Dietary intake is essential, but absorption depends on bioavailability, not just content