NCLEX - Introduction to Acids & Bases

Acids are substances that release hydrogen ions (H⁺) when dissolved in water.

• Example: Hydrochloric acid (HCl) dissociates into H⁺ and Cl⁻.

Bases (alkalis) are substances that accept hydrogen ions (H⁺) or release hydroxide ions (OH⁻) into a solution.

• Example: Sodium hydroxide (NaOH) releases OH⁻, which binds H⁺ to form water.

The stronger the acid, the more H⁺ it releases; the stronger the base, the more OH⁻ it releases or H⁺ it accepts.

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The pH scale measures the concentration of hydrogen ions in a solution. It ranges from 0 (very acidic) to 14 (very basic/alkaline). A solution with pH = 7 is neutral (equal H⁺ and OH⁻).

• Acidic: pH < 7 (more H⁺ than OH⁻)
• Basic: pH > 7 (more OH⁻ than H⁺)

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• Gastric juice: pH \~2 (very acidic)
• Saliva: pH \~6.5 (slightly acidic)
• Pure water: pH 7 (neutral)
• Arterial blood: pH \~7.4 (slightly basic)

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The pH scale is logarithmic, meaning each unit change = 10× change in \[H⁺]. A shift from pH 7.4 to 7.3 doubles the concentration of H⁺. Therefore, small pH changes = large physiological effects.

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Buffers help maintain a stable pH by donating or accepting H⁺ ions. The bicarbonate buffer system is the body’s primary extracellular pH buffer:

• When blood becomes acidic: H⁺ + HCO₃⁻ → H₂CO₃ (carbonic acid)
• When blood becomes basic: H₂CO₃ → H⁺ + HCO₃⁻

This reversible reaction helps maintain blood pH near 7.4.

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The normal blood pH range is 7.35–7.45.

• Below 7.35 = acidosis
• Above 7.45 = alkalosis

Enzymes and cellular processes are pH-sensitive.

• Deviations can impair oxygen transport, enzyme function, and electrolyte balance.

pH regulation involves: Buffer systems (immediate response)

• Respiratory compensation (via CO₂ control)
• Renal compensation (via H⁺ and bicarbonate excretion/reabsorption)