Anatomy and Physiology 2e · Fluid, Electrolyte, and Acid-Base Balance
Acid-Base Balance
On this page 9 sections
In 30 seconds
Acid-base balance is the process by which the body keeps the hydrogen ion concentration of its fluids — reflected in pH — within a narrow, life-compatible range. The pH scale runs 0–14, with 7 neutral, below 7 acidic, above 7 alkaline. Metabolism constantly produces acids: carbon dioxide becomes carbonic Acid Substance that donates H⁺ (lowers pH) Full entry →, and the breakdown of proteins and fats adds other acids. The body defends against this acid load with three lines of defense: chemical buffers (fractions of a second), the lungs (minutes, adjusting CO₂ and thus carbonic acid), and the kidneys (hours to days, adjusting bicarbonate and hydrogen ion handling). A commonly taught reference range for arterial blood pH is about 7.35–7.45 (verify against current texts).
Why this matters
Enzyme activity, membrane potentials, and oxygen delivery all depend on pH staying in range; even small deviations impair cell function, and severe deviations are life-threatening. Acid-base reasoning explains why a person who hyperventilates feels dizzy and tingly, why uncontrolled diabetes can produce deep rapid breathing, and why patients with kidney failure accumulate acids. For clinicians, interpreting blood gases — pH, PCO₂ Partial pressure of carbon dioxide in the blood Full entry →, and bicarbonate — is a daily skill built on the concepts here.
The college version
Core Concepts
pH and the acid load of metabolism
pH is a logarithmic measure of hydrogen ion concentration: each whole-number step is a tenfold change in acidity. Cellular respiration generates carbon dioxide (CO₂), which dissolves in water and forms Carbonic acid (H₂CO₃) Weak acid formed from CO₂ and water Full entry → — the "volatile" acid the lungs can eliminate by exhaling CO₂. Metabolism of proteins and other nutrients also generates nonvolatile (fixed) acids such as phosphoric and sulfuric acids, and under some conditions lactic acid and ketone bodies; the daily fixed-acid load must be excreted by the kidneys. Because pH is logarithmic, a drop from 7.40 to 7.30 reflects roughly a 25% increase in hydrogen ion concentration.
The bicarbonate buffer system: the workhorse
The most important Buffer Chemical pair that absorbs or releases H⁺ to resist pH change Full entry → of the extracellular fluid is the bicarbonate buffer system, built from carbonic acid (the weak acid) and bicarbonate (its conjugate base):
H⁺ + HCO₃⁻ ⇌ H₂CO₃ ⇌ CO₂ + H₂O
When acid (H⁺) is added, bicarbonate binds it, forming carbonic acid, which can be blown off as CO₂ by the lungs. When base is added, carbonic acid releases H⁺ to neutralize it. The system's genius is that its two components are regulated by different organs: the lungs control CO₂ via ventilation, and the kidneys control bicarbonate. A buffer only delays pH change — it does not remove acid or base — so the lungs and kidneys must ultimately dispose of the excess.
Chemical buffers beyond bicarbonate
Bicarbonate handles much of the ECF, but other buffers protect other compartments. Protein buffers — including hemoglobin inside red blood cells — use the ionizable groups of amino acids to soak up or release H⁺; hemoglobin buffering is central to carbon dioxide transport. Phosphate buffers are important in the intracellular fluid and in the urine, where the kidney uses them to carry H⁺ out of the body.
Respiratory regulation: fast but limited to CO₂
The lungs regulate acid-base balance by adjusting ventilation, which controls the partial pressure of CO₂ (PCO₂). CO₂ is in equilibrium with carbonic acid, so more CO₂ means more acid. When PCO₂ rises, chemoreceptors stimulate deeper, faster breathing, which blows off CO₂; when PCO₂ falls, ventilation slows and CO₂ is retained. This response starts within minutes but can only dispose of the volatile acid (CO₂) — it cannot remove fixed acids or change bicarbonate directly.
Renal regulation: slow but complete
The kidneys are the only organs that remove fixed acids and replenish bicarbonate. In the proximal tubule they reabsorb filtered bicarbonate; in the distal tubule and collecting duct they secrete H⁺ into the tubular fluid, where it is buffered (notably by phosphate and ammonia) and excreted in urine. Each H⁺ secreted is matched by a bicarbonate ion returned to the blood, so the kidneys simultaneously remove acid and regenerate buffer. This is powerful but slow — significant adjustments take hours to days — which is why kidney failure produces a gradual, persistent acidosis.
The balance concept: ratio, not just numbers
What matters most for blood pH is the ratio of bicarbonate to carbonic acid — roughly 20:1 in the commonly taught reference model (verify against current texts). Because carbonic acid tracks PCO₂, the body can defend pH by adjusting either component. This ratio logic is the foundation for interpreting acid-base disorders: pH tells the direction, and bicarbonate and PCO₂ tell which component changed.
How It Works / Step-by-Step Process
Trace what happens when intense exercise adds lactic acid to the blood:
- H⁺ from lactic acid accumulates in the ECF; pH starts to fall.
- Buffers act instantly: bicarbonate binds H⁺ to form carbonic acid, which dissociates to CO₂ and water — the pH change is blunted, not prevented.
- The lungs respond within minutes: chemoreceptors sense rising CO₂ and falling pH; breathing deepens and CO₂ is exhaled.
- The kidneys respond over hours: distal tubules secrete H⁺ (buffered by phosphate and ammonia in urine) and return new bicarbonate to the blood.
- Balance returns: the fixed acid is excreted, bicarbonate replenished, pH back in range.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Acidosis | Acidemia | Acidosis is the process tending to lower pH; acidemia is the blood state (pH < 7.35). Compensation can keep pH normal despite ongoing acidosis |
| Respiratory regulation | Renal regulation | Lungs adjust CO₂ in minutes but cannot remove fixed acids; kidneys remove fixed acids and regenerate bicarbonate but take hours to days |
| Buffers | Corrective organs | Buffers resist pH change temporarily but do not remove acid or base; lungs and kidneys dispose of the excess |
| CO₂ | Carbonic acid | CO₂ is a gas in equilibrium with carbonic acid in solution; ventilation controls CO₂, and the acid forms after CO₂ dissolves and reacts with water |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Your blood is like a swimming pool that needs the right chemical balance. Every time your cells work, they drop "acid lemons" into the pool, so the body has three helpers: a fast-acting sponge (buffers), a fan (your lungs) that blows away acid gas, and a cleanup crew (your kidneys) that carries leftover acid out in your pee. If one helper gets lazy or overworks, the water drifts too sour or too soapy.
Worked example
Consider a person who panics and hyperventilates. Each breath blows off more CO₂ than the body produces, so PCO₂ falls. Because CO₂ is in equilibrium with carbonic acid, removing CO₂ pulls the buffer equation to the left — H⁺ is consumed as the reaction shifts, so blood pH rises toward alkalinity: respiratory alkalosis. The classic symptoms (tingling fingers, lightheadedness) reflect the pH shift and reduced ionized calcium on excitable tissues. Once breathing slows, PCO₂ rises back and pH normalizes — no kidney involvement needed for a short episode. This scenario shows why the lungs are called the fast regulator.
Key takeaways
- Normal arterial pH ≈ 7.35–7.45 (commonly taught reference; verify against current texts); below is acidemia, above is alkalemia.
- Three lines of defense: chemical buffers (instant), lungs (minutes, CO₂ only), kidneys (hours to days, fixed acids + bicarbonate).
- Bicarbonate buffer: H⁺ + HCO₃⁻ ⇌ H₂CO₃ ⇌ CO₂ + H₂O — the lungs control CO₂, the kidneys control HCO₃⁻.
- Hemoglobin and phosphate buffers protect the ICF and the urine; phosphate lets the kidney excrete H⁺.
- The ratio matters: pH depends on the HCO₃⁻:H₂CO₃ ratio (~20:1 in the commonly taught model), not absolute amounts.
- The lungs cannot remove fixed acids (lactic, sulfuric, phosphoric); only the kidneys can excrete them.
- pH is logarithmic: small numeric changes equal large changes in hydrogen ion concentration.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
What are the three lines of defense against pH change, and how fast does each act?
Show answer
Buffers act instantly; the lungs adjust CO₂ within minutes; the kidneys excrete fixed acid and regenerate bicarbonate over hours to days.
Why can the lungs only remove "volatile" acid?
Show answer
The lungs can only exhale CO₂ — the volatile acid. Fixed acids have no gas phase and must be excreted by the kidneys.
What does the bicarbonate buffer equation look like, and which organ controls each end?
Show answer
H⁺ + HCO₃⁻ ⇌ H₂CO₃ ⇌ CO₂ + H₂O. Lungs control the CO₂ side via ventilation; kidneys control bicarbonate.
Why does kidney failure tend to cause acidosis?
Show answer
The kidneys are the only route for excreting fixed acids and regenerating bicarbonate; when they fail, acids accumulate and pH drifts down.
What happens to pH when someone hyperventilates, and why?
Show answer
pH rises (respiratory alkalosis): hyperventilation blows off CO₂, shifting the buffer equation so H⁺ is consumed.
Why is a pH change of 0.1 unit (7.40 → 7.30) a big deal despite looking small?
Show answer
pH is logarithmic — a 0.1 unit change reflects roughly a 25% change in hydrogen ion concentration.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- pH
- Logarithmic measure of hydrogen ion concentration
- Acid
- Substance that donates H⁺ (lowers pH)
- Base (alkali)
- Substance that accepts H⁺ (raises pH)
- Buffer
- Chemical pair that absorbs or releases H⁺ to resist pH change
- Carbonic acid (H₂CO₃)
- Weak acid formed from CO₂ and water
- Bicarbonate (HCO₃⁻)
- The main ECF buffer anion
- PCO₂
- Partial pressure of carbon dioxide in the blood
- Fixed (nonvolatile) acid
- Acid that cannot be exhaled (sulfuric, phosphoric, lactic)
Sources & references
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