Atlas MCAT Course
Systems & Physiology → Acid–Base Physiology
Acid–Base Physiology
Coordinating lungs, blood, and kidneys.
01The idea
Cells are exquisitely sensitive to pH. Enzymes, ion channels, and transporters depend on precise protonation states. Yet metabolism constantly produces CO₂, generates acids, and consumes or releases protons.
This requires coordination across organs.
THREAD · Regulation vs. capacity
02Build the model
AThe core variable: CO₂ is an acid
In physiology, CO₂ is not neutral. When CO₂ dissolves in water, it forms carbonic acid, which dissociates into H⁺ and bicarbonate.
This is why the lungs play a central role in pH control.
BThe bicarbonate buffer system: fast, reversible, dominant
Blood pH is primarily buffered by the bicarbonate system:
Why this system is ideal: its components are abundant, its reactions are reversible, CO₂ can be removed by the lungs, and bicarbonate can be adjusted by the kidneys.
CHenderson–Hasselbalch, now with physiology
Rather than the equation, focus on ratios. pH depends on bicarbonate (the base component) and CO₂ (the acid component).
- ↑ CO₂ → ↓ pH (more acidic)
- ↑ bicarbonate → ↑ pH (more basic)
DThe lungs: rapid pH control
Ventilation determines how much CO₂ is exhaled.
| Increased ventilation | Decreased ventilation |
|---|---|
| CO₂ removal increases | CO₂ retention |
| Blood pH rises | Blood pH falls |
EThe kidneys: slow, powerful pH control
Kidneys regulate pH by excreting H⁺ and reabsorbing or generating bicarbonate. This allows long-term correction and fine-tuning beyond respiratory limits.
FMetabolic vs. respiratory disturbances (conceptual)
| Disturbance | Primary problem | Compensation |
|---|---|---|
| Respiratory | CO₂ handling | Kidneys adjust bicarbonate |
| Metabolic | Acid or base load | Lungs adjust ventilation |
GWhy compensation is never perfect
Compensation moves pH toward normal but does not fully correct it. This is because systems have limits and the underlying pathology persists.
HCO₂ transport: supporting acid–base control
CO₂ is transported as dissolved gas, as bicarbonate, and bound to hemoglobin. This flexibility enhances transport capacity and stabilizes blood pH during transit.
03See the connections
The lesson names these links itself. Read the map left to right.
Acid side
- CO₂ (from metabolism)
- Set by lungs, fast
Buffer
- CO₂ ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
- Open system
Base side
- Bicarbonate
- Set by kidneys, slow but decisive
Next: renal physiology.
04Reason with it
Four questions. Answer in your head first, then choose. You’ll see the reasoning either way.
Think first.
A patient hypoventilates after an opioid overdose. What happens to blood pH?
Think first.
A patient develops a metabolic acidosis. What is the fastest compensation?
Think first.
A chronic respiratory acidosis has been present for days. What compensation do you expect?
Think first.
After compensation, where does the pH end up?
05MCAT lens
Predict the direction
Hypoventilation → CO₂ retention → acidosis
Hyperventilation → CO₂ loss → alkalosis
Metabolic acid load → increased ventilation
Chronic disturbance → renal compensation
No numbers required — only direction and system logic.
06Common traps
Trap 1: Buffers prevent pH change entirely
Misconception
They reduce magnitude.
Correct model
Control systems restore balance.
Trap 2: Kidneys act faster than lungs
Misconception
Renal compensation is slow.
Correct model
Lungs respond immediately.
Trap 3: CO₂ is just a waste gas
Misconception
It’s a key acid variable.
Correct model
CO₂ links respiration to pH.
07Dependency check
Can you reconstruct this concept without looking at the page? You should now be able to reason through:
Why CO₂ regulates pH
Check against 02 · A: CO₂ forms carbonic acid, which releases H⁺.
Why lungs and kidneys have distinct roles
Check against 02 · C through E: lungs set CO₂ fast; kidneys set bicarbonate slowly.
How compensation works directionally
Check against 02 · F: the other organ adjusts the other side of the ratio.
Why acid–base control is distributed
Check against 02 · B: an open buffer system with two control handles.
If something feels unclear, the missing idea is usually thispH is controlled by regulating flux of CO₂ and bicarbonate, not by neutralizing acid directly.
08Remember
REMEMBER · if you keep only a few things
- CO₂ is an acid.
- CO₂ ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻: an open buffer system.
- ↑ CO₂ → ↓ pH; ↑ HCO₃⁻ → ↑ pH.
- Lungs set CO₂ (fast); kidneys set bicarbonate (slow).
- Respiratory problem → renal compensation; metabolic problem → respiratory compensation.
- Compensation is never complete.
09Forward hook
You now have transport logic, circulation, gas exchange, and acid–base integration. The next question is:
How do kidneys use transport, pressure, and gradients to regulate volume and composition?
That leads directly to renal physiology: filtration, reabsorption, and secretion.
Next lessonRenal Physiology→