What the body is doing when the numbers stop making sense — plus four gases to work yourself
Compensation is the body playing tug of war with its own pH. When one side pulls too hard, the other side leans back — not to win, just to drag the pH back to common ground. This page is the concept, the three states, the two speeds, and the four practice gases from the question sheet, plotted and ready for you to answer. The method itself lives on NG-092; the full worked key is on NG-173.
📄 Simple Nursing original — opens in Drive →
Nobody is trying to cure anything. The body is only trying to keep the pH alive.
The rope is the pH. The lungs pull with CO₂ (acid), the kidneys pull with HCO₃⁻ (base). Compensation is the losing side digging in and leaning back to drag the flag toward the middle.
Answer first: enzymes. Almost every enzyme and protein in the body only works in a narrow pH window.
The organ that caused the problem is never the organ that compensates.
Left column: the lungs caused it, so the kidneys rescue — slowly. Right column: the problem is metabolic, so the lungs rescue — fast.
Answer first: they trade hydrogen for bicarbonate.
One patient, three days. The disorder never changes — only how far the rescue has got.
Watch the HCO₃⁻ pin walk out of its normal window while the pH pin crawls back into its own. The CO₂ never improves — this patient is not getting better, they are getting balanced.
pH abnormal · culprit abnormal · rescuer still NORMAL.
pH STILL abnormal · culprit abnormal · rescuer ALSO abnormal, the other way.
pH back INSIDE 7.35–7.45 · both partners abnormal.
The lungs are near-maximal within minutes to hours. The kidneys take days. Read the curve backwards: how far compensation has got tells you roughly how old the problem is.
| State | pH | The culprit | The rescuer | What it means |
|---|---|---|---|---|
| Uncompensated | Abnormal | Abnormal | Normal | Acute. No help has started. |
| Partially compensated | Still abnormal | Abnormal | Abnormal (opposite way) | Rescue under way, not finished. All three numbers off. |
| Fully compensated | Normal 7.35–7.45 | Abnormal | Abnormal (opposite way) | Balance bought, cause untreated. Usually chronic. |
| Genuinely normal | Normal | Normal | Normal | Nothing to report. |
Three abnormal numbers = partial. Normal pH with two abnormal partners = full.
These are the four gases from the question sheet. Each one is plotted for you. Say your answer out loud, then open the box.
All three pins are outside their normal windows — pH and CO₂ on their acid sides, HCO₃⁻ far over on its base side.
Step 1 — pH. 7.32 is below 7.35 → ACID.
Step 2 — the match. PaCO₂ 55 is above 45 → acid. Match in the lungs → respiratory acidosis. HCO₃⁻ 42 is way above 26 → that is a base, the opposite direction — so it is not the culprit.
Step 3 — compensation. HCO₃⁻ has moved a long way out of range in the opposite direction, so the kidneys are hoarding base to pull the patient out of acidosis. But the pH is still 7.32, outside 7.35–7.45 — so the rescue has not finished.
Respiratory acidosis · PARTIALLY compensated
Why it looks like this: a long-standing CO₂ retainer whose kidneys have had time to react but whose lungs have got worse faster than the kidneys can keep up.
pH and HCO₃⁻ pins are both on their base sides. The CO₂ pin has drifted onto its acid side — the opposite direction, which is the signature of a rescuer.
Step 1 — pH. 7.55 is above 7.45 → BASE (alkalosis).
Step 2 — the match. PaCO₂ 49 is above 45 → that is acid, the wrong way, so no match. HCO₃⁻ 35 is above 26 → base. Match in the kidneys → metabolic alkalosis.
Step 3 — compensation. The high CO₂ is the lungs deliberately hypoventilating to hold acid gas in and pull the pH back down. It is helping — but the pH is still 7.55, well outside the range.
Metabolic alkalosis · PARTIALLY compensated
Watch out: the source sheet has a typo here and says “a match in the lung area” — the match is in fact the bicarbonate, which is the kidney number. The stated answer, metabolic alkalosis, is correct.
The pH pin has landed inside the green window — but both partner pins are outside theirs, and on opposite sides. That combination has only one name.
Step 1 — pH. 7.37 is inside 7.35–7.45 → normal. Do not stop here. Both partners are abnormal, so something is being compensated. Ask which side of 7.40 it landed on: 7.37 is on the acidic side → this began as an acidosis.
Step 2 — the match. PaCO₂ 52 is high → acid, which agrees with “acidosis”. Respiratory acidosis. HCO₃⁻ 32 is high → base, the opposite way — the rescuer.
Step 3 — compensation. The pH is back inside the normal range → the rescue succeeded.
Respiratory acidosis · FULLY compensated
Why it looks like this: the textbook chronic COPD gas. The kidneys have had days to bank bicarbonate and have bought a normal pH at the price of two abnormal partners.
Only one pin is outside its window this time. The CO₂ pin is inside the green zone but sitting right at its top edge.
Step 1 — pH. 7.43 is inside 7.35–7.45 → normal, and it is on the alkaline side of 7.40 → this began as an alkalosis.
Step 2 — the match. HCO₃⁻ 33 is above 26 → base, which agrees. Metabolic alkalosis. PaCO₂ 43 is still technically inside 35–45.
Step 3 — compensation. Using the rule the question sheet teaches — full compensation = a fully normal pH — this one is fully compensated. The pH has made it home.
Metabolic alkalosis · FULLY compensated
The honest nuance: the compensating CO₂ of 43 is sitting at the very top of normal — the lungs are hypoventilating slightly, but not enough to push the number out of range. Some references would call that an incomplete respiratory response. For this course, judge the state by the pH: normal pH = fully compensated.
| Q | Values | Answer | The skill it drills |
|---|---|---|---|
| 1 | 7.32 · 55 · 42 | Respiratory acidosis, partial | Three abnormal numbers = partial compensation |
| 2 | 7.55 · 49 · 35 | Metabolic alkalosis, partial | Don't grab the first abnormal partner — check which one agrees with the pH |
| 3 | 7.37 · 52 · 32 | Respiratory acidosis, full | A normal pH is not a normal gas |
| 4 | 7.43 · 43 · 33 | Metabolic alkalosis, full | Use the side of 7.40 to name a compensated gas |
Compensation questions are only hard in four specific ways. Here they are.
In Question 1 the HCO₃⁻ of 42 is the most dramatic number on the page — and it is not the answer. It moved the opposite way to the pH.
Fix: always ask “does this one agree with the pH?” before you get excited about how abnormal it is.
Questions 3 and 4 both have a normal pH. Both still have a real acid–base disorder.
Fix: if a normal pH comes with any abnormal partner, keep going. Use the 7.40 midpoint to decide which disorder it was.
Both have an abnormal pH. The difference is the rescuer.
If both partners are abnormal in the same acid–base direction, nobody is rescuing anyone — both are pushing the pH the same way.
Example: pH 7.10 · PaCO₂ 60 · HCO₃⁻ 14 — high CO₂ (acid) and low HCO₃⁻ (acid). That is a combined respiratory and metabolic acidosis, and it is a peri-arrest picture.
Never call that “partially compensated”. Nothing is compensating.
Weak: “Her gas is a bit off but the pH is fine.”
Strong: “pH 7.37, CO₂ 52, bicarb 32 — a fully compensated respiratory acidosis, which fits her known
COPD. It matches last month's gas, so I think this is her baseline. She's alert and her work of breathing
is unchanged.”
| Page | What it gives you |
|---|---|
| NG-092 · ABG Answers (1 of 3) | THE METHOD — the 3-step read, ROME, the decision tree, the pH scale, which organ fixes what, and three fully worked uncompensated gases. |
| NG-138 · Compensation Questions (2 of 3) | PRACTICE — what compensation actually is, uncompensated vs partial vs full, and four compensation gases plotted for you to answer before you peek. |
| NG-173 · Compensation Answers (3 of 3) | ANSWER KEY — the filled-in decision path for all four gases, plus the causes of each disorder, the expected-compensation check, oxygenation, and the traps. |