20.6 Metabolic Acidosis from a Failing Kidney

Three short movements: the case, the anatomyThe study of the structure of the human body. and physiologyThe study of how the body functions., then the tie back to Stina. Each links to the scene where you’ll see it move.

I · Four numbers, filed

An arterial gas is a needle in an artery, not a vein, because venous blood is carbon-dioxide-rich and oxygen-poor by design and would tell you nothing. Hers came back pH 7.33, carbon dioxide 33, bicarbonate 19, oxygen 94, and was filed without a word. The notes read it in three steps. Is it acidosis or alkalosisA condition where blood pH rises above 7.45.? Below 7.35: acidosis. Is it respiratory or metabolic? Carbon dioxide below 35: metabolic, and the respiratory system is compensating. Is the bicarbonate low? Nineteen, below 22: metabolic acidosis. Partially compensated, because the pH is still outside the range. Mr. Wu, the site’s case patient, is the same shape further down the road — 7.28, 14, twenty-two breaths a minute — a man whose lungs are doing the job of his kidneys as hard as lungs can.

See it move: Scene 1 · Three Questions, Four Numbers · Scene 3 · Compensation: the Healthy Organ Steps Up

II · Four disturbances, and the healthy organ steps up

The ABG lecture says it plainly: different references use different steps; pick one and stick to it. The notes’ three questions with their numbers are the course’s reference, and the four disturbances are named for the end that moved first. Respiratory acidosis: CO₂ up, pH down — hypoventilation, pneumoniaA lung infection causing inflammation and fluid buildup in the alveoli., emphysemaA lung disease in which alveoli are damaged, reducing surface area for gas exchange and causing shor, heart failure, an inhaled toy. Respiratory alkalosis: CO₂ down, pH up — hyperventilation from fear, fever, hypoxiaA deficiency of oxygen in the tissues., an aspirin overdose’s first act. Metabolic acidosis: bicarbonate down, pH down — diarrhea losing bicarbonate; ketones in a starving or diabetic body, with the deep fruity breathing of ketoacidosis; lactate in a sprint or a shock; methanol, ethylene glycol; and uremia, the phosphoric, sulfuric and lactic acids a failing kidney cannot clear. Metabolic alkalosis: bicarbonate up, pH up — vomiting, antacids, diuretics, Cushing’s. The lecture’s shortcut: in a renal disturbance every number moves the same way; in a respiratory one the pH moves opposite to the CO₂. Then compensation: the organ not at fault drags the pH back. In a metabolic acidosis the low pH drives the respiratory centres within minutes — Kussmaul respirations when severe, a sign you can see — and CO₂ falls. In a respiratory disturbance the kidney compensates over days. The rule the lecture insists on: compensation stops at the edge of the range, not the middle; it buys time, it does not cure.

See it move: Scene 2 · Four Disturbances, Their Causes · Scene 3 · Compensation: the Healthy Organ Steps Up · Scene 4 · The ABG Game

III · When the slow partner fails

Her problem is that the slow, durable partner is the organ that is failing. A kidney with amyloid stiffening its glomeruli and crowding its interstitium is losing two acid–base jobs at once. It cannot excrete the day’s fixed acid: the phosphate, sulfate and organic acids of ordinary metabolismThe sum of all chemical reactions in the body., forty to seventy milliequivalents a day, which only a kidney removes. And it cannot make new bicarbonate fast enough: the glutamine machine of Chapter 5 runs slow, ammonium in the urine falls, and the alkaline reserve is not refilled. The acid is buffered where it can be — by bicarbonate, by hemoglobinThe oxygen-carrying protein in red blood cells that gives blood its red color., by the phosphate inside cellsThe basic structural and functional units of life., and, over years, by bone, which gives up calcium carbonate to soak a chronic acid load and thins as it does; the Skeletal module’s low scan at thirty-eight has one more author. Potassium rides along: hydrogen ionsCharged atoms or molecules. move into cells and potassium moves out, and a kidney that struggles to excrete potassium at the best of times is now handed more of it — her 4.9. Her lungs do what lungs do: breathe a little deeper, blow off CO₂, pull the pH from where it would be to 7.33. They can work the CO₂ end for as long as she breathes. They cannot manufacture a single bicarbonate ion or excrete a single sulfate. The compensation is partial by design: a patch over a hole the lungs were never built to fill. The fix is not a bag of bicarbonate, though one may be hung; it is the organ. Protect the filter, slow the amyloid, and the acid load stops growing.

See it move: Scene 4 · The ABG Game · Scene 5 · When the Slow Partner Fails

5 scenes. Each one has a short animation or a slide, something to explore, a true/false spot-check of your thinking, and a quick check. Every scene gives you an evidence card for your board.

Every scene gave you an evidence card, and the chart adds more. Sort each card into the column it supports: A partially compensated metabolic acidosis from a failing kidney, Neither on its own, or A respiratory disturbance, or no disturbance at all. Some cards can honestly go in more than one place — that’s allowed, and the feedback tells you which cards decide the case.

Time to decide. Look back at your bet, then build your verdict from statements that hold up — no writing needed.