The kidneys cannot excrete carbonic acid, the volatile acid formed when carbon dioxide dissolves in the blood. This job belongs entirely to the lungs. Every day your body produces roughly 13,000 millimoles of carbonic acid from normal metabolism, and the only way to clear it is by converting it back into carbon dioxide and breathing it out. The kidneys handle a much smaller but equally important category: non-volatile (fixed) acids like sulfuric acid and phosphoric acid, which total about 70 to 100 millimoles per day.
Why Carbonic Acid Requires the Lungs
Carbonic acid is called “volatile” because it can convert into a gas. In your tissues, carbon dioxide produced by cells enters red blood cells and reacts with water to form carbonic acid. That reaction is reversible. When blood reaches the lungs, the process runs in reverse: carbonic acid breaks apart into carbon dioxide and water, and you exhale the carbon dioxide. Under normal conditions, the volume of carbon dioxide eliminated at the lungs equals the amount produced by the tissues.
The kidneys have no mechanism to perform this gas exchange. They work by filtering blood and adjusting the chemical composition of urine, which is a liquid pathway. A volatile acid that needs to become a gas and leave the body simply cannot exit through urine. This is why lung disease can directly cause acid buildup in the blood: if you can’t exhale carbon dioxide efficiently, carbonic acid accumulates regardless of how well your kidneys function.
What the Kidneys Actually Excrete
Your kidneys specialize in clearing fixed, or non-volatile, acids. These are the byproducts of digesting and metabolizing food, particularly protein and phospholipids. The two most significant are sulfuric acid and phosphoric acid. Unlike carbonic acid, these cannot be converted into a gas, so the lungs are useless against them. They must be neutralized and removed through urine.
Under certain conditions the body also produces abnormal fixed acids. During uncontrolled diabetes, for example, the liver generates ketoacids. During intense exercise or shock, cells produce lactic acid. These, too, fall on the kidneys to clear. The daily fixed acid load is small compared to the volatile load (roughly 70 to 100 milliequivalents versus 15,000), but failing to clear it leads to a dangerous drop in blood pH over days to weeks.
How the Kidneys Handle Fixed Acids
The kidneys use two main strategies. First, they reclaim bicarbonate, the body’s primary chemical buffer. The proximal tubule, the first stretch of the kidney’s filtering system, reabsorbs about 80% of the bicarbonate filtered from the blood. This prevents the body from losing its main defense against acidity.
Second, the kidneys actively pump hydrogen ions into the urine. But there’s a limit: urine can only get so acidic before the pumping machinery stalls. The median pH of human urine over 24 hours is around 6, and it rarely drops much below that. To work around this ceiling, the kidneys rely on two urinary buffers, ammonia and phosphate, that soak up hydrogen ions and carry them out. Without these buffers, even small amounts of acid secretion would drive urine pH too low for the system to keep working. During periods of high acid stress, the kidneys ramp up production of both buffers to increase their acid-clearing capacity.
When the Kidneys Fall Behind
In chronic kidney disease, the loss of functional kidney tissue directly impairs the body’s ability to excrete fixed acids. One of the earliest problems is a drop in ammonia production. With fewer working cells to generate this critical buffer, the kidneys can’t neutralize the daily acid load. The result is metabolic acidosis: a slow accumulation of hydrogen ions and a measurable decline in blood pH.
As kidney disease advances, the excretion of phosphate and other anions also decreases, compounding the problem. Phosphate that should have been cleared into the urine instead builds up in the blood. This creates a persistent positive acid balance, meaning the body takes in and produces more acid than it can eliminate.
The lungs try to compensate. Sensors in the brain detect the rising acidity and trigger faster, deeper breathing within minutes. By exhaling more carbon dioxide, the lungs pull carbonic acid out of the blood, partially offsetting the fixed acid that the kidneys can’t clear. This respiratory compensation helps but cannot fully correct the problem, because the underlying fixed acids remain in the body.
The Division of Labor
Acid-base balance depends on three systems working together: the lungs, the kidneys, and the body’s chemical buffers. The lungs handle the enormous daily volume of volatile acid (carbonic acid) with every breath. The kidneys handle the smaller but chemically distinct fixed acids that have no gaseous escape route. Buffers, primarily bicarbonate in the blood, act as a bridge, neutralizing acids moment to moment until the lungs or kidneys can remove them permanently.
This division is absolute. The lungs cannot excrete sulfuric acid. The kidneys cannot excrete carbonic acid. Each organ covers for the other when things go wrong, through respiratory compensation or changes in bicarbonate handling, but neither can fully replace the other’s core job. That’s why severe lung disease and severe kidney disease both lead to dangerous acid-base disturbances through entirely different mechanisms.