Functions of the Kidneys 3: Acid-Base Balance

Functions of the Kidneys 3: Acid-Base Balance

How the kidneys keep blood pH stable

Acid–base balance is essential for normal body function. Even small changes in blood pH can affect enzymes, heart rhythm, muscle strength, and brain function.

The kidneys play a key role in maintaining this balance by controlling acid removal and bicarbonate levels over the long term.

This article explains how acid–base balance works and why healthy kidney function is critical.

What is acid–base balance?

Normal blood pH

• Normal arterial pH: 7.35–7.45
• pH reflects the concentration of hydrogen ions (H⁺) in the blood. These are acidic substances
• More H⁺ = more acidic, and pH lower
• Less H⁺ = more alkaline, and pH higher

The body tightly regulates pH because most biochemical reactions only work within this narrow range.

Why this matters in kidney disease

Chronic Kidney Disease (CKD) and Acute Kidney Injury (AKI) commonly lead to acidosis (too much acid in the blood) as one of the kidney’s primary roles is to remove acid from the body.

Where does acid come from?

Acid is continuously produced by normal metabolism:

• Carbon dioxide (CO₂) – oxygen that you breathe is concerted to CO₂, after the oxygen is used for energy production
• Fixed (non-volatile) acids from protein metabolism (e.g. sulfuric and phosphoric acid)
• Organic acids such as lactate or ketones in certain conditions

These acids must be neutralised and removed from the body to prevent acidosis (high acid levels in the blood). This is where the kidneys (and other mechanisms) come in.

The three systems that regulate pH

1. Chemical buffers (bases/alkalis) (work seconds)

• Bicarbonate, proteins, and phosphate limit rapid pH changes, by ‘buffering’ (neutralising) acid
• Buffers do not remove acid, they only contain it temporarily

2. Lungs (minutes to hours)

• Control CO₂ levels through breathing
• Faster breathing → lower CO₂ → higher pH
• Slower breathing → higher CO₂ → lower pH

This is respiratory regulation.

3. Kidneys (hours to days)

• Remove acids
• Preserve and make the alkali bicarbonate (which acts a buffer/neutraliser, see above)
• Adjust urine acidity

The kidneys provide permanent acid removal, making them essential for long-term balance.

How the kidneys regulate acid–base balance

Reabsorbing bicarbonate

The kidneys filter large amounts of bicarbonate (a base/alkali, opposite of an acid) every day. Almost all of it must be reabsorbed by the kidney to maintain its ability to neutralise acid.

• Most bicarbonate reabsorption occurs in the proximal convoluted tubule (PCT)
• Loss of bicarbonate in urine would quickly cause (metabolic) acidosis (high acid levels in the blood)

Excreting (removing) acid

The kidneys excrete hydrogen ions in two main forms:

1. Titratable acid
   • Hydrogen ions bind to urinary buffers (bases/alkalis) such as bicarbonate and phosphate
   • Limited by available buffer
2. Ammonium (NH₄⁺) – the main system
   • Produced from amino acids in kidney tubule cells
   • Each ammonium excreted adds new bicarbonate to the blood
   • Increases significantly during chronic (longterm)acidosis
   • This system allows large acid loads to be handled safely.

Key points for patients

• Acid–base balance is critical for normal organ function
• The kidneys are central to long-term pH control
• They:
  • Preserve bicarbonate
  • Remove acids
  • Generate new bicarbonate
• Chronic Kidney Disease (CKD) and Acute Kidney Injury (AKI) commonly lead to acidosis, as the kidneys become less able to remove acid from the body.