Renal Regulation of Acid-Base Balance AND Acid-Base Imbalances

How do kidneys conserve bicarbonate

They reabsorb filtered HCO₃⁻ so it is not lost in urine.

How do kidneys remove acid

They secrete H⁺ into tubular fluid for excretion.

Why is H⁺ secretion alone insufficient

Free H⁺ concentration in tubular fluid becomes too high unless secreted H⁺ is buffered.

Urinary buffers

Filtered phosphate and secreted ammonia.

Filtered phosphate buffer

Phosphate in tubular fluid binds secreted H⁺, allowing more acid to be excreted.

What happens when phosphate buffering is exceeded

Tubular fluid becomes more acidic, and ammonia buffering becomes important.

Ammonia (NH₃) buffer mechanism

NH₃ combines with H⁺ to form ammonium (NH₄⁺).

Ammonium excretion

NH₄⁺ is excreted in urine, helping remove H⁺ from the body.

How does ammonia buffering support H⁺ secretion

Binding H⁺ reduces free H⁺ in tubular fluid, allowing more H⁺ to be secreted.

Why are kidneys the third line of defence

Their responses take hours to days, although they provide powerful long-term regulation.

How do kidneys compare with the respiratory system in compensation

Kidneys can often restore pH more completely, while respiratory compensation is limited.

Normal bicarbonate-to-CO₂ ratio in the slides

Approximately 20:1.

What does a ratio below 20:1 indicate

An acidotic tendency.

What does a ratio above 20:1 indicate

An alkalotic tendency.

Respiratory acidosis

A decreased bicarbonate-to-CO₂ ratio caused by increased CO₂.

Respiratory alkalosis

An increased bicarbonate-to-CO₂ ratio caused by decreased CO₂.

Metabolic acidosis

A decreased bicarbonate-to-CO₂ ratio caused by decreased HCO₃⁻.

Metabolic alkalosis

An increased bicarbonate-to-CO₂ ratio caused by increased HCO₃⁻.

Primary change in respiratory acid-base disorders

A change in CO₂.

Primary change in metabolic acid-base disorders

A change in bicarbonate (HCO₃⁻).

What causes respiratory acidosis

CO₂ retention, often associated with inadequate ventilation.

What causes respiratory alkalosis

Excessive CO₂ loss, often associated with increased ventilation.

What causes metabolic acidosis

A decrease in bicarbonate or an increase in acid load.

What causes metabolic alkalosis

An increase in bicarbonate or loss of acid.

How can the lungs compensate for metabolic acidosis

Increased ventilation lowers CO₂.

How can the lungs compensate for metabolic alkalosis

Reduced ventilation retains CO₂.

How can the kidneys compensate for respiratory acidosis

They increase acid excretion and conserve or generate bicarbonate.

How can the kidneys compensate for respiratory alkalosis

They reduce bicarbonate conservation and excrete more bicarbonate.

Compensation

A physiological response that moves pH toward normal without necessarily removing the original cause.