Acid-Base Physiology

Hydrogen regulation
Depletion of chloride and K⁺ from the blood causes H⁺ loss

H⁺ is secreted into the renal tubule lumen, causing metabolic alkalosis. Specifically, chloride loss is accompanied by H⁺ to preserve charge (rather than Na⁺, which cannot accompany chloride in reabsorption). K⁺ loss decreases aldosterone release to allow more K⁺ into the blood. K⁺ is secreted into the plasma in exchange with H⁺ into the cell.

Sodium depletion of the blood causes H⁺ loss

Low Na⁺ causes more aldosterone to be secreted, and it stimulates potassium loss, which in turn stimulates H⁺ secretion in the same mechanism above. Another reason is that sodium depletion causes increased sodium reabsorption into the blood, where H⁺ exchanges for sodium during reabsorption and there is increased H⁺ secretion into the renal tubules. 

Buffering systems: HB ↔ H⁺ + B^- 

A common system involves a weak acid (HB), a strong acid (H⁺) and a weak conjugate base (B^-). The addition of a strong acid or base is partially neutralized by B^- or H⁺, and does not drastically change the pH.

1. Bicarbonate buffer: (CO₂ + H₂O) ↔ H₂CO₃ ↔ H⁺ + HCO₃^-
• The main extracellular buffering system. 
• Addition of a strong acid will shift the equation to the left, whereas strong base to the right. 
• To restore pH, the lungs may expire carbon dioxide and decrease blood acidity. The kidneys decrease blood acidity by secreting bicarbonate (which, in the acidotic state, is in lowered concentrations) to the blood, and excreting H⁺. 
2. Phosphate and ammonia buffers: H₂PO₄^- ↔ H⁺ + HPO₄^-2, NH₄⁺ ↔ H⁺ + NH₃
• Phosphate is an intracellular buffer, ammonia is an extracellular buffer. 
• Using these buffers, excess H⁺ can be neutralized and excreted as a weak acid. 
3. Protein buffer: H-protein (weak acid) ↔ H⁺ + protein (weak conjugate base)
• An intracellular buffering system. 

Respiratory acidosis 

May occur in advanced pulmonary disease, where lungs do not adequately remove carbon dioxide, and the blood becomes acidic. The kidney may compensate by reabsorbing more bicarbonate to the blood. 

Metabolic acidosis 

The addition of an acid to the body, other than carbonic acid, or the loss of bicarbonate from the body.  Bicarbonate is markedly decreased. The body responds by increasing respiration to remove carbon dioxide. 

Respiratory alkalosis

Too much carbon dioxide is removed by respiration. The kidneys compensate by excreting more bicarbonate. 

Metabolic alkalosis

An increase in plasma bicarbonate causes the body to decrease respiratory activity and preserve carbon dioxide.