L42: Acid-Base Balance

What is an acid?

a substance that releases hydrogen ions (H⁺) when dissolved in water

• pH level: < 7

• taste: sour

What is a base?

a molecule that can accept hydrogen ions (H⁺) from an acid

• pH level: > 7

• taste: bitter

What is the pH value of stomach acid?

1

What is the pH of distilled water?

7

What is the pH of drain cleaner?

14

What is the normal pH for body fluids?

7.4

_____________ occurs when the body loses more H⁺ (acid) than it gains, causing the blood to become too basic (alkaline).

Alkalosis occurs when the body loses more H⁺ (acid) than it gains, causing the blood to become too basic (alkaline).

___________ occurs when the body gains more H⁺ (acid) than it looses, causing the blood to become too acidic

Acidosis occurs when the body gains more H⁺ (acid) than it looses, causing the blood to become too acidic

What is the kidneys’ role in acid-base balance?

They can adjust how much H⁺ (acid) or HCO₃⁻ (base) they excrete or reabsorb.

• If the body gains acid → kidneys must increase base (HCO₃⁻) reabsorption or secretion

• If the body gains base → kidneys must increase acid (H⁺) reabsorption or secretion

The kidneys eliminate or replenish hydrogen ions (H⁺) from the body by increasing OR decreasing the plasma _____________ concentration.

The kidneys eliminate or replenish hydrogen ions (H⁺) from the body by increasing OR decreasing the plasma bicarbonate (base) concentration.

How does buffering minimize changes in H⁺ concentration?

• Buffers are usually negatively charged anions.

• When H⁺ increases (more acidic), buffers bind the excess H⁺.

• When H⁺ decreases (more alkaline), buffers release H⁺ back into the fluid.

• A buffer is a molecule that stabilizes pH by reversibly binding H⁺ ions, preventing large changes in acidity.

How do the kidneys reabsorb filtered bicarbonate (HCO₃⁻)?

1. HCO₃^- ions (base) are freely filtered into Bowman’s space (tubular lumen)

2. The kidney cannot directly reabsorb HCO₃⁻ from the lumen b/c the apical membrane does not have HCO₃⁻ transporters, so the body converts HCO₃⁻ into forms that can cross the membrane.

3. Tubular cells secrete H⁺ into the lumen (via Na⁺/H⁺ exchanger or H⁺ pumps). This H⁺ meets filtered HCO₃⁻ in the lumen.They combine to form: H⁺ + HCO₃⁻ → H₂CO₃ → H₂O + CO₂

4.  H₂O and CO₂ are highly permeable through the plasma membranes.

5. The overall effect is reabsorption of the filtered HCO₃^- ions (base) into the blood

6. The body keeps the HCO₃^- (base) to buffer acids in the ECF (and plasma) → prevents acidosis 

How do the kidneys respond to acidosis?

1. Make NEW Bicarbonate Using Urinary Buffers (Phosphate System)

   • Tubular cells produce new HCO₃⁻ (a base) in the nephron epithelial cell

   • This new HCO₃⁻ is added to the blood → raises pH (less acidic).

   • To get rid of excess acid, the kidney secretes H⁺ into the urine.

   • The H⁺ binds to urinary buffers other than bicarbonate (mainly phosphate).

   • The bound H⁺ is excreted in urine.

2. Make NEW Bicarbonate Using Glutamine (Ammonium System)

   • Kidney cells take up glutamine in the proximal tubule cell

   • Glutamine is broken down into:

     • HCO₃⁻ (goes to blood → raises pH)

     • NH₄⁺ (ammonium, an acid)

   • NH₄⁺ is secreted into the urine and excreted

How do the kidneys respond to alkalosis

1. decrease H⁺ ion (acid) secretion into the tubular fluid

2. decrease reabsorption of the filtered HCO₃^- (base)

3. decrease excretion of protonated phosphate (acid) and ammonium (acid)

4. increase renal HCO₃^- excretion in the urine

How do the kidneys maintain stable plasma pH?

• Excrete bicarbonate (response to alkalosis)

• Contribute new bicarbonate to blood (response to acidosis)

  • Excrete filtered phosphate

  • Excrete ammonium ions