T12. REGULATION OF ACID BASE BALANCE

Question 1

What is one of the principal functions of homeostasis?

Answer 1

Regulation of pH, since metabolism depends on pH-sensitive enzymes.

Question 2

What is the normal pH range of blood and tissue fluid?

Answer 2

7.35 - 7.45.

Question 3

Which organs maintain normal pH?

Answer 3

Lungs and kidneys.

Question 4

What can a slight deviation from normal pH cause?

Answer 4

Shut down of metabolic pathways and altered macromolecule structure/function.

Question 5

What condition occurs when pH < 7.35?

Answer 5

Acidosis (respiratory or metabolic).

Question 6

What condition occurs when pH > 7.45?

Answer 6

Alkalosis (respiratory or metabolic).

Question 7

Why is acid-base balance difficult to maintain?

Answer 7

Many metabolic reactions produce acid by-products.

Question 8

What are examples of acid by-products from metabolism?

Answer 8

Lactic acid, phosphoric acid, fatty acids, ketones, carbonic acid.

Question 9

Which acid is considered volatile and how is it removed?

Answer 9

Carbonic acid; converted to CO₂ and exhaled.

Question 10

What happens when CO₂ concentration increases in the body?

Answer 10

H+ and HCO₃⁻ concentrations also increase.

Question 11

How can pH be regulated via CO₂?

Answer 11

By adjusting ventilation to alter CO₂ levels.

Question 12

What are examples of non-volatile acids?

Answer 12

Lactic acid, fatty acids, ketones.

Question 13

How are non-volatile acids neutralized?

Answer 13

By buffers like bicarbonate, phosphate, ammonia, and proteins (e.g. hemoglobin).

Question 14

Which organs have the largest effect on acid-base balance?

Answer 14

The kidneys.

Question 15

How do kidneys neutralize more acid and base than other systems?

Answer 15

Through excretion of waste products.

Question 16

How do renal tubules contribute to acid-base balance?

Answer 16

They secrete H⁺ into the tubular fluid.

Question 17

What happens to secreted H⁺ in renal tubules?

Answer 17

Most binds to bicarbonate, ammonia, and phosphate buffers.

Question 18

How is H⁺ removed from the body by the kidneys?

Answer 18

Bound and free H⁺ are excreted in the urine.

Question 19

What is the limiting pH of tubular fluid?

Answer 19

4.5; below this, H⁺ secretion stops.

Question 20

How is the limiting pH prevented?

Answer 20

By buffers in the tubular fluid.

Question 21

What happens to bicarbonate ions in the tubular fluid?

Answer 21

They are all consumed in neutralizing H⁺, so none appear in urine.

Question 22

What happens to sodium as more acid is secreted by kidneys?

Answer 22

Less sodium appears in the urine.

Question 23

What is the phosphate buffer system in the kidneys?

Answer 23

Dibasic sodium phosphate reacts with H⁺, replacing Na⁺ and forming NaH₂PO₄.

Question 24

What is the reaction equation for the phosphate buffer system?

Answer 24

Na₂HPO₄ + H⁺ → NaH₂PO₄ + Na⁺.

Question 25

What is the role of ammonia (NH₃) in renal buffering?

Answer 25

It acts as a base to neutralize H⁺.

Question 26

What is the ammonia buffering reaction?

Answer 26

NH₃ + H⁺ + Cl⁻ → NH₄Cl (a weak acid).

Question 27

How do kidneys maintain blood pH overall?

Answer 27

By reabsorbing HCO₃⁻ and secreting H⁺.

Question 28

Is urine normally acidic or basic?

Answer 28

Acidic.

Question 29

What mechanism is used by the proximal tubule for acid-base regulation?

Answer 29

Na⁺/H⁺ pumps exchange Na⁺ out and H⁺ in.

Question 30

What happens to secreted H⁺ in the proximal tubule?

Answer 30

Used for HCO₃⁻ reabsorption.

Question 31

Why must HCO₃⁻ be converted to CO₂ for reabsorption?

Answer 31

Because HCO₃⁻ cannot cross the inner tubule membrane.

Question 32

How is CO₂ formed from HCO₃⁻ in the tubule?

Answer 32

Using carbonic anhydrase to form CO₂ and H₂O from H₂CO₃.

Question 33

What happens to CO₂ inside tubule cells?

Answer 33

It is converted back to HCO₃⁻ and H⁺.

Question 34

What does H⁺ do after being reformed in tubule cells?

Answer 34

It is secreted into the tubular fluid to repeat the process.

Question 35

What does the proximal tubule use to transport Na⁺ and HCO₃⁻ into capillaries?

Answer 35

Na⁺/K⁺ pumps and cotransporters.

Question 36

Is there net secretion of H⁺ in the proximal tubule?

Answer 36

No, but there is net reabsorption of HCO₃⁻.

Question 37

How is a new HCO₃⁻ formed in proximal tubule cells?

Answer 37

CO₂ and H₂O combine to form H₂CO₃, which dissociates into H⁺ and HCO₃⁻.

Question 38

What pump is used in the distal convoluted tubule to secrete H⁺?

Answer 38

H⁺ ATPase pumps.

Question 39

What transporter helps reabsorb bicarbonate in the distal tubule?

Answer 39

HCO₃⁻/Cl⁻ antiporter.

Question 40

How are H⁺ ions buffered in the distal tubule?

Answer 40

By phosphate and ammonia.

Question 41

Where do phosphates come from in the nephron?

Answer 41

Filtration at the glomerulus.

Question 42

Where does ammonia come from?

Answer 42

Amino acid catabolism.

Question 43

How is ammonia synthesized in tubular cells?

Answer 43

From amino acids such as glutamine.

Question 44

What does glutamine metabolism produce in renal tubule cells?

Answer 44

New bicarbonate ions.

Question 45

How does phosphate buffering work in the distal tubule?

Answer 45

Sodium phosphate reacts with H⁺ to form NaH₂PO₄ and Na⁺.

Question 46

How does the ammonia buffering system work in the distal tubule?

Answer 46

NH₃ reacts with H⁺ and Cl⁻ to form NH₄Cl, lost in urine.

Question 47

What happens when tubular fluid pH reaches 4.5?

Answer 47

H⁺ secretion stops — this is the limiting pH.

Question 48

In the distal tubule, what is the balance between H⁺ and HCO₃⁻?

Answer 48

Secretion of H⁺ equals reabsorption of HCO₃⁻.