Costanzo Renal Pysiology: Acid-Base balance

Question 1

What are the two kinds of acids produced by the human body?

Answer 1

(1) volatile acids (CO2)

2) Non-volatile acids (fixed acids eg sulfuric or phophoric acid

Question 2

What is carbonic anhydrase?

Answer 2

An enzyme that converts CO2 and H2O to HCO3- H+

Question 3

What is the major extracellular buffer and what is its pKa?

Answer 3

HCO3- is the major extracellular buffer with a pK of 6.1.

Question 4

What is the most important urinary buffer?

Answer 4

Phosphate is the most important urinary buffer with pK of 6.8.

Question 5

What are some major intracellular buffer?

Answer 5

(1) organic phosphates (ADP, ATP, 2,3-DPG, etc)
(2) imidazole and amino groups on proteins
(3) Hemaglobin

Question 6

Is hemaglobin a better buffer when its oxygenated or non-oxygenated?

Answer 6

Non-oxygenated hemoglobin is a better buffer

Question 7

What is the Henderson Hasselbach equation?

Answer 7

pH = pKa + Log( [A-] / [HA] )

Question 8

Where in the nephron is HCO3- primarily reabsorbed?

Answer 8

HCO3- is primarily reabsorbed in the proximal tubule.

Question 9

What happens to HCO3 in alkalosis?

Answer 9

There is too much filtered HCO3- to be reabsorbed therefore HCO3- is excreted in the urine. Additionally, there is lower H+ concentration. This will decrease HCO3- reabsorption because H+ is required for the reabsorption of HCO3- .

Question 10

What happens to filtered HCO3- if there is an increase in Pco2?

Answer 10

An increase in Pco2 (respiratory acidosis) will result in an increase in H+ ions which will after being secreted into the lumen, facilitate the reabsorption of HCO3-.

Question 11

What happens to filtered HCO3- if there is a decrease in Pco2?

Answer 11

A decrease in Pco2 (respiratory alkalosis) will result in less H+ available for secretion and therefore there will be less HCO3- taken up from the tubules.

Question 12

What is the effect of ECF volume expansion on HCO3- reabsorption?

Answer 12

HCO3- reabsorption is decreased by ECF volume expansion.

Question 13

What is the effect of ECF volume contraction on HCO3- reabsorption?

Answer 13

HCO3- reabsorption is increased when there is ECF volume contraction.

Question 14

What is the effect of angiotensin II on HCO3- reabsorption?

Answer 14

Angiotensin II often released secondary to volume contraction will increase the Na+/H+ exchange thus increasing HCO3- reabsorption.

Question 15

What two methods are used to excrete fixed H+?

Answer 15

(1) titratable acids

(2) NH4+

Question 16

How is H+ excreted as a titratable acid?

Answer 16

A portion of the H+ that is secreted binds with HCO3- to be converted back to CO2 and water to be reabsorbed. However, the excess H+ binds with filtered HPO4– to form
H2PO4- which is not reabsobed but excreted. This proressively lowers urine’s pH.

Question 17

What is the minimum urinary pH?

Answer 17

4.4 is the minimum urinary pH.

Question 18

What determines the amount of H+ excreted?

Answer 18

The amount and pK of the urinary buffer.

Question 19

How is NH3 used to excrete H+?

Answer 19

NH3 diffuses into the acidic environment of the tubule. In the lumen NH3 binds with H+ to make NH4+ which is not reabsorbed but excreted.

Question 20

How is potassium related to NH3 synthesis?

Answer 20

(1) hyperkalemia inhibits NH3 synthesis

(2) Hypokalemia stimulates NH3 synthesis.

Question 21

What is metabolic acidosis?

Answer 21

Overproduction or ingestion of H+ leading to acidemia.

Question 22

What happens to HCO3- during metabolic acidosis?

Answer 22

Arterial HCO3- decreases as it takes up extra H+

Question 23

What is the respiratory compensation for metabolic acidosis?

Answer 23

hyperventilation (Kassmaul breathing) is the respiratory compensation for metabolic acidosis.

Question 24

How is metabolic acidosis corrected?

Answer 24

(1) increased excretion of H+at titratable acid and NH4+

(3) increased reabsorption of HCO3- to bind up more H+

Question 25

What adaptive changes occur in response to metabolic acidosis?

Answer 25

NH3 synthesis can be increased in metabolic acidosis.

Question 26

What is the anion gap?

Answer 26

The unmeasured anions in the serum.

[Na+] - ( [Cl-] + [HCO3-] )

Question 27

What is the normal anion gap?

Answer 27

12mEq/L

Question 28

What is metabolic alkalosis?

Answer 28

the loss of H+(vomiting) through the loss of fixed H+ or increase in base.

Question 29

What is the respiratory compensation for metabolic alkalosis?

Answer 29

Hypoventilation

Question 30

How is metabolic alkalosis corrected?

Answer 30

By excretion of excess HCO3-

Question 31

Why does vomiting exacerbate metabolic alkalosis?

Answer 31

Because the volume contraction leads to increased angiotensin II and aldosterone which leads to increased HCO3- reabsorption.

Question 32

What causes respiratory acidosis?

Answer 32

A decrease in respiratory rate leading to an increase in retained CO2.

Question 33

What is the renal compensation for respiratory acidosis?

Answer 33

increased H+ secretion leads to increased HCO3- reabsorption. There is also an increase is H+ excretion via titratable acids and NH4+

Question 34

What is the difference between acute and chronic respiratory acidosis?

Answer 34

In acute respiratory acidosis renal compensation has not occured yet.
In chronic respiratory acidosis renal compensation has already begun.