Acid-Base Balance

Question 1

Effect of [H+] ions on pH?

Answer 1

Increase in [H+] reduced pH; small changes in pH reflext large changes in [H+]

Question 2

Consequences of fluctutating [H+]?

Answer 2

1. Acidosis can lead to CNS depression; alkalosis can lead to over-excitability of the peripheral NS and later of the CNS
2. [H+] exert a marked influence on enzyme activity
3. Changes in [H+] influence K+ levels in the body

Question 3

Sources of H+ ions in the body?

Answer 3

Continually added to the body fluids as a result of metabolic activity:

1. Carbonic acid formation
2. Inorganic acids produced during breakdown of nutrients
3. Organic acids resulting from metabolism

Diabetes mellitus

Question 4

Dissociation of strong and weak acids?

Answer 4

Strong acids dissociate completely in solution

Weak acids only do so partially

Question 5

Components of a buffer system?

Answer 5

Consists of a pair of substances:
• One can yield free H+ as the [H+] decreases
• Other can bind free H+ when [H+] increases

Question 6

What happens when H+ ions (acid) are added to this system:

HA dissociates to form H+ and A-

?

Answer 6

Protons are “mopped” up by A-, leading to formation of more HA

[HA] rises, [A-] falls

i.e: the rise in [H+], and fall in pH, has been limited (buffered) by the formation of HA

Question 7

What happens when base (B-) is added to this system:

HA dissociates to form H+ and A-

?

Answer 7

Base combines with H+, allowing more HA to dissociate

[HA] falls, [A-] rises

i.e: the rise in pH (fall in [H+]) has been limited (buffered) by further dissociation of HA

Question 8

What does this mean:

“For X, the pK = 6.8”?

Answer 8

At a pH of 6.8, this aprticular chemical reaction is at equilibrium

Question 9

Henderson-Hasselbalch equation?

Answer 9

pH = pK + log ( [A-] / [HA] )

Question 10

Most important physiological buffer system?

Answer 10

CO2 - HCO3 buffer

CO2 + H2O combines to form H2CO3 (carbonic acid), due to carbonic anhydrase; this dissociates to form H+ and HCO3-

Question 11

What else is the conc. of H2CO3 equal to?

Answer 11

Can rewrite:
• PCO2 x 0.03 (solubility coefficient)

This can be used in the Henderson-Hasselbalch equation

Question 12

Which systems control products of the buffer system?

Answer 12

[HCO3-] is controlled by the kidneys

PCO2 is controlled by the lungs

Question 13

Role of the kidney in control of [HCO3-] in the plasma?

Answer 13

1. Variable reabsorption of filtered HCO3-
2. Kidneys can add “new” HCO3- to the blood, i.e: the [HCO3-] in the renal vein is greater than in the renal artery

THESE BOTH DEPEND ON H+ SECRETION INTO THE TUBULE, i.e: this drives HCO3- reabsorption

Question 14

Mechanism of HCO3- reabsorption in the PCT?

Answer 14

There is no transport mechanism at the apical membrane to reabsorb HCO3- ions from the tubular fluid; so an indirect method is used

Question 15

In addition to conserving filtered HCO3-, how else can the kidneys allow more reabsorption of HCO3-?

Answer 15

Kidneys can generate new HCO3-, to regenerate buffer stores depleted by an acid load

When [HCO3-] in the tubular fluid is low (reabsorption), secreted H+ combines with the next most plentiful buffer in the filtrates, which is PHOSPHATE

In this way, acid is excreted and there is net gain of HCO3-

Question 16

Use of titratable acid measurement?

Answer 16

Amount of H+ excreted as largely H2PO4 can be measured by measuring the amount of strong base added to titrate the urine back to 7.4

This reverses the addition of H+ that has occurred as the fluid flows along the tubule

Maximum amount of titratable acid that can be excreted is approx. 40 mmol/day; thus, if this is added as titratable acid, 40 mmol/day “new” HCO3- is simultaneously gained by the circulation

Question 17

How is ammonia used as a tubular buffer?

Answer 17

With increasing acidosis, the kidneys break down glutamine to form ammonial this enters the tubular fluid and combines with H+, to form ammonium ions, which are excreted via urine (excreting the H+ ions)

Bicarbonate ions are still reabsorbed

Question 18

How is NH4+ (ammonium) measured)?

Answer 18

Measurement of titratable acid does NOT measure NH4+; a separate ammonium ion determination is necessary

Question 19

3 things that H+ secretion by the tubule does?

Answer 19

1. Drives reabsorption of HCO3-, preventing acidosis
2. Forms “acid-phosphate”
3. Forms ammonium ion

Question 20

How to calculate HCO3- reabsorption?

Answer 20

REFER TO POWERPOINT (slide 23)

Question 21

How to calculate formation of acid-phosphate?

Answer 21

REFER TO POWERPOINT (slide 24)

Question 22

How to calculate ammonium ions?

Answer 22

REFER TO POWERPOINT (slide 25)