Acid-Base Balance I

Question 1

What is the average ph of blood?

Answer 1

7.4 (arterial blood is 7.45 and venous blood is 7.35)

Question 2

What is the pH of the ECF?

Answer 2

Tightly controlled close to 7.4 (40 x 10^-9 micromoles)

Question 3

What do small changes in pH reflect?

Answer 3

Large changes in H+ concentration

Question 4

What does an increase in H+ concentration cause?

Answer 4

Reduces pH

Question 5

What do fluctuations in H+ concentration alter?

Answer 5

Nerve, enzyme and K+ activity

Question 6

How do changes in [H+] affect the CNS?

Answer 6

Acidosis can lead to depression of the CNS

Alkalosis can lead to over-excitability of the peripheral nervous system and later the CNS

Question 7

How does metabolic activity affect [H+]?

Answer 7

Causes H+ to be continually added to the body fluid = input must equal output to maintain constant H+

Question 8

Where is H+ continually added from?

Answer 8

Carbonic acid formation
Inorganic acids produced during breakdown of nutrients
Organic acids resulting from metabolism

Question 9

What happens to acids when they are in solution?

Answer 9

Strong acids fully dissociate

Weak acids partially dissociate

Question 10

What is the equilibrium equation for acids?

Answer 10

HA = H+ + A-

Question 11

What does a buffer system consist of?

Answer 11

A pair of substances = one can yield free H+ as the [H+] decreases, the other can bind free H+ when [H+] increases

Question 12

What happens to the equilibrium equation when an acid is added to solution?

Answer 12

Equilibrium shifts to the left = protons mopped up by A- leading to formation of more HA, [HA] rises and [A-] falls
Rise in [H+] has been limited by the formation of HA

Question 13

What happens to the equilibrium equation when a base is added to solution?

Answer 13

Equilibrium shifts to right = base tied up by combining with H+ allowing HA to dissociate, [HA] falls and [A-] rises, rise in pH has been limited by further dissociation of HA

Question 14

What is the dissociation constant (K) at equilibrium according to the Law of Mass Action?

Answer 14

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

Question 15

What is the Henderson-Hasselbach equation?

Answer 15

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

Question 16

What is the most important physiological buffer?

Answer 16

Co2-HCO3 buffer:

CO2 + H2O = H2CO3 = H+ + HCO3

Question 17

How is H2CO3 formed?

Answer 17

Formed from CO2 and water, catalysed by carbonic anhydrase

Question 18

What is the role of the kidneys in the control of [HCO3-]?

Answer 18

Variable reabsorption of filtered HCO3-
Kidneys can add new HCO3- to the blood
Both depend upon H+ secretion into the tubule

Question 19

What is the rate of filtration of HCO3-?

Answer 19

GFR x [HCO3-]plasma = 4320 mmol/day

Question 20

What happens to HCO3- in the fluid of the kidneys?

Answer 20

Disappears from tubular fluid and appears in the interstitial fluid

Question 21

Why is the reabsorption of HCO3- considered unorthodox?

Answer 21

The same HCO3- ion doesn’t cross the epithelium

Question 22

How is new HCO3- produced by the kidneys?

Answer 22

When [HCO3-] of the tubular fluid is low, secreted H+ combines with the next most plentiful buffer in the filtrate (phosphate) = allows gain of HCO3-

Question 23

What is the purpose of generating new HCO3-?

Answer 23

To regenerate buffer stores

Question 24

What is titratable acid?

Answer 24

The amount of H+ excreted as H2PO4-

Question 25

How is titratable acid measured?

Answer 25

Measured amount of NaOH added to titrate urine pH back to 7.4 = reverses addition of H+ that has occurred as fluid flows along the tubule

Question 26

What is the maximum amount of titratable acid that can be excreted?

Answer 26

40 mmol/day = 40 mmol of new HCO3- is simultaneously gained by the circulation

Question 27

What other base besides phosphate can act as a tubular buffer?

Answer 27

Ammonia = acid excreted as NH4+ and new HCO3- is generated

Question 28

How is NH4+ measured?

Answer 28

Not measured as part of titratable acid = a separate ammonium ion determination is needed

Question 29

When may NH4+ excretion increase?

Answer 29

May rise from its usual 20 mmol/day to between 500-600 mmol/day during acidosis

Question 30

What does H+ secretion by the tubule do?

Answer 30

Drives reabsorption of HCO3-
Forms acid phosphate
Forms ammonium ion

Question 31

What is H+ excretion equal to?

Answer 31

The amount of new HCO3- generated

Question 32

What is the total H+ secretion into the tubule?

Answer 32

4360 mmol/day

Question 33

What are the components that make up the total H+ secretion into the tubule?

Answer 33

H+ excretion = 4300 mmol/day
NH4+ excretion = 40 mmol/day
TA excretion = 20 mmol/day

Question 34

What is the total tubular H+ excretion?

Answer 34

60 mmol/day

Question 35

What are the components that contribute to total tubular H+ excretion?

Answer 35

TA excretion = 20 mmol/day

NH4+ excretion = 40 mmol/day

Question 36

What is the vast majority of H+ secretion used for?

Answer 36

HCO3- reabsorption = to prevent acidosis

Question 37

What does the excretion of NH4+ and TA achieve?

Answer 37

Simultaneously rids the body of acid load and regenerates buffer stores (alkanises body)