Acid-Base Balance 1

Question 1

What is the normal pH of the arterialised blood?

Answer 1

7.4 = free [H+] of 40x10^-1 mole/L

Question 2

What type of ions contribute to pH?

Answer 2

Only free H+ ions

Question 3

What are the two categories of sources of H+ ions?

Answer 3

Respiratory acid and metabolic acid

Question 4

Describe the formation of H+ ions via respiratory acid

Answer 4

Due to aerobic respiration formation of CO2:
Formation of carbonic acid is not normally a net contributor to ↑ acid because any ↑ in production → ↑ in ventilation.
• Problems occur if lung function is impaired.

Question 5

Describe the formation of H+ ions via metabolic acid (non-respiratory acid

Answer 5

Via metabolism

a) Inorganic acids: eg S-containing amino acids → H2SO4 and phosphoric acid is produced from phospholipids
b) Organic acids: fatty acids, lactic acid

Question 6

Name three coping mechanism for increase in [H+]

Answer 6

1. Buffers
2. Ventilation
3. Renal regulation

Question 7

Define respiratory acidosis

Answer 7

State of respiratory acidosis occurs when alveolar hypoventilation results in CO2 retention and elevated plasma PCO2.

Question 8

Define metabolic acidosis

Answer 8

Disturbance of mass balance that occurs when dietary and metabolic input of H+ exceeds H+ excretion.

Question 9

What is the most important extracellular buffer?

Answer 9

Bicarbonate buffer system

H2CO3 → H+ + HCO3- ,

Question 10

What is the Henderson-Hasselbalch Equation?

Answer 10

pH=pK+log⁡〖([A-])/([HA])〗

(HA → H+ + A-)

OR

pH = [HCO3]/PCO2

Question 11

What does the quantity of H2CO3 in the bicarbonate buffer system depend on?

Answer 11

Amount of CO2 in plasma and therefore the solubility of CO2 and Pco2 (as it is originally a gas)

Question 12

At pH of 7.4 (normal) what is the ratio of bicarbonate to carbonic acid?

Answer 12

20:1

Question 13

State the normal value and range for pH

Answer 13

pH = 7.4
Range = 7.37-7.43

Question 14

State the normal value and range for pCO2

Answer 14

pCO2 = 5.3kPa or 40mmHg
Range = 4.8-5.9 kPa or 36-44mmHg

Question 15

State the normal value and range for [HCO3]

Answer 15

24mmoles and range 22-26

Question 16

At normal PCO2 of 40mmHg, what is the concentration of bicarbonate?

Answer 16

24mmoles/l (as carbonic acid is 1.2 and ratio is 20:1)

Question 17

What is the action of the bicarbonate mechanisms if there is an increase in H+ ions in ECF?

Answer 17

Drives the reaction to the right, so that some of the additional ↑ H+ ions are removed from solution via ventilation and therefore a change in pH is reduced.

Question 18

What is the action of the bicarbonate mechanisms if there is a decrease in H+ ions in ECF?

Answer 18

↑CO2 as ↓ ventilation which pushes equation to the left to produce more H+ ions and bicarbonate

Question 19

How are H+ ions eliminated from the body?

Answer 19

By the kidneys, which is coupled to the regulation of plasma [HCO3-]

pH = [HCO3] (renal regulated)/Pco2 (resp. regulated)

Changes is these levels are compensated by the changes in other levels

Question 20

Name two other buffer systems in the ECF other than bicarbonate buffer

Answer 20

Plasma proteins: Pr- + H+ → HPr

Dibasic phosphate HPO4^2- + H+ → H2PO4- monobasic phosphate.

Question 21

What are the primary intracellular buffers?

Answer 21

Proteins, organic and inorganic phosphates and, in the erythrocytes, haemoglobin

Also bone carbonate

Question 22

How is an increase in intracellular H+ ions buffered?

Answer 22

Changes plasma electrolytes, so in order to maintain electrochemical neutrality, movement of H+ must be accompanied by Cl or exchanged for another cation, K+.

Question 23

What is the clinical aspect of acidosis of the ICF?

Answer 23

The movement of K+ out of cells into plasma can cause hyperkalaemia → depolarization of excitable tissues → ventricular fibrillation and death.

In chronic renal failure → bone carbonate is used to buffer causing wasting of bones

Question 24

How much buffer is received via the diet?

Answer 24

50-100 moles H+ per day

Question 25

What is the effect of buffering metabolic acid?

Answer 25

[H+] increases because of the H+ contributed by the metabolic acids. This increase shifts the equilibrium to the LEFT, increasing CO2 levels and using up HCO3 43% buffered in plasma and 57% buffered within the cells

Question 26

Where is most of respiratory acid buffered in the cells?

Answer 26

97% in the cells (as that's where CO2 is mostly produced) Hb particularly important and the rest occurs with plasma proteins.

Question 27

How does the kidney regulate [HCO3]?

Answer 27

1. Reabsorbing filtered HCO3 2. Generating new HCO3 for increased H+ ions These processes depend on active H+ ion secretion form the tubule cells into the lumen

Question 28

Describe the mechanism of reabsorption of bicarbonate

Answer 28

Bulk HCO3 reabsorption occurs in prox. tubule: 1. Active H+ secretion from the tubule cells 2. Coupled to passive Na+ reabsorption 3. Filtered HCO3- reacts with the secreted H+ to form H2CO3. In the presence of carbonic anhydrase on the luminal membrane  CO2 and H2O 4. CO2 is freely permeable and enters the cell 5. Within the cell, CO2  H2CO3 in the presence of carbonic anhydrase (present in all tubule cells) which then dissociates to form H+ and HCO3- 6. The H+ ions are the source of the secreted H+ 7. The HCO3- ions pass into the peritubular capillaries with Na+

Question 29

What is the importance of the HCO3 reabsorption?

Answer 29

GFR = 180l/day [HCO3- ] = 24mmoles/l = 4320 mmoles HCO3- filtered per day. Must be reabsorbed, since failure to do so = to adding H+ to the ECF (causing acidosis) There is no excretion of H+ ions during HCO3- reabsorption.

Question 30

Why is the pH of urine not 1?

Answer 30

If H+ were present as free ions in the urine it would be, but as it is buffered by weak acids and bases, pH = 4.5-5

Question 31

What weak acids and bases buffer H+ ions in the urine?

Answer 31

Dibasic phosphate, H2SO4, mainly as well as uric acid and creatinine

Question 32

Why is dibasic phosphate called a titratable acid?

Answer 32

In the urine, is it able to titrate pH of urine back up to plasma pH of 7.4 by measuring the amount of NaOH needed to titrate the urine

Question 33

What is the importance of titratable acidity?

Answer 33

Generated new HCO3 and excretes H+

Question 34

Describe the steps in buffering pH of the urine

Answer 34

1. Na2HPO4 in the lumen. One Na+ is reabsorbed in exchange for secreted H+. This monobasic phosphate removes H+ from the body. 2. Within the distal tubule cell, CO2 (which has come form the blood) combines with H2O to form carbonic acid, in the presence of carbonic anhydrase, which then dissociates to yield H+, used for secretion, and new HCO3- , which passes with Na+ into the peritubular capillaries. This process is dependent on PCO2 of the blood to allow CO2 to diffuse into the lumen of the tubule

Question 35

What is the adaptive response to an increase in acid load?

Answer 35

Generate new HCO3 AND excrete H+ (whereas to reabsorb HCO3 it is converted to CO2 in lumen and then back into HCO3 in cell, which then diffuses out, there is NO excretion of H+)

Question 36

Why is the distal tubule the site of tittle acidity?

Answer 36

Unreabsorbed dibasic phosphate becomes highly concentrated by the reabsorption of fluid in the loop of henle

Question 37

What is another process which is used during chronic acidosis to excrete more protons and produce more bicarbonate?

Answer 37

Elimination of ammonium

Question 38

How does ammonia help in acid loading?

Answer 38

1. Ammonia (NH3) produces by deamination of AA by renal glutaminase within renal tubule cells and is LIPID SOLUBLE so diffuses into renal tubule 2. CO2 from blood enters tubule cells → H+ and HCO3 (bicarbonate moves into blood) 3. H+ ions move into renal tubule and join NH3 → ammonium (NH4) 4. NH4 join Cl- (from NaCl) → NH4Cl which is excreted Ammonium can also enter tubule via NH4/Na exchanger in the prox. tubule

Question 39

Where does ammonia secretion occur?

Answer 39

Distal tubule

Question 40

What regulates renal glutaminase activity (used to form NH3)

Answer 40

When intracellular pH falls → increase renal glutaminase activity and therefore more NH4+ produced and excreted (decrease pH as it uses up H+).