Acid Base Balance

Question 1

What is the normal range for pH for arterial blood?

Answer 1

7.37-7.43

Question 2

What are sources of hydrogen ions?

Answer 2

Respiratory acids from respiratory pathology.
Metabolic acids- either inorganic like sulfuric (from amino acids) and phosphoric (from phospholipids) or organic like lactic or fatty acids.

Question 3

What is the normal range for bicarbonate?

Answer 3

22-26

Question 4

Through what mechanism does the acidic equation for CO2 and bicarb not get pulled to the right and rendered useless?

Answer 4

Ventilation blows off CO2

Question 5

What are two buffer systems in the ECF other than the bicarb one?

Answer 5

Plasma proteins Pr- + H+  HPr

Dibasic phosphate HPO42- + H+  H2PO4- monobasic phosphate

Question 6

What are some intracellular buffers but what is the problem with these?

Answer 6

Primary intracellular buffers are proteins, organic and inorganic phosphates and, in the erythrocytes, haemoglobin.
Buffering of H+ ions by ICF buffers cause changes in plasma electrolytes, since to maintain electrochemical neutrality, movement of H+ must be accompanied by Cl- as in red cells or exchanged for a cation, K+ .
In acidosis, the movement of K+ out of cells into plasma can cause hyperkalaemia  depolarization of excitable tissues  ventricular fibrillation and death.

Question 7

What buffer is stored in the bones and what effect does this have on chronic renal patients?

Answer 7

Carbonate

Bone wasting and breakdown due to buffer stores having to be used

Question 8

What is the mechanism for reabsorption of Bicarbonate by the kidneys?

Answer 8

Active H+ secretion from the tubule cells coupled to passive Na+ reabsorption
Filtered HCO3- reacts with the secreted H+ to form H2CO3. In the presence of carbonic anhydrase on the luminal membrane it is converted toCO2 and H2O.
CO2 is freely permeable and enters the cell.
Within the cell, CO2 is turned to H2CO3 in the presence of carbonic anhydrase (present in all tubule cells) which then dissociates to form H+ and HCO3- The H+ ions are the source of the secreted H+
The HCO3- ions pass into the peritubular capillaries with Na+
Bulk of HCO3- reabsorption occurs in the proximal tubule

Question 9

What is the pH of urine?

Answer 9

4.5.-5.0

Question 10

What is urine buffered by?

Answer 10

dibasic phosphate, HPO42-, also uric acid and creatinine.

Question 11

What is the process of creating new bicarbonate by the kidney?

Answer 11

Na2HPO4 in the lumen. One Na+ is reabsorbed in exchange for secreted H+. This monobasic phosphate removes H+ from the body.
The source of the new HCO3- is indirectly CO2 from the blood. It enters the tubule cells, combining 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.
Occurs principally in the distal tubule. This is where, phosphate ions, not reabsorbed by the proximal tubule Tm mechanism, become greatly concentrated because of removal of up to 95% of the initial filtrate.
Process is dependent on Pco2 of the blood.

Question 12

What is the process by which Hydrogen ions are excreted in the urine?

Answer 12

NH3 is produced by deamination of amino acids, primarily glutamine, by the action of renal glutaminase within the renal tubule cells. NH3 moves out into the tubule lumen, where it combines with secreted H+ ions to form NH4+ which combines with Cl- ions (from NaCl) to form NH4Cl which is excreted. (Distal tubule mechanism)
The source of the secreted H+ is again CO2 from the blood.
The new HCO3- passes with Na+ ions into the peritubular capillaries.
In the proximal tubule, there is an NH4+/Na+ exchanger so NH4+ ions formed within the cells pass out into the lumen. Net effect is the same.

Question 13

What are some causes of respiratory acidosis and what is the body’s response to this?

Answer 13

Acute: drugs which depress the medullary respiratory centres, such as barbiturates and opiates.
Obstructions of major airways.
Chronic: lung disease eg bronchitis, emphysema, asthma.
Response: Need to protect pH so need to increase [HCO3-].

Question 14

What are some causes of respiratory alkalosis and what is the body’s response to this?

Answer 14

Acute: voluntary hyperventilation, aspirin, first ascent to altitude
Chronic: long term residence at altitude, reduced Po2 to < 60mmHg (8kPa) stimulates peripheral chemoreceptors to increase ventilation.
To protect pH, [ HCO3-] should decrease.

Question 15

What are some causes of metabolic acidosis and what is the body’s response to this?

Answer 15

Increased H+ production, as in ketoacidosis of a diabetic (acetoacetic acid, hydroxybutyric acid) or in lactic acidosis.
Failure to excrete the normal dietary load of H+ as in renal failure.
Loss of HCO3- as in diarrhoea ie failure to reabsorb intestinal HCO3-
To protect the pH, Pco2 must be decreased.

Question 16

What are some causes of metabolic alkalosis and what is the body’s response to this?

Answer 16

Increased H+ ion loss- vomiting loss of gastric secretions
Increased renal H+ loss- aldosterone excess, excess liquorice ingestion
Excess administration of HCO3- is unlikely to produce a metabolic alkalosis in subjects with normal renal function, but may do so if renal function impaired.
Massive blood transfusions can lead to metabolic alkalosis because bank blood contains citrate to prevent coagulation, which is converted to HCO3-, but need at least 8 units to have this effect.
[HCO3-] must have increased and Pco2 will have to increase to protect the pH.

Question 17

What is the anion gap?

Answer 17

Anion Gap = The difference between the sum of the principal cations ( Na+ and K+) and the principal anions in the plasma (Cl- and HCO3- ).
Normally 14-18mmoles/L
ie (140 +4) – (104 +24) =16mmoles/L, (due to plasma proteins)

Question 18

What are the two types of metabolic acidosis in relation to the anion gap?

Answer 18

If the acidosis is due for example to a loss of bicarbonate from the gut, then the reduction of bicarbonate is compensated by an increase in chloride and so there is no change in anion gap.

However in eg lactic or diabetic acidosis, the reduction in bicarbonate is made up by other anions such as lactate, acetoacetate, -OH butyrate and so the anion gap is increased.