Acid-Base Physiology

Question 1

Why is the maintenance of hydrogen ions within tight limits important?

Answer 1

Protein folding is highly affected by pH - thus pH affects enzyme activity
pH influences the excitability of nerve and muscle cells
Hydrogen ion concentration influences potassium ion concentration also

Question 2

What physiological processes provide a threat to pH homeostasis?

Answer 2

Generation of carbon dioxide from aerobic respiration
Metabolism of food which generates acid or alkali
Anaerobic respiration
Loss of alkali in still
Loss of acid in vomiting

Question 3

It is possible to have an acid/base disturbance but a normal pH. T/F?

Answer 3

True - in this case pH is compensated by abnormal bicarbonate or carbon dioxide

Question 4

How much of the total body buffering capacity doe bicarbonate account for?

Answer 4

80%

Question 5

Plasma proteins are an important physiological buffer. T/F?

Answer 5

True - especially haemoglobin

Question 6

If the concentration of hydrogen ion increases then there is an increase in the consumption of bicarbonate buffer and an increase in the formation of carbonic acid, carbon dioxide and water. How is this increased carbon dioxide expelled?

Answer 6

Exhalation

Question 7

At a physiological pH, what is the ratio of bicarbonate ions to hydrogen ions?

Answer 7

20:1

Question 8

Bone acts as a long-term pH buffer. T/F?

Answer 8

True

Question 9

Why are dietary acids termed ‘fixed’ acids?

Answer 9

They cannot be converted to carbon dioxide

Question 10

How are fixed acids secreted by the kidneys?

Answer 10

Through hydrogen excretion through ammonia or phosphate

Question 11

Which enzyme is involved in the reaction of hydrogen and bicarbonate to form carbonic acid?

Answer 11

Carbonic anhydrase

Question 12

The excretion of ammonia in the kidneys is much better able to be unregulated than phosphate. T/F?

Answer 12

True

Question 13

Describe the excretion of hydrogen ions with phosphate in the kidneys?

Answer 13

water and carbon dioxide in the tubular cell combine to form a hydrogen ion and a bicarbonate ion. the bicarbonate ion is transported out of the cell across the basolateral membrane and the hydrogen ion is transported out of the cell across the apical membrane. in the lumen of the tubule the hydrogen ion combines with phosphate and then is excreted in the urine

Question 14

Describe the excretion of hydrogen ions with ammonia in the kidneys?

Answer 14

hydrogen ions are formed from the reaction of water with carbon dioxide and the bicarbonate ions which are also formed in this reaction are pumped out of the tubular cell across the basolateral membrane. hydrogen is pumped out of the cell into the tubular lumen. Here it combines with ammonia which is formed by the degradation of the amino acid glutamine by the enzyme glutaminase to form ammonia and bicarbonate

Question 15

Ammonia excretion by the kidneys can be increased up to…?

Answer 15

300mml/day

Question 16

In addition to the kidneys, what other organ is primarily involved in ammonia metabolism?

Answer 16

The liver

Question 17

What are the general causes of metabolic acidosis?

Answer 17

Addition of extra acid - generation of extra acid, ingestion of acid
Failure to excrete acid
Loss of bicarbonate (in stool and/or urine)

Question 18

What is the compensatory response to metabolic acidosis?

Answer 18

Decrease pCO2 by increasing respiration

Question 19

What are the effects of metabolic acidosis on the body?

Answer 19

Arrythmias
Decrease in cardiac contractility
Vasodilation
Increased ventilation
Protein wasting
Resorption of calcium from bone
Neutrophilia

Question 20

What is the normal anion gap?

Answer 20

6-12mmol/l

Question 21

An anion gap of greater than ….. should be investigated?

Answer 21

greater than 25mmol/l

Question 22

Explain the concept of the anion gap

Answer 22

The difference between the major cation (sodium) and the major anions (chloride and bicarbonate) which reflects the presence of unmeasured anions

Question 23

Why is the anion gap important in metabolic acidosis?

Answer 23

It can help determine the cause of the metabolic acidosis

Question 24

By how much should pC02 fall for every 1 mol/l fall in bicarbonate?

Answer 24

0.125kpa

Question 25

What factors can cause lactic acidosis?

Answer 25

Hypoperfusion leading to an increase in anaerobic metabolism and reduced hepatic clearance of lactic acid

Question 26

Give an example of a drug which can cause lactic acidosis?

Answer 26

Metformin

Aspirin

Question 27

What is the compensatory response to metabolic alkalosis?

Answer 27

Hypoventilation to increase pCO2

Question 28

What are the general causes of metabolic alkalosis?

Answer 28

Volume depletion - gastric acid loss (vomiting), diuretics
Volume repletion - mineralocorticoid, hyeraldosteronism, barter’s syndrome, Cushing’s syndrome, profound potassium depletion

Question 29

How doe shyperaldosteronism cause metabolic alkalosis?

Answer 29

Aldosterone promotes sodium reabsorption and sodium reabsorption promotes bicarbonate reabsorption to remain electroneutral. This can lead to an excess of bicarbonate

Question 30

What is the treatment for metabolic alkalosis?

Answer 30

Fluid resuscitation with physiological dextrose

Question 31

What acid base disturbance is caused by hypoventilation?

Answer 31

Respiratory acidosis

Question 32

What acid base disturbance is caused by hyperventilation?

Answer 32

Respiratory alkalosis

Question 33

How is respiratory acidosis compensated?

Answer 33

Retention of bicarbonate by the kidneys

Question 34

How is respiratory alkalosis compensated?

Answer 34

Secretion of bicarbonate ions by the kidney

Question 35

How does respiratory alkalosis lead to tetany?

Answer 35

This increases the binding of calcium to albumin, decreasing ionised calcium and this results in tetany

Question 36

Other than the production of carbon dioxide in respiration, what sources in the body produce acid?

Answer 36

Breakdown of nutrients to inorganic ions (especially from the breakdown of dietary proteins)
production of organic acids from intermediary metabolism e.g. lactic acid production

Question 37

What are the three lines of defence against changes in [H]?

Answer 37

Chemical buffer system
Respiratory control of pH
Renal control of pH

Question 38

Carbonic acid/bicarbonate is the major component of the chemical buffering system of the body. However, what other buffering systems exist?

Answer 38

haemoglobin buffering system
phosphate buffering system
protein buffering system

Question 39

Urine is normally acidic. T/F?

Answer 39

True

Question 40

What is the lowest possible pH of the urine?

Answer 40

4.5

Question 41

Hydrogen ions are produced within the kidney tubules for secretion from water and carbon dioxide. Where does this carbon dioxide come from?

Answer 41

Diffuses from the plasma or tubular fluid

From tubular cell metabolism

Question 42

In the PCT, how is hydrogen transported across the apical membrane for secretion into the urine?

Answer 42

By active transport with a H-ATPase or by secondary active transport with a sodium/hydrogen anti porter

Question 43

Describe the reabsorption of bicarbonate ions in the kidney tubules

Answer 43

Bicarbonate ions are not freely permeable to the apical tubule membrane so combine with secreted hydrogen ions under the influence of carbonic anhydrase on the apical membrane of the tubular cells to form carbon dioxide and water which diffuse into the cell. In the cell, carbonic anhydrase catalyses the conversion of carbon dioxide and water into hydrogen and bicarbonate ions. hydrogen ions are secreted into the lumen and bicarbonate ions can move into the plasma via a bicarbonate/chloride anti porter on the basolateral membrane

Question 44

When pH is low, less bicarbonate ions are filtered at the glomerulus as more bicarbonate ions are being used up in buffering the excess hydrogen in the plasma. How does this affect excretion of hydrogen in the urine?

Answer 44

Increased excretion of hydrogen in the urine as less of the secreted hydrogen is being use dup in reabsorbing bicarbonate