Acid-base regulation

Question 1

What are the 2 sources of CO2 from the body?

Answer 1

Lungs (aerobic metabolism) and Kidneys (metabolic processes)

Question 2

What are the 3 main mechanisms to minimise changes in pH? and which organs do these happen in?

Answer 2

Buffer systems
Lung (excretion of CO2)
Kidneys (excretion of H+ in urine altering HCO3- levels)

Question 3

What is the chemical equation catalysed by carbonic anhydrase

Answer 3

H+ + HCO3- ↔ H2CO3 ↔ H2O + CO2

Kidneys Lungs

Question 4

What is the Henderson-Hasselbalch Equation? and what is it used to calculate?

Answer 4

pH = pK + log 10 [HCO3-]/[CO2]

Calculate pH

Question 5

What is the constant in the Henderson-Hasselbalch Equation?

Answer 5

pK

Question 6

In response to change in pH, which organ provides a rapid response restore pH?

Answer 6

Lungs - rapid response (inc/dec ventilation)

Kidneys - slow response (alter HCO3- production and H+ excretion)

Question 7

To give a pH of 7.4 (normal), what is the ratio of [HCO3-] : [CO2]?

Answer 7

20:1

[HCO3-] : [CO2]

Question 8

What are the two main processes by which kidneys regulate extracellular fluid pH? Urine is usually acidic

Answer 8

1) Reabsorption of filtered HCO3-
2) Excretion of H+ (Production of new HCO3-)
loss of H+ is equivalent to gain of HCO3-

Question 9

How much H+ is excreted by kidneys into tubules?

Answer 9

70 - 100 mmol

Question 10

How do kidneys do reabsorption of filtered HCO3-?

Answer 10

Proximal convoluted tubule (85 - 90%)

Question 11

H+ is secreted in the late distal and collecting tubules, what transported are used and how is this activity stimulated?

Answer 11

Use H+ (H+/K+) ATPases in type A intercalating cells to pump H+ into tubular lumen.
Activity can be stimulated aldosterone and hypokalemia
H+ ATPase can generate an 800 fold H+ gradient (4.5 pH)

Question 12

What are the 2 main urinary buffers?

Answer 12

Phosphate and ammonia

Question 13

What are the 2 forms of filtered phosphate (give chemical form) that create a buffered pair in renal tubular fluid?

Answer 13

HPO42- (monoprotic)
H2PO4- (diprotic)
HPO42- + H+ ↔ H2PO4-

Question 14

How is H+ excreted by urinary phosphate buffer?

Answer 14

H+ is excreted in combination with NaHPO4-

H+ is excreted in the urine, HCO3- passes into the interstitial fluid

Question 15

Describe how urinary ammonia buffer works.

Answer 15

Ammonium (NH4+) is synthesised from glutamine mainly in PCT cells
Ammonia is secreted in collecting duct – ‘picks up’ excess secreted H+ and excretes it in urine as ammonium
Leads to production of HCO3-

Question 16

If there is a decrease in pH in the body, how will the urinary ammonia buffer work?

Answer 16

A decrease in pH stimulates renal glutamine metabolism leading eventually to increased H+ excretion (and vice versa)
(hence slower process in lungs due to metabolism)

Question 17

If a patient has respiratory acidosis, what could have caused it? How does the body compensate?

Answer 17

Any disorder affecting the lungs; chest wall; nerves, muscles; or CNS - reduction in ventilation
Compensate - Slowly (days) by kidney to increase the production of bicarbonate

Question 18

If a patient has respiratory alkalosis, what could have caused it? How does the body compensate?

Answer 18

Inappropriate increase in ventilation e.g. anxiety and hyperventilation; high altitude
Compensate - Slowly by kidneys to decrease the production of bicarbonate

Question 19

If a patient has metabolic acidosis, what could have caused it? How does the body compensate?

Answer 19

Addition of acid – exogenous (e.g. methanol) or endogenous (e.g. lactic or keto acids) – failure of H+ excretion or loss of HCO3- (e.g. severe prolonged diarrhoea)
Compensate - Rapidly by lungs to increase ventilation and thus decrease [CO2]

Question 20

If a patient has metabolic alkalosis, what could have caused it? How does the body compensate?

Answer 20

Addition of alkali; or excess loss of H+ (e.g. severe, prolonged vomiting); excess aldosterone e.g. due to dehydration (stimulates H+ secretion in distal tubule)
Compensate - Rapidly by lungs to decrease ventilation and thus increase [CO2]