Acid-Base Homeostasis

Question 1

What is the total CO2 produced in a day on average?

Answer 1

25mol/day

Question 2

Name 6 buffering systems in the body?

Answer 2

1) Hb
2) Bicarbonate
3) Phosphate
4) Proteins
5) Ammonia
6) Misc organic acids

Question 3

What is the reference range for blood pH?

Answer 3

7.45-7.35

Question 4

What is the reference range for plasma [H+]?

Answer 4

35-45mmol/L

Question 5

What is the Henderson-Hesselbalch equation and whatr does it show?

Answer 5

pH = pK + log([HCO3]/[pCo2}xa)
Says the pH and thus [H+] is determined by the balance between H+ and HCO3 so [H+] homeostasis requires a balance between H+ production and regeneration of HCO3-

Question 6

Give 4 sites of acid-base metabolism?

Answer 6

Lungs
Kidneys
GI tract
Liver

Question 7

Which are the 2 main sources of excretion of acid from the body?

Answer 7

Lungs (as CO2)

Kidneys (H+)

Question 8

Describe how reactions of CO2 within the RBC cause O2 to be released to tissues in tissue gas exchange?

Answer 8

CO2 is released from the tissues and is non polar so passes straight across the RBC membrane
CO2 reacts within the RBC to become H+ and HCO3-
H+ sticks to the Hb molecule (a protein), this acidifies the protein causing it to change shape and release the O2
Thus O2 is released from the RBC to the tissues

Question 9

Describe how reactions within the RBC cause O2 to be taken up and CO2 to be released by RBC’s at the lungs?

Answer 9

More O2 outside the RBC enters the RBC
The O2 binds to the Hb molecule causing H+ to be released which binds to HCO3- to form H20 and CO2 which is released into the lungs

Question 10

Name the 3 things which cause a right shift of the O2-Hb dissociation curve?

Answer 10

You get a right shift (O2 released more readily) with:

1) Increased temp
2) Acidosis
3) Increased 2,3 DPG

Question 11

Which 2 buffering systems are considered to be most important in the body?

Answer 11

1) Bicarbonate

2) Hb

Question 12

What role does the kidneys play in acid-base homeostasis (3 mechanisms)?

Answer 12

1) Renal reclamation of bicarbonate
2) Renal generation of bicarbonate
3) Mineralocorticoid action in kidney in excreting H+

Question 13

What is the mechanism of renal reclamation of bicarbonate?

Answer 13

1) Bicarbonate is a small molecule so is filtered out of the blood into the renal tubule
2) Its combined with H+ pumped into the renal tubule in exchange for Na+ to form CO2 and H2O
3) CO2 is reabsorbed and converted back into bicarbonate

Question 14

What is the mechanism for renal generation of bicarbonate?

Answer 14

Renal tubular cells produce bicarbonate and in doing so produce H+ which is lost in the urine

Question 15

How does mineralocorticoid action in the kidney affect acid-base homeostasis?

Answer 15

Occurs in the distal tubules
Under endocrine control via the hormone aldosterone
Either K+ or H+ can be excreted from the kidneys in exchange for Na+
If [H+] is high then H+ is excreted under the control of aldosterone

Question 16

What role does the GI tract have in acid base homeostasis?

Answer 16

Stomach excretes acid, this needs to be balanced by alkaline excretion
Bicarbonate is excreted in pancreatic juices to neutralise this acid, this has to be balanced to prevent disturbances

Question 17

What role does the liver have in acid-base homeostasis?

Answer 17

1) Dominant site of lactate metabolism

2) Only sight of urea synthesis

Question 18

What role does the urea cycle have in acid-base homeostasis?

Answer 18

1) Proteins catabolised to NH4+ and HCO3-
2) The average protein produces an excess of NH4+
3) The role of the urea cycle is to combine NH4+ and CO2 to neutralise HCO3-
HCO3- can be converted to H+ and Urea, this is inhibited by acidosis
NH4+ is converted to NH3 which is excreted in the urine (loses a H+) and this is stimulated by alkalosis

Question 19

How can severe liver failure lead to metabolic alkalosis and NH4+ toxicity?

Answer 19

Because these 2 reactions do not occur:

1) NH4+ + oxo-glutarate -> glutamine
2) NH4+ + CO2 -> urea + H+

Question 20

Which are the 2 important reversible reactions involved in acid-base homeostasis?

Answer 20

CO2 HCO3- + H+
2NH4+ + HCO3- (NH2)2CO2 + H+
acidosis alkalosis

Question 21

What are the 2 kind of acid base states that can occur?

Answer 21

Metabollic acidosis/alkalosis

Respiratory acidosis/alkalosis

Question 22

What are the 3 main compensatory mechanisms for acidosis/alkalosis?

Answer 22

1) Respiratory
2) Renal bicarbonate regeneration
3) Hepatic shift between urea synthesis and ammonia excretion

Question 23

What are the 5 possible causes of metabolic acidosis?

Answer 23

1) Increased H+ formation
2) Acid ingestion
3) Reduced renal H+ excretion
4) Loss of bicarbonate
5) Inability to excrete H+ through expiration of CO2

Question 24

What happens to H+ levels, pCO2 and pO2 in metabolic acidosis?

Answer 24

H+ increases (because its acidosis)

pCO2 decreases and pO2 increases (respiratory compensation causes hyperventilation)

Question 25

Give 3 possible basic reasons for metabolic alkalosis?

Answer 25

1) Generation of bicarbonate by gastric mucosa
2) Renal generation of HCO3- in hypokalaemia
3) Administration of bicarbonate

Question 26

What happens to H+ levels, pCO2 and pO2 in metabolic alkalosis?

Answer 26

H+ decreases (because its alkalosis)

pCO2 increases and pO2 decreases (breathing rate decreased as respiratory compensation so blow off less CO2)

Question 27

What are the 3 main consequences of metabolic alkalosis?

Answer 27

1) K+ moves into the cells and urine
2) PO4 moves into cells
3) Respiratory suppression

Question 28

What acid-base disorder does excessive vomiting lead to?

Answer 28

Metabolic alkalosis

Question 29

What kind of acid-base disorder to ectopic ACTH lead to and why?

Answer 29

Metabolic alkalosis
Causes body to produce too much cortisol from the adrenals which stimulates the mineralcorticoid receptors which leads to loss of potassium and H+ and absorption of Na+

Question 30

Give 3 examples of conditions which lead to respiratory acidosis?

Answer 30

CO2 retention due to:

1) inadequate ventilation
2) parenchymal lung disease
3) inadequate perfusion

Question 31

What happens to the levels of H+ and pCO2 and pO2 in respiratory acidosis?

Answer 31

increased H+ (because its acidosis)
increased pCO2 (its Co2 retention which is leading to the acidosis)
pO2 decreased because of reduced ventilation

Question 32

What causes respiratory alkalosis?

Answer 32

Increased CO2 excretion due to excessive ventilation producing alkalosis

Question 33

What happens to the levels of H+, pCO2 and pO2 in respiratory alkalosis?

Answer 33

H+ decreases because its alkalosis
pCO2 decreases (its the excessive excretion leading to alkalosis)
pO2 increases because of hyperventilation

Question 34

Give 4 possible basic causes of metabolic acidosis?

Answer 34

1) Increased H+ formation
2) Acid ingestion
3) Reduced renal H+ excretion
4) Loss of bicarbonate

Question 35

Give 4 possible reasons for increased H+ formation leading to metabolic acidosis?

Answer 35

1) Ketoacidosis, diabetic or alcoholic
2) lactic acidosis
3) poisoning
4) inherited organic acidoses

Question 36

Give the 4 mechanisms leading to acidosis in diabetic keto-acidosis?

Answer 36

1) Hyperglycaemia
2) osmotic diuresis -> to pre-renal uraemia
3) hyperketonaemia
4) increase FFA
All 4 lead to acidosis

Question 37

Give the 2 causes of lactic acidosis?

Answer 37

Type a = shock

Type b = metabolic and toxic causes

Question 38

How does acidosis occur in an alcoholic?

Answer 38

Get NAD+ depletion and thiamine deficiency
Enhanced glycolysis for ATP formation
Keto-acids to counter regulatory hormones
Profuse vomiting

Question 39

Can high lactate occur in alkalosis?

Answer 39

Yes
Get increased glycolysis
Reduced O2 delivery due to shift in O2 dissociation curve
Lactate induced vasoconstriction
Impaired mitochondrial respiration
Also get a O2 debt leading to further anaerobic lactate production leading to hyperventilation

Question 40

Give 2 reasons for reduced H+ excretion?

Answer 40

1) Renal tubular acidosis

2) Generalised renal failure

Question 41

What 4 steps lead to metabolic acidosis in renal failure?

Answer 41

Due to reduced volume of nephrons:

1) Increased bicarbonate loss
2) Reduced NH4+ excretion
3) NH4+ to liver for urea and H+ synthesis
4) Only fraction of NH4+ derived from glutamine (normally approx. 100%)