Mechanisms of acid-base balance Flashcards

1
Q

What is the normal extracellular concentration of H+?

A

40 nmol/L

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2
Q

What is the normal plasma pH range?

A

7.35-7.45

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3
Q

Outside what range of plasma pH is considered incompatible with life?

A

6.8-7.8

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4
Q

What is the urine pH range?

A

4 - 8.5

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5
Q

What controls the PCO2?

A

Alveolar ventilation

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6
Q

What controls plasma HCO3- concentration?

A

Renal excretion of H+ and reabsorption of HCO3-

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7
Q

What are the two main buffers: intracellular and extracellular?

A

Intracellular: H2PO4- —–> H+ + HPO42-
Extracellular: H2CO3 —–> H+ + HCO3-

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8
Q

Describe where most of the buffering takes place in the four kinds of acid-base imbalance.

A

Metabolic Acidosis - 80-85% intracellular
Metabolic Alkalosis - 30-35% intracellular
Respiratory Acidosis + Alkalosis - almost ALL intracellular

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9
Q

What are the four sources of H+ ions in the body?

A

Physiological - carbohydrate and fat metabolism, sulfur containing amino acids
Pathological - hypoxia —-> anaerobic respiration —-> lactic acid
Or diabetes —–> ketoacids

Volatile acids - from carbohydrate and fat metabolism (results in CO2 production - CO2 is lost through respiration)
Non-Volatile acids - from protein metabolism (these H+ are excreted by the kidneys)

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10
Q

In which three regions of the nephron does renal H+ excretion take place?

A

Proximal convoluted tubule
Thick ascending limb of the loop of Henle
Outer medullary collecting duct

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11
Q

What transporters are involved in the renal excretion of H+ in the proximal convoluted tubule and in the collecting duct?

A

PCT
Na+/H+ exchanger : H+ secreted into lumen

HCO3-/Na+ cotransporters (basolateral membrane): bicarbonate ions are returned to systemic circulation

Collecting Duct
H+ ATPase
Cl-/HCO3- exchanger (basolateral membrane)

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12
Q

Describe the process of excretion of H+ and retention of HCO3-.

A

H+ goes out of the cell via a Na+/H+ exchanger
H+ reacts with HCO3- in the filtrate to form H2CO3
Carbonic anhydrase converts H2CO3 to H2O + CO2
H2O + CO2 are absorbed into the tubular cell and react to form H2CO3
Carbonic anhydrase in the cell converts H2CO3 —-> H+ + HCO3-
H+ moves out via the Na+/H+ exchanger
HCO3- is reabsorbed into the blood

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13
Q

Where does bicarbonate reabsorption take place and which areas reabsorb more bicarbonate?

A

80% takes place in the proximal convoluted tubule

20% takes place in the thick ascending limb of the loop of Henle and in the outer medullary collecting duct

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14
Q

State two primary stimuli for increase in H+ secretion.

A

Decrease in plasma bicarbonate concentration

Increase in PaCO2

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15
Q

State two secondary stimuli for an increase in H+ secretion.

NOTE: secondary means that it is not directed at maintaining acid-base balance

A
Angiotensin II secretion 
Aldosterone secretion 
Decrease ECF volume 
Hypokalaemia 
Increase in filtered load of bicarbonate
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16
Q

How are the stimuli for a decrease in H+ secretion different?

A

They are the opposite

17
Q

Describe bicarbonate production.

A

Amino acids are broken down in the liver to produce glutamine and urea
Glutamine is taken to the kidneys and broken down to produce ammonium and alpha ketoglutarate.
Alpha-ketoglutarate is then converted to HCO3-.

18
Q

What happens to the ammonia produced in the tubular cells from the breakdown of glutamine?

A

It moves into the tubular fluid either as ammonium or as NH3 + H+
Further down the nephron, H+ will enter the tubular fluid and react with the NH3 to form NH4+

19
Q

What could cause metabolic acidosis?

A

Low pH and bicarbonate.

Addition of non-volatile acids (ketoacidosis in diabetes)
Loss of non-volatile alkalis (diarrhoea)
Failure to reabsorb sufficient HCO3- (renal failure)

20
Q

What is the most common cause of metabolic alkalosis?

A

Loss of non-volatile acids (e.g. vomiting)

raised aldosterone.

21
Q

What two things can the kidney change to compensate for respiratory acidosis or alkalosis?

A

NH4+ excretion and HCO3- reabsorption

Respiratory acidosis: increasing bicarbonate and ammonium secretion