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Urinary System > Acid Base Balance - Pt 1 > Flashcards

Acid Base Balance - Pt 1 Flashcards

1
Q

Describe acid base balance

A

Metabolic reactions are exquisitely sensitive to pH of fluid in which they occur
Relates to high reactivity of H+ ions with protons

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2
Q

What does acid/ base disturbances lead to?

A

All sort of metabolic disturbances
pH of ECF is closely regulated

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3
Q

What is the pH of arterialised blood?

A

Around 7.4 so free H+ conc. of 40 x 10-9moles/l or 40 x10-6mmoles/l

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4
Q

What H+ ions contribute to pH?

A

Only free H+ ions
H+ conc. is one millionth that of other plasma constituents
Body produces H+

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5
Q

What are the sources of H+?

A

Respiratory acid
Metabolic acid via metabolism

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6
Q

Describe source of H+ ions from respiratory acid

A

CO2 + H2O - H2CO3 - H+ + HCO3
Formation of carbonic acid is not usually a net contribution to increase acid because any increase in production leads to increase ventilation

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7
Q

What are the sources of H+ in metabolic acid?

A

Inorganic acids - S containing amino acids
Organic acids - fatty acids and lactic acid (normal diet - 50-100mmoles of H+)

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8
Q

What are buffers?

A

Minimise changes in pH when H+ ions are added or removed

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9
Q

Describe the Henderson-Hasselbalch equation

A

Defines the pH in terms f the ration of A-/HA not the absolute amounts
pH = pK + log A-/HA

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10
Q

What is extracellular buffers?

A

Most important bicarbonate buffer system
Quantity of H2CO3 depends on amount o CO2 dissolved in plasma, depends on solubility of CO2 and PCO2

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11
Q

What is the ratio of (HCO3)/(H2CO3) in blood at 7.4?

A

20:1

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12
Q

What is the solubility of CO2 in blood at 37 degrees?

A

0.03 mmoles/l/mmHg

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13
Q

What is the normal PCO2?

A

40mmHg
5.3kPa

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14
Q

What is standard bicarbonate?

A

24mmoles/l
At normal PCO2 40mmHg H2CO3 conc. is 40 x 0.03mmoles/l x 0.225mmoles/l = 1.2 mmoles/l
20:1 ratio so 2.4 mmoles/l

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15
Q

What is the simple Henderson Hasselbalch equation?

A

pH is proportional to HCO3-/PCO2

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16
Q

What does increase H+ in ECF lead to in equation?

A

Drives reaction to right so additional H+ are removed from solution so therefore a change in pH is reduced

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17
Q

What happens when reaction is driven to the right?

A

Increases production of CO2 which increases ventilation and decreases CO2
Greatly increases the buffering capacity of bicarbonate

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18
Q

Does H+ get eliminated from the body?

A

No - instead bicarbonate has buffered the H+ ions and respiratory has compensated greatly so free H+ ions are prevented from contributing to pH

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19
Q

What happens if there is a decrease in H+ in ECF?

A

Pulls reaction to left - decreases CO2 which decreases ventilation so increases CO2

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20
Q

What is the aim of the acid base reaction?

A

Protecting arterial pH

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21
Q

How is H+ ions eliminated from the body?

A

By the kidneys
This excretion is couples to the regulation of plasma HCO3- conc.

22
Q

How is plasma conc. of HCO3- regulated?

A

Renal regulation

23
Q

What are the other buffers in the ECF?

A

Plasma proteins Pr- + H+ - HPr
Diabetic phosphate HPO42- + H+ - H2PO4 monobasic phosphate

24
Q

Describe intracellular buffers

A

Primary - proteins, organic and inorganic phosphates, and haemoglobin (erythrocytes)
Bone carbonate provides additional source of buffer - chronic acid loads in chronic renal failure

25
Q

What does buffering of H+ by ICF cause?

A

Changes in plasma electrolytes so maintain neutrality - movement of H+ is accompanied by Cl- as in red cells or exchanged for K+

26
Q

In acidosis, what does movement of K+ out of cells into plasma lead to?

A

Depolarisation of excitable tissues leading to ventricular fibrillation and death
Increase in H+ leads to hyperkalaemia

27
Q

Why are buffers incredibly important?

A

50-100 mmoles of H+ from diet and if present as free H+ in body water then pH of 1.2-2.4
H+ is successfully buffered until kidney excretes it

28
Q

In metabolic acid, how much is buffered in plasma and cells?

A

43% in plasma - primarily with HCO3-
57% in cells

29
Q

In respiratory acid, how much is buffered in plasma and cells?

A

97% in cells
Hb particularly important, rest within plasma proteins

30
Q

How does the kidney regulate conc. of HCO3-?

A

Reabsorbing filtered HCO3-
By generating new HCO3-

31
Q

What do the processes of kidney regulation of HCO3- depend on?

A

Both depend on active H+ ion secretion from tubule cells into lumen

32
Q

What is the mechanism of reabsorption of HCO3-?

A

Active H+ secretion from tubule cells coupled to passive Na+ reabsorption
Filtered HCO3- reacts with secreted H+ to form H2CO3
In presence of carbonic anhydrase on luminal membrane - CO2 and H2O
CO2 freely permeable and enters cell

33
Q

During the mechanism of reabsorption of HCO3-, what happens to CO2 in cell?

A

CO2 makes H2CO3 in presence of carbonic anhydrase which dissociates to form H+ and HCO3-
H+ ions are source of secreted H+ ions
HCO3- passes into peritubular capillaries with Na+

34
Q

Where does the bulk of HCO3- reabsorption occur?

A

In proximal tubule - 90%

35
Q

What is the importance of HCO3- reabsorption?

A

GFR = 180l/day
HCO3- conc. = 24mmoles/l
4320mmoles HCO3- filtered a day
Must be reabsorbed as failure = adding H+ to ECF
No excretion of H+ ions

36
Q

What is the minimum and maximum urine pH?

A

Min. = 4.5-5
Max. = 8
Usually net production of 50-100mmoles of H+ a day

37
Q

What happens if free H+ ions in urine volume of 1l?

A

pH of 1
But they are buffered

38
Q

What acts as buffers in urine?

A

Weak acids and bases - mostly dibasic phosphate HPO42-, also uric acids and creatine

39
Q

Why is the process called titratable acidity?

A

As its extent is measured by amount of NaOH needed to titrate urine pH back to 7.4

40
Q

What is the importance of titratable acidity?

A

Generates new HCO3- and excretes H+
Only used for acid loads

41
Q

Describe titratable acidity in distal tubule

A

Na2HPO4 in lumen - Na reabsorbed in exchange for secreted H+
Monobasic phosphate removes H+ from body
Source of new HCO3- is indirectly CO2 from blood - CO2 enters tubule cell
New HCO3- passes with Na into peritubular capillaries

42
Q

How is new HCO3- made in distal tubule cells?

A

CO2 enters tubule cells and combine with H2O to form carbonic acid
Carbonic anhydrase - dissociates H2CO3 into H+ and HCO3-
H+ for secretion and HCO3- leaves with Na

43
Q

Where does titratable acidity mainly occur?

A

Distal tubule - phosphate ions are not reabsorbed by proximal tubule Tm mechanism
They become greatly concentrated as removal of 95% initial filtrate

44
Q

What is the titratable acidity process dependant on?

A

PCO2 in blood
Generation of new HCO3- and net excretion of H+

45
Q

What is the response of ammonium excretion?

A

Adaptive response to acid load - generates new HCO3- and excretes H+
Only used for acids loads

46
Q

Describe the process of ammonium excretion

A

NH3 is lipid soluble but NH4+ is not
NH3 moves out into tubule lumen where combines with excreted H+ to form NH4+
Combines with Cl- ions to form NH4Cl which is excreted (distal)

47
Q

What is the source of H+ in ammonium excretion?

A

CO2 in blood

48
Q

What is the difference in distal and proximal tubule ammonium excretion?

A

Proximal - there is an NH4+/Na+ exchanger so NH4+ ions formed within cells are passed into lumen so net effect is the same

49
Q

What happens to new HCO3- in ammonium excretion?

A

Passes into peritubular capillaries with Na ions

50
Q

What is NH3 produced by?

A

Deamination of amino acids, primarily glutamine, by action of renal glutaminase within renal tubule cells

51
Q

Describe the activity of renal glutaminase

A

pH dependant
When intracellular pH falls there is increase in renal glutaminase activity so more NH4+ produced and excreted

52
Q

What is the main adaptive response of kidney to acids loads?

A

Ability to augment NH4+ production
Takes 4-5 days to reach maximal effect because requirements of increased protein synthesis
Takes time to switch off ability to make NH4+ when excess alkali

Urinary System (26 decks)

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