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RESP > Lecture 8: Control of plasma pH > Flashcards

Lecture 8: Control of plasma pH Flashcards

1
Q

What are the 2 major organ systems involved in the maintenance of acid-base homeostasis?

A

Respiratory system: lungs are rapid responders (change breathing)
-primarily controls CO2
Urinary system: kidneys help with longer term homeostasis (full compensation could take days)
-primarily controls HCO3-
Both essential

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2
Q

What is plasma pH regulated by?

A

Very low concentration of H+ ions, but these are tightly regulated
-44.5-35.5 nmol/l
(pH 7.35-7.45)

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3
Q

Which is more dangerous: alkalosis or acidosis?

A

Alkalosis

  • lowers free calcium by causing Ca2+ ions to come out of solution and bind elsewhere
  • this increases neuronal excitability (bivalent cations: charge sheilding, which protects the membranes from becoming too excitable)
  • leads to paraesthesia and tetany

45% mortality: pH rises to 7.55
80% mortality: pH rises to 7.65

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4
Q

What is the problem with acidosis?

A
  • increase in plasma potassium ion conc
  • this effects excitability: cause arrythmias
  • increase in H+ ions also effects enzymes: denatures proteins and effects muscle contractility, glycolysis, hepatic function

Effects severe below pH 7.1
Life threatening below pH 7.0

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5
Q

How do the lungs and kidneys work together to control plasma pH?

A

Kidneys: control pH- variable recovery of HCO3- and active secretion of H+ ions

Lungs: alveolar ventilation controls pO2 and pCO2, and the rate of ventilation is controlled by chemoreceptors

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6
Q

Where is HCO3- made?

A

HCO3- is made in the RBC’s
-conc is controlled by the kidneys

Normal conc in arterial blood is: 25 mmol/l
Range: 22-26
But can be changed to maintain pH

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7
Q

What do we produce due to metabolism?

A

Acid in the form of H+ ions (continuous production)

  • react with HCO3- to form CO2 and water
  • HCO3- does not deplete because:
  • the kidneys recover all filtered HCO3-
  • proximal tubule makes HCO3- from AA’s, putting NH4+ into the urine
  • distal tubule makes HCO3- from CO2 and H2O, and the H+ is buffered by phosphate and ammonia in the urine
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8
Q

What is the renal control of HCO3- in the proximal tubule?

A
  • HCO3- is filtered at the glomerulus
  • most is recovered in the PCT
  • CO2 and H2O in the tubular cell as they are metabolically active and these are converted to H+ and HCO3- in the cell, creating high conc of H+ for the Na/H exchanger
  • the HCO3- are pumped into the ECF via the Na/HCO3- symporter
  • Na/K ATPase on basolateral side of tubular cell pumps Na+ out and K+ in
  • this creates a sodium gradient allowing the Na/H exchanger to pump Na into the cell and H+ out of the cell
  • H+ reacts with HCO3- in the lumen to form H2O and CO2
  • CO2 diffuses into the tubular cell

Via this 80% of HCO3- is recovered

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9
Q

How is HCO3- created in the proximal tubule?

A
  • glutamine is converted to NH4+ and alpha-ketoglutarate
  • ammonium (NH4+) dissociates into NH3 (ammonia) and H+
  • NH3 is uncharged so can freely diffuse out of the cell into the lumen
  • in lumen it combines with the H+ from the Na/H exchanger to form NH4+, which buffers the H+ ions
  • alpha-ketoglutarate breaks down into two HCO3- which are moved out into ECF via Na/HCO3- symporter
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10
Q

What happens to HCO3- in the distal tubule and where does this occur?

A

Occurs in the alpha-intercalated cells

  • CO2 and H2O react in the cell to form H+ and HCO3-
  • distal tubule and collecting ducts secrete H+ produced from reaction of CO2 and H2O
  • these are actively secreted via H+ ATPase
  • H+ is buffered by ammonia (this can freely move from the PCT) and phosphate to produce NH4+ and H2PO4- which are excreted as they are charged (trapped in lumen)
  • HCO3- leaves via HCO3-/CL- exchanger on basolateral membrane
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11
Q

Why do we excrete ammonium in our urine?

A

Major adaptive response to an increased acid load in healthy individuals due to metabolism
-ammonium generation from glutamine in PCT can be increased in response to low pH

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12
Q

What is the minimum pH of urine and why doesn’t it get any lower than that?

A
  1. 5
    - not lower due to the buffering system (ammonium and hydrogen phosphate buffering systems)
    - no HCO3- in urine
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13
Q

How much H+ do we excrete per day?

A

50-100 mmol

-needed to keep HCO3- normal

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14
Q

What are reciprocal ion shifts?

happens in cells throughout the body

A 
Acidosis: 
-K+ moves out of the cells 
-H+ move into the cell
=hyperkalaemia and decreased K+ excretion in distal 
nephron

Alkalosis:
-K+ moves into cells
-H+ move out of cells
=hypokalaemia and enhanced excretion of K+ in distal nephron

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15
Q

How does hyperkalaemia disturb the acid base balance?

A

Hyperkalaemia makes intracellular pH of tubular cells more alkaline
-H+ ions moves out of cell as K+ moves into cell down its conc gradient (this tends to favour HCO3- excretion)
=metabolic acidosis

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16
Q

How does hypokalaemia disturb the acid base balance?

A

Hypokalaemia makes the intracellular pH of tubular more acidic
-H+ ions move into cells
-this favours H+ excretion and HCO3- recovery
=Metabolic alkalosis

17
Q

What is respiratory acidosis?

A

-hypoventilation
-hypercapnia (high pCO2)
-causes a fall in plasma pH
(normal HCO3-)

18
Q

What is respiratory alkalosis?

A

-hyperventilation
-hypocapnia (fall in pCO2)
-rise in pH
(normal HCO3-)

19
Q

What is compensation?

A

Plasma pH depends on the ratio of HCO3- to pCO2
-therefore changes in pCO2 can be compensated by changes in HCO3-

-kidneys increase HCO3- to compensate for respiratory acidosis
-kidneys decrease HCO3- to compensate for respiratory alkalosis
This takes time (2-3 days)

20
Q

What is compensatory respiratory acidosis characterised by?

A
  • high pCO2
  • raised HCO3-
  • normal pH
21
Q

What is compensatory respiratory alkalosis characterised by?

A
  • low pCO2
  • lowered HCO3-
  • normal pH
22
Q

What is metabolic acidosis?

A
  • tissues produce acid abnormally, this reacts with and removes HCO3-, producing CO2 and water
  • CO2 breathed off at lungs so there is no increase in arterial pCO2
  • fall in HCO3-
  • fall in pH
23
Q

What is the anion gap?

A

Used to determine the cause of metabolic acidosis (measured in an ABG)
-difference between measured cations and anions
((Na+) + (K+)) - ((Cl-) + (HCO3-))
-normally 10-18 mmol/l because there are other anions present

24
Q

When would you get an increase in the anion gap?

A

If metabolic acid reacts with HCO3-, the anion of the acid replaces the HCO3-, and this is not a measured anion, therefore the anion gap increases.

If the HCO3- is replaced by Cl- ions, then you won’t get a change in the anion gap
(in renal causes of acidosis the anion gap will remain unchanged, as you aren’t making enough HCO3-, but this is replaced by Cl-)

25
Q

What is compensated metabolic acidosis characterised by?

A

Peripheral chemoreceptors detect drop in pH
-stimulate ventilation leading to decreased pCO2

  • low HCO3-
  • low pCO2
  • near normal pH
26
Q

What is metabolic alkalosis?

A

If the HCO3- increases

  • normal pCO2
  • raised HCO3-
  • increased pH

Can’t normally be compensated to a great extend by reduced breathing because we need to maintain pO2
-easier for kidney to correct by losing more HCO3-

27
Q

Name some conditions that lead to respiratory acidosis:

A

Type 2 respiratory failure
-low pO2 as not properly ventilating the alveoli
-high pCO2
e.g. severe COPD, severe asthma, drug overdose, neuromuscular disease
= can be compensated for by increase in HCO3- (chronic conditions can be well compensated)

28
Q

Name some conditions that lead to respiratory alkalosis:

A

Hyperventilation

  • anxiety/panic attacks (acute setting)
  • causes low pCO2, and a rise in pH

Hyperventilation due to type 1 respiratory failure (not getting enough oxygen, but ventilation is normal)

  • low pCO2 with initial rise in pH
  • chronic hyperventilation can be compensated for by reducing the amount of HCO3-
29
Q

What conditions lead to metabolic acidosis with an increased anion gap?

A

Anion gap is increased (indicates a production of a metabolic acid)

  • keto-acidosis in diabetes
  • lactic acidosis due to exercise exhaustion or poor tissue perfusion
  • uraemic acidosis due to advanced renal failure
30
Q

What conditions lead to metabolic acidosis with a normal anion gap?

A

HCO3- is replaced with Cl-

Renal tubular acidosis (rare)
-problems with transport mechanisms in tubules
Type 1: inability to pump out H+ (distal)
Type 2: problems with HCO3- reabsorption (proximal)

Severe persistent diarrhoea
-due to loss of HCO3-, which is replaced by Cl-

31
Q

How does non-renal metabolic acidosis effect potassium?

A
  • cause increased reabsorption of K+ by the kidneys
  • causing hyperkalaemia

(in diabetic ketoacidosis there may be a total body depletion of K+: K+ moves out the cells due to acidosis and lack of insulin, but osmotic diuresis means K+ is lost in the urine)

32
Q

What conditions lead to metabolic alkalosis?

A

Stomach is major site of HCO3- production

  • it is the byproduct of H+ secretion
  • severe prolonged vomiting-lose H+ ions
  • this means the acidic chyme doesn’t enter the duodenum so the HCO3- isn’t released to neutralise it, so it stays in the blood
33
Q

What is metabolic alkalosis correct by?

A

-fall in H+ excretion
-reduction in HCO3- recovery
BUT
-problem if there if also volume depletion
-capacity to lose HCO3- is reduced due to high rate of Na+ recovery, because recovering Na+ favour H+ secretion and HCO3- recovery

34
Q

How does metabolic alkalosis effect potassium?

A

-less H+ excretion in nephron leads to more K+ excreted
-alkalosis also causes movement of K+ into cels
=hypokalaemia

35
Q

What is it when HCO3- is low, pCO2 is low and pH is normal?

A

Could be compensated metabolic acidosis or compensated respiratory alkalosis
To see which one it is (you can usually tell by the pH)
-check anion gap (if change it is metabolic acidosis)
-onyl resp alkalosis if resp disease or altitude exposure

36
Q

What does afebrile mean?

A

No fever

37
Q

What does vomiting and diarhhoea lead to?

A

Vomiting: alkalosis (H+ loss)
Diarrhoea: acidosis (HCO3- loss)

RESP (18 decks)

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