Session 5: Acid-Base Balance Flashcards
Give the range of normal plasma pH
7.35-7.45
Above what plasma pH is alkalaemia?
7.45
Below which plasma pH is acidaemia?
7.35
Which condition is more life threatening: alkalaemia or acidaemia?
Alkalaemia
If pH rises to 7.55 what is the mortality?
45%
If pH rises to 7.65 what is the mortality?
80%
How does alkalaemia increase neuronal excitability?
Lowers free calcium by making Ca2+ come out of solution
What are the two major symptoms of alkalaemia?
Parasthesia
Tetany
How does acidaemia cause arrhythmia?
Increased plasma potassium ion concentration
How does acidaemia affect enzymes?
Proteins damaged by increased [H+]
Below what pH is considered life threatening?
7.0
How do the kidneys control pH?
Vary the recovery of HCO3- and actively secrete H+ ions
How do the lungs control pO2 and pCO2?
Altering rate of ventilation
What organ controls the [HCO3-]?
The kidneys
What process produces acid in the body?
Metabolism
In which region of the kidney is HCO3- mostly recovered?
PCT
Which membrane is carbonic anhydrase located on in the tubular cells?
Apical membrane
What role does the Na+K+ ATPase have in renal recovery of HCO3-?
Sets up a Na+ concentration gradient across the basolateral membrane
What does the HCO3-Na+ transporter do?
Transports 3 HCO3- and an Na+ ion out into the ECF
How can HCO3- be created in the kidneys?
Breakdown of glutamine to α-ketoglutarate
What else apart from HCO3- is released from breakdown of amino acids?
Two NH4+ ions
Why does NH4+ get ‘trapped’ in the lumen?
It is charged so cant cross the membrane
What two compounds act to buffer H+ in the kidneys?
Ammonia and phosphate
Why is H+ produced in the kidneys?
Reaction between CO2 and water
How does a fall in pH affect ammonium production?
Increases it
What buffering system is mainly used in the proximal tubule?
Ammonium
What buffering system is used in the distal tubule of the kidney?
Phosphate
Where is ammonium mostly created?
In the PCT
What is the minimum safe pH of urine?
4.5
What is the range of average total acid secretion a day?
50-100mmol
How does acidosis cause hyperkalaemia?
Causes K+ to move out of the cells and so less is excreted at the distal nephron
How does alkalosis cause hypokalaemia?
Cause enhanced excretion of K+ at the distal nephron as it moves into the cells
How is metabolic alkalosis related to hypokalemia?
H+ moves into the tubular cells and lowers the pH
How does hyperkalaemia cause metabolic acidosis?
H+ ions move out of the cells and into the blood
What is respiratory acidosis characterised by?
Low pH
High pCO2
Normal HCO3-
How will hyperventilation affect pH?
Causes a rise in pH due to hypocapnia
What is respiratory alkalosis characterised by?
Raised pH
Low pCO2
Normal HCO3-
How will the kidneys respond to compensate for respiratory acidosis?
Increase [HCO3-]
How is compensated respiratory acidosis characterised?
High pCO2
Raised [HCO3-]
Relatively normal pH
How is compensated respiratory alkalosis characterised?
Low pCO2
Lowered [HCO3-]
Relatively normal pH
Why is metabolic acidosis different to respiratory acidosis?
There is no change in pCO2 but a fall in [HCO3-] as it reacts with the acid produced
How do you calculate the anion gap?
([Na+]+[K+]) - ([Cl-]+[HCO3-])
Give the normal range for the anion gap
10-18mmol/L
When will the anion gap increase?
When HCO3- is replaced by other anions
Why do renal causes of acidosis not change the anion gap?
Cl- replaces the fall in HCO3- production
What are the characteristics of metabolic acidosis?
Low pH
Low HCO3-
Normal pCO2
What is compensated metabolic acidosis characterised by?
Near normal pH
Low pCO2
Low HCO3-
What are the characteristics of metabolic alkalosis?
Raised pH
Raised HCO3-
Normal pCO2
Why can’t a reduction in breathing compensate for metabolic alkalosis?
Need to maintain pO2
What conditions can lead to respiratory alkalosis?
Type 1 respiratory failure
Type 2 respiratory failure
Hyperventilation
What is type 1 respiratory failure?
Hyperventilation in response to long term hypoxia
What happens to the pCO2 in type 2 respiratory failure?
Increases
How does pCO2 change in type 1 respiratory failure?
Falls
How is chronic hyperventilation compensated for?
Drop in [HCO3-]
What does an increase in the anion gap indicate?
Metabolic production of an acid
Give three ways in which there may be metabolic production of an acid
Ketoacidosis
Lactic acidosis
Uraemic acidosis
What is type 1 renal tubular acidosis?
Inability to pump out H+ (affects distal tubule)
What does type 2 renal tubular acidosis cause?
Problems with HCO3- reabsorption
How will the anion gap be changed in renal tubular acidosis?
Unaltered
How can severe persistent diarrhoea lead to metabolic acidosis and why is there no change to the anion gap?
Loss of HCO3- but this is replaced by Cl-
How do non renal causes of metabolic acidosis affect K+ at the kidneys?
Increases their reabsorption
Why is there a total body depletion of K+ in diabetic ketoacidosis?
K+ moves out of cells and is lost in urine due to osmotic diuresis
What effect does insulin have on movement of K+?
Causes it to move into the cells of the tubule
What may severe prolonged vomiting lead to and why?
Metabolic acidosis because of loss of H+
How is volume depletion avoided in metabolic acidosis?
Na+ H+ exchanger continues to operate (Na+ recovered and H+ excreted)
