Acid-base balance Flashcards
What is normal plasma pH?
7.35 - 7.45
What is the relative concentration of hydrogen ions in the plasma with a pH of 7.35 - 7.45?
Low concentration of hydrogen ions
What are the three systems involved in controlling plasma pH?
Bicarbonate buffer system
Respiratory system
Kidneys
How is the bicarbonate buffer system set up?
Dissolved carbon dioxide in red blood cells and in plasma
carbon dioxide + water bicarbonate ion + hydrogen ion
What does the direction of the bicarbonate buffer reaction depend on?
The concentrations of reactants and products
How do the concentrations of reactants and products affect the direction of the bicarbonate buffer reaction?
Higher concentration of reactants favours forwards direction
Higher concentration of products favours backwards direction
What is the concentration of bicarbonate ions in the plasma?
25mmol/L
Why is the concentration of bicarbonate ions in the plasma so high?
Because in red blood cells
hydrogen ions bind to haemaglobin, removing them from solution, reducing their concentration
favours forward reaction
more bicarbonate ions produced
more bicarbonate ions transported into plasma
Why do hydrogen ions bind to haemaglobin?
Because hydrogen ions are positively charged and haemaglobin is negatively charged
What is the concentration of carbon dioxide in the plasma?
1.2mmol/L
How does the higher concentration of bicarbonate ions than carbon dioxide in the plasma affect the bicarbonate buffer reaction?
Favours backwards reaction
How does the backwards reaction of the bicarbonate buffer reaction affect plasma pH?
Fewer hydrogen ions produced, more hydrogen ions used up
increases plasma pH to slightly alkaline
What controls the concentration of bicarbonate ions in the plasma?
The kidneys
How do the kidneys control the concentration of bicarbonate ions in the plasma?
Variable reabsorption, production and excretion
What does the concentration of carbon dioxide in the plasma depend on?
pCO2 of plasma
What does the pCO2 of plasma depend on?
pCO2 of alveoli
Metabolism producing CO2
What does the pCO2 of alveoli depend on?
Alveolar ventilation
What is the equation for calculating pH?
Henderson Hasselbach equation
What is the Henderson Hasselbach equation for plasma pH?
pH = pK + log [HCO3-] / [CO2]
What is the value of pK at 37C?
6.1
What is the order in which the systems controlling pH act when there is a change in pH?
Bicarbonate buffer system
then respiratory system
then kidneys
What has happened to hydrogen ion concentration when there is a decrease in pH?
Increased hydrogen ion concentration
What happens to the direction of the bicarbonate buffer reaction when there is decrease in plasma pH?
Favours backwards reaction
What happens to the hydrogen and bicarbonate ion concentration and pCO2 in response to a decrease in plasma pH?
Hydrogen and bicarbonate ions react, reducing their concentrations
Carbon dioxide is produced, increasing plasma pCO2
What happens to the plasma pCO2 after the response to a decrease in plasma pH?
Stimulates central chemoreceptors
stimulates increased alveolar ventilation
more carbon dioxide expired
decreases plasma pCO2
What happens to the plasma bicarbonate ion cocnentration after the response to a decrease in plasma pH?
Stimulates kidneys to reabsorb more, produce more, excrete less
increases plasma bicarbonate ion concentration
What has happened to hydrogen ion concentration when there is an increase in pH?
Decreased hydrogen ion concentration
What happens to the direction of the bicarbonate buffer reaction when there is an increase in plasma pH?
Favours forwards reaction
What happens to the plasma hydrogen and bicarbonate ion concentration and pCO2 in response to an increase in plasma pH?
Carbon dioxide and water react, decreasing plasma pCO2
Hydrogen and bicarbonate ions produced, increasing their plasma concentration
What happens to the reduced plasma pCO2 after the response to an increase in plasma pH?
Stimulates central chemoreceptors
reduce alveolar ventilation
less carbon dioxide expired
increase in plasma pCO2
How effective is response to reduced plasma pCO2? Why?
Not very effective
reducing alveolar ventilation is limited
because this reduces plasma pO2
which then stimulates peripheral chemoreceptors…
What happens to the increased plasma bicarbonate ion concentration after the response to an increase in plasma pH?
Kidneys reabsorb less, produce less and excrete more bicarbonate ions
reduce their plasma concentration
How do the kidneys control plasma pH?
Variable reabsorption, production and excretion of bicarbonate ions
Variable secretion of hydrogen ions
What part of the nephron does bicarbonate reabsorption occur in?
Proximal convoluted tubule
Later distal convoluted tubule
Collecting ducts
How are bicarbonate ions reabsorbed in the proximal convoluted tubule?
Sodium bicarbonate in the tubular lumen dissociates into sodium ion and bicarbonate ion
NHE on apical domain of epithelial cell
Moves sodium ions down their concentration gradient from the lumen into the cell, whilst moving hydrogen ions against their concentration from the cell into the lumen, so is secondary active transport
Hydrogen ions in the lumen combine with bicarbonate ions in the lumen to form carbonic acid
Carbonic anhydrase converts carbonic acid into carbon dioxide and water
Carbon dioxide and water move from the lumen into the cell by simple diffusion
Carbonic anhydrase converts carbon dioxide and water into carbonic acid
Carbonic acid dissociates into bicarbonate ion and hydrogen ion
Na-HCO3- co-transporter on basolateral domain of epithelial cell
Moves sodium ion and bicarbonate ion from epithelial cell into interstitial fluid
What is the difference between bicarbonate ion reabsorption in the late distal convoluted tubule and collecting ducts compared to proximal convoluted tubule?
AE on basolateral domain of epithelial cell
moves bicarbonate ion from cell into interstitial fluid, whilst moving chloride ion from interstitial fluid into cell
What part of the nephron does hydrogen ion secretion occur in?
Later distal convoluted tubule
Collecting ducts
What type of cell in the late distal convoluted tubule and collecting ducts does hydrogen ion secretion occur in?
Intercalated cells
type A
How does hydrogen ion secretion occur in the later distal convoluted tubule and collecting ducts?
Carbon dioxide diffuses from interstitial fluid into epithelial cell
carbon dioxide + water hydrogen ion + bicarbonate ion
H+ K+ ATPase on basolateral domain of epithelial cell
actively transports one hydrogen ion from the cell into the lumen whilst actively transporting one potassium ion from the lumen into the cell
H+ ATPase on basolateral domain of epithelail cell
actively transports one hydrogen ion from the cell into the lumen
Why are bicarbonate ions normally reabsorbed whilst hydrogen ions are normally secreted?
Because bicarbonate ions in the plasma are used up
by reacting with hydrogen ions from acids produced by metabolism
Hydrogen ions from acids produced by metabolism are secreted
What are some examples of acids produced by metabolism?
Lactic acid
Keto acids
How does bicarbonate ion reabsorption and hydrogen ion secretion affect the urine?
Makes it acidic, lower pH
What is the minimum pH of urine?
4.5
What happens to hydrogen ions in the lumen of the nephron?
Bicarbonate buffer system
When this is not present
phosphate buffer system
ammonia buffer system
What is the phosphate buffer system?
HPO4– + H+ —–> H2PO4-
What is the ammonia buffer system?
NH3 + H+ —–> NH4+
What part of the nephron does bicarbonate production occur in?
Proximal convoluted tubule
Later distal convoluted tubule
How are bicarbonate ions produced in the proximal convoluted tubule?
Glutamine —–> a-ketoglutarate + NH3+ + H+
A-ketoglutarate —–> 2HCO3-
What happens to the bicarbonate ions produced in the proximal convoluted tubule?
Na-HCO3- transporter on basolateral domain of epithelial cell
moves both sodium ion and bicarbonate ion from epithelial cell into interstitial fluid
What happens to the ammonium produced by bicarbonate ion production?
Diffuses across apical domain from epithelial cell into tubular lumen
NH3 + H+ —–> NH4+
cell membrane is impermeable to NH4+
remains in lumen
What type of cell in the later distal convoluted tubule does bicarbonate ion production occur in?
Intercalated cell
type A
How are bicarbonate ions produced in the later distal convoluted tubule?
Carbon dioxide + water bicarbonate ion + hydrogen ion
catalysed by carbonic anhydrase
What happens to the bicarbonate ions produced in the later distal convoluted tubule?
AE on basolateral domain of epithelial cell
moves bicarbonate ion from cell into interstitial fluid, whilst moving chloride ion from interstitial fluid into cell
How does the pH change along the nephron? Why?
Decreases across proximal convoluted tubule
due to bicarbonate ion reabsorption
Does the phosphate buffer system or the ammonia buffer system bind more hydrogen ions?
Ammonia buffer system
