Acid Base Balance Flashcards
State the blood buffering systems in an equation and the [H+] equation for each
Blood buffering systems -
- Bicarbonate
H+ + HCO3- -> H2CO3 -> CO2 + H2O
<- <-
[H+] = K1 [CO2] / [HCO3-]
- Phosphate
H+ + HPO4 2- -> H2PO4 -
<-
[H+] = K2 [H2PO4 -] / [HPO4 2-]
- Protein (including haemoglobin)
H+ + Pr- -> HPr
<-
[H+] = K3 [HPr] / [Pr-]
Compare the buffering abilities of the CO2/ HCO3- system and H2PO4- and HPO4 2-
When 50% is protonated and 50% is not protonated, the CO2 / HCO3- system has a pK of 6.1 - can buffer between pH’s of 5.1-7.1
When 50% is protonated and 50% is not protonated, the H2PO4- / HPO4 2- system has a pK of 6.8 - closer to the physiological pH - can buffer between pH’s of 5.8-7.8
Describe the bicarbonate buffering system with more detail
[H+] = K1 [CO2] / [HCO3-]
pH = pK + log10 [HCO3-] / [CO2]
- Most important is the [HCO3-] : [CO2] ratio
- Plasma [CO2] proportional to the partial pressure of CO2 in the plasma
- Mathematical constant to convert p CO2 mmHg to [CO2] mmol/L is 0.03
Pros and cons of bicarbonate buffering after stating what a good buffer should be like
A good buffer -
- pK is the equilibrium constant of a reaction
- Buffer solutions resist change in pH when [base] = [acid]
- Buffer is most effective 1 pH on either side of the pK value
- At 6.1 the pK of the CO2 - HCO3- buffer is not close enough to the desire plasma pH of 7.4 so it isn’t a great buffer choice
- It is an abundant source as CO2 is made in metabolism
- It has independent regulation- alveolar ventilation controls PCO2 and kidneys control [HCO3-]
State the renal mechanisms involved in acid base levels
Renal control of acid base levels -
- Kidneys control acid base levels by excretion of acidic or basic urine
Primary renal mechanisms involved -
- Reabsorption and secretion of HCO3-
- Formation of new HCO3- -> released into plasma at a controlled rate
- Secretion of [H+] into the tubular fluid - acidic urine is produced
- Kidney produces NH3 that contributes to buffering and phosphate ions are reabsorbed from the tubule
Buffer systems within the tubule that react with secreted [H+]-
HCO3- : H2CO3
HPO4 2- : H2PO4 -
NH3 : NH4 +
Describe the renal control of [H+] and [HCO3-]
- In the epithelial cells of the tubule CO2 and H2O react to form carbonic acid (H2CO3) catalysed by carbonic anhydrase (enzyme activity governed by [H+] in the extracellular fluid)
- The carbonic acid dissociates into HCO3- and H+
- H+ ions are transported to the lumen by a H+/Na+ antiporter or by ATP dependent proton pumps - hence why urine is acidic
- The HCO3- ion transported via a HCO3-/Na+ symporter out of the epithelium and into the ECF
Describe the renal mechanisms of CO2/HCO3- buffer in the proximal tubule
- In the epithelial cells of the tubule CO2 and H2O react to form carbonic acid (H2CO3) catalysed by carbonic anhydrase (enzyme activity governed by [H+] in the extracellular fluid)
- The carbonic acid dissociates into HCO3- and H+
- H+ ions are transported to the lumen by a H+/Na+ antiporter or by ATP dependent proton pumps - hence why urine is acidic
- HCO3 in the tubular fluid reacts with the H+ entering to form H2CO3 which via carbonic anhydrase forms CO2 and H2O - decreases pH
- CO2 can pass transcellularly into the ECF and can also reenter epithelial cells to form more carbonic acid
- The HCO3- ion transported via a HCO3- / Na+ symporter out of the epithelium and into the ECF
- This is why we say HCO3 is ‘reabsorbed’ as the HCO3 ultimately transports from the tubular lumen back to the ECF
Describe the CO2/HCO3- buffer in the intercalating cells of the DCT and CD
- The [HCO3-] is low for H+ reacts with other buffers
- H+ ATPase pump is the most important
- pH decreases significantly from around 7.0 to acidic pH of around 4 by the time it reaches the collecting duct - urine is acidic
Describe the phosphate buffer in the DCT/CD intercalating cells
- Further H+ secretion into the lumen is buffered by HPO4 2-
- Very effective buffer because pK = 6.8 which is close to the pH of the filtrate
- In the epithelial cells of the tubule CO2 and H2O react to form carbonic acid (H2CO3) catalysed by carbonic anhydrase (enzyme activity governed by [H+] in the extracellular fluid)
- The carbonic acid dissociates into HCO3- and H+
- H+ ions are transported to the lumen by ATP dependent proton pumps - hence why urine is acidic
- The HPO4 2- ion is in the tubular fluid and reacts with the H+ to form H2PO4 - ions which are excreted in the urine
- HCO3- ions are removed by a HCO3- / Cl- antiporter (predominant in intercalated cells) - the HCO3- enters the ECF and Cl- enters but diffuses back into ECF after
Describe the ammonia buffer in the DCT/CD intercalating cells
- Tubular epithelium produces NH3 from glutamine with the enzyme glutaminase
- Glutamine forms alpha ketoglutarate and 2NH3 using glutaminase as the enzyme
- Alphaketoglutarate enters krebs cycle and via metabolism leads to formation of 2 H2CO3 molecules which then split to form 2H+ and 2HCO3- ions
- The 2NH3- and 2H+ bond to form 2 NH4+ which is then excreted into the lumen via a NH4+/ Na+ antiporter
- The 2HCO3- ions are transported into the ECF via a HCO3-/Na+ symporter
Describe the role of the respiratory system in acid base balance
Role of respiratory system -
- Chemosensitive area in the medulla oblongata regulates respiration
- It monitors [H+] of plasma indirectly via cerebrospinal fluid
- Charged ions cannot cross the blood brain barrier but CO2 can
- If plasma pCO2 increases it decrease the pH of plasma and cerebrospinal fluid
- pH of plasma is detected by peripheral chemoreceptors in aortic arch and carotid bodies while the pH of the cerebrospinal fluid is detected in the medulla oblongata
- They both cause respiratory ventilation to increase which decreases plasma pCO2 and creates a normal ECF pH
State what metabolic acidosis is and what causes it
Metabolic acidosis -
Characterised by a low pH resulting from a high ECF [H+] or low ECF [HCO3-]
Caused by -
- Severe sepsis of shock due to lactic acid production
- Uncontrolled diabetes due to overproduction of ketoacids
- Diarrhoea - loss of HCO3- from GI tract
Describe how acidosis is couteracted
- When [H+] increases the ICF and ECF buffering kicks in but eventually the HCO3- ions will have been used and [H+] will still be high
- The lungs will increase their rate of ventilation after the [H+] is detected in the medulla oblongata which will decrease the pCO2 in the plasma which will increase pH
- The kidney will increase H+ secretion by increasing NH4 + secretion so that there is new HCO3- formation and increased HCO3- reabsorption
State what metabolic alkalosis is and what causes it
Metabolic alkalosis:
Characterised by high pH caused by high ECF [HCO3-] or low ECF [H+]
Caused by:
- Excessive diuretic use
- Chronic loss of Cl-, Na+ and K+ which increase H+ secretion
- Vomiting as this causes loss of H+ from the GI tract
- Ingestion of alkaline antacids
- Hypokalaemia
Describe how alkalosis is couteracted
- When [H+] decreases the ICF and ECF buffering kicks in but eventually the H+ ions will have been used and [HCO3-] will still be high
- The lungs will decrease their rate of ventilation after the [HCO3-] is detected in the medulla oblongata which will increase the pCO2 in the plasma which will decrease pH
- The kidney will decrease H+ secretion by decreasing NH4 + secretion so that there is less new HCO3- formation and decreased HCO3- reabsorption
