Regulation of plasma pH Flashcards
pH =
-log10[H+]
Normal blood pH =
7.4 +/- 0.4
Normal blood [H+] =
~40nM (35-45nM/L)
Physiological changes due to pH acidosis
Renal ammoniagenesis
Pulmonary vasoconstriction
Systemic vasodilation
Hyperventilation
Outside of what pH range do pathological effects associated with acidosis/alkalosis occur??
7.35-7.45
Pathological changes due to pH acidosis
Hyperkalemia (K+ leaves cells) Reduced cardiac contractility Bone reabsorption (H+ replaces Ca2+) Cerebral palsy in newborns Death
Physiological changes due to pH alkalosis
Hypoventilation
Systemic vasoconstriction
Pulmonary vasodilation
Renal bicarbonate secretion
Pathological changes due to pH alkalosis
Tetany from low Ca2+ (calcium binding to albumin is altered at high pH) Fainting from low cerebral blood flow Hypokalemia Cardiac dysrhythmia Haemolysis Death
What is the SID?
Difference between the sum of all positive ions and the sum of all strong negative ions
What is the SID value?
~40mM
What controls SID?
The kidneys
What are the two basic ways to change the H+ concentration in the blood?
Metabolic change
Respiratory change
If you plot [H+] against pCO2 what does the graph look like?
As pCO2 increases, [H+] increases
If you plot [H+] against pCO2, how does changing the SID change the graph?
Lower SID, graph moves steeper, higher SID graph less steep (lower SID means greater [H+] as the sum of SID and H+ = sum of negative ions)
If you plot [H+] against pCO2, how does changing the protein conc change the graph?
Lower protein concentration graph becomes less steep, raise protein concentration, steeper graph (H+ + SID = negative ions incl protein-)
How do you view a metabolic pH change on the H+/pCO2 graph?
Move up and down graphs, stay at constant pCO2 (changing the SID or protein conc i.e metabolic changes)
How do you view a respiratory pH change on the H+/pCO2 graph?
Slide up and down a single line at set SID/protein level, as pCO2 changes (respiratory) H+ changes
What is the davenport diagram?
Plot [HCO3-] against pH (horizontal) then lines on constant pCO2 create bands (isobars) on the graph.
How are metabolic changes shown on a davenport diagram?
are represented through a movement up and down the pCO2 lines (as pCO2 remains constant, but pH rises or falls)
How are respiratory changes represented on a davenport diagram?
are represented through movements along the respiratory line that pass through different pCO2 values
What can change the slope of the davenport line?
Presence of protein makes the line more extreme, (i.e Hb concentration)
Causes of metabolic acidosis
Exercise: rapid production of lactic acid (pK=~4) (especially in anaerobic exercise)
Diabetic ketoacidosis: insulin deficiency leads to production of acetoacetic acid and B-hydroxybutyric acid, ketone bodes have pK=~4, act as strong negative ions
Diarrhoea: loss of sodium and bicarbonate - lower SID
Renal failure: changing chloride concertation in blood, retain Cl- and lower SID
Acetazolamide: CA inhibitor, reduces reabsorption of HCO3-, retains more Cl-, lower SID
Causes of metabolic alkalosis
Antacid excess: ingestion of alkaline metabolic substance
Vomiting: HCl loss from stomach
Aldosteronism: excessive aldosterone leads to extreme sodium retention, raises SID
Diuretic therapy: losses of K+ and H+, raise SID
Causes of respiratory alkalosis (hyperventilation)
Anxiety: increased drive to breathe
Hypoxia: as at high altitude
Voluntary overbreathing
Causes of respiratory acidosis (hypoventilation)
Narcotics and anaesthetics: reduced drive to breathe
Severe asthma or COPD: airway obstruction
Kyphoscoliosis: airway restriction
CO2 breathing
What does compensation to metabolic acidosis relate to?
Increase in respiratory rate to change pH
What does compensation to respiratory acidosis relate to?
Improved regeneration/reabsorption of HCO3- (via H+ secretion)
What does compensation to metabolic alkalosis relate to?
Reduction in respiratory rate to change pH
What does compensation to respiratory alkalosis relate to?
Increase in HCO3- secretion (type B intercalated cells) (or reduction of regeneration/reabsorption of HCO3-)
According to the Stewart concept of acid-alkali balance, the independent variables that determine the pH of a body fluid are as follows:
PCO2, SID and Atot (Total protein content)
The anion gap is used to aid in the differential diagnosis of
Organic and inorganic metabolic acidosis
In an inorganic metabolic acidosis (e.g. due HCl infusion), the infused Cl- replaces HCO3 and the anion gap remains normal.
In an organic acidosis, the lost bicarbonate is replaced by the acid anion which is not normally measured. This means that the AG is increased.
By convention only the following are used for calculation of the anion gap in clinical
settings.
Anion gap = [Na+] - [Cl-] - [HCO3-] (Can include K+ but has little clinical significance)
Acidosis in the presence of a normal anion gap indicates
Inorganic metabolic acidosis (e.g. due HCl infusion), the infused Cl- replaces HCO3 and the anion gap remains normal
In the Henderson-Hasselbalch equation, pH represents
log(1/[H+])
In the Henderson-Hasselbalch equation, pK represents
log10[CO2]/([H+]x[HCO -])
In the Henderson-Hasselbalch equation, alpha represents
Aqueous solubility coefficient of CO2
In the Henderson-Hasselbalch equation, PCO represents
Aqueous partial pressure of CO2
In the Henderson-Hasselbalch equation, HCO represents
Arterial bicarbonate conc
What is the Henderson-Hasselbalch?
pH = pK + log10 [HCO3-]/ alpha PCO2
