Acid Base Balance Flashcards
What is the target pH of the blood?
7.4
Why are metabolic reactions so sensitive to the pH of the fluid in which they occur?
Due to the high reactivity of hydrogen ions with negatively charged proteins/segments of protein resulting in changes in configuration and function, especially in enzymes
What are the sources of hydrogen ions in the body?
Respiratory acid (Co2 + water results in production of carbonic acid) Metabolic acid- via metabolism of dietary constituents that result in production of inorganic acids or organic acids
When does respiratory acid contribute to acid/base balance?
In some cases where lung function is impaired. In health, the body ensures that respiratory acids do not have an effect
What is the function of a buffer?
To minimise the changes in pH when hydrogen ions are added or removed
What does the Henderson-Hasselbalch equation define?
Defines pH in terms of a ratio of acid:base
What is the most important extracellular buffer?
The bicarbonate buffer
What is the normal range of blood pH?
7.37-7.43
What is the normal range of blood pCO2?
4.8-5.9kPa OR 36-44mmg
What is the normal range of concentration of carbonic acid in the blood?
22-26mmoles/L
How does the bicarbonate buffer system act as a buffer?
An increased concentration of H+ in the ECF drives the reaction to the right to remove some additional H+ from the solution and therefore a change in pH is reduced
How are hydrogen ions eliminated from the body?
Through the kidneys through an excretion that is coupled with the regulation of carbonic acid concentration in the plasma
What are the possible intracellular buffers?
Proteins
Organic and inorganic phosphates
Haemoglobin (in erythrocytes only)
How does buffering of hydrogen ions by ICF buffers cause a change in plasma electrolytes?
To maintain electrochemical neutrality, movement of hydrogen ions must be accompanied by chlorine ions in red cells or exchanged for a cation- K+
What organs is regulation of blood pH reliant on?
Lungs and kidneys
How does the kidney regulate [HCO3-]?
Reabsorbing filtered HCO3-
Generating new HCO3-
Describe the mechanism of reabsorption of HCO3- in the kidneys
H+ is actively secreted from the tubule cells in a process that is coupled to passive sodium reabsorption. Filtered HCO3- reacts with the secreted H+ to form H2CO3, which is converted to carbon dioxide and water in the presence of carbonic anhydrase. CO2 is freely permeable and enters the cell, where it is converted to H2CO3 in the presence of carbonic anhydrase which then dissociates to form H+ and HCO3-. The H+ ions are the source of secreted H+ and HCO3- ions pass into peritubular capillaries with sodium.
Where does most (>90%) of the carbonic acid reabsorption take place?
In the proximal tubule
Why is HCO3- reabsorption vital to life?
Failure to reabsorb results in addition of hydrogen ions to the ECF
What is the buffer for most of the buffering of urine?
Dibasic phosphate
What is the process of buffering urine known as and why is it called this?
Titratable acidity- extent is measured by the amount of NaOH needed to titrate urine pH back to 7.4 for a 24hr urine sample
Why is the formation of titratable acidity important?
It generates new HCO3- and excretes H+
How does Na2HPO4 remove H+ from the body?
One Na+ is reabsorbed in the lumen in exchange for secreted H+
What is the source of new HCO3- in titratable acidity?
CO2 enters the tubule cells and combines with H20 to form carbonic acid (in presence of carbonic anyhdrase), dissociating to yield H+ (used for secretion) and new HCO3-, which passes with sodium ions into the peritubular capillaries
Where is the site of formation of titratable acidity and why is it there?
Distal tubule- not reabsorbed dibasic phosphate becomes highly concentrated by removal of volume of filtrate
What is the function of ammonium excretion?
Major adaptive response to an acid load and generates new HCO3- and excretes H+
Describe the mechanism of ammonium excretion
NH3 is produced by deamination of amino acids (primarily glutamine by action of renal gutaminase) within the renal tubule cells. NH3 moves into tubule lumen and combines with secreted H+ to form NH4+, which combines with Cl- ions to form NH4Cl, which is excreted
What terms refer to an increase and decrease in blood pH
Decrease in pH = acidosis
Increase in pH = alkalosis
What disorders affect PCO2 and what disorders affect [HCO3]?
Respiratory disorders affect PCO2
Renal disorders affect [HCO3]
Describe the pathway of respiratory acidosis?
pH has fallen due to respiratory change so PCO2 must have increased. Respiratory acidosis results from reduced ventilation and so retention of CO2
What are the possible causes of respiratory acidosis?
Acute: -Drugs that suppress medullary respiratory centres (ie barbiturates and opiates) - Obstructions of major airways Chronic: - Lung disease
What are the causes of respiratory alkalosis?
Acute: -Voluntary hyperventilation -Aspirin -First ascent to altitude Chronic: -Long term residence at altitude
What are the causes of metabolic acidosis?
Increased H+ production (ie DKA)
Failure to excrete normal dietary load of H+ (ie renal failure)
Loss of HCO3- (as in diarrhoea)
What are the causes of metabolic alkalosis?
Increased hydrogen ion loss (vomiting)
Increased renal H+ loss (aldosterone excess)
Excess administration of HCO3-
Massive blood transfusions
What acid/base disorder is an increased H+ and decreased pH with a primary disturbance of increased PCO2 indicative of?
Respiratory acidosis
What acid/base disorder is an decreased H+ and increased pH with a primary disturbance of decreased PCO2 indicative of?
Respiratory alkalosis
What acid/base disorder is an increased H+ and decreased pH with a primary disturbance of decreased HCO3- indicative of?
Metabolic acidosis
What acid/base disorder is an decreased H+ and increased pH with a primary disturbance of increased HCO3- indicative of?
Metabolic alkalosis
What is the anion gap?
The difference between the sum of the principal cations and the principal anions in the plasma
In what acid/base balance disorder can the anion gap be useful and why?
Metabolic acidosis
Some causes of metabolic acidosis (ier DKA) will cause an increase in the anion gap and some causes (ie loss of bicarbonate from gut) there will be no change
