Acid-base Balance Flashcards
What is an acid ?
A substance that can release or donate hydrogen ions
What is a base ?
A substance that can combine with or accept hydrogen ions
What is the pH of blood ?
Blood is maintained at approximately pH 7.35 to 7.45 under normal conditions in the arterial blood
What is the pH of the plasma/ECF ?
7.4
What is the the narrow physiological pH range ?
7.35 - 7.45
What is the narrow vital pH range ?
6.8 - 7.7
What does it mean when acids and bases are weak or strong ?
Strength of acids and bases depends on how completely they dissociate in solution
- strong = complete dissociation
- weak = partial dissociation
What is the equation to calculate pH ?
pH = -log [H+]
What is the relationship between H+ concentration and pH ?
- concentrations of acids and bases is proportional to [H+]
- as solutions get more acidic pH decreases
- a pH change of 1 reflects a 10 fold change in [H+]
What is meant by a neutral solution ?
The concentration of hydrogen ions and hydroxyl ions is equal
H2O > OH- + H+
Why do small pH changes cause big problems ?
- most enzymes only function within narrow pH ranges
- acid base balance can also affect electrolytes (Na+, K+, Cl-)
- proteins can be denatured
- nerve and cardiac function altered
- can also affect hormones
Why does the body produce more acids than bases ?
- acids are taken in with foods e.g. in meat
- acids are produced by the metabolism of lipids and proteins
- cellular metabolism produces carbon dioxide
CO2 + H2O <> H2CO3 <> H+ + HCO3-
What are volatile acids ?
All acids produced in the body are non-volatile except for carbonic acid
What are non volatile acids ?
An acid produced from sources other than carbon dioxide
- produced from the incomplete metabolism of carbohydrates, lipids and proteins
- examples include lactic acid, phosphoric acid, beta-hydroxybutyrate and acetoacetic acid
What are the sources of hydrogen ion loss ?
- loss of H+ in vomit
- loss of H+ in urine
(cause an increase in pH)
What are the sources of hydrogen ion gain ?
- generation from carbon dioxide
- from metabolic products : phosphoric acid, lactic acid and ketones
- loss of bicarbonate ions in diarrhoea
- loss of bicarbonate ions in urine
(cause a decrease in pH)
What type of food is ideal to keep an acid base balance ?
60% alkaline foods and 40% acidic foods ideal
What is the normal value of HCO3 ?
22 - 26 mEq/L
What controls the partial pressure of carbon dioxide in alveolar gas ?
- the rate of carbon dioxide production by the body
- the rate of alveolar ventilation
What causes arterial partial pressure of carbon dioxide to increase in healthy individuals ?
- increase in carbon dioxide production
- insufficient alveolar ventilation
What is hypercapnia ?
- a decrease in ventilation without a similar decrease in oxygen consumption or carbon dioxide production
- ventilation does not meet the metabolic needs of the body
What causes hypercapnia ?
Can occur :
- in disease states which increase physiological dead space
- when respiratory muscles are paralysed or the chest wall is damaged
- when tidal volume is reduced by CNS depression
What are the consequences of hypercapnia ?
- excess carbon dioxide in arterial blood
- primarily affects acid base balance
- elevated production of carbon dioxide generated H+
What is respiratory acidosis ?
An acidic environment as a result of excess carbon dioxide
What is hypocapnia ?
When ventilation is in excess of metabolic needs or carbon dioxide is exhaled at a greater rate than production
What causes hypocapnia ?
Can occur :
- during acute asthma attacks
- under conditions of stress
- at altitude
What are the consequences of hypocapnia ?
- primarily affects acid base balance
- decreased production of carbon dioxide generated H+
What is respiratory alkalosis ?
Alkaline environment due to a decrease in H+ because of a decrease in carbon dioxide
What is acidemia ?
- pH less than 7.35
- increased H+ concentration
- happens due to an increase in non volatile acid > metabolic acidosis
- happens due to failure to remove carbon dioxide from the blood > respiratory acidosis
What is alkalaemia ?
- pH greater than 7.45
- decreased H+ concentration
- can happen due to an increase in base > metabolic alkalosis
- can happen due to decreased carbon dioxide > respiratory alkalosis
Summarise respiratory acidosis
- increased partial pressure of carbon dioxide of plasma
- inadequate ventilation or significant carbon dioxide in the air
- increased formation of carbonic acid which dissociates
Summarise respiratory alkalosis
- decreased partial pressure of carbon dioxide of plasma
- increased alveolar ventilation
- decreased formation of carbonic acid
Summarise metabolic acidosis
- fall in plasma pH accompanied by a fall in plasma bicarbonate
- increase in metabolically derived acids (ketoacidosis)
- loss of base
- failure of renal tubes to excrete H+
Summarise metabolic alkalosis
- excess of non volatile base in the plasma
- vomiting > loss of HCl
- partial pressure of carbon dioxide is unchanged
- increase in plasma bicarbonate
How is blood pH kept closely regulated at 7.35 - 7.45 ?
There are 3 lines of defence :
- blood buffers
- respiration
- renal acid excretion
At what time scale do the 3 lines of defence operate ?
• buffers - seconds
• respiration - minutes
• renal acid excretion - hours/days
What is a buffer ?
A mixture of substances in solution that can resist changes in [H+] when strong acids or bases are added
Summarise the function of buffers in resisting pH changes
- buffering is the first stage in maintaining acid base balance
- buffer systems are located in ICF and ECF compartments
- the most important plasma buffer systems are the bicarbonate-carbonic acid system and haemoglobin
- phosphate and plasma proteins are the most important intracellular buffers
Describe the bicarbonate - carbonic acid buffer
- in the lung and kidney
- renal mechanisms don’t act as quickly as in the lungs
- when it absorbs H+, bicarbonate converts to carbonic acid which dissociates to form carbon dioxide and water
- excess carbon dioxide will stimulate central chemoreceptors
- alveolar ventilation will increase and excess carbon dioxide is excreted via the lungs
Describe how haemoglobin acts as a buffer
- bicarbonate ions are exported out of the cell in exchange for chloride ions so haemoglobin buffers the resulting hydrogen ions
- bicarbonate ions are used as a buffer throughout the body
Hb + H+ > HHb
Describe how plasma proteins act as buffers
- in extracellular fluid, a small amount of buffering is made possible by plasma proteins
- this works in tandem with the bicarbonate buffering system
- buffering occurs at specific amino acids
- amphoteric molecules will release or bind hydrogen ions
Describe the role of phosphate in acting as a buffer
- plays a minor role in the ECF due to low concentrations
- plays a larger role in intracellular buffering
- phosphate buffering plays a major role in the kidney where it is responsible for buffering H+ fluctuations in the urine
Summarise respiration as a compensation mechanism
- the respiratory system compensates for metabolic acidosis and alkalosis by excreting or preserving carbon dioxide
- body pH can be adjusted by changing the rate and depth of breathing
- only works with volatile acids
Describe renal compensation as a line of defence
- the renal system compensates for respiratory acidosis and alkalosis by excreting or preserving H+
- can eliminate large amounts of acid
- can also excrete base
- can conserve and produce bicarbonate ions
- most effective regulator of pH
- if kidneys fail pH balance fails
What are the chemical buffer systems ?
- bicarbonate buffer system
- phosphate buffer system
- protein buffer system
What are the physiological buffers ?
- respiratory mechanism
- renal mechanism
