Acid Base Flashcards
Define pH
A measure of the molar concentration of hydrogen ions in solution
pH = -log10 [H+]
What is increasing H+ concentration responsible for
Increasing acidity
What is increasing OH- concentration responsible for?
Increasing basicity
What are the two ley properties of the pH system?
As pH decreases the [H+] increases
The pH scale is a log scale, it is not linear
What is the pH reference range for plasma?
7.35 to 7.45
What is the H+ reference range for plasma
35-45 nmol/L
Why is acid base balance needed?
(3)
There is always production of acid by the body’s metabolic processes
To maintain balance, these acids need to be excreted or metabolised
The various acids produced by the body are classified as respiratory/volatile acids and as metabolic/fixed acids
How are acids produced by the body classified
Respiratory/volatile acids
Metabolic/fixed acids
What is acidosis?
(3)
pH below the normal range
Trying to push your pH into an acid range
Less than 7.35
What is alkalosis
(3)
pH above the normal range
Trying to push your pH into alkaline range
pH greater than 7.45
What two organs have roles in maintaining acid-base balance
Kidneys
Lungs
How do the kidneys regulate acid base balance
(2)
They regulate hydrogen ion concentration
They generate and recycle buffers that enter the bloodstream
How do the lungs regulate acid base balance
Eliminate carbon dioxide which is acidic
What is considered acidaemia
Blood pH less than 7.35
What is considered alkalaemia?
Blood pH greater than 7.45
How is carbon dioxide broken down?
(3)
CO2 (acid) + H2O
Converted to H2CO3 (Carbonic Acid)
Converted to H+ + HCO3- (bicarbonate ion)(base)
What molecule is acidic?
CO2
What molecule is basic
HCO3-
Why is carbon dioxide so important in acid-base balance?
(3)
Regulation of the amount of CO2 in blood, or more precisely the ratio of bicarbonate to dissolved carbon dioxide concentration is essential for maintaining acid-base balance
CO2 is a major determinant of blood pH because of it’s conversion to carbonic acid
As CO2 concentration rises, so does H+ concentration
What is pCO2?
Partial pressure of Carbon dioxide
What controls respiration rate?
(2)
pCO2 sensitive chemoreceptors in the brain stem and carotid artery
Respiration rate is increased if pCO2 is rising and decreased if PCO2 is declining
How does increased respiratory rate affect CO2 concentration
This increases the rate of CO2 elimination
How does decreased respiratory rate affect CO2
This promotes CO2 retention
What might a low CO2 be associated with
(2)
Metabolic acidosis
Compensated respiratory alkalosis
What might a high CO2 level be associated with?
(2)
Metabolic alkalosis
Compensated respiratory acidosis
What happens to CO2 in red blood cells when they reach the lungs
(3)
When the rbcs reach the pulmonary circulation, CO2 diffuses from the blood cells to the alveoli for it to be exhaled
As the rbc is oxygenated, bicarbonate passes from plasma to the red blood cells, buffering hydrogen ions released from haemoglobin
Reversal of the carbonic anhydrase reaction results in production of CO2, this then diffuses from red cells to plasma and ultimately to alveoli
Explain in your own words CO2 diffusion in blood
(3)
Two forms of CO2 present, CO2 in red blood cells and bicarbonate in plasma.
CO2 must move from red blood cells to alveoli to be exhaled, therefore CO2 in the form of bicarbonate must first be converted back to CO2
Therefore, bicarbonate pass from the plasma to the red blood cell where it is converted back to CO2 through the reverse of the carbonic anhydrase reaction
Compare the CO2 content of mixed venous blood arriving at the lung vs blood leaving the lungs
23.5 mmol/L upon entering
21.5 mmol/L when leaving
What are the five different ways carbon dioxide can be transported in the body?
90% is transported as bicarbonate
- 65% of this is in plasma
- 25% of this is in the red blood cells themselves
5% is physically dissolved in the plasma and red cell cytoplasm i.e. as CO2
5% is loosely bound to haemoglobin and plasma proteins
Less than 0.1% is transported as carbonic acid
What is arterial blood gas analysis?
(2)
This analysis includes measurement of parameters related to the carbon dioxide content of blood
Partial pressure of carbon dioxide (pCO2) and
Plasma bicarbonate concentration (HCO3-) is measured
What is the only concentration actually measured from arterial blood gas analysis?
pCO2
How is HCO3- determined
It is calculated from pCO2 and pH
When might CO2 be measured?
(2)
Can be measured from plasma or serum by chemical methods
It is included in all chemistry panels containing electrolytes
What is partial pressure of CO2?
A measure of the pressure exerted by that small portion (5%) of total carbon dioxide in blood that is dissolved in the aqueous phase of plasma and blood cell cytoplasm
Measure of the CO2 in plasma and rbcs
What happens when there is any deviation from normal pH range?
(4)
This alters protein structure and therefore function e.g it affect plasma membrane stability and enzyme activity
The central nervous system function deteriorates
Cardiac contractions become weak and irregular (heart failure)
Peripheral vasodilation (low blood pressure, circulation fails)
When are acids produced by the body?
(5)
Amino acid breakdown
Release of fatty acids from triacylglycerols
Lactic acid
Ketone bodies
Carbon dioxide
How many defence systems does the body have to respond to an acid-base alteration
Three, first, second and third
What is the body’s first defence to an acid-base alteration?
Buffering [HCO3-, Hb, Phosphate]
What is the body’s second defence to an acid-base alteration?
Respiratory: alteration in arterial pCO2 by increasing/decreasing respiration
What is the body’s third defence to an acid-base alteration?
(2)
Renal: alteration in HCO3- excretion
Increase or decrease in HCO3- excretion
Why must H+ be buffered in the blood
Buffers temporarily neutralise the acid in order to transport H+ to the kidneys and lungs
H+ combines with the base (B-) to form a weak acid HB
What is a buffer
A substance which resists changes in pH
What is a buffer system
A buffer system consists of a weak acid (which donate H+) and a weak base (that accept/absorb excess H+)
What is the Henderson-Hasselbach equation and what is it used for?
(3)
Used to determine what the final pH will be
pH = PKa + log [B-]/[HB]
Ka is the acid dissociation constant
What is the basis of the Henderson-Hasselbach equation?
pH depends on the ratio of the conjugate base (B-) to the undissociated acid (HB)
What does a high Ka mean?
Mostly dissociated acid
What does a low Ka mean
Partially dissociated acid
What are the three main buffer systems in the body
Plasma proteins such as haemoglobin
Phosphate (HPO4-) and ammonia (NHs)
Bicarbonate
Write about how bicarbonate acts as a buffer
(5)
It’s the principle buffer system in the blood
Based on carbonic acid and bicarbonate
Carbonic acid (H2CO3) forms spontaneously when CO2 dissolves in H2O
Gas exchange occurs at the lungs which removes CO2 (and thus acid)
Basis for how the body controls the pH of the extracellular fluid
What does the concentration of a gas in a solution depend on?
The partial pressure of that gas (pCO2)
The solubility of the gas in the solvent (S)
What in ratio in blood determines the acidity
The ratio of bicarbonate to pCO2
What is the equation for the pH of blood
6.1 + log [HCO3-/(0.03) x (pCO2)]
6.1 = dissociation constant
How is CO2 produced in the body
Aerobic metabolism
What happens when CO2 is picked up by red blood cells?
(4)
CO2 + H2O is converted to H2CO3 (carbonic acid) by carbonic anhydrase
Carbonic acid then dissociates to H+ and HCO3- (bicarbonate)
The H+ produced is buffered by haemoglobin in the rbc which releases O2
In oxygenated tissue, O2 binds haemoglobin preferentially
In oxygenated tissue, O2 binds haemoglobin preferentially, what does this do?
(3)
Prevents haemoglobin from buffering H+
Instead H+ must be buffered by bicarbonate (HCO3-)
Bicarbonate must be present for buffering to continue
Why is recovery of bicarbonate from glomerular filtrate needed?
(2)
The glomerular filtrate will contain the same [HCO3-] as blood
Therefore this needs to be reabsorbed in the tubules
How is HCO3- reabsorbed by the renal tubules
(4)
Renal tubular cells are impermeable to HCO30
Therefore bicarbonate combines with H+ to form carbonic acid which dissociates to CO2 and H2O
H2O is then excreted to get rid of excess H+
CO2 is taken up into the tubular cells
What happens when CO2 is taken up into tubular cells?
(4)
It combines with H2O to form carbonic acid
Carbonic acid then dissociates to form HCO3- and H+
The H+ is then secreted into the lumen where it combines with HCO3-
Any HCO3- produced is reabsorbed back into the blood
What are the five steps of bicarbonate filtration in renal tubular cells?
CO2 from the tubular blood supply diffuses into tubular cells
CO2 combines with H2O to form carbonic acid (H2CO3)
Carbonic acid dissociates to HCO3- and H+
HCO3- is taken up by the tubular cells (into blood supply)
H+ is actively secreted into urine
What is the role of buffers in acid-base balance?
(5)
They provide a temporary solution to acid base imbalance
But these buffer systems are limited in the body
Ultimately excess H+ must be removed
Buffer systems must be linked with renal and respiratory mechanisms
Generation/recycling of buffers is necessary
How are acids removed?
Excess H+ tied up in water molecules (H2O): excretion via the kidneys
Removal of CO2 by the lungs
How does the respiratory system affect acid base balance?
(3)
When breathing is inadequate carbon dioxide (respiratory acid) accumulates
The extra CO2 molecules combine with water to form carbonic acid which contributes to an acid pH
The treatment, if all else fails is to lower PCO2 by breathing for the patient using a ventilator
How does the metabolic system affect acid base balance?
(3)
When normal metabolism is impaired - acid forms, e.g. poor blood supply stops oxidative metabolism and lactic acid forms
This acid is not respiratory so, by definition, it is “metabolic acid”
If severe, the patient may be in shock and require treatment, possible by neutralising this excess acid with bicarbonate, possible by allowing time for excretion/metabolism.
What is respiratory compensation
Lungs become involved to correct the pH
What is metabolic compensation
Kidneys become involved to correct the pH
What compensation is there if the primary problem is metabolic (renal)
(2)
There will be respiratory compensation
Compensation is immediate (within minutes)
Respiration rate is altered to retain or expire CO2
What compensation is there if the primary problem is respiratory?
(3)
The compensation mechanism will be metabolic (renal)
Acute respiratory problem metabolic compensation is delayed (hours to days)
Chronic respiratory problem metabolic compensation will have kicked in (kidneys will regulate H+ and HCO3- retention and secretion)
Write about respiratory compensation
(7)
Change in respiration to compensate for a change in plasma pH
The pH can be lowered or raised by changing the rate of respiration
Retention of CO2 will cause a decrease in pH (more acidic)
Therefore, a respiratory acidosis will compensate for a rise in plasma pH (alkalosis)
Increased expiration of CO2 will cause an increase in pH (more basic)
Therefore a respiratory alkalosis will compensate for a fall in plasma pH (acidosis)
This compensation allows plasma pH to recover to normal levels
Write about metabolic (renal) compensation
(6)
Change in the rates of H+ and HCO3- secretion or reabsorption by the kidneys in response to changes in plasma pH
Decline in H+ secretion and HCO3- reabsorption
Therefore, a metabolic acidosis will compensate for a rise in plasma pH (alkalosis)
Secretion of H+ and reabsorption of HCO3-
Therefore, a metabolic alkalosis will compensate for a fall in plasma pH (acidosis)
This compensation allows plasma pH to recover to normal levels
List four causes of metabolic acidosis
Increased formation of acids
Increased ingestion of acids
Decreased excretion of H+
Increased loss of bicarbonate
What two processes may cause an increased formation of acids?
Increased ketone production (diabetic ketoacidosis)
Increased lactic acid formation
The ingestion of what three things will cause metabolic acidosis?
Ethanol
Methanol
Ethylene glycol
What may cause decreased excretion of H+
Renal failure
What may cause increased loss of bicarbonate
Diarrhoea
Renal failure
What are the two causes of metabolic alkalosis?
Loss of H+
Gain of HCO3-
What might cause the loss of H+
Gastrointestinal loss via vomiting or diarrhoea
Renal loss via excess amounts of aldosterone or mediations such as diuretics
What might cause the increased gain of HCO3-
Chronic alkali ingestion
Inappropriate correction of an acidosis
What causes respiratory acidosis?
Retention of CO2
What six things may cause the retention of CO2?
Airway obstruction
Pulmonary disease e.g. asthma, pneumonia
Thoracic musculoskeletal problems
Neuromuscular conditions or neurotoxins
Depression of respiration through medications
Choking
What causes respiratory alkalosis?
Increased expiration of CO2
What are the causes of increased expiration of CO2?
(8)
Low O2 (hypoxia)
- high altitude
- severe anaemia
- pulmonary disease
Increased respiratory drive
- Hyperventilation (artificial or panic)
- Medications e.g. Aspirin
- Trauma, infection
How are blood gases measured
Using specific electrodes on blood gas analysers
What blood gases are measured
(4)
[H+] measured and pH calculated
Partial pressure of CO2 (pCO2)
Partial pressure of O2 (pO2)
Bicarbonate (calculated from [H+] and pCO2)
What are the six steps to acid-base analysis?
Is the pH normal?
Is the CO2 normal?
Is the HCO3 normal?
Match the CO2 or the HCO3 with the pH
Does the CO2 or the HCO3 go the opposite direction of the pH?
Are the pO2 and the O2 saturation normal?
How would you investigate blood gas results?
Examine the [H+] or pH -> is there acidosis or alkalosis
Examine the pCO2 -> if abnormal it must be respiratory involvement
Examine the HCO3- -> if abnormal it must be metabolic involvement
Is there evidence of compensation?
Define compensation
The attempt by the body to maintain homeostasis by correcting the pH
Comment on compensation in a patient experiencing shock
(3)
A patient in shock will undergo anaerobic metabolism which produces lactic acid
The production of lactic acid will bind or use up available HCO3 and will be manifested by a decrease in the HCO3 level.
Therefore, the HCO3 level is an indicator of metabolic acid-base balance
