acid base regulation Flashcards
define alkalaemia
Refers to high-than-normal pH of blood
define acidaemia
Refers to lower-than-normal pH of blood
define alkalosis
Describes circumstances that will decrease [H+] and increase pH
*osis causing a change in the pH
define acidosis
Describes circumstances that will increase [H+] and decrease pH
explain the relationship between pH and hydrogen ion concentration
An acid is any molecule that has a loosely bound H+ ion that it can donate
H+ ions are also called protons (because an H atom with a +1 valency has no electrons or neutrons)
PARADOX: A greater concentration of H+ ions refers to a lower pH (discussed next)
The acidity of the blood must be tightly regulated, marked changes will alter the 3D structure of proteins (enzymes, hormones, protein channels)
A base is an anionic (negatively charged ion) molecule capable of reversibly binding protons (to reduce the amount that are ‘free’)
H+A- H+ and A-
This relationship is in an equilibrium. Increasing something on one side will push the equation in the opposite direction
H2O + CO2 H2CO3 H+ + HCO3-
(le chatalier’s principle)
what impacted the Pitts and Swan experiment
the blood has an enormous buffering capacity that can react almost immediately to imbalances
[H+] = ?
10^-pH
pH = ?
-log10 [H+]
What is the H+ concentration
4 * 10^-8
where does acid come from?
Respiratory acid: CO2 - produced 100x more than metabolic acids - so affects pH more
Metabolic acid: pyruvic, lactic, hydrochloric acids etc
Henderson’s equation
Henderson equation: K = [H+][HCO3-]/[CO2][H2O]
Henderson-Hasselbach equation:
pH = pK + log10([HCO3-]/[CO2])
Hypoxaemia: based on PaO2
> 10kPa Normal
8-10kPa Mild
6-8kPa Moderate
<6kPa Severe
Compensatory mechanisms:
Acidosis needs an alkalosis to correct
Alkalosis needs an acidosis to correct
Changes in ventilation can lead to rapid compensatory response to change CO2 elimination
Changes in HCO3- and H+ retention/secretion in kidneys stimulate slow compensatory response to change pH
define acid base homeostasis
Acid-Base homeostasis: acid production and clearing is equal leading to ECF pH 7.4 optimum (needed for receptor/enzyme binding and membrane transport)
consequences of respiratory acidosis
may result from hypoventilation causing reduced diffusion gradient for CO2, leading to a greater PCO2 in post-alveolar blood, decreased pH and normal base excess (bicarbonate normal for pCO2)
partial compensation in respiratory acidosis
will have lower pH, high PCO2 and high base excess
Acute phase: CO2 moves into erythrocytes, combines with H2O in presence of carbonic anhydrase to form bicarbonate, which moves out of cell by AE1 transporter; increased bicarbonate leads to raised base excess, shifting equilibrium backwards to carbonic acid and reducing [H+]
Chronic phase: increases bicarbonate reabsorption in kidneys to stabilise pH
Full compensation of acidosis
Full Compensation: will normalise pH with large PCO2 and base excess
respiratory alkalosis
may result from hyperventilation causing an increased gradient for CO2, leading to a lower PCO2 in post-alveolar blood, increased pH and normal base excess
partial compensation of alkalosis
Partial Compensation: will have higher pH, low PCO2 and low base excess
Acute phase: none
Chronic phase: reduces bicarbonate from nephrons and increases secretion in collecting duct, causing more carbonic acid dissociation, reducing base excess
Full compensation of alkalosis
will normalise pH with low PCO2 and base excess
metabolic acidosis
may result from diarrhoea (or H+ gaining/bicarb losing) as will lose bicarbonate in faeces, leading to increased dissociation of carbonic acid, causing pH reduction with normal PCO2 and low base excess
partial compensation of metabolic acidosis
will have a lower pH, low PCO2 and low base excess; occurs by increasing ventilation rate to increase diffusion gradient and reduce PCO2, causing shift to left on equilibrium, forming carbonic acid, and then CO2
full compensation of metabolic acidosis
will normalise pH with low PCO2 and base excess
metabolic alkalosis
may result from vomiting (or H+ losing/bicarb losing) as will lose protons in stomach acid, leading to increased bicarbonate, leading to high pH, normal PCO2 and high base excess
partial compensation of metabolic alkalosis
will have high pH, high PCO2 and high base excess; reducing ventilation rate to increase arterial PCO2 drives equation to right to increase protons and bicarbonate
full compensation of metabolic alkalosis
will normalise pH with high PCO2 and base excess
base excess range
-2 to +2 mmol
pCO2 normal range
4.7 to 6.4 kPa
what is base excess?
BE – base excess: Describes the concentration of bases compared to the ‘expected concentration’. An exact match is zero.
normal pH values for intracellular fluid extracellular fluid arterial blood venous blood stomach
Intracellular fluid: 7.0. § Extracellular fluid: 7.4. § Arterial blood: 7.4. § Venous blood: 7.36. § Stomach: 2.4.
changes in base excess due to?
A rise in base excess is due to an increase in renal excretion of acid, ingestion/administration of a base or loss of acid from vomiting. The result is a metabolic alkalosis.
A fall in base excess is due to the overproduction of metabolic acids, the ingestion of acid, a reduction/failure of acid excretion by the kidney or excessive loss of alkali from intestines with diarrhoea. The result is a metabolic acidosis.
Method of Interpretation
Remember the above with the CADO acronym: Compensation, Aetiology, Disturbance, Oxygenation
is acidosis extablished what are the next steps?
If acidosis has been established:
- Assess the PaCO2:
a. Elevated = respiratory acidosis.
b. Low = metabolic acidosis. - Assess the BE:
a. Low:
i. With low PaCO2 = partially compensated metabolic acidosis.
ii. With normal PaCO2 = uncompensated metabolic acidosis.
iii. With high PaCO2 = uncompensated mixed acidosis.
b. Normal:
i. With low PaCO2 = N/A to this lecture.
ii. With high PaCO2 (this is usually associated) = uncompensated respiratory acidosis.
c. High:
i. With low PaCO2 = N/A to this lecture.
ii. With high PaCO2 (this is usually associated) = partially compensated respiratory acidosis
is alkalosis established what are the next steps?
- Assess the PaCO2:
a. Elevated or normal = metabolic alkalosis.
b. Low = respiratory alkalosis. - Assess the BE:
a. Low:
i. With low PaCO2 (this is usually associated) = partially compensated respiratory acidosis.
ii. With high/normal PaCO2 = N/A to this lecture.
b. Normal:
i. With low PaCO2 (this is usually associated) = uncompensated respiratory alkalosis.
ii. With high/normal PaCO2 = N/A to this lecture.
c. High:
i. With low PaCO2 = uncompensated mixed alkalosis.
ii. With normal PaCO2 = uncompensated metabolic alkalosis.
iii. With high PaCO2 = partially compensated metabolic alkalosis.
if normal pH has been established then?
Assess the PaCO2 AND BE together:
a. Both within range = patient is normal.
b. Both low – one of either:
i. Fully compensated respiratory alkalosis.
ii. Fully compensated metabolic acidosis.
c. Both high – one of either:
i. Fully compensated respiratory acidosis.
ii. Fully compensated metabolic alkalosis.
