acid base regulation

Question 1

define alkalaemia

Answer 1

Refers to high-than-normal pH of blood

Question 2

define acidaemia

Answer 2

Refers to lower-than-normal pH of blood

Question 3

define alkalosis

Answer 3

Describes circumstances that will decrease [H+] and increase pH

*osis causing a change in the pH

Question 4

define acidosis

Answer 4

Describes circumstances that will increase [H+] and decrease pH

Question 5

explain the relationship between pH and hydrogen ion concentration

Answer 5

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)

Question 6

what impacted the Pitts and Swan experiment

Answer 6

the blood has an enormous buffering capacity that can react almost immediately to imbalances

Question 7

[H+] = ?

Answer 7

10^-pH

Question 8

pH = ?

Answer 8

-log10 [H+]

Question 9

What is the H+ concentration

Answer 9

4 * 10^-8

Question 10

where does acid come from?

Answer 10

Respiratory acid: CO2 - produced 100x more than metabolic acids - so affects pH more
Metabolic acid: pyruvic, lactic, hydrochloric acids etc

Question 11

Henderson’s equation

Answer 11

Henderson equation: K = [H+][HCO3-]/[CO2][H2O]

Question 12

Henderson-Hasselbach equation:

Answer 12

pH = pK + log10([HCO3-]/[CO2])

Question 13

Hypoxaemia: based on PaO2

Answer 13

> 10kPa Normal
8-10kPa Mild
6-8kPa Moderate
<6kPa Severe

Question 14

Compensatory mechanisms:

Answer 14

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

Question 15

define acid base homeostasis

Answer 15

Acid-Base homeostasis: acid production and clearing is equal leading to ECF pH 7.4 optimum (needed for receptor/enzyme binding and membrane transport)

Question 16

consequences of respiratory acidosis

Answer 16

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)

Question 17

partial compensation in respiratory acidosis

Answer 17

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

Question 18

Full compensation of acidosis

Answer 18

Full Compensation: will normalise pH with large PCO2 and base excess

Question 19

respiratory alkalosis

Answer 19

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

Question 20

partial compensation of alkalosis

Answer 20

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

Question 21

Full compensation of alkalosis

Answer 21

will normalise pH with low PCO2 and base excess

Question 22

metabolic acidosis

Answer 22

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

Question 23

partial compensation of metabolic acidosis

Answer 23

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

Question 24

full compensation of metabolic acidosis

Answer 24

will normalise pH with low PCO2 and base excess

Question 25

metabolic alkalosis

Answer 25

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

Question 26

partial compensation of metabolic alkalosis

Answer 26

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

Question 27

full compensation of metabolic alkalosis

Answer 27

will normalise pH with high PCO2 and base excess

Question 28

base excess range

Answer 28

-2 to +2 mmol

Question 29

pCO2 normal range

Answer 29

4.7 to 6.4 kPa

Question 30

what is base excess?

Answer 30

BE – base excess: Describes the concentration of bases compared to the ‘expected concentration’. An exact match is zero.

Question 31

normal pH values for 
intracellular fluid
extracellular fluid
arterial blood
venous blood
stomach

Answer 31

Intracellular fluid: 7.0. 
§ Extracellular fluid: 7.4. 
§ Arterial blood: 7.4. 
§ Venous blood: 7.36. 
§ Stomach: 2.4.

Question 32

changes in base excess due to?

Answer 32

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.

Question 33

Method of Interpretation

Answer 33

Remember the above with the CADO acronym: Compensation, Aetiology, Disturbance, Oxygenation

Question 34

is acidosis extablished what are the next steps?

Answer 34

If acidosis has been established:

1. Assess the PaCO2:
   a. Elevated = respiratory acidosis.
   b. Low = metabolic acidosis.
2. 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

Question 35

is alkalosis established what are the next steps?

Answer 35

1. Assess the PaCO2:
   a. Elevated or normal = metabolic alkalosis.
   b. Low = respiratory alkalosis.
2. 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.

Question 36

if normal pH has been established then?

Answer 36

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.