acid base physiology

Question 1

define an acid

Answer 1

a substance that acts as a proton/H+ donator.

Question 2

define a base

Answer 2

a substance that acts as a proton/H+ acceptor

Question 3

how are acids and bases ionised in respect to their pKa?

Answer 3

Acids are more ionised Above their pKa

Bases are more ionised Below their pKa

Question 4

Define pH

Answer 4

pH is the negative logarithm (base 10) of the hydrogen ion activity in a solution.

Question 5

what is the “gold standard” of pH measurement?

Answer 5

the harned cell; composed of two half-cells, one with the standard platinum-hydrogen electrode and the other with a silver/silver chloride electrode.

The two electrodes are immersed in the same reference buffer, the substance which is being evaluated as a reference standard.

The spontaneous cell reaction which takes place can be described by the following equation:

½ H2 + AgCl →Ag(s) + H+ + Cl-

This reaction generates a current E, which together with the known concentration variables can be plugged into the Nernst equation to arrive at the pH value.

Question 6

how does an ABG machine measure pH?

Answer 6

pH measurement is achieved by a potentiometric method, which determines the concentration of a substance on the basis of how it affects the potential difference between two electrodes.
pH is proportional to the activity of hydrogen ions in solution.

The current machines in use are solid state, ion sensitive field-effect transistors.

Question 7

how does temperature effect pH?

Answer 7

The more heat is available in the system, the more the system will trend towards autoionisation of the molecules.

i.e heat breaks down molecules into its ionised components.

at -35 degrees water has a pH of 8.5.

at 300 degrees (needing 50megapascals of pressure to remain liquid) waters pH is 6.0

Question 8

what is the normal pH of a human

Answer 8

extracellular- as measured on a ABG- 7.35-7.45
intracellular- 6.8 (at body temperature) - this is approximately the pH of distilled water at body temp (where the pH is 7/neutral at 25 degrees).

Question 9

how much acid is produced in the body each day?

Answer 9

different sources have different units of measurement and can be off by significant amounts, but about 150g/day.

it is composted of;
ingestion, infusion and inhalation of acidic substances
loss of buffering substances
Acids created during metabolism.
Acidifying effects of ATP usage.

Question 10

what are the sources of buffers in the human body?

Answer 10

For the whole system, if it were closed (anephric and apnoeic), the buffer capacity (β) is probably about 10mEq/pH/kg body mass, consisting of:

60% intracellular buffering
    Intracellular proteins (25%)
    Intracellular peptides (25%)
    Intracellulr phosphate (10%)

40% extracellular buffering
    Haemoglobin (20%)
    Bicarbonate (10%)
    Plasma proteins (9.4%)
    Phosphate (0.6%)

Question 11

Define a buffer

Answer 11

“A buffer is a solution that can resist pH change upon the addition of an acidic or basic components”

it weak acid and congugate base
HB <=> H+ + B-

Question 12

how does Hb act as a buffer?

Answer 12

Hb contains a large number (38) of histidine residues which have an imidazole side chain which acts as both good donator and acceptor of hydrogen.

pKa of histidine is 6.8 (oxygenated), but with conformational change to deoxyhaemoglobin the pKa changes to 8.2 making it more effective as a buffer to acids.

additionally CO2 and H2CO3 bind with amino groups to form carbamino compound- but this process results in the release of H+.