Physiological Buffering

Question 1

Define the terms acid

Answer 1

A proton donor (increases H+ concentration in a solution

Question 2

Define the term base

Answer 2

A proton acceptor (decreases H+ concentration in a solution)

Question 3

Outline the dissociation constant

Answer 3

ka = [H+][A-]/[HA]

Question 4

Define the term pKa

Answer 4

The pH at which 50% of the molecules are ionised – ie there are equal numbers of ionised and non-ionised.

pKa = -log ka

Question 5

Define the terms buffer solution

Answer 5

A buffer is a substance (solution) that resists change in pH when small amounts of acids or alkalis are added

Question 6

Appreciate the importance of the modified Henderson equation in buffering and acid base physiology

Answer 6

Many drugs as weak acids or bases as unionised drugs cross the membranes and enter the bloodstream more easily.
It is important to note that the environment (being either acidic or basic) will impact upon the ratio of ionised (A-) to unionised (AH)

These are weak acids or bases that exist at equilibrium with the conjugate

For biological solutions it’s vital to maintain a steady pH

Question 7

Outline the Henderson Hasselbalch Equation

Answer 7

pH = pKa + log ([A-] / [HA])

Question 8

Outline some clinical examples of the importance of the HH equation

Answer 8

Aspirin is a weak acid with pKa 3.4
Stomach pH 1.4
-Meaning Aspirin dissociates by 50% at pKa 3.4, so less than 3.4 less of the acid will be dissociated meaning there is a higher number of non ionised molecules which can then be more easily absorbed.

But weak bases in an acidic environment – don’t work so well

Bupivacaine (local anaesthetic) is a weak base with pKa 8.1
-Acidic (such as infected tissue) tissue has a pH 4, which is not optimal for bupivacaine – so don’t use local anaesthetics in infected tissue
-Can change the pH of the area to optimise effect of Bupivacaine

Question 9

Indicate the major buffering systems present in mammalian blood

Answer 9

ISF
-Bicarbonate
-Phosphate
-Protein

Blood
-Bicarbonate
-Haemoglobin
-Plasma protein

ICF
-Proteins
-Phosphate

Urine
-Phosphate
-Ammonia

Bone
-Ca Carbonate

Question 10

How do small pH fluctuations (e.g. 0.5) cause so much damage?

Answer 10

pH is a logarithmic scale – a small change in pH is BIG change in [H+]

Question 11

Understand the role of protein buffers with an increase of pH

Answer 11

Increase in pH
Protein molecule will buffer by donating H+ to bring H+ levels back up to normal

Sites
-The carboxyl group (-COOH) can dissociate and act as a weak acid releasing H+ becoming as carboxylate ion (-COO-)

-The R group contain an imidazole ring which can donate H+ f pH climbs too high

Question 12

Identify the important protein molecules utilised in the buffer system in response to increase of pH

Answer 12

Histidine and cysteine are very important within ECF and ICF

Histidine can be a proton donor or acceptor at physiological pH
Albumin contains more histidine residues than globulin

Question 13

Understand the role of protein buffers with an decrease of pH

Answer 13

Decrease in pH
Protein molecule will buffer by accepting H+ to bring H+ levels back down to normal

Sites
-The carboxylate ion (-COO-) acts as weak base and accept H+ forming carboxyl group (-COOH)

-The amino group (NH2) acts as weak bases and accept H+ forming an amnio ion (-NH3+)

Question 14

Understand the role of a haemoglobin buffer

Answer 14

Hb accounts for the majority (75%) of the overall protein buffering capacity of the body.

CO2 diffuses into the blood stream where 7% remains in the plasma and 93% is diffused into RBCs

23% of this binds to Hb forming Hb.CO2 (carbaminohaemoglobin)

70% is converted to H2CO3 by carbonic anhydrase

H2CO3 dissociates into H+ and HCO3-

H+ buffered by Hb

HCO3- moves into the ECF in exchange for Cl-

Question 15

Explain the Bohr effect and the important role Haemoglobin buffer system plays within it

Answer 15

Capacity of hemoglobin to form ionic bonds with H+ ions is also related to haemoglobin’s ability to transport oxygen

When a hemoglobin molecule ionically bonds with H+ ions it tends to favour a consequent dissociation from oxygen

Therefore, in the presence of increased hydrogen ions, hemoglobin has less affinity for oxygen and this means that hemoglobin will “unload” oxygen more readily in the capillaries of metabolically active tissues which are liberating H+ ions and CO2, both of which contribute to a more acidic pH environment;

Question 16

Understand the role of the phosphate buffer system

Answer 16

Within bones/cells and urine system
Closed (chemical) buffer system

Important buffer in ICF and urine
pKa for H2PO4- is 6.8

Phosphates as buffers
-Inorganic – high concentrations intracellularly

-Especially in skeletal muscle cells

Question 17

Understand the role of the Bicarbonate buffer system

Answer 17

Open (physiological) buffer system

Quantitatively the most important ECF buffer system

The pka is 6.1, meaning its chemical buffering capacity at pH 7.4 is poor. BUT as it is open at both ends, it is a highly effective system

Question 18

Outline the equation defining the bicarbonate buffer system

Answer 18

CO2 + H2O ~ H2CO3 ~ H+ + HCO3

Carbon dioxide dissolves in water catalysed by carbonic anhydrase (CA)

Carbonic acid present in many cells including red blood cells, renal tubular cells, alveoli

Primary role to prevent changes in pH caused by organic and fixed acids in the ECF

Reaction is almost instantaneous

Question 19

Define an open system

Answer 19

Open at both ends – lungs (exhale CO2) and kidney (excrete urine)