physiological buffering

Question 1

why are the hydrogen ion concentrations in the body so important

Answer 1

they are highly reactive - about 1 millionth of the concentration of other ions in the body but because so reactive small changes can lead to devestating consequences

Question 2

what is an acid

Answer 2

• a proton donor
• increases H+ concentration in solution
• acidity is directly related to the conentration of H+ where temperature is kept constant (body temps)

Question 3

what is a base

Answer 3

• a proton acceptor
  -decreases H+ concentration in solution

Question 4

the mathematical relation to pH

Answer 4

pH is the negative logarithm of the hydrogen ion concentration (mol/L)
- as concentration of H increases, pH decreases and vice versa

Question 5

what is a weak acid

Answer 5

an acid not completely dissociated into component parts

Question 6

what is the dissociation constant

Answer 6

ka
ka=concentration of H X concentration of base/concentration of acid
stronger acids have higher Ka

Question 7

what is pKa

Answer 7

pKa=-log ka
pKa is independent of concentration and is the rendency to dissociate into bases H+ and acid
- pKa is the pH at which 50% OF THE MOLECULES ARE IONISED

Question 8

what Ka and pKa values constitute weak and strong acids

Answer 8

strong acids:
Ka more than 1
pKa negative (less than 0)
weak acids
Ka is less than 1
pKa is positive (more than 0)

Question 9

what is the henderson hasselbalch equation and why is it relevant

Answer 9

an equation relating the pH, the pKa and the ratio of concentrations of the proton acceptor and the proton donor species in a solution

clinically:
- many drugs are weak acids or bases as easier for them to cross membranes
- un-ionised drugs cross membranes and enter blood stream more easily (also impacted by surrounding pH)
- the environment (pH) will impact upon the ration of ionised to unionsied drug

Question 10

why is aspirin best absorbed in the stomach

Answer 10

• weak acid (pKa=3.4) = 50% ionised so 50% can easily cross
• stomach pH is 1.4
• when pKa is closer to pH promotes crossing

Question 11

why are buffers important

Answer 11

resists changes in pH when amounts of acid or alkalis are added
- weak acids or bases that exist in equilibrium with the conjugate

Question 12

what is considered a fatal blood pH fluctuation? why are small fluctuations so fatal

Answer 12

+/- 0.5
- fatal because pH is logarithmic scale so a small change in pH leads to an even larger change in H+ concentration

Question 13

what are volatile acids

Answer 13

an acid which can leave solution and enter the atmosphere (carbonic acid in the lungs is broken into CO2 and H20, CO2 is then exhaled)

Question 14

what is a fixed acid

Answer 14

an acid which cannot leave solution and must be eliminated via the kidney (phosphoric acid)

Question 15

what are organic acids

Answer 15

by products or participants in aerobic metabolism (lactic acid)

Question 16

whata re important characteristics of buffers

Answer 16

• compounds with pKa values in the range of 6.4-8.4 are most useful in biological systems
• the pH associated with the mid range has the greatest buffering capacity
• region of best buffer capacity extends 1.0 pH unit either side if pKa
• the relative importance depends upon concentration in the biological fluid, pKa, and the prevailing H+ concentration

Question 17

what are the most important buffers in the blood

Answer 17

• bicarbonate
• haemoglobin
• plasma protein

Question 18

explain the function of protein buffers

Answer 18

a decrease in pH
- the carboxylate ion and the amino group can act as weak bases and accept H+
- forms the carboxyl group and amino ion

an increase in pH
- the carboxyl group can dissociate and act as a weak acid releasing H+
- the carboxyl group becomes a carboxylate ion

Question 19

describe the importance of histidine as a protein buffer

Answer 19

• important H+ donor. the R group contains an imidazole ring which can donate H+ if pH climbs too high
• very important within ECF and ICF
• can be a proton donor or acceptor based on pH
• albumin contains more histidine residues than globulin

Question 20

explain the haemoglobin buffer system

Answer 20

• RBCs are tightly packed with haemoglobin and cytoplasm contains carbonic anhydrase
• Hb accounts for the majority (75%) of the overall buffering capacity in the body
• Hb is rish in histidine residues

Question 21

describe the phosphate buffer system

Answer 21

• impotant buffer in the icf and urine
• a CLOSED buffer system

Question 22

explain the bicarbonate buffer system

Answer 22

• the most important ECF buffer system
• functions as an open system ( one end breath out CO2 other end urine excretes bicarb)
• the pKa is about 6.1 meaning its chemical buffering capacity at pH 7.4 is poor BUT as it is open at both ends it is highly effective
• carbon dioxide dissolves in water catalysed by carbonic anhydrase
• CA present in many cells including RBCs, renal tubular cells and alveoli
• primary role to prevent changes in pH caused by organic and fixed acids in the ECF
• reaction almost instant

Question 23

describe the modified henderson equation

Answer 23