physiological buffering Flashcards
why are the hydrogen ion concentrations in the body so important
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
what is an acid
- a proton donor
- increases H+ concentration in solution
- acidity is directly related to the conentration of H+ where temperature is kept constant (body temps)
what is a base
- a proton acceptor
-decreases H+ concentration in solution
the mathematical relation to pH
pH is the negative logarithm of the hydrogen ion concentration (mol/L)
- as concentration of H increases, pH decreases and vice versa
what is a weak acid
an acid not completely dissociated into component parts
what is the dissociation constant
ka
ka=concentration of H X concentration of base/concentration of acid
stronger acids have higher Ka
what is pKa
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
what Ka and pKa values constitute weak and strong acids
strong acids:
Ka more than 1
pKa negative (less than 0)
weak acids
Ka is less than 1
pKa is positive (more than 0)
what is the henderson hasselbalch equation and why is it relevant
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
why is aspirin best absorbed in the stomach
- 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
why are buffers important
resists changes in pH when amounts of acid or alkalis are added
- weak acids or bases that exist in equilibrium with the conjugate
what is considered a fatal blood pH fluctuation? why are small fluctuations so fatal
+/- 0.5
- fatal because pH is logarithmic scale so a small change in pH leads to an even larger change in H+ concentration
what are volatile acids
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)
what is a fixed acid
an acid which cannot leave solution and must be eliminated via the kidney (phosphoric acid)
what are organic acids
by products or participants in aerobic metabolism (lactic acid)
what are the most important buffers in the blood
- bicarbonate
- haemoglobin
- plasma protein
explain the haemoglobin buffer system
- 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
explain the bicarbonate buffer system
- 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
describe the modified henderson equation
