L9 Acid/Base Physiology Flashcards
Acid/base physiology describes the regulation of …
H+ concentrations in the ECF
The three main components of the homeostatic regulation of [H+] in the body
Buffered (fast): bicarbonate, proteins, phosphates, etc
Respiratory compensation: alters CO2 levels
Renal compensation (slow): alters HCO3- levels
____ is a short-hand method of expressing [H+]
pH
pH = -log[H+]
Plasma [H+] = 0.00000004 moles/L = 0.00004 mEq/L
So pH = -log[0.00000004] = 7.4
A 10-fold increase in [H+] = 1 unit change in pH
Normal blood pH = ?
7.4 (pH = -log[H+] = -log[0.00000004])
Normal range is 7.37-7.42 so [H+] can vary by +7 and -5 % around it’s mean of 40 nEq/L
pH and [H+] are related ________
Logarithmically
Increasing pH from 7.4 to 7.6 will decrease [H+] by 15 nEq/L
Decreasing pH from 7.4 to 7.2 will increase [H+] by 23 nEq/L
Normal [H+] ranges in different parts of the body
Gastric HCl [H+] = 0.15mol/L —> pH = 0.8
Max urine acidity [H+] = 3x10^-5 —> pH = 4.5
Normal plasma [H+] = 4x10^-8 —> pH = 7.4
Extreme acidosis pH = 7
Extreme alkalosis pH = 7.7
Pancreatic juice [H+] = 1 x 10^-8 —> pH = 8.0
13,000 to 20,000 mEq/day of H+ is produced from …
Respiratory CO2
Almost entirely handled by lungs
Known as volatile acid
Also, significant amounts of non-volatile, or fixed, acid arise from normal and abnormal processes:
• Degradation of certain amino acids
• Additional acid loads from exercise (lactate), diabetic kenos is (ß-hydroxy butyric & acetoacetic acides), and ingestion of acids
We produce ____ mEq/day of fixed (non-volatile) acid
50 mEq/day
From gluconeogenic utilization of AAs in the liver
Sulfuric acid from methionine and cysteine catabolism (75%); phosphoric acid from phospholipid degradation (25%)
Represent major acid load that kidney must eliminate
Acids are H+ ________ and bases are H+ ______
Acid = donor, base = acceptor
HA H+ + A- (A- = conjugate base)
The constant K can be defined as
The product of [H+] and [A-] divided by [HA]
K = [A-][H+]/[HA]
Rearranged:
[H+] = K[HA]/[A-]
What is the Henderson-Hasselbach equation?
Log[H+] = log K + log [HA]/[A-]
Since pH = -log[H+], we can rearrange:
pH = pK + log[A-]/[HA]
________ have lower affinities for hydrogen ions which dissociate easily from the conjugate base
Strong acids
They also have lower pK.
________ have higher affinities for hydrogen ions which do not dissociate as easily from the conjugate base
Weak acids
Have high pKs than strong acids
The first line of defense against pH changes?
Buffers
Located in the ECF, ICF, and bone
The effectiveness of a buffer is proportional to:
Its concentration and its pK
The most important buffer system in ECF?
Bicarbonate, due to its high concentration (22-26 mEq/L).
Both CO2 and HCO3- are tightly regulated by the following equation:
CO2 +H2O H2CO3 H+ + HCO3-
________ consist of acid and conjugate base pairs (ie HA/A-)
Buffers
At low pH’s, [HA] > [A-]; at high pH’s, [A-] > [HA]
Under base load, HA can contribute H+; under acid load, A- can absorb H+. pH changes little
Because titration curves for acids are not linear, most effective buffering is …
+/- one pH unit from pK
What are the acid/conjugate base pairs for the bicarbonate and phosphate buffers systems?
HCO3-/H2CO3 (CO2)
HPO4^2-/H2PO4-
Since the pH of blood is 7.4, which would you THINK would be the better buffer, phosphate (pK = 6.8) or bicarb (pK = 6.1)?
You would think phosphate because its pK (6.8) is within 1 pH unit of the homeostatic pH.
Bicarb is still more important in the blood though…
Well then what ARE the important buffers of the blood?
1) Bicarbonate (53% of total buffering capacity) - pK is low (6.1) but it’s effective due to its concentration and because both the acid (H2CO3) and base (HCO3-) are regulated
2) Hemoglobin (35%): Hb- + H+ HHb
Imidazole groups on histidine and alpha amino groups are the primary buffer sites on all proteins
3) Proteins (7%): Prot- + H+ HProt
Have good pKs (6.4-7.9) but concentrations are too low
4) Phosphate (5%)
Unimportant in blood due to low concentration
VERY important in urine where concentrations are higher
The primary INTRACELLULAR buffers are …
Proteins: high [IC], pK’s close to 7.4
Phosphate has the same advantages as proteins
Bicarb is a secondary intracellular buffer because concentration is low
Why are bones a special case in the world of buffering?
They take up [H+] in exchange for Na+ and K+
Bone minerals may account for a significant amount of body buffering capacity during acute acid loads.
The __________ buffer system is the most important blood buffer for pH regulation
Bicarbonate
