14. Acid Bases Physiology Flashcards
How do lungs and kidneys regulate blood acid-base balance?
Lungs and kidneys regulate blood acid-base balance by independently controlling 2 major components of the body’s major buffering system:
CO2 (lungs)
HCO3 (kidneys)
What is a pH buffer?
A substance that reversibly consumes or releases H+
Minimizes size of pH changes
Slide 2
What are the weak acids and conjugate weak acids involved in body?
(3 of each)
Weak acids:
NH4+ (ammonium)
H2CO3 (carbonic acid)
H2PO4- (“monobasic” inorganic phosphate)
Conjugate weak bases:
NH3 (ammonia)
HCO3- (bicarbonate)
HPO4- - (“dibasic” inorganic phosphate)
Each buffer reaction is governed by a dissociation constant, K
Slide 3
What happens if we add a small amount of strong acid to a physiological solution?
Strong acid (HCl) buffers in the solution consume almost all added H+
For each H buffered, one B is consumed
The amount of H that is not buffered remains free in solution and is responsible for a decrease in pH
H+Cl+B -> HB+Cl
Slide 4
What happens if we add a small amount of strong base to a physiological solution?
Strong base (NaOH) H derived from HB neutralizes almost all the added OH-
Na+OH+HB -> Na+B+H2O
Slide 4
What is buffering power (β)
Useful measure of strength of a buffer
Number of moles of strong base (NaOH) that must add to a litre of solution to increase pH by 1 pH unit
Equivalent to the amount of strong acid (HCl) one must add to decrease the pH by 1 pH unit
β= Δ[strong base]/ΔpH = -Δ[strong acid]/ΔpH
Slide 5
What is the most important physiological buffer pair?
CO2 and HCO3
The impressive strength of this buffer pair is due to the volatility of CO2
Allows lungs to maintain stable CO2 conc in the blood plasma despite ongoing metabolic and buffer reactions that produce or consume CO2
Slide 6
How do you derive Henderson-Hasselbalch equation from K of CO2 and HCO3 reaction?
K= [H][HCO3]/[CO2]
LogK=log[H] + log[HCO3]/[CO2]
pH=pK + log [HCO3]/[CO2]
pH = pK + log[HCO3]/s•PCO2
Slide 7
What are the 3 factors buffering power depends on?
- Total concentration of the buffer pair- β is proportional to total concentration
- The pH of the solution
- Whether the system is open or closed- one member of the buffer pair equilibrate between the “system” (the solution in which the buffer is dissolved) and the “environment” (everything else)
CO2/HCO3 has a far higher buffering power in an open system than in a closed system
What is a closed system?
Neither member of the buffer pair can enter or leave the system
HB can become B and vice versa
Total concentration [TB] is fixed
Ex: inorganic phosphate in beaker of water
Most non-HCO3 buffers in biological fluids behave as if they are in a closed system
Slide 9
What is the total β(non-HCO3) value?
Sum of the β closed values
Slide 9
What is an open system?
One buffer species (ex: CO2) equilibrates between three system and the environment
Blood plasma dissolved in CO2 equilibrates with gaseous CO2 in alveoli
[CO2]+[HCO3-] can vary widely since CO2 equilibrates with environment
Slide 10-11
What happens in the absence of buffers?
Doubling PCO2 causes pH to fall by 0.3 -> no change in [HCO3-]
Doubling [HCO3-] causes pH to increase by 0.3
Slide 12
What exactly do biological systems contain interns of acid bases?
(Buffers)
Mixtures of CO2/HCO3- buffer and many non-HCO3 buffers
Acid-base depends on multiple competing equilibria
Obtaining a precise solution is impossible
Intuitive approach is a graphical tool for interpreting acid-base disturbances in blood
Slide 13
What is the Davenport diagram?
Graphical tool for interpreting acid-base disturbances in blood
Slide 14-17
