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Chapter 5

BIOLOGY 3U03 Chapter 5: 5. Acid Base Balance


Department
Biology
Course Code
BIOLOGY 3U03
Professor
Michael O' Donnell
Chapter
5

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H+ Regulation
- Precisely regulated
- Maintained at a low level
o Variation varies only one millionth as much as Na+
o Also regulated at much lower concentrations than sodium
- Acids: H+ donor (Hcl, H2CO3 carbonic acid)
- Bases: H+ acceptors (HCO3-)
- Most acids and bases involved in normal acid-base regulation in ECF are weak
o Do’t full dissoiate – high proportion remains in undissociated state
- pH = log (1/[H+]) = -log[H+]
o inversely related to H+ concentration
o [H+] expressed in equivalents
- Blood concentration of H = 40 +- 4Eg/L
o Therefore a normal pH of 7.4
- pH of venous blood and interstitial fluids is slightly lower
o amount of CO2 released from tissues forms H2CO3 (weak acid) in these fluids
o What about intracellular pH? cells are metabolising producing CO2 and cell potential
is egatie i respet to hat’s outside - favors accumulation of cations (H) favoring a
slightly more acidic pH
Acid-Base Balance
- Control of ECF and ICF pH
- Focus on ECF pH normal arterial blood plasma pHa ~ 7.4
- ICF (cytoplasmic pH ~ 7.00) depnds critically on ECF pH ~ 7.40
Three Primary Systems that Regulate H+ Concentration
- Buffer systems (acts within seconds)
- Respiratory centre (within a few minutes)
- Kidneys (within several hours)
Two Major Buffer Systems in the Blood
- Act to minimize changes in free [H+] and [OH-]
1. Protein system (hemoglobin [Hb], plasma
proteins)
H+ + Pr- HPr
OH- + HPr H2O + Pr-
2. CO2/HCO3- system
H+ + HCO3- CO2 + H2O
OH- + CO2 HCO3-
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- While the protein system is quantitatively the
most important, CO2/HCO3- system is
mechanistically most improtant
o CO2/HCO3- is under active physiological
control
Isohydric Principle
- Protein and CO2/HCO3- buffer systems are in
equilibrium with each other, and with all other
less important systems (phosphate, ammonia)
o By the isohydric principle
- NOTE: only the CO2/HCO3- buffer system is subject to active physiological regulation others
follow passively
CO2/HCO3- System
- Major principles of acid-base regulation can be understood by following this system
- If CO2 produce = excretion, there is no net acid-base effect
o (i.e. equilibrium)
- Respiratory Acidosis if CO2 production > excretion by ventilated
o Net H+ and HCO3- buildup
- Respiratory Alkalosis CO2 excretion > production
o Net H+ and HCO3- loss
- Metabolic Acidosis if an acid H+ other than CO2 is added to the blood (e.g. lactic acid), reaction
is driven to the left and HCO3- is lost
o If HCO3- is lost directly (e.g. diarrhea), reaction is pulled to the right & an H+ ion is
added to the blood
- Metabolic Alkalosis if a base (OH-, HCO3-) is added to the blood, it forms or adds HCO3- and the
reaction is driven to the left, & an H+ ion is lost
o If H+ is lost directly (vomiting), reaction is pulled to the right, and a HCO- is added to
the blood
For These Metabolic Disturbances
- Net H+ loss = Net HCO3- gain
- Net H+ gain = Net HCO3- loss
- Metabolic disturbances due to a disturbance of plasma [HCO3-], which is regulated by
metabolism & kidney function slow (hours days)
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