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Acid-Base Homeostasis – outline notes

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Acid-base imbalance will affect all systems including the cardiovascular system (heart) and nervous system.

I. Acids and Bases

A. Acids – release H+ (proton donors)
-Strong acids – completely dissociate in water
HCl —-> H+ + Cl-
-Weak acids – do not completely dissociate in water
H2CO3 <—-> H+ + HCO3-

B. Bases – accept H+ (proton acceptors)
– HCO3- (bicarbonate ion) acts as a base because it accepts H+

C. Buffers – resist changes in pH by releasing H+ when solution is challenged by
a base and binds H+ when the solution is challenged by an acid

Example:       Carbonic Acid – Bicarbonate buffer system
CO2 + H2O <—-> H2CO3 <—-> H+ + HCO3-

D. pH = -log [H+]  or  pH = log 1 / [H+]
– increase [H+] —–increase acidity —- decrease pH
– increase PCO2 —decrease pH
– increase HCO3- — increase pH

E. pH scale
– ranges 0 to 14
– pH = 7 is neutral
– >7 is basic
– <7 is acidic

II. Physiological Abnormalities of Acid-Base Balance
-Normal pH range = 7.35 – 7.45
Acidosis – pH < 7.35
Alkalosis – pH > 7.45
-absolute limits –
-low of 7.0 (depresses CNS; coma and death)
-high of 7.8 (overexcites CNS; muscle tetany, extreme nervousness,
convulsions, respiratory arrest)

III. Classes of Acidosis and Alkalosis

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A. Respiratory – pH imbalance due to failure of the respiratory system to
perform its normal pH balancing role.
*Normal range — Pco2 = 35 – 45 mm Hg

B. Metabolic – pH imbalance due to all abnormalities of acid-base
imbalance except those caused by CO2 in the blood.(Kidneys / Renal)
*Normal range — HCO3- = 22 – 26 mEq/L

Four Major Classes

1. Respiratory acidosis
-Possible causes
-anything impairing gas exchange or lung ventilation (emphysema,
cystic fibrosis)
-depress respiratory center (overdose of barbiturate)
-shallow breathing
-pH < 7.35
-PCO2 > 45 mm Hg

2. Respiratory alkalosis
-Possible causes
-hyperventilation in asthma, pneumonia, anxiety
-pH > 7.45
-PCO2 < 35 mm Hg

3. Metabolic acidosis
-Possible causes
-severe diarrhea – loss of bicarbonates
-renal disease – failure of kidneys to rid the body of metabolic acids
-diabetes mellitus – ketoacidosis
-starvation – acidic metabolites are formed as body proteins and fat
reserves are used for energy
-excess alcohol ingestion – results in excess acids in the blood
-high ECF [K+] – K+ is secreted at kidneys instead of H+
-pH < 7.35
-HCO3- < 22 mEq/L

4. Metabolic alkalosis
-Possible causes
-vomiting – loss of HCl
-diuretics – cause low K+ – stimulates tubule cells of kidneys to
secrete H+
-ingestion excess antacids (NaHCO3 )  –increases HCO3- in ECF
-constipation – prolonged retention of feces results in increased
amounts of HCO3- to be reabsorbed
-pH > 7.45
-HCO3- > 26 mEq/L


IV. Respiratory and Renal Compensations
-when one system (respiratory or kidneys) is failing the other will attempt

V. Rules to determine acidosis/alkalosis, respiratory/metabolic, compensation

A. Note the pH
-greater than 7.45 = alkalosis
-less than 7.35 = acidosis

B. Check the PCO2
—-If the pH indicates acidosis and
1. the PCO2 is over 45 mm Hg = Respiratory acidosis
2. the PCO2 is below 35 mm Hg = Metabolic acidosis with
respiratory compensation
3. the PCO2 is 35-45 mm Hg = Metabolic acidosis with no
respiratory compensation

C. Check HCO3-
-In the example above (2 and 3), since the respiratory system is not the
cause of the acidosis, the cause must be metabolic; reflecting a decreased HCO3- level (below 22 mEq/L)


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