PA 502: Medical Physiology- Acid Base Physiology

-H+ donors
-strong acids dissociate completely in solution
-weak acids do not dissociate completely
-H+ acceptors
-can be classified as strong or weak
-measure of H+ concentration
How can the pH of the ECF be measured?
Henderson-Hasselbach equation
pH=pKa + log[HCO3-]/[CO2]
What are the cells of the body constantly producing?
How much H+ do healthy individuals make a day?
50-100 mEq/L
What are the 3 ways to control pH?
1. Buffer System
2. Respiratory Regulation
3. Renal Regulation
What is the Buffer System?
-takes a strong base or acid and converts it to weak base or acid
-reaction occurs within seconds
-prevent changes in pH by binding H+ (acting like bases) whenever the pH of the ECF drops below normal and releasing H+ (acting like acids) whenever the pH of the ECF climbs above normal
What are the 3 main buffer systems?
1. Bicarbonate Buffer System
2. Phosphate Buffer System
3. Protein/Amino Acid Buffer System
Bicarbonate Buffer System
-If ECF is acidotic, weak acid created:
HCL +HCO3- = HCL and carbonic acid

-If ECF is alkalotic, weak base created:
NaOH + carbonic acid = H2O and sodium bicarbonate

Phosphate Buffer System
-If ECF is acidotic, weak acid created:
HCL + disodium hydrogen Phosphate (Na2HPO4) = Salt (NaCl) + Sodium dihydrogen phosphate (Na2H2PO4)

-If ECF is alkalotic, weak base created:
Sodium Hydroxide (NaOH) + Sodium dihydrogen (NaH2PO4) + H2O and Disodium Hydrogen Phosphate (Na2HPO4)

Protein/Amino Acid Buffer System
-If ECF is acidotic:
the amine group acts as a BASE and picks up an H+ from the fluid

-If ECF is alkalotic:
carboxyl group acts as an ACID by donating H+ to the fluid

Respiratory Regulation of pH *memorize this equation!*
-increases or decreases the amount of CO2 expelled from lungs depending on what is needed by the body
-reaction occurs in minutes
-Using the equation:
CO2 + H2O H2CO3 H+ + HCO3-

Hypoventilation: increased CO2, pushing the equation to the right and increased H+ concentration

Hyperventilation: decreased CO2, reversing the equation to the left and decreased H+ concentration

Renal Regulation of pH
-only process that physically removes H+ or adds HCO3- in the body
-reactions occurs in hours to days
-If someone is in metabolic acidosis and it won’t regulate fast enough, they will die before it gets controlled
-Renal Tubules Functionals in 3 ways
What are the 3 ways Renal Tubules function?
1. Regulating H+ levels
2. Regulating HCO2- levels
3. Synthesizing HCO3-
Acidotic conditions:
-kidneys secret H+ and reabsorb HCO3-
-causes urine to become more acidic and blood more basic
-Synthesizing HCO3-: renal cells lining the proximal convoluted tubule can also synthesize HCO3- to a certain extent which is then released into the blood stream
Alkalotic conditions:
-kidneys reabsorb H+ and secrete HCO3-
-causes blood to become more acidic
decrease in arterial pH below 7.35
condition resulting from acidemia
increase in arterial pH above 7.45
condition resulting from alkalemia
Arterial Blood Gases: What is this test used to assess the adequacy of?
-Oxygenation, pO2
-Ventilation, pH and CO2
-Acid-Base Balance, pH, CO2, HCO3-
What is the normal range of pH?
What is less that 7.35 pH considered?
What is more than 7.45 pH considered?
What is a pH of less than 7.0 or more than 7.8 considered?
incompatible with life
What is pCO2?
-partial pressure of CO2 in arterial blood
-the level controlled by the respiratory system
-tells how the patient is ventilating
normal is: 45-45mmHg
What is hypocarbia?
less than 35 pCO2
What is hypercarbia?
more than 45 pCO2
What is Respiratory Alkalosis?
Increased Respirations, Decreased CO2
-causes hyperventilation (fear, pain, anxiety, crying in babies, exercise, etc.)
-stimulation of respiratory center (brain injury, overdose, encephalitis, etc.)
-mechanical over-ventilation
What is Respiratory Acidosis
Decreased Respirations, Increased CO2
-COPD, Pneumonia, Atelectasis- collapsed alveoli, Respiratory muscle wall weakness
Mechanical under-ventilation, Barbiturate or Sedative OD, Near drowning, Suffocation (choking), Chest wall abnormality (kyphosis, obesity)
Metabolic Alkalosis
patient is alkalotic and it is not respiratory alkalosis
-accumulation of too much base
-loss of too much acid
-CO2 should be normal or high, >45
-HCO3- should be high >28
*If the pH is off, but the oxygen level is okay, you need to check!
*excessive gastric suctioning
*severe vomiting of stomach contents only
*Potassium deficit
*Medications: Antacids, Diuretics
Metabolic Acidosis
-patient is acidotic and it is not respiratory acidosis
-accumulation of too much acid and loss of too much base
-CO2 normal or low <35 -HCO3- should be low <22 *If the pH is off, but the oxygen level is okay, you need to check! *Diabetic Ketoacidosis- body is starving, even though they do have enough glucose, body moves to ketones, Lactic Acidosis- not enough oxygen, Renal tubular acidosis, Renal failure, Severe diarrhea- body losing stool so fast, body has to replace enzymes in there, Starvation, Poisoning (e.g. methanol, ethylene glycol)
What is HCO3-?
-measure of bicarbonate ion in arterial blood
-a Base
-regulated by kidneys
-Normal range: 22-28 mmol/L
-leads to tissue hypoxia
-causes vasoconstriction
-causes O2 to bind tighter to Hgb, not released to tissues
-Remember Oxygen-Hemoglobin Disassociation curve
pO2, PaO2
-partial pressure of oxygen in arterial blood
-Range 80-100mmHg, age and altitude dependent
-values increase with supplemental O2
-tells how well patient is oxygenated
O2 Saturation, SaO2
-percentage of hemoglobin actually binding, saturated with oxygen
90-100% age dependent
fraction inspired air that is oxygen
-FiO2 of room air is 21%
2 liters = 28%
3 liters = 32%
4 liters = 36%
What is the FiO2 of a simple face mask?
What is the FiO2 of a Nonrebreather mask?
(for seizures, no oxygen into their blood…)
What is the FiO2 of a Bag-Mask Ventilation?
Respiratory Failure
pCO2 greater than 50
pO2 less than 50
Chronic Respiratory Failure
pCO2 greater than 50, pH 7.35-7.45
pO2 less than 50, pH 7.35-7.45
Acute Respiratory Failure
-pCO2 greater than 50, pH less than 7.35 or greater than 7.45
-pO2 less than 50, pH less than 7.35 or greater than 7.45

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