PA 502: Medical Physiology- Acid Base Physiology

Acids
-H+ donors
-strong acids dissociate completely in solution
-weak acids do not dissociate completely
Bases
-H+ acceptors
-can be classified as strong or weak
pH
-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?
H+
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
Acidemia
decrease in arterial pH below 7.35
Acidosis
condition resulting from acidemia
Alkalemia
increase in arterial pH above 7.45
Alkalosis
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?
7.35-7.45
What is less that 7.35 pH considered?
Acidosis
What is more than 7.45 pH considered?
Alkalosis
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
-hypoventilation
-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
Alkalosis
-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
FiO2
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?
40-60%
What is the FiO2 of a Nonrebreather mask?
60-100%
(for seizures, no oxygen into their blood…)
What is the FiO2 of a Bag-Mask Ventilation?
100%
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

Get instant access to
all materials

Become a Member