howMed Know Yourself
pH Review:
pH = – log [H+]
H+ is really a proton
Range is from 0 – 14
If [H+] is high, the solution is acidic; pH < 7
If [H+] is low, the solution is basic or alkaline ; pH > 7
Acids are H+ donors.
Bases are H+ acceptors, or give up OH- in solution.
Acids and bases can be:
Strong – dissociate completely in solution
HCl, NaOH
Weak – dissociate only partially in solution
Lactic acid, carbonic acid
The Body and pH:
Homeostasis of pH is tightly controlled
Extracellular fluid = 7.4
Blood = 7.35 – 7.45
< 6.8 or > 8.0 death occurs
Acidosis (acidemia) below 7.35
Alkalosis (alkalemia) above 7.45
Small pH changes produce major disturbances:
Most enzymes function only with narrow pH ranges
Acid-base balance can also affect electrolytes (Na+, K+, Cl-)
Can also affect hormones
Body produces more acids than bases:
Acids take in with foods
Acids produced by metabolism of lipids and proteins
Cellular metabolism produces CO2.
CO2 + H20 ↔ H2CO3 ↔ H+ + HCO3-
Control of Acids:
- Buffer systems
Take up H+ or release H+ as conditions change
Buffer pairs – weak acid and a base
Exchange a strong acid or base for a weak one
Results in a much smaller pH change
2. Bicarbonate Buffer
Sodium Bicarbonate (NaHCO3) and carbonic acid (H2CO3)
Maintain a 20:1 ratio : HCO3- : H2CO3
HCl + NaHCO3 ↔ H2CO3 + NaCl
NaOH + H2CO3 ↔ NaHCO3 + H2O
3. Phosphate Buffer
Major intracellular buffer
H+ + HPO42- ↔ H2PO4-
OH- + H2PO4- ↔ H2O + H2PO42-
4. Protein buffers
Includes hemoglobin, work in blood and ISF
Carboxyl group gives up H+
Amino Group accepts H+
Side chains that can buffer H+ are present on 27 amino acids.
2) Respiratiory Mechanisms:
Exhalation of carbon dioxide
Powerful, but only works with volatile acids
Doesn’t affect fixed acids like lactic acid
CO2 + H20 ↔ H2CO3 ↔ H+ + HCO3-
Body pH can be adjusted by changing rate and depth of breathing
3) Kidney Excretion
Can eliminate large amounts of acid
Can also excrete base
Can conserve and produce bicarb ions
Most effective regulator of pH
If kidneys fail, pH balance fails
Rates of Correction
Buffers function almost instantaneously
Respiratory mechanisms take several minutes to hours
Renal mechanisms may take several hours to days
Acid Base Imbalances
pH< 7.35 acidosis
pH > 7.45 alkalosis
The body response to acid-base imbalance is called compensation
May be complete if brought back within normal limits
Partial compensation if range is still outside norms.
Compensation
If underlying problem is metabolic, hyperventilation or hypoventilation can help : respiratory compensation.
If problem is respiratory, renal mechanisms can bring about metabolic compensation.
Acidosis
Principal effect of acidosis is depression of the CNS through ↓ in synaptic transmission.
Generalized weakness
Deranged CNS function the greatest threat
Severe acidosis causes
› Disorientation
› coma
› death
Alkalosis
Alkalosis causes over excitability of the central and peripheral nervous systems.
Numbness
Lightheadedness
It can cause :
› Nervousness
› muscle spasms or tetany
› Convulsions
› Loss of consciousness
› Death
Respiratory Acidosis
Carbonic acid excess caused by blood levels of CO2 above 45 mm Hg.
Hypercapnia – high levels of CO2 in blood
Chronic conditions:
› Depression of respiratory center in brain that controls breathing rate – drugs or head trauma
› Paralysis of respiratory or chest muscles
› Emphysema
Acute conditons:
› Adult Respiratory Distress Syndrome
› Pulmonary edema
› Pneumothorax
Compensations for Respiratory Acidosis
Kidneys eliminate hydrogen ion and retain bicarbonate ion
Signs and Symptoms of Respiratory Acidosis
Breathlessness
Restlessness
Lethargy and disorientation
Tremors, convulsions, coma
Respiratory rate rapid, then gradually depressed
Skin warm and flushed due to vasodilation caused by excess CO2
Treatment of Respiratory Acidosis
Restore ventilation
IV lactate solution
Treat underlying dysfunction or disease
Respiratory Alkalosis
Carbonic acid deficit
pCO2 less than 35 mm Hg (hypocapnea)
Most common acid-base imbalance
Primary cause is hyperventilation
Conditions that stimulate respiratory center:
Oxygen deficiency at high altitudes
Pulmonary disease and Congestive heart failure – caused by hypoxia
Acute anxiety
Fever, anemia
Early salicylate intoxication
Cirrhosis
Gram-negative sepsis
Compensation of Respiratory Alkalosis
Kidneys conserve hydrogen ion
Excrete bicarbonate ion
Treatment of Respiratory Alkalosis
Treat underlying cause
Breathe into a paper bag
IV Chloride containing solution – Cl- ions replace lost bicarbonate ions
Metabolic Acidosis
Bicarbonate deficit – blood concentrations of bicarb drop below 22mEq/L
Causes:
› Loss of bicarbonate through diarrhea or renal dysfunction
› Accumulation of acids (lactic acid or ketones)
› Failure of kidneys to excrete H+
Symptoms of Metabolic Acidosis
Headache, lethargy
Nausea, vomiting, diarrhea
Coma
Death
Compensation for Metabolic Acidosis
Increased ventilation
Renal excretion of hydrogen ions if possible
K+ exchanges with excess H+ in ECF
( H+ into cells, K+ out of cells)
Treatment of Metabolic Acidosis
IV lactate solution
Metabolic Alkalosis
Bicarbonate excess – concentration in blood is greater than 26 mEq/L
Causes:
› Excess vomiting = loss of stomach acid
› Excessive use of alkaline drugs
› Certain diuretics
› Endocrine disorders
› Heavy ingestion of antacids
› Severe dehydration
Compensations for Metabolic Alkalosis
Alkalosis most commonly occurs with renal dysfunction, so can’t count on kidneys
Respiratory compensation difficult – hypoventilation limited by hypoxia
Symptoms for Metabolic Alkalosis
Respiration slow and shallow
Hyperactive reflexes ; tetany
Often related to depletion of electrolytes
Atrial tachycardia
Dysrhythmias
Treatment of Metabolic Alkalosis
Electrolytes to replace those lost
IV chloride containing solution
Treat underlying disorder
Diagnosis of Acid Base Disorder
- Note whether the pH is low (acidosis) or high (alkalosis)
- Decide which value, pCO2 or HCO3- , is outside the normal range and could be the cause of the problem. If the cause is a change in pCO2, the problem is respiratory. If the cause is HCO3- the problem is metabolic.
- Look at the value that doesn’t correspond to the observed pH change. If it is inside the normal range, there is no compensation occurring. If it is outside the normal range, the body is partially compensating for the problem.
