Acid-base homeostasis Dr. MALIK ALQUB MD. PHD.
pH pH is a measure of the acidity or basicity of an aqueous solution.
The importance of pH control • Blood pH Must be Kept Close to 7.4 • Hydrogen ion is extremely reactive and effects many molecules which regulate physiological processes • Blood pH is set at a slightly alkaline level of 7.4 (pH 7.0 is neutral) • A small change of pH in either direction is considered serious • Blood pHs below 6.9 or above 7.9 are usually fatal if they last for more than a short time
The importance of pH control • The pH of the ECF remains between 7.35 and 7.45 • If plasma levels fall below 7.35 (acidemia), acidosis results • If plasma levels rise above 7.45 (alkalemia), alkalosis results • Alteration outside these boundaries affects all body systems • Can result in coma, cardiac failure, and circulatory collapse
Mechanisms to maintain body pH 1. Chemical buffers Instantaneous physicochemical reactions that limit changes in [H+]. Their capacity is limited and cannot fully correct pH abnormalities. 2. Respiratory regulation The respiratory system can respond to changes in plasma pH by altering the excretion of CO2. The response is rapid (within minutes) and the system has a large reserve capacity. 3. Renal regulation Plasma bicarbonate is controlled via the secretion of H+ as well as the reabsorption and formation of HCO3, allowing complete correction of acid–base disorders.
1. Chemical Buffer • A buffer is a solution which contains a mixture of a weak acid and its conjugate base (or a weak base and its conjugate acid). A buffered solution resists drastic changes in pH by neutralizing any acid or base which is added to the solution.
Acids and Bases • Acid – increases the [H+] of a solution • Base – decreases the [H+] of a solution • May form OH- ion or absorb H+ ions • Strong acid/base • Completly dissociated • Weak acid/base • Partialy dissociated
Titration a method of estimating the amount of solute in a solution. The solution is added in small, measured quantities to a known volume of a standard solution until a reaction occurs, as indicated by a change in color or pH or the liberation of a chemical product
Titration of acetic acid OH- H2O CH3COO− + H+ CH3COOH OH- added Area of maximum buffering; important increase of OH added increases the PH slightly pKa = 4.8 pH 3 4 5 6 7
Carbonic Acid – Bicarbonate Buffer System CO2 +H2O H2CO3H + + HCO3– Principal buffer system in the body. Provides 95% of buffering capacity in plasma.
Weak acids - HA H + A + Weak acids Conjugate base
Henderson–Hasselbalch equation - + HA H + A Ka , acid dissociation constant (also known as acidity constant, or acid-ionization constant)
Henderson–Hasselbalch equation log log log log
Henderson–Hasselbalch equation X log log + log log log
Henderson–Hasselbalch equation - - + log log pKa = -log Ka pH = -log
Bicarbonate Buffer System CO2 +H2O H2CO3H + + HCO3– Carbonic anhydrase
Bicarbonate Buffer System H2CO3depends on pCO2, then we can replace H2CO3 by pCO2 H2CO3 is almost completely dissociated then the equation;
Henderson–Hasselbalch/Bicarbonate Buffer System The amount of CO2 in the plasma is determined by its partial pressure and solubility coefficient. pKa has the value 6.1 at 37 °C. then the equation
Carbonic Acid-Bicarbonate Buffering System CO2 + H2O H2CO3 H+ + HCO3– Respiratory regulation Renal regulation
2. Respiratory regulation • A normal by-product of cellular metabolism is carbon dioxide (CO2). CO2 is carried in the blood to the lungs, where excess CO2 combines with water (H2O) to form carbonic acid (H2CO3). • The blood pH will change according to the level of carbonic acid present. • This triggers the lungs to either increase or decrease the rate and depth of ventilation until the appropriate amount of CO2 has been re-established. • Activation of the lungs to compensate for an imbalance starts to occur within 1 to 3 minutes
3. Renal regulation • In an effort to maintain the pH of the blood within its normal range, the kidneys excrete or retain bicarbonate (HCO3) • As the blood pH decreases, the kidneys will compensate by retaining HCO3 and as the pH rises, the kidneys excrete HCO3 through the urine. • Although the kidneys provide an excellent means of regulating acid-base balance, the system may take from hours to days to correct the imbalance.
Objectives 1. Describe the physiology involved in the acid/base balance of the body. 2. Compare the roles of PaO2, pH, PaCO2 and Bicarbonate in maintaining acid/base balance. 3. Review causes and treatments of Respiratory Acidosis, Respiratory Alkalosis, Metabolic Acidosis and Metabolic Alkalosis. 4. Identify normal arterial blood gas values and interpret the meaning of abnormal values. 5. Interpret the results of various arterial blood gas samples. 6. Identify the relationship between oxygen saturation and PaO2 as it relates to the oxyhemoglobin dissociation curve. 7. Interpret the oxygenation state of a patient using the reported arterial blood gas PaO2 value.
Getting an arterial blood gas sample Arterial Blood Gas Drawn from artery- radial, brachial, femoral It is an invasive procedure. Caution must be taken with patient on anticoagulants. Arterial blood gas analysis is an essential part of diagnosing and managing the patient’s oxygenation status, ventilation failure and acid base balance.
Blood Gas Report • Acid-Base Information • pH • PCO2 • HCO3 • Oxygenation Information • PO2 [oxygen tension] • SO2 [oxygen saturation]
Detection of acidosis and alkalosis • Diagnostic blood tests • Blood pH • PCO2 • Bicarbonate levels • Distinguish between respiratory and metabolic
Normal values • pH Measurement of acidity or alkalinity, based on the hydrogen (H+) ions present. The normal range is 7.35 to 7.45 • PaCO2 The amount of carbon dioxide dissolved in arterial blood. The normal range is 35 to 45 mm Hg. • HCO3The calculated value of the amount of bicarbonate in the bloodstream. The normal range is 22 to 26 mEq/liter
Steps to an Arterial Blood Gas Interpretation • Step One Assess the pH to determine if the blood is within normal range, alkalotic or acidotic. If it is above 7.45, the blood is alkalotic. If it is below 7.35, the blood is acidotic. Acidosis Vs. alkalosis
Step Two If the blood is alkalotic or acidotic, we now need to determine if it is caused primarily by a respiratory or metabolic problem. To do this, assess the PaCO2 level. Remember that with arespiratory problem, as the pH decreases below 7.35, the PaCO2 should rise. If the pH rises above 7.45, the PaCO2 should fall. Compare the pH and the PaCO2 values. If pH and PaCO2 are indeed moving in opposite directions, then the problem is primarily respiratory in nature.
Step Three Finally, assess the HCO3 value. Recall that with a metabolic problem, normally as the pH increases, the HCO3 should also increase. Likewise, as the pH decreases, so should the HCO3. Compare the two values. If they are moving in the same direction, then the problem is primarily metabolic in nature.
Clinical cases • pH: 7.35 - 7.45 • PaCO2: 35 to 45 mm Hg. • HCO3: 22 to 26 mEq/liter
pH: 7.35 - 7.45 • PaCO2: 35 to 45 mm Hg. • HCO3: 22 to 26 mEq/liter
Respiratory Acidosis • Respiratory acidosis is defined as a pH less than 7.35 with a PaCO2 greater than 45 mm Hg. • Acidosis is caused by an accumulation of CO2 which combines with water in the body to • produce carbonic acid, thus, lowering the pH of the blood. Any condition that results in • hypoventilation can cause respiratory acidosis.
Causes of hypoventilation • Central nervous system depression related to head injury • Central nervous system depression related to medications such as narcotics, sedatives, or • anesthesia • Impaired respiratory muscle function related to spinal cord injury, neuromuscular diseases, or neuromuscular blocking drugs • Pulmonary disorders such as atelectasis, pneumonia, pneumothorax, pulmonary edema, or bronchial obstruction • Massive pulmonary embolus • Hypoventilation due to pain, chest wall injury/deformity, or abdominal distension
Respiratory Alkalosis • Respiratory alkalosis is defined as a pH greater than 7.45 with a PaCO2 less than 35 mm Hg. • Any condition that causes hyperventilation can result in respiratory alkalosis. These conditions include: • Psychological responses, such as anxiety or fear • Pain • Increased metabolic demands, such as fever, sepsis, pregnancy, or thyrotoxicosis • Medications, such as respiratory stimulants. • Central nervous system lesions
Metabolic Acidosis • Metabolic acidosis is defined as a bicarbonate level of less than 22 mEq/L with a pH of less than 7.35. Metabolic acidosis is caused by either a deficit of base in the bloodstream or an excess of acids, other than CO2. Diarrhea and intestinal fistulas may cause decreased levels of base. Causes of increased acids include: • Renal failure • Diabetic ketoacidosis • Anaerobic metabolism • Starvation • Salicylate intoxication
Metabolic Alkalosis • Metabolic alkalosis is defined as a bicarbonate level greater than 26 mEq/liter with a pH greater than 7.45. Either an excess of base or a loss of acid within the body can cause metabolicalkalosis. • Excess base occurs from ingestion of antacids, excess use of bicarbonate. • Loss of acids can occur secondary to protracted vomiting, gastric suction, • hypochloremia, excess administration of diuretics, or high levels of aldosterone.
Compensation When a patient develops an acid-base imbalance, the body attempts to compensate. Remember that the lungs and the kidneys are the primary buffer response systems in the body. The body tries to overcome either a respiratory or metabolic dysfunction in an attempt to return the Ph into the normal range. A patient can be uncompensated, partially compensated, or fully compensated. When an acidbase disorder is either uncompensated or partially compensated, the pH remains outside the normal range. In fully compensated states, the pH has returned to within the normal range, although the other values may still be abnormal. Be aware that neither system has the ability to overcompensate.
In order to look for evidence of partial compensation, review the following three steps
1. Assess the pH. This step remains the same and allows us to determine if an acidotic or alkalotic state exists.
2. Assess the PaCO2. In an uncompensated state, we have already seen that the pH and PaCO2 move in opposite directions when indicating that the primary problem is respiratory. But what if the pH and PaCO2 are moving in the same direction? That is not what we would expect to see happen. We would then conclude that the primary problem was metabolic. In this case, the decreasing PaCO2 indicates that the lungs, acting as a buffer response, are attempting to correct the pH back into its normal range by decreasing the PaCO2 (“blowing off the excess CO2”). If evidence of compensation is present, but the pH has not yet been corrected to within its normal range, this would be described as a metabolic disorder with a partial respiratory compensation.
3. Assess the HCO3. In our original uncompensated examples, the pH and HCO3 move in the same direction, indicating that the primary problem was metabolic. But what if our results show the pH and HCO3 moving in opposite directions? That is not what we would expect to see. We would conclude that the primary acid-base disorder is respiratory, and that the kidneys, again acting as a buffer response system, are compensating by retaining HCO3, ultimately attempting to return the pH back towards the normal range.