Chemistry - Acid-Base Metabolism
Which is the most predominant buffer system in the body? A. Bicarbonate/carbonic acid B. Acetate/acetic acid C. Phosphate/phosphorus acid D. Hemoglobin
A. Because of its high concentration in blood, the bicarbonate/carbonic acid pair is the most important buffer system in the blood. This buffer system is also effective in the lungs and in the kidneys in helping to regulate body ph. The other buffers that also function to help maintain body pH are the phosphate, protein, and hemoglobin buffer systems. The acetate buffer system is not used by the body to regulate pH.
Which of the following statements about carbonic anhydrase (CA) is true? A. Catalyzes conversion of CO2 and H2O to HHCO3 in red blood cells B. Causes shift to the left in oxygen dissociation curve C. Catalyzes formation of H2CO3 from CO2 and H2O in the tissues D. Inactive in renal tubular cells
A. Carbonic anhydrase (CA) is an enzyme found in red blood cells that catalyzes the reversible hydration of CO2 to bicarbonate and a proton: H2O + CO2 <-CA-> HHCO3 The proton, in turn, is buffered by the histidine portion of the hemoglobin molecule that activates the release of oxygen. It is at this point that oxyhemoglobin is converted to deoxyhemoglobin. In the alveoli of the lungs, CA catalyzes the conversion of H2CO3 to CO2 and H2O. The CO2 is then exhaled. Carbonic anhydrase is an intracelular enzyme of erythrocytes and renal tubular cells, and it is not found normally in any significant concentration in the plasma. It is not associated with the oxygen dissociation curve.
What is the term that describes the sum of carbonic acid and bicarbonate in plasma? A. Total CO2 B. Standard bicarbonate C. Buffer base D. Base excess
A. The concentration of total CO2 (ctCO2) or carbon dioxide content is a measure of the concentration of bicarbonate, carbonate, carbamino compounds, carbonic acid and dissolved carbon dioxide gas (PCO2) in the plasma. Bicarbonate makes up approximately 95% of the total CO2 content, but most laboratories are not equipped to directly measure bicarbonate. Therefore, total CO2 is generally quantified. The bicarbonate concentration may be estimated by subtracting the H2CO3 concentration (measured in terms of PCO2 and converted to H2CO3) from the total CO2 concentration.
Which of the following will cause a shift of the oxygen dissociation curve to the right, resulting in a decreased affinity of hemoglobin for O2? A. Low plasma pH level B. Low PCO2 level C. Low concentration of 2,3-bisphosphoglycerate D. Low temperature
A. There is a wide variety of conditions that will cause a shift of the dissociation curve of oxyhemoglobin to the left or to the right. A shift to the left will mean an increase in the affinity of hemoglobin for oxygen. Because of this increased affinity, there is also less oxygen delivered to the tissue for a given percent saturation of hemoglobin. When the curve is shifted to the right, there is a decrease in the affinity of hemoglobin for oxygen. Hence there is increased oxygen delivered to tissues for a given hemoglobin oxygen saturation. Oxyhemoglobin is a stronger acid than deoxyhemoglobin. Both exist in equilibrium in the blood. Increased hydrogen ion concentration shifts the equilibrium toward the deoxygenated form. This shift results in increased oxygen delivery to the tissue. The higher the concentration of 2,3-bisphosphoglycerate in the cell, the greater is the displacement of oxygen, thus facilitating the release of oxygen at the tissue level. Increased PCO2 and increased temperature will also have this same effect.
If a blood gas specimen is left exposed to air, which of the following changes will occur? A. PO2 and pH increase; PCO2 decreases B. PO2 and pH decrease; PCO2 increases C. PO2 increases; pH and PCO2 decrease D. PO2 decreases; pH and PCO2 increase
A. When a blood specimen is drawn for gas analysis, it is important to avoid exposure of the specimen to air because of the differences in the partial pressures of carbon dioxide and oxygen in air and in blood. The PCO2 in blood is much greater than the PCO2 in air. Hence on exposure of blood to air, the total CO2 and the PCO2 both decrease, causing an increase in pH. Similarly, the PO2 of air is much greater than that of blood, thus, the blood PO2 increases on exposure to air.
Which of the following statements best describes "base excess"? A. Primarily refers to carbonic acid concentration B. Positive values reflect metabolic alkalosis C. Created through metabolism of carbohydrates D. Negative values represent a respiratory imbalance
B. Base excess is a measure of the nonrespiratory buffers of the blood. They are hemoglobin, serum protein, phosphate and bicarbonate. Therefore, base excess reflects an abnormality in the buffer base concentration. Bicarbonate has the greatest influence on base excess, which is an indicator of metabolic function. The normal range for base excess is +/- 2.5 mmol/L. A quick estimation of base excess is to subtract the average "normal" reference bicarbonate level set by the laboratory from the measured bicarbonate level (e.g. if laboratory reference bicarbonate= 25 and patient's bicarbonate =30, then base excess = (30-25)= +5; if patient's bicarbonate=20, then base excess = (20-25)= -5). As demonstrated, a positive base excess is associated with metabolic alkalosis, and a negative base excess is associated with metabolic acidosis.
What is the anticoagulant of choice for blood gas analysis? A. EDTA B. Heparin C. Sodium fluoride D. Citrate
B. Heparin is the best anticoagulant to use in drawing blood for blood gas analyses because it does not affect the value of the blood pH. This is also critical to PO2 measurements because alterations in blood pH will cause concomitant changes in PO2 values. Several heparin salts are available for use as anticoagulants. Sodium heparinate, 1000 U/mL, is commonly used. Ammonium heparinate may be substituted for the sodium salt when it is necessary to perform additional testing, such as electrolyte analysis, on the blood gas sample.
A 24-year-old drug abuser is brought into the emergency department unconscious. He has shallow breaths, looks pale and is "clammy". Blood gases show the following results: pH= 7.29, PCO2 = 50 mmHg, HCO3- =25 mmol/L. What condition is indicated by these results? A. Metabolic alkalosis, partially compensated B. Respiratory acidosis, uncompensated C. A dual problem of acidosis D. An error in one of the blood gas measurements
B. Here the pH is decreased indicating acidosis. The PCO2 is increased, which indicates that the problem is respiratory in nature. The HCO3- is unchanged from the reference range, which indicates that there is no compensation; thus the patient has uncompensated respiratory acidosis.
In acute diabetic ketoacidosis, which of the following laboratory findings would be expected? A. Fasting blood glucose elevated, pH elevated, ketone bodies present B. Fasting blood glucose elevated, pH low, ketone bodies present C. Fasting blood glucose elevated, pH normal, ketone bodies absent D. Fasting blood glucose decreased, pH low, ketone bodies absent
B. In the diabetic patient, diabetic ketoacidosis is one of the complications that may require emergency therapy. Blood glucose levels are usually in the range of 500-700 mg/dL but may be higher. The result is severe glycosuria that produces an osmotic diuresis, leading to loss of water and depletion of body electrolytes. Lipolysis is accelerated as a result of insulin deficiency. The free fatty acids produced are metabolized to acetyl-coenzyme A units, which are converted in the liver to ketone bodies. Hydrogen ions are produced with ketone bodies (other than acetone), contributing to a decrease in blood pH. Ketoacids are also excreted in the urine, causing a decrease in urinary pH.
What is the specimen of choice for analysis of acid-base disturbances involving pulmonary dysfunction in an adult? A. Venous blood B. Arterial blood C. Capillary blood D. Urine
B. It is possible to use arterial, venous or capillary blood for blood gas analysis. The specimen of choice for determining pulmonary dysfunction in adults is arterial blood. Analysis of arterial blood is the best indicator of pulmonary function, the capacity of the lungs to exchange carbon dioxide for oxygen. PO2 and PCO2 measurements from capillary blood are usually confined to infant sampling, and they are dependent on the patient preparation and sampling site. Venous blood should not be used for blood gas studies involving pulmonary problems because venous blood gas values also reflect metabolic processes. Furthermore, the reference range for PO2 in venous blood varies drastically from arterial blood. Urine cannot be used to determine the acid/base status of a patient.
To maintain a pH of 7.4 in plasma, it is necessary to maintain a A. 10:1 ratio of bicarbonate to carbonic acid B. 20:1 ratio of bicarbonate to carbonic acid C. 1:20 ratio of bicarbonate to carbonic acid D. 20:1 ratio of carbonic acid to bicarbonate
B. The most important buffer pair in the plasma is bicarbonate with carbonic acid. Use of the Henderson-Hasselbalch equation pH = pK' + log ([salt]/[acid]) shows that the pH changes with the ratio of salt to acid- that is, bicarbonate to carbonic acid- because pK' is a constant. For this buffer pair, apparent pK' = 6.1. When the ratio of the concentrations of bicarbonate to carbonic acid is 20:1 (log of 20= 1.3), the pH is 7.4; that is, pH= 6.1+ log20 7.4= 6.1+1.3 The carbonic acid designation represents both the undissociated carbonic acid and the physically dissolved carbon dioxide found in the blood. Because the concentration of the undissociated carbonic acid is negligible compared to the concentration of physically dissolved carbon dioxide, the expression for carbonic acid concentration is usually written (PCO2 X 0.03).
How would blood gas parameters change if a sealed specimen is left at room temperature for 2 or more hours? A. PO2 increases, PCO2 increases, pH increases B. PO2 decreases, PCO2 decreases, pH decreases C. PO2 decreases, PCO2 increases, pH decreases D. PO2 increases, PCO2 increases, pH decreases
C. Glycolysis and other oxidative metabolic processes will continue in vitro by red blood cells when a whole blood specimen is left standing at room temperature. Oxygen is consumned during these processes, resulting in a decrease in PO2 levels. A decrease of 3-12 mmHg/hr at 37 degrees Celsius has been observed for blood specimens exhibiting normal PO2 ranges. This rate of decrease is accelerated with elevated PO2 levels. Additionally, carbon dioxide is produced as a result of continued metabolism. An increase in PCO2 levels of approximately 5 mmHg/hr at 37 degrees Celsius has been demonstrated. The increased production of carbonic acid and lactic acid during glycolysis contributes to the decrease in blood pH.
The measurement of the pressure of dissolved CO2 (PCO2) in the blood is most closely associated with the concentration of what substance? A. pH B. Bicarbonate (HCO3-) Carbonic acid (H2CO3) D. PO2
C. PCO2 is an indicator of carbonic acid (H2CO3). The PCO2 millimeters of mercury value (mmHg) multiplied by the constant 0.03 equals the millimoles per liter (mmol/L) concentration of H2CO3 (PCO2 X 0.03 = H2CO3). PCO2 can be measured using a pH/blood gas analyzer.
Which is a compensatory mechanism in respiratory acidosis? A. Hypoventilation B. Decreased reabsorption of bicarbonate by the kidneys C. Increased Na+/H+ exchange by the kidneys D. Decreased ammonia formation by the kidneys
C. Respiratory acidosis is a disturbance in acid-base balance that is caused by the retention of CO2 by the lungs. This imbalance is associated with such conditions as bronchopneumonia, pulmonary emphysema, pulmonary fibrosis and cardiac insufficiency. Respiratory acidosis is characterized by a primary excess in physically dissolved CO2, which is quantified by measuring the blood PCO2 level. The primary problem leading to an increase in the PCO2 level is hypoventilation. This retention of CO2 alters the normal 20:1 ratio of cHCO3-/PCO2, causing a decrease in blood pH level. In respiratory acidosis, because the initial defect is associated with the lungs, the kidneys respond as the major compensatory system. The production of ammonia, the exchange of Na+ for H+ with the excretion of H+ and the reabsorption of bicarbonate are all increased in the kidneys to compensate for the malfunction of the lungs. In cases where the defect is not within the respiratory center, the excess of PCO2 in the blood can actually have a stimulatory effect on the center, causing an increase in the respiration rate. Thus compensation can also occur through CO2 elimination by the lungs.
Given the following information, calculate the blood pH. PCO2= 44 mmHg Total CO2= 29 mmol/L A. 6.28 B. 6.76 C. 7.42 D. 7.44
C. The acid-base equilibrium of the blood is expressed by the Henderson-Hasselbalch equation: pH=pK' + log (cHCO3-/ (PCO2 X 0.03)) For the stated problem, convert PCO2 in mmHg to dissolved CO2, multiplying by the solubility coefficient of CO2 as: 44 mmHg X 0.03 mmol/L/mmHg=1.32 mmol/L. Next, determine the difference between the total CO2 and dissolved CO2 concentrations: 29 mmol/L -1.32 mmol/L=27.68 mmol/L. pK' for the bicarbonate buffer system is 6.1. Therefore, pH=6.1+log(27.68/1.32) pH=6.1+log 20.97 pH=6.1+ log 21 pH= 6.1+1.32 pH=7.42
Which of the follwoing characterizes respiratory acidosis? A. Excess of bicarbonate B. Deficit of bicarbonate C. Excess of dissolved carbon dioxide (PCO2) D. Deficit of dissolved carbon dioxide (PCO2)
C. The normal ratio of bicarbonate ions to dissolved carbon dioxide is 20:1 and pH=6.1+log 20/1. An excess of dissolved CO2 (e.g. increase in PCO2) will increase the denominator in the equation or decrease the ratio of bicarbonate ions to dissolved CO2. The pH will decrease; that is, the plasma becomes more acidic. The amount of dissolved CO2 (PCO2) in the blood is related to respiration. Hence, this condition is termed "respiratory acidosis".
Blood gases are drawn on a 68-year-old asthmatic who was recently admitted for treatment of a kidney infection. Blood gas results are as follows: pH = 7.25, PCO2 = 56 mmHg, HCO3- = 16 mmol/L. What condition is indicated by these results? A. Metabolic alkalosis, partially compensated B. Respiratory acidosis, uncompensated C. A dual problem of acidosis D. An error in one of the blood gas measurements
C. The pH clearly indicates acidosis. Both the metabolic (decreased HCO3-) and respiratory (increased PCO2) components, however, indicate acidosis. There is no compensation seen in the results. Thus the patient has a double or mixed problem of acidosis.
In order to maintain electrical neutrality in the red blood cell, bicarbonate leaves the red blood cell and enters the plasma through an exchange mechanism with what electrolyte? A. Sodium B. Potassium C. Chloride D. Phosphate
C. The red blood cell membrane is permeable to both bicarbonate and chloride ions. Chloride ions participate in buffering the blood by difusing out of or into the red blood cells to compensate for the ionic change that occurs when bicarbonate enters or leaves the red blood cell. This is called the chloride shift.
A mother brings her daughter, a 22-year old medical technology student, to her physician. The patient is hyperventilating and has glossy eyes. The mother explains that her daughter is scheduled to take her final course exam the next morning. The patient had been running around frantically all day in a worried state and then started to breathe heavily. Blood gases are drawn in the office with the following results: pH= 7.58, PCO2 = 55 mmHg, HCO3- = 18 mmol/L. What do these data indicate? A. Metabolic alkalosis, partially compensated B. Respiratory acidosis, uncompensated C. A dual problem of acidosis D. An error in one of the blood gas measurements
D. Here the pH and case information indicate alkalosis, but both the metabolic (decreased HCO3-) and respiratroy (increased PCO2) components indicate acidosis. Most likely there is a problem/error in one or more of the measurements.
Which of the following statements is true about partially compensated respiratory alkalosis? A. PCO2 is higher than normal B. HCO3- is higher than normal C. More CO2 is eliminated through the lungs by hyperventilation D. Renal reabsorption of HCO3- is decreased.
D. Laboratory results from arterial blood gas studies in partially compensated respiratory alkalosis are as follows: pH slightly increased, PCO2 decreased, HCO3- decreased, and total CO2 decreased. Respiratory alkalosis is a disturbance in acid-base balance that is caused by hyperventilation associated with such conditions as fever, hysteria and hypoxia. Respiratory alkalosis is characterized by a primary deficiency in physically dissolved CO2 (decreased PCO2). This decrease in the level of PCO2 is due to hyperventilation, causing the accelerated loss of CO2 by the lungs. This loss of CO2 alters the normal 20:1 ratio of cHCO3-/PCO2, causing an increase in the blood pH level. In respiratory alkalosis, because the initial defect is in the lungs, the kidneys respond as the major compensatory system. Ammonia production in the kidneys is decreased, Na+-H+ exchange is decreased with the retention of H+, and bicarbonate reabsorption is decreased. By decreasing the bicarbonate reabsorption into the bloodstream, the kidneys attempt to reestablish the 20:1 ratio and normal blood pH. In a partially compensated state, as the blood bicarbonate level decreases, the blood pH begins to return toward normal but continues to be slightly alkaline. In a fully compensated state the blood pH is normal.
Which of the following is a cause of metabolic alkalosis? A. Late stage of salicylate poisoning B. Uncontrolled diabetes mellitus C. Renal failure D. Excessive vomiting
D. One of the primary reasons for metabolic alkalosis, especially in infants, is vomiting. Hydrogen ions are lost in the vomit, and the body reacts to replace them in the stomach. Consequently, hydrogen is lost from the plasma. This loss of hydrogen is due to a metabolic as opposed to a respiratory reason. Salicylate poisoning, uncontrolled diabetes mellitus and renal failure all lead to metabolic acidosis either through an overproduction of ketone bodies, such as acetoacetic acid and beta-hydroxybutyric acid, or because of a reduced excretion of acid by the kidneys.
In the plasma, an excess in the concentration of bicarbonate without a change in PCO2 from normal will result in what physiological state? A. Respiratory acidosis B. Respiratory alkalosis C. Maetabolic acidosis D. Metabolic alkalosis
D. The acid-base equilibrium of the blood is expressed by the Henderson-Hasselbalch equation: pH=pK' + log (cHCO3-/ (PCO2 X 0.03)) In this buffer pair, pK'=6.1. Normally, the ratio of the concentration of bicarbonate ions cHCO3- to the concentration of carbonic acid expressed as (PCO2 X 0.03) in the plasma is 20:1. The bicarbonate component of the equation is considered to be the "metabolic" component, controlled by the kidneys. The carbonic acid component is considered the "respiratory" component, controlled by the lungs. An excess of bicarbonate without a change in PCO2 will increase the ratio of bicarbonate to carbonic acid. Therefore, the pH will increase; that is, the plasma becomes more alkaline.
The bicarbonate ion concentration may be calculated from the total CO2 and PCO2 blood levels by using which of the following formulas? A. 0.03 X (PCO2 - total CO2) B. (total CO2 + 0.03) X PCO2 C. 0.03 X (total CO2 - PO2) D. total CO2 - (0.03 X PCO2)
D. The solubility coefficient of CO2 gas (dissolved CO2) in normal blood plasma at 37 degrees Celsius is 0.03 mmol/L/mmHg. The concentration of dissolved CO2 found in plasma is calculated by multiplying the PCO2 blood level by the solubility coefficient (0.03). The predominant components of total CO2 are bicarbonate (95%) and carbonic acid (5%). The bicarbonate ion concentration in millimoles per liter can be calculated by subtracting the product of (0.03 mmol/L/mmHg X PCO2 mmHg), which represents carbonic acid, from the total CO2 concentration (millimoles per liter).
A 75-year-old woman comes to her physician complaining of abdominal pain. She says she has had a sore stomach for the last 3 weeks and has been taking increasing doses of antacid pills to control it. Now she is taking a box of pills a day. Blood gases are drawn with the following results: pH= 7.49, PCO2 =59 mmHg, HCO3- =25 mmol/L. What do these data indicate? A. Metabolic alkalosis, partially compensated B. Respiratory acidosis, uncompensated C. A dual problem of acidosis D. An error in one of the blood gas measurements
In evaluating acid-base balance, the pH, PCO2 and total CO2 of an arterial blood specimen are measured. The reference values of arterial whole blood at 37 degrees Celsius for adults are as follows: pH= 7.35-7.45 PCO2= 35-45 mmHg cHCO3- =22-26 mmol/L ctCO2= 23-27 mmol/L PO2= 80-110 mmHg Respiratory acidosis is characterized by an increase in blood PCO2, whereas respiratory alkalosis is characterized by a decrease of blood PCO2. Metabolic acidosis is characterized by a decrease in the blood bicarbonate levels, whereas metabolic alkalosis is related to an increase in blood bicarbonate levels. A. In this case the pH is increased indicating alkalosis. HCO3- is increased, which means it is a metabolic problem. The PCO2 is also increased, which indicates that the lungs are trying to compensate by retaining PCO2 thus bringing the pH closer to 7.4.
