Lesson 2 Unit 6 – Acid-Base Homeostasis and Imbalances

Lesson 2 Unit 6 – Acid-Base Homeostasis and Imbalances 1 Unit 6 Study Guide Acid-Base Balance - Normal metabolism produces H+ (acidity) Three Homeostatic mechanisms: o Buffer systems – instantaneous; temporary o Exhalation of CO2 – operates within minutes; cannot completely correct serious imbalances o Kidney excretion – can completely correct any imbalance (eventually) Buffer Systems - Consist of a weak acid and the salt of that acid which functions as a weak base. - Carbonic Acid – Bicarbonate Buffer o A weak base (carbonic anhydrase) o H+ + HCO3- <-> H2CO3 <-> H20 + CO2 Phosphate Buffer o NaOH + NaH2PO4 <-> H20 + Na2HPO4 o HCl + Na2HPO4 <-> NaCl + NaH2PO4 Protein Buffer (respiratory hemoglobin and albumin) o Most abundant buffer in body cells and plasma. Amino acids have amine group (proton acceptor = weak base) and a carboxyl group (proton donor = weak acid) - - Lesson 2 Unit 6 – Acid-Base Homeostasis and Imbalances Buffers: • • • First line of defense against pH changes in all body fluids Chemicals that help control pH of body fluids Components – Weak acids that release hydrogen ions when a fluid is too alkaline – A base that takes up hydrogen when fluid is too acidic Acid-Base Balance - CNS and peripheral chemoreceptors control changes in blood pH - Increased [H+] causes immediate hyperventilation and later increased renal secretion of [H+] and [NH4+] - Decreased [H+] causes immediate hypoventilation and later decreased renal secretion of [H+] and [NH4+] Some stimulus (stress) disrupts homeostasis by causing a decrease in 2 Lesson 2 Unit 6 – Acid-Base Homeostasis and Imbalances 3 Controlled condition Blood pH (increase in H+ concentration) Receptors Central chemoreceptors in medulla oblongata. Return to homeostasis when response brings blood pH or H+ concentration (controlled condition) back to normal. Peripheral chemoreceptors in aortic and carotid bodies. Input Nerve impulses Control center Inspiratory area in medulla oblongata Output Nerve Impulses Effectors Diaphragm contracts more forcefully and frequently so more CO2 is exhaled Response As less H2CO3 forms and fewer H+ are present, blood pH increases (H+ concentration decreases) Lesson 2 Unit 6 – Acid-Base Homeostasis and Imbalances Response to an Acid-Base Perturbation The body’s response to a change in acid-base status has three components:  First defence: Buffering [as noted above this page]  Second defence: Respiratory : alteration in arterial pCO2  Third defence: Renal : alteration in HCO3- excretion The word 'defence' is used because these are the three ways that the body 'defends' itself against acid-base disturbances. This is not the complete picture as it neglects some metabolic responses (eg changes in metabolic pathways) that occur. Respiratory Contribution • • • Second defense against acid-base disorders Lungs excrete CO2 and water from body – Rid the body of carbonic acid Changes in rate and depth of respiration – Result from stimulation of chemoreceptors that sense PaCO 2 and pH of blood – Exert an influence on and serve to alter amount of PaCO2 and carbonic acid in blood Respiratory Compensation • • Respiratory response to an imbalance of any metabolic acid (except carbonic acid) Compensatory response – Does not correct a pH disorder – Does compensate for it by adjusting the pH back toward normal 4 Lesson 2 Unit 6 – Acid-Base Homeostasis and Imbalances • • • – Other blood chemistry values are made abnormal in the process Deficit of any acid except carbonic acid is hypoventilation Excess of any acid except carbonic acid is hyperventilation For an imbalance of metabolic acid – Begins in minutes – Requires at least several hours for full effectiveness Renal Contribution • Third defense against acid-base disorders • Can excrete any acid from the body except carbonic acid (solely excreted by the lungs) • Excrete metabolic acids produced continuously during normal metabolism Increased Excretion of Hydrogen • Increase production of ammonia (NH3) • Hydrogen combines with ammonia and is excreted in the urine Slow Excretion of Hydrogen • Decrease production of ammonia • Excrete less H; reabsorb hydrogen back into the blood • Excrete bicarbonate HCO3− in Plasma • Decreased HCO3− – Indicates a relative excess of metabolic acids – Metabolic acids are accumulating in the blood – Buffered by HCO3− – HCO3− concentration will drop below normal – Increased HCO3− – Indicates a relative deficit of metabolic acids – In other words, a relative excess of base • Renal response for carbonic acid imbalances • Compensatory response – For high carbonic acid levels • Increase the excretion of metabolic acids and H+ – For low carbonic acid levels • Decrease the excretion of metabolic acids and H+ • Excrete HCO3– – Requires several days to be fully operative 5 Lesson 2 Unit 6 – Acid-Base Homeostasis and Imbalances Acid-Base Imbalances Four primary disorders  Metabolic acidosis  Respiratory acidosis  Metabolic alkalosis  Respiratory alkalosis Another disorder  Mixed acid-base imbalances Acidosis - High blood [H+] Low blood pH, <7.35 Alkalosis - Low blood [H+] High blood pH, >7.45 6 Lesson 2 Unit 6 – Acid-Base Homeostasis and Imbalances 7 Acid-Base imbalances may be due to problems with ventilation or due to a variety of metabolic problems: - Respiratory Acidosis (pCO2 > 45mm Hg) Respiratory Alkalosis (pCO2 < 35 mm Hg) Metabolic Acidosis (HCO3- < 23 mmol/L) Metabolic Alkalosis (HCO3- > 26 mmol/L) Compensation: the physiological response to an acid-base imbalance begins with adjustments by the system less involved. Acid-Base Imbalances Ordinarily, chemical and physiological buffer systems maintain the hydrogen ion concentration of body fluids within very narrow pH ranges. Abnormal conditions may disturb the acid-base balance. For example, the pH of arterial blood is normally 7.35-7.45. A value below 7.35 produces acidosis. A pH above 7.45 produces alkalosis. Such shifts in the pH of body fluids may be life threatening. In fact, a person usually cannot survive if the pH drops to 6.8 or rises to 8.0 for more than a few hours. Acidosis results from an accumulation of acids or a loss of bases, both of which cause abnormal increases in the hydrogen ion concentrations of body fluids. Conversely, alkalosis results from a loss of acids or an accumulation of bases accompanied by a decrease in hydrogen ion concentrations. The two major types of acidosis are respiratory acidosis and metabolic acidosis. - Respiratory acidosis: Factors that increase carbon dioxide, also increasing the concentration of carbonic acid (the respiratory acid) - Metabolic acidosis is due to an abnormal accumulation of any other acids in the body fluids or to a loss of bases, including bicarbonate ions. Similarly, the two major types of alkalosis are respiratory alkalosis and metabolic alkalosis. - Respiratory alkalosis: Excessive loss of carbon dioxide and consequent loss of carbonic acid. - Metabolic alkalosis is due to excess loss of hydrogen ions or gain of bases. Lesson 2 Unit 6 – Acid-Base Homeostasis and Imbalances 8 Since in respiratory acidosis carbon dioxide accumulates, it can result from factors that hinder pulmonary ventilation. These include the following: - Injury to the respiratory center of the brain stem, decreasing rate and depth of breathing. - Obstructions in air passages that interfere with air movement into the alveoli. - Diseases that decrease gas exchanges, such as pneumonia, or those that reduce surface area of the respiratory membrane, such as emphysema. Any of these conditions can increase the level of carbonic acid and hydrogen ions in body fluids, lowering pH. Chemical buffers, such as hemoglobin, may resist this shift in pH. At the same time, increasing concentrations of carbon dioxide and hydrogen ions stimulate the respiratory center, increasing breathing rate and depth and thereby lowering carbon dioxide concentration. Also, the kidneys may begin to excrete more hydrogen ions. Eventually, thanks to these chemical and physiological buffers, the pH of the body fluids may return to normal. When this happens, the acidosis is said to be compensated. The symptoms of respiratory acidosis result from depression of central nervous system function, and include drowsiness, disorientation, and stupor. Evidence of respiratory insufficiency, such as labored breathing and cyanosis, is usually also evident. In uncompensated acidosis, the person may become comatose and die. Metabolic acidosis is due to either accumulation of nonrespiratory acids or loss of bases. Factors that may lead to this condition include: Kidney disease that reduces glomerular filtration and fails to excrete the acids produced in metabolism (uremic acidosis). Prolonged vomiting that loses the alkaline contents of the upper intestine and stomach contents. (Losing only the stomach contents produces metabolic alkalosis.) Prolonged diarrhea in which excess alkaline intestinal secretions are lost (especially in infants). Diabetes mellitus, in which some fatty acids are converted into ketone bodies. These ketone bodies include acetoacetic acid, betahydroxybutyric acid, and acetone. Normally these molecules are produced in relatively small quantities, and cells oxidize them as energy sources. However, if fats are being utilized at an abnormally high rate, as may occur in diabetes mellitus, ketone bodies may accumulate faster than they can be oxidized. At such times, these compounds may be excreted in the urine (ketonuria); in addition, acetone, which is volatile, may be excreted by the lungs and impart a fruity odor to the breath. More seriously, Lesson 2 Unit 6 – Acid-Base Homeostasis and Imbalances 9 the accumulation of acetoacetic acid and betahydroxybutyric acid may lower pH (ketonemic acidosis). These acids may also combine with bicarbonate ions in the urine. As a result, excess bicarbonate ions are excreted, interfering with the function of the bicarbonate acid-base buffer system. In each case, the pH tends to shift toward lower values. However, the following factors resist this shift: chemical buffer systems, which accept excess hydrogen ions; the respiratory center, which increases the breathing rate and depth; and the kidneys, which excrete more hydrogen ions. Respiratory alkalosis develops as a result of hyperventilation. Hyperventilation is accompanied by too great a loss of carbon dioxide and consequent decreases in carbonic acid and hydrogen ion concentrations. Hyperventilation may occur during periods of anxiety, although it may also accompany fever or poisoning from salicylates, such as aspirin. At high altitudes, hyperventilation may be a response to low oxygen pressure. Also, musicians, such as bass tuba players, who must provide a large volume of air when playing sustained passages, sometimes hyperventilate. In each case, rapid, deep breathing depletes carbon dioxide and the pH of body fluids increases. Chemical buffers, such as hemoglobin, that release hydrogen ions resist this pH change. Also, the lower concentrations of carbon dioxide and hydrogen ions stimulate the respiratory center to a lesser degree. This inhibits hyperventilation, thus reducing further carbon dioxide loss. At the same time, the kidneys decrease their secretion of hydrogen ions, and the urine becomes alkaline as bases are excreted. The symptoms of respiratory alkalosis include lightheadedness, agitation, dizziness, and tingling sensations. In severe cases, impulses may be triggered spontaneously on peripheral nerves, and muscles may respond with tetanic contractions. Metabolic alkalosis results from a great loss of hydrogen ions or from a gain in bases, both of which are accompanied by a rise in the pH of the blood (alkalemia). This condition may occur following gastric drainage (lavage), prolonged vomiting in which only the stomach contents are lost, or the use of certain diuretic drugs. Because gastric juice is very acidic, its loss leaves the body fluids with a net increase of basic substances and a pH shift toward alkaline values. Metabolic alkalosis may also develop as a result of ingesting too much antacid, such as sodium bicarbonate, medication to relieve the symptoms of indigestion. The symptoms of metabolic alkalosis include a decrease in the breathing rate and depth, which, in turn, results in an increased concentration of carbon dioxide in the blood. Lesson 2 Unit 6 – Acid-Base Homeostasis and Imbalances 10 Aterial Blood Gases (ABG) An arterial blood gas (ABG) test measures the acidity (pH) and the levels of oxygen and carbon dioxide in the blood from an artery. This test is used to check how well your lungs are able to move oxygen into the blood and remove carbon dioxide from the blood. As blood passes through your lungs, oxygen moves into the blood while carbon dioxide moves out of the blood into the lungs. An ABG test uses blood drawn from an artery, where the Lesson 2 Unit 6 – Acid-Base Homeostasis and Imbalances 11 oxygen and carbon dioxide levels can be measured before they enter body tissues. An ABG measures:  Partial pressure of oxygen (PaO2). This measures the pressure of oxygen dissolved in the blood and how well oxygen is able to move from the airspace of the lungs into the blood.  Partial pressure of carbon dioxide (PaCO2). This measures the pressure of carbon dioxide dissolved in the blood and how well carbon dioxide is able to move out of the body.  pH. The pH measures hydrogen ions (H+) in blood. The pH of blood is usually between 7.35 and 7.45. A pH of less than 7.0 is called acid and a pH greater than 7.0 is called basic (alkaline). So blood is slightly basic.  Bicarbonate (HCO3). Bicarbonate is a chemical (buffer) that keeps the pH of blood from becoming too acidic or too basic.  Oxygen content (O2CT) and oxygen saturation (O2Sat) values. O2 content measures the amount of oxygen in the blood. Oxygen saturation measures how much of the hemoglobin in the red blood cells is carrying oxygen (O2). Blood for an ABG test is taken from an artery. Most other blood tests are done on a sample of blood taken from a vein, after the blood has already passed through the body's tissues where the oxygen is used up and carbon dioxide is produced. Interpreting Blood Gas Results 1) Look at the pH number on your results. The pH level in a healthy human should be between 7.35 to 7.45. Your body is constantly striving to keep pH in balance. • If the pH level is below 7.35, then the condition of acidosis is present, indicating a buildup of carbonic acid in the blood. • If the pH level is above 7.45, then the condition of alkalosis is present, which indicates a buildup bicarbonate (bases) in the blood. 2) Examine the PCO2 number on your report. This number is the amount of carbon dioxide in the blood. It is the respiratory component in acid-base balance. It is measured in millimeters of mercury (mmHg). The normal level for PCO2 is between 35 to 45 mmHg. • • The condition of respiratory alkalosis is present if the PCO2 number is below 35 mmHg. This means there is too little carbon dioxide in the blood. A person with this condition can be breathing very fast (hyperventilating). The condition of respiratory acidosis is present if the PCO2 number is above 45 mmHg. There is too much carbon dioxide in the blood. A person with this Lesson 2 Unit 6 – Acid-Base Homeostasis and Imbalances 12 condition can be confused or restless and have a low heart rate. 3) Check your blood gas report for the bicarbonate (HCO3) level. It is the renal component in the acid-base balance. The normal level for these ions is between 24 to 26 milliEquivalents Per Liter (mEq/L). • An HCO3 level is below 24 mEq/L indicates metabolic acidosis. The body cannot produce enough bicarbonates to keep up with the carbonic acid in the blood. It can be caused by many conditions including dehydration and kidney disease. • An HCO3 level above 26 mEq/L indicates metabolic alkalosis. There are too many bicarbonates in the blood. A common cause is hyperventilation, but it is also seen in many other conditions including Cushing's disease and long term steroid therapy. 4) Determine whether there is compensation for the pH imbalance. Generally, the pulmonary and renal systems compensate for each other to return the pH to normal. The lungs will compensate for metabolic instability by changing CO2 excretion and the kidneys will compensate for the respiratory instability by altering bicarbonate retention and H+ secretion. • To look for compensation in respiratory acidosis (↓ pH, ↑ CO2), look at the HCO3 values. There is compensation when the HCO3 level is increased (excess hydrogen is excreted in the urine in exchange for bicarbonate ions, therefore making the blood more alkaline). However, in respiratory alkalosis (↑pH, ↓CO2), there is compensation when the HCO3 level is decreased (the renal excretion of bicarbonate increases to retain the hydrogen ions, therefore, making the blood more acidic). • To look for compensation in metabolic acidosis (↓pH, ↓HCO3), look at the PaCO2 levels. There is compensation when the PaCO2 level is decreased (due to hyperventilation, thereby raising the blood's pH value, making it more alkaline.) However, in metabolic alkalosis (↑pH, ↑HCO3), there is compensation when the PaCO2 level is increased (due to hypoventilation, therefore decreasing the blood's pH value, making it more acidic) 5) Inspect the PO2 number in your blood gas test results. This is the amount of oxygen that is dissolved in the blood. The normal PO2 level should be between 80 to 100 mmHg. If they are below limits, there is evidence of hypoxemia, therefore, no compensation is done to restore the pH balance.