Buffers and Acid Base Balance Lecture Notes PDF

BUFFERS AND ACID BASE BALANCE 1 Learning Objectives 1. Definition of buffers and component of buffer system 2. What are the factors affecting acid base balance ? 3. Define and classify acidosis and alkalosis. 4. Mechanisms of the body to compensate acid base disturbances 5. Comment on clinica...

BUFFERS AND ACID BASE BALANCE 1 Learning Objectives 1. Definition of buffers and component of buffer system 2. What are the factors affecting acid base balance ? 3. Define and classify acidosis and alkalosis. 4. Mechanisms of the body to compensate acid base disturbances 5. Comment on clinical conditions associated with disturbances of the acid–base balance. 2  The Body and pH  Homeostasis of pH is tightly controlled  Extracellular fluid = 7.4  Blood = 7.35 – 7.45  < 7.35: Acidosis (acidemia)  > 7.45: Alkalosis (alkalemia) 3 4  Maintenance of blood pH  Three lines of defense to regulate the body’s acid-base balance 1. Blood buffers 2. Respiratory mechanism 3. Renal mechanism  Rates of correction  Buffers function: almost suddenly  Respiratory mechanisms: take several minutes to hours 5  Renal mechanisms: may take several hours to days BUFFERS Buffers are solutions that resist changes in pH in spite of addition of moderate amounts of acids or alkalies (bases). Buffer consist of : 1. weak acid Salt with strong base Or 2. weak base Salt with strong acid Factors Affecting Acid Base Balance: Lungs Diet Acids Buffered Bases Kidneys Excreted Metabolic reactions Intestines 6  Type of buffer 1. Bicarbonate buffer Bicarbonate (HCO3) and carbonic acid (H2CO3)  HCO3- : H2CO3: Maintain a 20:1 ratio H2CO3 H+ + HCO3- 2. Phosphate buffer  Major intracellular buffer 3. Protein Buffers  Include plasma proteins and hemoglobin 7 8  Importance of Keeping pH within Narrow Range 7.35-7.45 Maintenance of stable pH is essential because 1. It affects the ionization of proteins and, consequently, the activity of many enzymes and other biologically active molecules such as ion channels 2. Changes in pH together with the partial pressure of carbon dioxide (pCO2) affect the shape of the hemoglobin saturation curve, and thus tissue oxygenation. 3. Decrease in pH increases sympathetic tone and may lead to cardiac dysrhythmias. 9 Reference Ranges of Blood Gases The measured values in ‘blood gases’ are pH, pCO2 and pO2; the bicarbonate concentration is calculated from pH and pCO2 values; pH below 7.0 or above 7.7 is life threatening. 1 0  RESPIRATORY AND METABOLIC COMPONENTS OF ACID BASE BALANCE The partial pressure of carbon dioxide [pCO2] is controlled by the lungs. For this reason it is called ‘the respiratory component of the acid–base balance’. On the other hand, plasma bicarbonate concentration [HCO3-] is controlled by the kidneys and erythrocytes and, consequently, it is called ‘the metabolic component of the acid–base balance The bicarbonate buffer. Blood pH is proportional to the ratio of plasma bicarbonate to the partial pressure of carbon dioxide (pCO2). The components of the bicarbonate buffer are thus the carbon dioxide and the bicarbonate. The pCO2 is the respiratory component of acid– base balance, and bicarbonate is the metabolic component. 1 1  Acid-Base Imbalances  Acidosis- a decline in blood pH ↓  Metabolic acidosis: due to a decrease in bicarbonate. ↓  Respiratory acidosis: due to an increase in carbonic acid. ↑  Alkalosis- a rise in blood pH ↑  Metabolic alkalosis: due to an increase in bicarbonate.↑  Respiratory alkalosis : due to a decrease in carbonic acid. ↓ 12 Disorders of the acid–base balance. A primary increase in pCO2, or a decrease in plasma bicarbonate concentration, lead to acidosis. A decrease in pCO2, or an increase in plasma bicarbonate, lead to alkalosis. If the primary change is in pCO2, the disorder is called respiratory, and if the primary change is in plasma bicarbonate (HCO3-), it is called metabolic. 1 3 ACIDOSIS ▪ Acidosis is a process that leads to the accumulation of hydrogen ion. ▪ Blood pH ˂ 7.35 ▪ The ratio of [HCO3- ] /[H2CO3] ˂ 20/1 ; the [HCO3 ] is decreased as in metabolic acidosis or [H2CO3] is increased as in respiratory acidosis 1. Respiratory Acidosis o Due to decreased CO2 excretion o Causes of respiratory acidosis:- A. Inhibition of respiratory center e.g.. Morphine poisoning B. Lung diseases e.g. Pneumonia and Emphysema C. Obstruction of respiratory passages e.g. Asphyxia o Compensation of respiratory acidosis Reabsorption of bicarbonate and this [HCO3 - ] Normal lead to [HCO3 ] ↑ By the Kidney ≈20/1 and pH ≈ [H2CO3] [H CO ] ↑ 7.35 2 3 Secretion of H+ + NH3 in the form of NH4+ which is excreted in urine 1 4 15 2. Metabolic Acidosis ❑ Due to decreased bicarbonate [HCO3 -] [H2CO3] Normal ❑ Causes A. Loss of bicarbonate through diarrhea or renal dysfunction B. Accumulation of acids (lactic acid or ketones) ❑ Compensation of metabolic acidosis Initial State Action of the lung After compensation [HCO3- ] [HCO3-] Hyperventilation ≈ 20/1; pH ≈ 7.35 [H2CO3] Normal [H2CO3] Loss of CO2 Later on , the kidney reabsorbs (HCO3 – ) for complete correction of acid base balance 1 6 17 ALKALOSIS Alkalosis causes a decrease in hydrogen ion concentration ↑----→ Metabolic alkalosis - [HCO3 ] It means the blood pH ˃ 7.45 ; ˃ 20/1 [H2CO3] ↓-→ Respiratory alkalosis 1. Respiratory Alkalosis ❑ Causes: Due to excessive loss of CO2 by hyperventilation as in:- 1) Fever 2) Encephalitis 3) High altitude Salicylate poisoning 4) Hysterical ❑ Compensation of Respiratory Alkalosis Initial state Action of kidney After compensation [HCO3- ] Normal Excretion of bicarbonate [HCO3- ] ↓ ≈ 20/1; pH ≈ 7.45 [H2CO3] ↓ [H2CO3] ↓ 1 HCO3 - 8 19 2. Metabolic Alkalosis ❑ Causes 1) Excess vomiting = loss of stomach acid 2) Excessive use of alkaline drugs 3) Certain diuretics 4) Endocrine disorders: aldosterone ↑ 5) Heavy ingestion of antacids ❑ Compensation of Metabolic Alkalosis Initial state Action of Lung After compensation [HCO3- ]↑ Hypoventilation [HCO3- ] ↑ ≈ 20/1; pH ≈ 7.45 [H2CO3] Normal Retention of CO2 [H2CO3] ↑ CO2 Later on , the kidney excrete (HCO3 – ) for complete correction of acid base balance 2 0 21