Acid-base Balance PDF

# Acid-base balance ## Defining Acid and Base * **Acid**: A substance that releases protons or hydrogen ions (H+). * examples: hydrochloric acid (HCl), carbonic acid (H2CO3) * Chemical reactions: * HCl → H+ + Cl- * H2CO3 → H+ + HCO3- * **Base**: A substance that acce...

# Acid-base balance ## Defining Acid and Base * **Acid**: A substance that releases protons or hydrogen ions (H+). * examples: hydrochloric acid (HCl), carbonic acid (H2CO3) * Chemical reactions: * HCl → H+ + Cl- * H2CO3 → H+ + HCO3- * **Base**: A substance that accepts hydrogen ions, for example, bicarbonate ion (HCO3-), and HPO₄^-2, or can yield hydroxyl ion (OH-). * Chemical reactions: * HCO3 + H+ → H2CO3 * HPO4 + H+→ H2PO4 ## Buffers * **Buffer**: A solution that consists of a weak acid, like H2CO3, and salt of its conjugated base (HCO3-) that resists changes in pH when acid or base is added to the solution. ## Notes on Acid-base balance 1. Change in one pH unit represents a 10-fold change in [H+]. 2. The normal pH of arterial blood is 7.4 3. The pH of venous blood and interstitial fluids is about 7.35 because of the extra amounts of (CO2) released from the tissues to form H2CO3 in these fluids. 4. Thus, the pH of blood is maintained within a constant level of 7.35-7.45. 5. **Acidemia**: An arterial blood pH is less than 7.35. 6. **Alkalemia**: An arterial blood pH is more than 7.45. 7. **Acidosis and alkalosis** refer to pathological states that lead to acidemia and alkalemia respectively. ## Regulation of blood pH To maintain the blood pH at 7.35-7.45 there are three systems that regulate the H+ ion concentration in the body fluids, these are: 1. **Buffer mechanism** 2. **The respiratory mechanism** 3. **Renal mechanism** **Buffer mechanism and their role in regulation the pH of the body fluids** ## Bicarbonate/Carbonic acid buffer system * H2CO3 is weak acid (because it doesn't completely dissociate into HCO3- and H+). * In contrast, a strong acid such as HCl completely dissociates to H+ and Cl- in a solution. ### How the buffer system functions * When an acid is added to this buffer system, the HCO3- combines with H+ from the acid to form H2CO3. * When a base is added, H2CO3 will combine with the OH- group to form H2O and HCO3-. * **In both cases, there is a smaller change in pH when adding acid or base.** * Reaction: HCO3- + H+ --------→ H2CO3 ## Protein buffer system * Most of circulating proteins have a net -ve charge and are capable of binding H+. * The most important buffer groups are the imidazole groups of histidines, where each albumin molecule contains 16 histidines. * Reaction: Pr- + H+--→HPr ## Haemoglobin buffer system * Reaction: HbO2 + H+---------→ HHb + O2 (oxygen moves to the peripheral tissue) ## Disturbances of hydrogen ion homeostasis ### Acidosis * **Metabolic Acidosis (HCO3 – Deficit or Fall in pH)**: * The primary abnormality is lost in HCO3- in the buffering of excess acid, resulting in a fall in pH. * The following are types of metabolic acidosis: * **Diabetic ketoacidosis**: Production of organic acid (acetoacetic acid and beta-hydroxy butyric acid) exceeds the rate of elimination. * **Lactic acidosis**: Production of lactic acid that exceeds the rate of elimination. * **Renal failure**: Reduced excretion of acid (H+) ion. * **Renal tubular acidosis**: Excessive renal loss of bicarbonate. * **Duodenal fluid loss**: e.g. severe diarrhea (Co2 + H20 -> H2CO3 in cells). ### Respiratory Acidosis (Excess CO2) * The primary abnormality is CO2 retention, usually due to impaired alveolar ventilation, resulting in a fall in pH and there is a fall in ratio [ HCO3-]: PCO2 * It occurs in: * COPD, like emphysema * Severe pneumonia * Asthma * Depression of respiratory center like CNS infection and drugs ### Alkalosis * **Metabolic Alkalosis (HCO3- Excess or rise in pH)**: * The primary abnormality is rise in [HCO3-] and increase the ratio HCO3: PCO2. * It can occur due to: * **Hypokalemia**: Hypokalemia causes K+ to move from cells into the extracellular fluid, and this transcellular K+ movement is accompanied by movement of H+ from the extracellular fluid into cells to maintain electroneutrality, which causes metabolic alkalosis. * **Prolonged vomiting or nasogastric suction**: When HCl loss occurs, this causes hypochloraemic alkalosis. * **Bicarbonate administration**: This occurs when large amounts of HCO3 - are administered, such as IV administration or chronic use of antacids for the treatment of indigestion. * **Respiratory Alkalosis (CO2 Deficit):** * The primary abnormality is a fall in PCO2, and an increase in the ratio HCO3 : PCO2. * It occurs when rapid or deep inspiration due to: * Hysterical overbreathing * Hypoxia * Pulmonary edema * Excessive artificial ventilation ## Anion gap * The anion gap is the difference between the total concentration of measured cations (Na+, K+) and measured anion (Cl-, HCO3-). * It is normally about 15-20 mEq/L. ## Laboratory findings in acid-base disturbances | | pH | pCO2 | HCO3 | HCO3/H2CO3 | | :----------------------- | :-------------- | :--------------- | :-------------------------- | :-------- | | **Normal** | 7.4 ± 0.05 | 40 mm Hg | 20 mm Hg | 20 | | **Metabolic acidosis** | Decreased | Normal | Decreased | Decreased | | **Metabolic alkalosis** | Increased | Normal | Increased | Increased | | **Respiratory acidosis** | Decreased | Increased | Normal | Decreased | | **Respiratory alkalosis** | Increased | Decreased | Increased | Increased |