Acid-Base Balance Lecture Notes PDF

ACID-BASE BALANCE LECTURER: NADA MAHMOUD  DEFINITIONS Acid is a substance that can yield a hydrogen ion (H_) or hydronium ion when dissolved in water. A base Base :is a substance that can yield hydroxyl ions (OH_) Buffer: the combination of a weak acid or weak base and its salt, is...

ACID-BASE BALANCE LECTURER: NADA MAHMOUD  DEFINITIONS Acid is a substance that can yield a hydrogen ion (H_) or hydronium ion when dissolved in water. A base Base :is a substance that can yield hydroxyl ions (OH_) Buffer: the combination of a weak acid or weak base and its salt, is a system that resists changes in pH(example in plasma: the bicarbonate– carbonic acid system  Maintenance of H IONS -The normal concentration of H+ in the extracellular body fluid ranges from 36–44 nmol/L (pH, 7.34– 7.44) -The buffering system , lungs and kidneys controls and excretes H+ in order to maintain pH homeostasis. -pH is the negative log of the conc H+, an increase in H+ concentration decreases the pH, whereas a decrease in H+ concentration increases the pH Acidosis ---- Less than 7.35 pH Alkalosis ---- more than 7.45 pH  Buffer Systems -The body’s first line of defense against extreme changes in H+ concentration -Present in all body fluids. All buffers consist of a weak acid, such- as carbonic acid (H2CO3), and its salt or conjugate base, bicarbonate(HCO3) -When an acid is added to the bicarbonate–carbonic acid system, the HCO3- will combine with the H+ from the acid to form H2CO3. -When abase is added, H2CO3 will combine with the OH- group to form H2O and HCO3  The interrelationship of hemoglobin in the red blood cells and the H+ from the bicarbonate buffering system. -Phosphate buffer system plays a role in plasma and red blood cells and is involved in the exchange of sodium ion in the urine H+filtrate. - Plasma protein, especially the imidazole groups of histidine, also forms an important buffer system in plasma. Most circulating proteins have a net negative charge and are capable of binding H+.  Regulation of Acid-Base Balance: Lungs and Kidneys -The interrelationship of the lungs and kidneys in maintaining pH is depicted by the Henderson- Hasselbalch equation -The numerator (HCO3-) denotes kidney function, whereas the denominator (pCO2, which represents H2CO3) denotes lung.function -The Henderson-Hasselbalch equation expresses acidbase relationships in a mathematical formula -pK’ is pH at which there is an equal concentration of protonated and unprotonated species - In plasma and at body temperature (37°C), the pK of the bicarbonate buffering system is 6.1  -The lungs (respiratory component )participate rapidly in the regulation of blood pH through hypoventilation or hyperventilation. -A change in the H concentration of blood that results from nonrespiratory disturbances causes the respiratory center to respond by altering the rate of ventilation in an effort to restore the blood pH to normal. -The kidneys the non respiratory or formerly known as the metabolic component, control the bicarbonate concentration. -Without reclamation , the loss of HCO3 in the urine would result in an excessive acid gain in the blood. The main site for HCO3 reclamation is the proximal tubules -In health, when the kidneys and lungs are functioning properly, a 20:1 ratio of HCO3 to H2CO3 will be maintained (resulting in a pH of 7.40). -Under normal conditions, the body produces a net excess (50–100 mmol/L) of acid each day that must be excreted by the kidney. -Because the minimum urine pH is approximately 4.5, the kidney excretes little nonbuffered H. -The remainder of the urinary H+ combines with dibasic phosphate (HPO4-) and ammonia (NH3) and is excreted as dihydrogen phosphate (H2PO4-) and ammonium (NH4-). How kidney reabsorbed HCO3 ?? in proximal tubule ANION GAP= (Na+K) - (Cl+HCO3) Normal Range: 10 – 20 mmol per L  Acid-Base Disorders Acidemia: which reflects excess acid or H concentration. Alkalemia : excess base or pH greater than.the reference range Respiratory acidosisor alkalosis :A disorder caused by ventilatory dysfunction (a change in the pCO2, the respiratory component) Nonrespiratory (metabolic): A disorder resulting from a change in the bicarbonate level (a renal or metabolic function) Compensation : the body tries to restore acid-base homeostasis by altering the factor not primarily affected Mixed respiratory and nonrespiratory disorders occasionally arise from more than one pathologic process and represent the most serious of medical conditions as compensation for the primary disorder is failing. Acid-Base Disorders -The lungs can compensate immediately, but the response is short term and often incomplete. -The kidneys are slower to respond (2–4 days), , but the response is long term and potentially complete. -Fully compensated implies that the pH has returned to the normal range (the 20:1 ratio has been restored) -Partially compensated implies that the pH is approaching normal. Metabolic acidosis: decrease in bicarbonate (less than 24 mmol/L), resulting in a decreased pH. Compensated by hyperventilation. Respiratory acidosis decrease in alveolar ventilation (hypoventilation), causing a decreased elimination of CO2 by the lungs ,which lead to hypercarbia (elevated pCO2). Compensated by kidneys increase the excretion of H and increase the reclamation of HCO3  Note Although the renal compensation begins immediately, it takes days to weeks for maximal compensation to occur. When the HCO3 in the blood increases as a result of the action of the kidneys, the base-to-acid ratio will be altered and the pH will return toward normal. Metabolic alkalosis: increase in bicarbonate , resulting in a increased pH. Compensated by hypoventilation(increase retention of CO2) Respiratory alkalosis increased rate of alveolar ventilation causes excessive elimination of CO2 by the lungs. Compensated the kidneys compensate by excreting HCO3 in the urine and reclaiming H to the blood.