Fluid and Electrolytes P A R T 4

Fluid and Electrolytes P A R T 4

Mechanisms that control acid-base homeostasis Acids and bases continually enter and leave body Hydrogen ions also result from metabolic activity

Acids Hydrogen ion donors Dissociation = H + and a conjugate base Bases Hydrogen ion acceptors ph determined by hydrogen ion concentration

Chemical buffer System of two or more compounds that act to resist ph changes when strong acid or base is added

Mechanisms for neutralizing or eliminating H + 1. Bicarbonate carbonic acid buffer system 2. Phosphate buffer system 3. Hemoglobin-oxyhemoglobin system 4. Protein buffer system

Strong acids Dissociate completely in water Can dramatically affect ph Weak acids Dissociate partially in water Efficient at preventing ph changes Buffering changes strong acids into weak ones

HCI H 2 CO 3 (a) A strong acid such as HCI dissociates completely into its ions. (b) A weak acid such as H 2 CO 3 does not dissociate completely. Figure 26.11

Buffers Sodium Bicarbonate-carbonic acid Mixture of H 2 CO 3 (weak acid) and NaHCO 3 (weak base) Buffers ICF and ECF Present in all body fluids

Buffers Sodium bicarbonate-carbonic acid If strong acid is added: HCO 3 ties up H + and forms H 2 CO 3 HCl + NaHCO 3 H 2 CO 3 + NaCl ph decreases only slightly H 2 CO 3 is easily converted to CO 2 and H 2 O Removed by respiratory system

Buffers Sodium bicarbonate-carbonic acid If strong base is added Causes H 2 CO 3 to dissociate and donate H + H + ties up the base (e.g. OH ) NaOH + H 2 CO 3 NaHCO 3 + H 2 O Creation of a weak base means that ph rises only slightly H 2 CO 3 supply is almost limitless from CO 2 released by respiration

Buffers Sodium bicarbonate-carbonic acid Normal body processes tend to acidify blood More base is needed Normal ratio is 20:1 (NaHCO 3 :H 2 CO 3 ) Produces ph of 7.4

Buffers Sodium bicarbonate-carbonic acid Plasma bicarbonate highly regulated by kidneys Plasma carbonic acid regulated by lungs

Buffers Phosphate buffer system Takes place in kidney tubular fluids and RBC s Action is nearly identical to the bicarbonate buffer

Buffers Phosphate buffer system Components are sodium salts of: Dihydrogen phosphate (H 2 PO 4 ) Weak acid Monohydrogen phosphate (HPO 2 4 ) Weak base Effective buffer in urine and ICF (RBC s) Where phosphate concentrations are high

Buffers Phosphate buffer system Addition of strong acid HCL + Na 2 HPO 4 NaH 2 PO 4 + NaCl Addition of strong base NaOH + Na 2 HPO 4 Na 2 HPO 4 + H 2 O Both reactions prevent large changes in ph

Buffers Hemoglobin-oxyhemoglobin buffer system Second buffer in RBC s

CO 2 O 2 Tissue cell CO 2 CO 2 Interstitial fluid CO 2 CO 2 CO 2 CO 2 CO 2 (dissolved in plasma) Slow CO 2 + H 2 O H 2 CO 3 HCO 3 + H + Fast CO 2 + H 2 O H 2 CO 3 Carbonic anhydrase HCO 3 + H + Cl CO 2 + Hb HbCO 2 (Carbaminohemoglobin) Red blood cell HbO 2 O 2 + Hb HHb HCO 3 Cl Binds to plasma proteins Chloride shift (in) via transport protein O 2 (a) Oxygen release and carbon dioxide pickup at the tissues O 2 (dissolved in plasma) Blood plasma Figure 22.22a

Buffers Protein buffer system Most abundant buffer of the body Active over wide ph range

Buffers Protein buffer system Protein molecules are amphoteric Can function as both weak acids and weak bases When ph rises Carboxyl (COOH) groups release H + When ph falls NH 2 groups bind H +

Buffers Protein buffer system A single protein molecule may function as an acid or a base Depends upon the ph in the solution

Respiratory and renal regulation Respiratory regulation Regulates CO 2 and H + Renal regulation Three actions 1. Acidification of urine 2. Reabsorption of bicarbonate 3. Buffering effects of phosphate and ammonia in filtrate

Refer to F&E 45 Read on Respiratory Acidosis Left column List causes of respiratory acidosis in your notes

Respiratory acidosis Carbonic acid excess, distinguished by CO2 Conditions which impair ventilation Drug overdose (sedatives) Chest or head injuries Pulmonary edema Sudden airway obstruction COPD

Respiratory acidosis Compensation by kidneys Work to reestablish 20:1 ratio of bicarbonate : carbonic acid CO2 diffuses into RBC to be buffered Chloride shift liberates bicarbonate into ECF Kidney increases formation of NaHCO 3 and excretion of ammonium ion Example ph = 7.30 pco 2 = 55 HCO 3 - = 27

Refer to F&E 45-46 Read on Respiratory Acidosis #1-6 List signs and symptoms in notes

Respiratory acidosis Temporary disturbance Increased respiratory rate Signs and symptoms of respiratory acidosis Dilation of cerebral blood vessels Cerebral edema and depressed CNS Increased acid and ammonia in urine Hyperkalemia Dysrhythmias

Refer to F&E 47 Read on Respiratory Alkalosis Left column List causes in notes

Respiratory alkalosis Carbonic acid deficit, characterized by fall in CO 2 Causes of hyperventilation Anxiety Early COPD Early aspirin toxicity Excessive mechanical ventilation High altitude

Respiratory alkalosis Compensation involves Na + and K + excretion H + and Cl - retention Example ph = 7.48 pco 2 = 30 HCO - 3 = 20

Refer to F&E 47-48 Read on Respiratory Alkalosis #1-6 List symptoms in notes

Respiratory alkalosis Symptoms Cerebral vasoconstriction Lightheadedness Lack of ability to concentrate Tingling in hands and feet Carpopedal spasms Altered conciousness

Metabolic acidosis Decreased bicarbonate concentration Loss of base Kidney disease Severe diarrhea or vomiting Excess acid accumulation Diabetes Lactic acid Impaired renal excretion

Metabolic acidosis Respiratory compensation Hyperventilation to blow of CO 2 Kussmal breathing Renal compensation If possible Reabsorb NaHCO 3 and secrete H + and NH 4 + Example ph = 7.32 pco 2 = 29 HCO 3 - = 17

Refer to F&E 49-50 Read on Metabolic Acidosis List symptoms in notes

Metabolic acidosis Signs and symptoms Headache, nausea, vomiting Confusion and drowsiness Increased respiration rate and depth Hyperkalemia Colic, diarrhea, muscular weakness, tingling and numbness in extremities Hypercalcemia More calcium is ionized

Metabolic alkalosis Distinguished by an increase in bicarbonate Excess accumulation of base Increased consumption (antacid abuse) Depletion of acid Excessive vomiting, gastric suction, diuretics

Metabolic alkalosis Compensation Respiratory Hypoventilation Hypoxic drive of chemoreceptors limits this response Kidneys If possible Secretes NaHCO 3 and retains H + Example ph = 7.50 pco 2 = 50 HCO - 3 = 32

Refer to F&E 51-52 Read on Metabolic Alkalosis List symptoms in notes

Metabolic alkalosis Signs and symptoms Tingling of fingers and toes Dizziness Tetany Carpopedal spasma Signs mainly due to decreased ionized calcium and increased membrane permeability

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