Blood pH, gases, related abnormalites

• Anoxia / Hypoxia


  absence or lack of oxygen reaching to the tissues
• Hypoxemia


  lack of oxygen in blood
• Hypercapnia / Hypocapnia


  high / low CO2 content
• Acid


  substance that can donate or release hydrogen ions (H+) when dissolved in water
• Base


  substance that can accept (H+), such as hydroxyl group (OH-), when dissolved in water
• Buffer


  substances or solutions that resist change in pH
• Effectiveness of a buffer depends on:

  1. pKa/dissociation constant of the buffering system
  2. pH of the environment
• pH


  negative log of the hydrogen ion concentration
• Ka


  dissociation constant (aka ionization constant); describes the relative strengths of acids and bases, their ability to dissociate in water
• pK / pka


  defined as the negative log of the dissociation constant of acid
• Total oxygen (ctO2) 


  oxygen content; combination of free O2 and bound O2 in hemoglobin
• Oxygen saturation (SO2)


  bound O2 / total number of O2 that hemoglobin could bind) x 100
• P50


  partial pressure
• pO2


  partial pressure exerted by O2 in the blood
• Total Co2


  (ctO2/CO2 content) - dissolved CO2
• pCO2


  partial pressure exerted by CO2 in blood
• pH Normal Value

  7.35 - 7.45
• pO2, pCO2


  blood exchange gas efficiency; do not reveal gas in blood
• pCO2 Normal Value

  35 - 45 mmHg
• HCO3- Normal Value

  22 - 29 mmol/L
• Total CO2 content Normal Value

  23 - 27 mmol/L
• pO2 Normal Value

  85 - 105 mmol/L
• SO2 Normal Value

  >95 %
• O2Hb Normal Value

  >95 %
• Clinical Importance of Acid-Base balance

  • body produces 15-20 moles of H+ per day
  • Normal concentration of H+ = 36-44 mmol/L (pH 7.35 - 7.45)
  • H+ concentration of >44 mmol/L = altered consciousness or comatose
  • H+ concentration of <36 mmol/L = neuromuscular irritability, tetany; loss of consciousness
• Buffer Systems

  • Blood buffer system: Bicarbonate-Carbonic Acid (open system), Hemoglobin-Oxyhemoglobin, Phosphate, Protein
  • Respiratory buffer system
  • Renal (kidney)buffer system
• Bicarbonate-Carbonic Acid Buffer System

  • 60-65% of buffering capacity of BLOOD
  • All catabolism (fats, protein, glucose) has CO2 as byproduct
  • INDEPENDENT to the rate and and depth of respiration
  • CO2 + H2O - Carbonic anhydrase - H2CO3 - HCO3 - H+
  • Carbonic anhydrase: can be found in renal tubular cells and inside RBC's
  • Normal HCO3: H2CO3 ratio is 20:1
• Hemoglobin-Oxyhemoglobin Buffer System

  • HPO42- H2PO4
  • plays a role in buffering the CO2 and is involved in exchange of sodium ion in urine filtrate
• Protein Buffer System


  most circulation proteins have a net negative charge and are capable of binding H+
• Respiratory Buffer System

  • Primary regulator of carbonic acid (H2CO3) and the elimination of CO2 via ventilation
  • Net effect: minimal change in H+ concentration between venous and arterial circulation
  • HYPOVENTILATION: INCREASE pCO2 and H2CO3, = retain CO2= low pH = acidic
  • HYPERVENTILATION: DECREASE pCO2 and H2CO3, = low CO2 in body = high pH = basic
• 
  

  Renal (Kidney) Buffer System

  • Kidneys regulate the excretion of both acid and base; specifically by reabsorption of bicarbonate ion from glomerular filtrate in proximal tubules
  • body produces a net excess (50 to 100 mmol/L) of acid (H+) each day that must be excreted by kidney
  • Bicarbonate ion is reabsorbed into blood along sodium
  • elevated pH: HCO3 EXCRETED
  • decreased pH: H+ EXCRETED (helped by mono hydrogen phosphate and ammonia)
• numerator-base (kidney) ; denominator-acid (lungs)
• Lungs

  • non-respiratory
  • short term compensatory (within the day)
• Kidney

  • rapid, sensitive, first line of defense
  • fast full maximal compensatory (2-4 days)
• Acid-Base Imbalances

  • Metabolic Acidosis
  • Metabolic Alkalosis
  • Respiratory Acidosis
  • Respiratory Alkalosis
• Acid-Base Imbalances (Causes)

  1. Respiratory Acidosis

  • Slow/non-removal of CO2: COPD, airway blockage, emphysema, pneumonia, asthma, anesthesia, CNS depressant

  2. Respiratory Alkalosis

  • Hyperventilation: Hypoxemia, pulmonary emboli, pulmonary fibrosis, increased environment temp, increase body temp, hysteria, aspirin
• Acid-Base Imbalances (Causes)

  3. Metabolic Acidosis

  • Loss of alkali: renal tubular acidosis, diarrhea
  • Gain of acids: acid-producing substances, diabetic ketoacidosis, starvation, organic acidosis

  4. Metabolic Alkalosis

  • Loss of acid: vomiting, diuretics
  • Bicarbonate excess: antacids
• Acid-Base Imbalances (Compensation)

  1. Respiratory Acidosis

  • Kidneys: INCREASED - HCO3 reabsorption, Na+-H+ exchange, NH4 formation

  2. Respiratory Alkalosis

  • Kidneys: DECREASED - HCO3 reabsorption, Na+-H+ exchange, NH4 formation
• Acid-Base Imbalances (Compensation)

  3. Metabolic Acidosis

  • Lungs: HYPERVENTILATE
  • Kidneys: INCREASED - HCO3 reabsorption, Na+-H+ exchange, NH4 formation
  • differentiate if it is due to: Loss of alkali (>15 mmol/L) or Gain of acid (<12 mmol/L)
  • use ANION GAP: measures undetermined anions

  4. Metabolic Alkalosis

  • Lungs: HYPOVENTILATE
  • Kidneys: DECREASED - HCO3 reabsorption, Na+-H+ exchange, NH4 formation