Acid Base Balance Flashcards
How do Acids and Bases behave?
Acids donate H+
- Strong acids readily give up H+
Bases accept H+
- Strong bases readily accept H+
What are conjugate acids and bases
- Conjugate acid - Formed by the addition of a proton to a base e.g. Ammonium ion, NH4+ (ammonia is the base)
- Conjugate base - Formed by the removal of a proton from an acid e.g. Hydrogen carbonate ion, HCO3- (carbonic acid is the acid
What is the definition of pH?
pH of a solution is defined as the negative logarithm of the hydrogen ion activity
pH = -log10[H+]
What is the pH of blood?
pH of blood is 7.35 – 7.45 (35 – 45 nmol/L)
- Blood [H+] > 45 nmol/L acidaemic
- Blood [H+] < 35 nmol/L alkalaemic
Describe the hydrogen ion production within the body?
- Normal metabolic processes cause the net formation of 40 – 80 mmol of H+ ions in 24hrs
- Temporary imbalances can be absorbed by buffering
- Disease states where there is an imbalance leads to acidosis or alkalosis
How does the buffer system work within the body?
- Consists of a weak acid i.e. not completely dissociated and conjugate base
- If H+ are added to a buffer some will combine with the conjugate base and convert it to the undissociated acid
- Buffer works most efficiently at H+ concentrations which result in approximately equal concentrations of undissociated acid and conjugate base
What is the pKa?
pKa represents the negative logarithm of the ionisation constant of an acid (Ka)
pKa is the pH at which a buffer exists in equal proportions with its acid and conjugate base
- Acids have pKa values < 7.0
- Bases have pKa values > 7.0
How is pH derived from the pKa?
Henderson-Hesselbach
What are the Buffering systems in the body?
Blood
- Bicarbonate
- Haemoglobin
- Plasma proteins
Bone
Urine
- Phosphate
- Ammonium
How does haemoglobin provide a buffering system?
- Anaerobic metabolism therefore little CO2 in red blood calls
- Hb is a more powerful buffer when in deoxygenated state and the proportion in this state increases during the passage of blood through capillary beds because oxygen is lost to tissues
- Hb combines with the Hydrogen ions
How do Plasma proteins provide a buffering system?
- Proteins contain weakly acidic and basic groups due to their amino acid composition
- Account for 95% of the non-bicarbonate buffering capacity of plasma. Also act as buffers intracellularly
- Albumin is the predominant plasma protein and is the main protein buffer in this compartment
- Albumin behaves as a weak acid due to high concentration of negatively charged amino acids
How does Bone protein buffering occur?
- Bone proteins play a major role in acid-base balance
- Protein buffering within bone matrix
- Increased H+ stimulates bone resorption (alkaline minerals act as buffers)
What are the renal processes to buffer plasma/serum?
- Excess H+ are excreted in the urine and, because the body is a net producer of acid, the urine is usually acidic. To achieve this, the body must reabsorb bicarbonate filtered at the glomerulus and excrete H+, usually against a steep concentration gradient
- The reactions linking these are mediated by carbonic anhydrase and occur in the renal tubular cell
- Glomerular filtrate contains bicarbonate at the same concentration as the plasma (18-24mmol/L)
- Urine is virtually bicarbonate-free (the kidney is not 100% efficient at reabsorption)
- The excreted H + is then buffered in urine by, e.g. phosphate and ammonia
What are the ways in which hydrogen ions are produced?
Turnover of H+ ions from normal metabolic processes
- Incomplete oxidation of energy substrates generates acid e.g. lactic acid by glycolysis
- Further metabolism of these intermediates consumes it e.g. gluconeogenesis from lactate
Temporary imbalances between rates of production and consumption may occur in health
- Accumulation of lactic acid during anaerobic exercise
Metabolism also produces CO2
- Greater than 15000 mmol of CO2 is produced every 24 hr and in water this converts to a weak acid (H2CO3, carbonic acid)
- H+ can be generated stoichiometrically therefore potentially equivalent to 15000 mmol H+ ions
- In health pulmonary ventilation is controlled so excretion matches rate of formation
How is Bicrabonate filtered and reabsorbed?
- Glomerular filtrate contains same concentration of bicarbonate ions as the plasma
- Virtually all filtered bicarbonate is reabsorbed
- Luminal surface of renal tubules is impermeable to bicarbonate therefore reabsorption must occur indirectly
How does Bicarbonate work as a buffering compound?
When dissolved in blood, CO2 becomes an acid
• The more carbon dioxide added to blood, the more carbonic acid (H2CO3) is produced, which readily dissociates to release H+
Blood pH depends, not on absolute amounts of CO2 or HCO3-, but the ratio of the two. (20:1 HCO3:CO2)
How does the Acidification of Urine take place?
Achieved by active secretion of H+ by the intercalated cells of the distal tubular cells and proximal collecting duct cells
- Minimal urine pH that can be achieved (4.5 or ~38 µmol/L) which Insufficient to remove daily H+ production, which is measured in mmol
- Significant acid excretion is achieved by H+ being buffered by phosphate, titrating HPO42- and H2PO4-
How is H+ buffered in the Urine?
Monohydrogen (HPO42-) and dihydrogen phosphate (H2PO4-) form a buffer pair with a pKa of 6.8
- In renal filtrate, these molecules are in higher concentrations and are an important buffer.
- 30 – 40 mmol hydrogen is excreted in this way daily
Ammonium allows for urea syntheis to allow excretion fo hydrogen ions without generating H+
- pKa of ammonium (NH4+) is ~100 x lower than the physiological [H+], so almost all of ammonia in the body is already in the ammonium form
What regulates respiratory rate?
Carbon dioxide is excreted by the lungs
- Respiratory control mechanisms are extremely sensitive to plasma CO2 concentrations.
- In health (excluding a conscious effort to hypo- or hyper-ventilate), the rate of CO2 elimination is equal to the rate of production
- Blood CO2 remains constant and is what regulates respiratory rate
How are Blood Gases measured?
- Heparin blood sample from radial artery and measured in blood gas analyser
- Capillary blood maybe used for pH and PCO2 in neonates
What are some measurements that can be made on Blood gases?
- pH: Measure of the level of hydrogen ion (H+)
- PO2: The partial pressure of O2 (the amount of oxygen gas dissolved in blood).
- PCO2: The partial pressure of CO2 (the amount of carbon dioxide gas dissolved in the blood)
- O2 saturation: A calculation of how much oxygen is bound to haemoglobin in the red blood cells and available to be carried through the arteries to nourish the body’s cells
- HCO3- (bicarbonate): Directly related to the pH level
- Base Excess: the amount of acid/base to titrate 1 litre of blood to a pH of 7.4
What are causes of acid base disorders?
Respiratory
- Primarily due to breathing
- Impaired respiratory function causes a build-up of CO2 in blood
- Hyperventilation can cause a decreased PCO2
Metabolic
- Primarily due to bicarbonate concentration e.g.diabetic ketoacidosis
How does H2CO3 relate to pCO2?
- H2CO3 is proportional to the dissolved carbon dioxide, which is in turn proportional to the partial pressure of CO2
- [H2CO3] can, therefore, be replaced in the equation by PCO2
Describe the process of compensation of acid base disorders?
- Body tries to compensate for derangements in acid–base balance by means of physiological mechanisms that try to return [H +] to normal.
- Metabolic compensation for respiratory disorders takes a longer time and happens through H+ excretion and simultaneous bicarbonate regeneration
- Respiratory compensation for primary metabolic disorders occurs quickly – it takes the form of increased ‘blowing off’ of CO2 by the lungs.
- If compensation is complete, [H +] returns to normal and the acid–base disorder is described as fully compensated . A good example is stable chronic obstructive pulmonary disease.
- Body will never overcompensate
What is the Anion Gap?
- Concentration of all the unmeasured anions in the plasma
- Negatively charged proteins account for ~10% of plasma anions and make up the majority of the unmeasured anion represented by the anion gap
([Na+] + [K+]) – ([Cl-] + [HCO3-])
Normal range: 6-16
What is the purpose of Anion Gap?
- Identifies the presence of a metabolic acidosis
- Assists in assessing the biochemical severity of the acidosis and follow the response to treatment
- Help differentiate between causes of a metabolic acidosis: high vs. normal anion gap
What causes changes in the anion gap?
- In an inorganic metabolic acidosis (e.g. due HCl infusion), the infused Cl- replaces HCO3-, the Anion Gap is normal
- In an organic acidosis, the lost HCO3- is replaced by the acid anion which is not normally measured then Anion Gap is increased
- Hypoalbuminaemia causes a low anion gap
