Acid Base Balance Flashcards
What is normal H+ conc in ECF?
35-45 nmol/l
What is normal pH and how do you calculate it?
pH 7.35 -7.46
pH = log 1/ [H+]
How is H+ made and excreted?
Metabolism of proteins, carbohydrates and fats produce carbon dioxide, water and hydrogen ions
Production of H+ (50-100 mmol/day)
Excretion by the kidney
Where do H+ ions travel?
Hydrogen ion produced in the tissues is transported via the circulation to the kidneys where it is excreted
What is buffering of H+ in the body?
One can add 14 mmol of hydrogen ion per litre of body water with an increase in hydrogen ion concentration of only 36 nmol/L.
ECF buffering of hydrogen is at the expense of bicarbonate
Bicarbonate buffering of H+ is only effective in the short term.
To maintain normal homeostasis the kidney needs to excrete hydrogen ions and regenerate bicarbonate.
How is bicarbonate handled in the nephron?
Bicarbonate reabsorption in the proximal tubule
H+ excretion & bicarbonate regeneration
How is CO2 handled in the body?
Production of CO2 (20,000-25,000 mmol/day)
Excreted by lungs
How does respiration control CO2?
Respiration is controlled by chemoreceptors in the hypothalamic respiratory centre
In health any increase in CO2 stimulates respiration thus tending to maintain a stable concentration of CO2
20-25,000 mmol/day of CO2 is excreted through the lungs in expired air.
In health CO2 produced in the tissues is transported via the circulation to the lungs where it is excreted
How does CO2 make H+?
By reacting with water to make bicarbonate and hydrogen ions (HHb)
How do the kidneys and lungs control acid base balance?
Kidneys- control bicarbonate
Lungs- control CO2
How do you calculate H+ using CO2 and bicarb?
k x (CO2 conc/ HCO3 conc)
What are the causes of metabolic acidosis?
- Increased H+ production e.g. diabetic ketoacidosis
- Decreased H+ excretion e.g. Renal tubular acidosis
- Bicarbonate loss e.g. intestinal fistula
What is respiratory compensation in metabolic acidosis?
This stimulates the respiratory centre
Identified by a fall in pCO2
H+ returns towards normal
What are the causes of acute respiratory acidosis?
Primary abnormality is increased CO2 producing increased H+ (decreased pH) and a slight increase in bicarbonate (2-4 mmol/L).
May be due to:
Decreased Ventilation
Poor Lung Perfusion
Impaired Gas Exchange
What happens in chronic respiratory acidosis?
Over the course of a few days this leads to increased renal excretion of H+ combined with generation of bicarbonate.
H+ may return to near normal but pCO2 and bicarbonate remain elevated
What are the causes of metabolic alkalosis?
H+ loss (e.g. pyloric stenosis)
Hypokalaemia
Ingestion of Bicarbonate
What is the compensation of metabolic alkalosis?
This tends to inhibit the respiratory centre
Identified by a rise in pCO2
H+ returns towards normal
What are the causes of respiratory alkalosis?
Due to Hyperventilation
Voluntary
Artificial ventilation
Stimulation of respiratory centre
What happens in chronic respiratory alkalosis?
If prolonged this leads to decreased renal excretion of H+ and less bicarbonate generation
H+ may return to near normal but pCO2 and bicarbonate remain low
How do you assess an ABG?
H+/pH tells us whether there is an overt acidosis or alkalosis
pCO2 tells us whether there is a respiratory disturbance (primary or secondary)
pO2 does not directly affect acid-base status but gives an indication of respiratory function and tissue oxygenation
Note: Bicarbonate predominantly reflects metabolic disturbances but is also affected by respiratory disturbances
Abnormality?
Metabolic Acidosis with partial compensation
64 year old female
3 week history of intermittent vomiting
abdominal pain
weight loss
O/E
Dehydrated
Jaundiced
Hypotensive
Oliguric
Urea 28.1 mmol/l (2.5-8.0)
Creatinine 387 mmol/l (60-125)
Sodium 129 mmol/l (135-145)
Potassium 1.6 mmol/l (3.5-5.5)
Bicarbonate 56 mmol/l (22-30)
Total protein 89 g/l (64-83)
Gas abnormality? Diagnosis?
Metabolic alkalosis with partial respiratory compensation
Hyperchloraemic hypokalaemia
Diagnosis: pyloric stenosis
How does vomiting cause a metabolic alkalosis?
Loss of HCl in vomit produces a metabolic alkalosis
(Low H+, high bicarbonate)
Loss of fluid produces dehydration
(Raised urea, creatinine and total protein)
Dehydration stimulates renin/angiotensin/aldosterone mechanism
Low potassium as it is lost in vomit and urine
pH 7. 55 (7.35-7.45)
H+ 28 nmol/l (35-46)
pCO2 3.0 kPa (4.7-6.0)
pO2 14.4 kPa (10.0-13.3)
Bicarbonate 20 mmol/l (22-30)
Abnormality?
Respiratory alkalosis (with little compensation)
pH 7. 41 (7.35-7.45)
H+ 39 nmol/l (35-46)
pCO2 10.4 kPa (4.7-6.0)
pO2 7.8 kPa (10.0-13.3)
Bicarbonate 47 mmol/l (22-30)
Abnormality?
Compensated Resp acidosis/ met alkalosis
Resp acidosis - e.g. COPD
Met Alkalosis- e.g. hypokalaemia, loss of H+, too many rennies ;)
A young woman was admitted to hospital 8 hours after she had taken an overdose of aspirin.
Arterial blood:
pH 7. 46 (7.35-7.45)
H+ 35 nmol/l (35-46)
pCO2 2.0 kPa (4.7-6.0)
pO2 17.8 kPa (10.0-13.3)
Bicarbonate 10 mmol/l (22-30)
Abnormality?
Respiratory alkalosis with partial compensation
Arterial blood:
pH 6. 93 (7.35-7.45)
H+ 116 nmol/l (35-46)
pCO2 9.7 kPa (4.7-6.0)
pO2 65.8 kPa (10.0-13.3)
Bicarbonate 15 mmol/l (22-30)
Abnormality?
Mixed respiratory and metabolic acidosis
