Acid Base Homeostasis Flashcards
What is the requirement for acid-base buffering?
70mEq of hydrogen ions is released into body fluids daily. A large amount of CO2 is produced, which combines with water to form carbonic acid.
Methods to eliminate this acid are necessary otherwise the pH of body fluids would fall rapidly
Name the acid-base buffer systems
Proteins
Haemoglobin
Phosphate
Bicarbonate
How is the bicarbonate buffer system regulated? Why is this important?
- CO2 is excreted in the lungs and can be regulated by changes in ventilation
- bicarbonate excretion is also regulated in the kidney.
What is the Henderson-Hasselbach equation?
The equation makes it clear that pH depends on the ratio of [HCO3 − ] to PCO2 , i.e. the buffer pair. [HCO3 − ] is controlled slowly by the kidneys while CO 2 is controlled rapidly by the lungs.
What does 0.03 represent in Henderson-Hasselbach equation?
Solubility of CO2 in mmol/L
How can the Henderson-Hasselbach equation be simplified?
Simplify the three ways in which acid-base balance is maintained
- excretion of H + and re-absorption of HCO 3 − by the kidneys
- excretion of CO 2 by the lungs through regulation of alveolar ventilation
- buffering of H + by other buffering systems within the body.
What are the causes of respiratory acidosis?
Any cause of hypoventilation:
CNS Depression:
- head injury
- drugs e.g. opiates, anaesthetics
- coma
- CVA
- encephalitis
Neuromuscular Disease:
- Myasthenia gravis
- Gillain-Barré syndrome
Skeletal Disease
- Kyphoscoliosis
- Ankylosing spodylitis
- Flail chest
Artificial ventilation (uncontrolled and unmonitored)
Impaired gas exchange:
- Thoracic injury e.g. pulmonary contusions
- Obstructive airway disease (acute and chronic)
- Alveolar disease
List the causes of respiratory alkalosis
Any cause of hyperventilation
Stimulation of respiratory centre
* High altitude (hypoxia)
* Pneumonia
* Pulmonary oedema
* Pulmonary embolism
* Fever
* Head injury
* Metabolic acidosis (overcompensation)
Increased alveolar gas exchange
* Hyperventilation, e.g. hysteria, pain, anxiety
* Artificial ventilation (uncontrolled)
List the causes of metabolic acidosis
Excessive production of H+
* Diabetic ketoacidosis
* Lactic acidosis secondary to hypoxia
* Septicaemia
* Starvation
Impaired excretion of H+
* Acute renal failure
* Chronic renal failure
Excess loss of base
* Diarrhoea
* Intestinal, biliary and pancreatic fistulae
List the causes of metabolic alkalosis
Excess loss of H+
* Vomiting
* Nasogastric aspiration
* Gastric fistula
* Diuretic therapy (thiazide or loop)
* Cushing’s syndrome
* Conn’s syndrome
Excessive intake of base
* Antacids, e.g. milk–alkali syndrome
Normal ABG values
pH 7.35-7.45
pCO2 4.8-5.6
HCO3- 22-28
Anion gap 10-19
How can the anion gap be used to identify the cause of metabolic acidoses?
Raised anion gap = increased H+ production
Ketoacidosis
Lactate raise
Renal failure
Normal anion gap = excessive HCO3- loss/Cl- gain
Diarrhoea/fistulae
Iatrogenic
What is the standard background fluids regimen for 24 hours?
1L saline
2L dextrose
40 mom KCL
Why do fluid/electrolyte requirements change after major surgery?
- Physiological response:
* catecholamines are released
* stress stimulates the hypothalamo–pituitary–adrenal axis with an increased secretion of cortisol and aldosterone
* these hormones produce conservation of sodium and water by the kidney, resulting in a reduction of urine volume and urine sodium concentration - Third spacing
* if renal perfusion falls, e.g. due to haemorrhage or fluid loss into other spaces, the renin–angiotensinaldosterone mechanism is activated
* this promotes reabsorption of sodium and water, and more potassium is lost in the urine
* ADH secretion from the posterior pituitary also leads to water conservation - Tissue damage
* despite loss of potassium in the urine, serum K+ does not usually fall but may even rise, due to release of potassium from tissue damage caused by trauma or surgery or administration of stored blood containing excessive potassium.
