L5 - Acid Base Balance Flashcards
Why is acid-base balance important?
A small change in pH can have a large effect on body function
Log scale – 1 unit change = x10 change in [H+]
What can fluctuations in plasma H+ concentrations have an effect on?
Excitability of muscle and nerve
Enzyme activities
K+ levels
pH at venous end of tubule?
7.35
pH at arterial end of tubule?
7.45
What are the different pH value around the body?
Gastric secretion – 0.7
Cerebrospinal fluid – 7.3
Pancreatic secretions – 8.1
Urine – 5.4
What are the different sources of acid and alkali?
Metabolism – large amounts of acid produced
- 15 moles/ day CO¬¬2
- 40 mmol/day H+ (western diet has an excess of H+)
Diet – alkali and acid in food
- 20 mmol/day H+
- Lose 10 mmol/day OH-
Overall excess 70 mmol/day H+
What are the 3 different systems involved in acid-base balance?
Blood and tissue buffers (seconds)
Respiration (minutes)
Renal (hours) – only mechanism for the extrusion of acid/alkali
Where are buffers present throughout the body?
Blood – plasma and red blood cells
Extracellular and intracellular fluid
Urine
E.g. haemoglobin, inorganic phosphate, weak acids/bases, HCO¬¬-3
Carbonic acid/bicarbonate ECF buffer
CO2 + H2O H2CO3 H+ + HCO3
What is the Henderson-Hasselbach equarion
pH = pK + log [HCO3]/[H2CO3]
pK is a constant of 6.1 at 37oC
Ratio of [HCO3]/[CO2] = 20:1
6.1+log 20 = 6.1+1.3 = 7.4
What does chemical control of ventilation control?
Controls blood gas composition – PO2, PCO2, pH
How is ventilation chemically controlled?
Peripheral and central chemoreceptors - negative feedback system
Stimulated by hypoxia, hypercapnia, acidosis – all increase ventilation
- Increase in PO2
- Decrease in PCO2
- Increase in pH
What are peripheral chemoreceptors triggered by?
Mainly stimulated by decreased PO2 – hypoxia
What do peripheral chemoreceptors do once triggered?
When activated send signals through the sinus, gloss-pharyngeal and the vagus nerve to the medulla
What is the structure of carotid receptors?
Small – 2mg
High blood flow (40X higher than the brain per unit mass)
High metabolic rate
Glomus cells
Parasympathetic/sympathetic cells – regulate blood flow Aortic receptors smaller and less well studied
What do glomus cells do?
Fire action potentials when O2 drops
Neural phenotype
Type II – supporting
What is the mechanism of glomus cells?
- Inhibition of BK K channels – decreased PO2, increased PCO2, decreased pH
- Depolarisation and action potential firing
- Ca channels open increasing intracellular Ca
- Ach, dopamine, 5-HT, substance P, noradrenaline released
- Some SIDS babies have higher concentration of carotid body noradrenaline and dopamine - Afferent nerve fibre stimulation
Sensitivity to PO2 changes with?
Acid/base status
Higher pH = less action potentials
Sensitivity to PCO2 changes with?
pH
Higher pH = less action potentials
What are central chemoreceptors stimulated by?
Increased CO2 - hypercapnia
Change in PCO2 from 40 - 45 mmHg - doubles ventilation
Same increase in ventilation only seen with 50% fall in PO2
What factor is actually measured that triggers central chemoreceptors?
pH
Perfused cerebral ventricles with acidic solution – observed hyperventilation
Where are central chemoreceptors located?
Within the brain parenchyma and bathed in brain extracellular fluid
Separated from arterial blood by the blood brain barrier
- BBB has poor ion permeability
- H+ and bicarbonate can’t cross
- CO2 can cross
Increase in arterial PCO2 = increase in brain extracellular fluid PCO2
Where are central chemoreceptor neurones located?
Ventrolateral medulla and other brainstem nuclei
What are the two types of central chemoreceptor neurone?
Acid activated – serotonin
Acid inhibited – GABA
