acid-base balance Flashcards
how is pH calculated?
pH=-log[H+]
what is the physiological level pH level in the blood?
should be kept between 7.35 and 7.45
what are the 2 main mechanisms that keep the pH at this level in the body?
- co2 production from respiration
- acid production from metabolism
how does CO2 metabolism help to maintain pH?
In health CO2 production is the same as excretion.
C02 diffuses from the cell through the plasma into the red blood cell
In the red blood cell C02 binds with water producing bicarbonate and H+
H+ binds to Hb so it releases its oxygen
Hco3- exits the RBC into the plasma and Cl- enters to balance charges
In the lungs the exact opposite happens so that C02 can be breathed out and 02 taken up
good explanation of the buffer systems and diagrams in notes
how does acid production from metabolism help manage pH levels?
Extracellular buffers - bicarbonate, phosphate and proteins
Bicarbonate is the main buffering system and it is actively reabsorbed in the kidneys in order to retain the buffer. This happens in the distal renal tubular cell.
However this is a lot slower than the lungs
good explanation of the buffer systems and diagrams in notes
outline the henderson- hasselbatch equation
full equation in notes
Easier to look at it like this pH = HCO3/PCO2
Acidosis can arise by: - A fall in HCO3 with a normal PCO2 - A rise in PCO2 with a normal HC03 Alkalosis can arise by: A riske in HCO3 with a normal PCO2 A fall in PCO2 with a normal HCO3
what is compensation and what are the mechanisms of compensation?
Compensation - if a disease process altered the concentration of one of the components, the ratio and therefore pH can be returned to normal if the other component concentration is sufficiently altered in the same direction. If this returns pH to normal this is complete compensation and if pH is improved but not normal this is partial compensation.
mechanisms of compensation:
- Metabolic acidosis - breathe quicker to decrease pCO2 by blowing out CO2 - takes 12 to 24 hours
-Metabolic alkalosis - breathe slower to increase pCO2 - takes 12-24 hours
- Respiratory acidosis - renal compensation to increase HC03- - takes 2-4 days
Respiratory alkalosis - renal compensation to reduce HC03- - takes 2 -4 days
what approach should you use in a patient with an acid-base disorder?
- Does the patient have an acidosis or alkalosis?
- Look at pH - low is acidosis and high is alkalosis - Is the primary process respiratory or metabolic? Look at pCO2 or HC03-
- In acidosis:
If PCO2 is high - respiratory
If Hco3- low - metabolic
- In alkalosis:
If PCO2 is low - respiratory
If Hco3- is high - metabolic - Is compensation appropriate? - note if it is partial or complete
- In acidosis:
If respiratory cause - compensation if HC03 is high
If metabolic cause - compensation if pCO2 is low
- In alkalosis:
If respiratory cause - compensation if HC03 is low
If metabolic cause - compensation if pCO2 is high - If acidosis, is there an anion gap?
- Calculate
- Helps you to determine the cause - Is more than one disorder present?
- Look at compensation tables
what causes metabolic acidosis?
increased H+ load
- ketoacidosis
- lactic acidosis
- drugs - salicylates, methanol, ethanol, Ethelene glycol
decreased H+ secretion
- renal failure
- renal tubular acidosis
- mineralocorticoid deficiency
loss of HCO3-
- GIT - diarrhoea
- renal - renal tubular acidosis
what are the causes of metabolic alkalosis?
loss of H+
- GIT - vommiting
- renal - mineralocorticoid excess, severe K+ deficiency
excessive HCO3- intake
- oral
- IV therapy
what are the causes of respiratory acidosis?
lung disease
- emphysema
- bronchitis
- asthma
neuromuscular disease
- Guillian Barre syndrome
central NS disease
- trauma
- infection
- tumour
drugs
- sedatives
- anaesthetics
mechanical ventillation (if ventalation settings are wrong)
