Potassium and pH Flashcards
What are levels of potassium like intracellularly?
High
What is potassium a key determinant of?
Resting membrane potential
What cells are particularly affected by abnormalities in K+?
Excitable cells in particular cardiac/muscle function because of its role in membrane potential
What does the ENaC/ROMK channel do?
Swaps Na+ and K+- Na+ in and K+ out (regulates K+ elimination
What activates ENaC/ROMK channels?
Aldosterone and it inserts them into the membrane
What is K+ excretion dependent on?
Aldosterone and plasma K+ conc
Higher K+ conc, higher K+ secretion
Increase in aldosterone, increase in K+ secretion
What is the Na+/K+ pump responsible for?
Maintaining difference in electrolyte conc between ICF and ECF
What influences Na+/K+ pump activity?
Beta adrenergic stimuli and thyroxine- disorders of these affect plasma K+ conc (adrenaline lowers plasma K+)
What happens after a meal with K+?
Kidneys will increase K+ excretion before plasma K+ rises
Where is K+ low?
ECF
What is the main cause of low K+?
Reduced intake (unlikely)
Renal losses
Intracellular shift
Give examples of renal losses of K+?
Mineralocorticoid excess (Conn’s)
Renal tubular disorder
Diuretics
Give examples of intracellular shift that cause low K+?
Glucose load- after glucose is ingested and enters the blood, it stimulates a release of insulin which drives K+ into cells
Adrenaline
Alkalosis
What is the most common cause of K+ loss?
Loss of K+ rich fluid particularly from GI tract- diarrhoea and vomiting
What causes of high plasma K+ are there?
Increased intake
Renal retention
Intracellular shift
Why is increased intake an unlikely cause?
We have the capacity to eliminate a large amount of K+ per day
What causes of renal retention are there?
Mineralocorticoid deficiency (Addisons)
ACE inhibitors
Renal failure
Potassium sparing diuretics (spironolactone)
What examples of intracellular shift cause increased plasma K+?
Insulin deficiency
Acidosis- H+ compete wih K+
Exercise
Cell lysis
Give a summary of factors that increase plasma K+
Renal failure
Mineralocorticoid deficiency
Cell lysis
ACE inhibitors
Give a summary of factors that decrease plasma K+
GI fluid losses
Diuretics
Mineralocorticoid excess
What is the main issue with high k+?
Abnormal ECG
What ECG abnormalities are there with hyperkalaemia?
Tented T waves
Loss of P waves
Broad QRS
Bradycardia
How do you treat hyperkalaemia?
Calcium chloride (IV)- no effect on plasma potassium but reduces effect of potassium on cardiac excitability and limits ECG changes. Fixes bradycardia 50% glucose- taken up by cells, shift of potassium from ECF to ICF. It will lower the plasma K+ within half an hour and will last for 4-6 hours- K+ will eventually leak back out again but it does buy time Sodium bicarbonate (NaHCO3)- works if patient is not fluid overloaded, affects movement of potassium between ECF and ICF
What is the important rule about Acid Base balance?
Whenever you have water, there will be a little dissociation into H+ and OH-
[H+] x [OH-] = 10^-14
What is the pH?
Negative log of [H+]
Why is pH important?
It alters the charge on amino acids which leads to altering the structure of proteins and hence their function
What are the consequences of change in pH?
Impaired ventricular function Arrythmias Vasodilation, catecholamine release Impaired oxygen delivery Bronchoconstriction Reduced hepatic/renal blood flow Impaired consciousness Respiratory muscle fatigue Protein catabolism Insulin resistance
What does it mean if you have a base excess of 0?
There is no excess of base or acid
What will happen if you have a base excess?
The pH will increase
What is neutral pH at body temperature?
6.7
What is the important weak acid physiologically?
Carbon dioxide
What can carbon dioxide be converted into?
Carbonic acid which can be converted into H+ and HCO3-
What is the Henderson-Hasselbach equation?
pH= pK + log( [A]/[HA])
What is the pKa of H2CO3?
6.1
What is HCO3- controlled by?
Kidneys
What is total CO2 controlled by?
Lungs
What does pH physiologically depend on?
Kidney function and lung function
What do the lungs regulate in regard to CO2?
They regulate partial pressure of carbon dioxide within the blood, not the amount of dissociated carbonic acid
What is normal pCO2?
4.5-6
What happens if your blood pH decreases?
You have acidosis- you will get a respiratory drive and you will breathe faster. This will mean that you blow off more CO2 and partial pressure of CO2 will go down and you will jump up the curves
What is the base excess regulated by?
Kidneys
What does a pCO2 below 4.5 cause?
Resp alkalosis
What does a pCO2 above 6.0 cause?
Resp acidosis
What does a HCO3 below 22 or a BE below -2 cause?
Met acidosis
What does a HCO3 above 28 or a
BE above 2 cause?
Met alkalosis
What happens when you breathe faster?
pCO2 decreases - respiratory alkalosis
What happens when you breathe slower?
pCO2 increases- respiratory acidosis
How do CO2 and oxygen differ in terms of solubility?
CO2- soluble in water
O2- not very soluble in water (need Hb)
With focal lung disorders like lobar pneumonia, what happens to oxygen and carbon dioxide?
Oxygen will go down but CO2 will not increase
How does focal lung disorder lower oxygen content of blood?
Blood that goes to bit of lung affected wont become fully saturated and blood that goes to normal part of lung will be fully saturated. Increase in breathing rate can increase saturation of oxygen going to good [arts of the lung to 100% but overall it will be lower than normal
Why does CO2 not increase in a focal lung disorder?
It doesn’t have the same complex sigmoid relationship so doesn’t saturate in the blood- it has a linear relationship with pCO2. In lobar pneumonia, blood going to damaged part won’t have CO2 removed but good parts of lung will have abnormally low CO2 as you’re breathing faster and removing more so overall there is normal CO2 concentration
What type of respiratory disorder is caused by focal lung disorders?
Type 1 respiratory failure
What causes type 1 respiratory failure?
Pneumonia
Pulmonary embolism
How do you differentiate between pneumonia and pulmonary embolism?
Pneumonia shows up on chest x-ray and pulmonary embolism doesn’t
What is normal pO2?
> 10.5kPa (room air- 21% oxygen)
What is pO2 when you’re breathing 100% oxygen?
50kPa
How can you work out what pO2 should be from fraction of inspired oxygen?
Partial pressure should be half the fraction of inspired oxygen
What is pulmonary fibrosis?
A diffuse lung abnormality- whole of lung isn’t normal
How will people with pulmonary fibrosis or where the whole lung is affected present?
They will have an abnormality in both O2 and CO2- O2 will be low and CO2 will be high- type 2 respiratory failure
What are the causes of type 2 respiratory failure?
Airways disease
Pulmonary fibrosis
What are the common causes of metabolic acidosis?
Renal failure
Mineralocorticoid deficiency
Diarrhoea
(Associated with high chloride, lactic acid or ketoacids)
What is the anion gap?
Na-Cl-HCO3
What is the normal range for anion gap?
12+/-4
What common anions aren’t measured in anion gap?
Phosphate
Proteins
Specific metabolic acids
What common causes of metabolic alkalosis?
Diuretics
Mineralocorticoid excess
Vomiting
What do things that cause high K+ tend to cause?
Metabolic acidosis
What do things that cause low K+ tend to cause?
Metabolic alkalosis
What does diarrhoea and vomiting tend to cause?
Diarrhoea- acidosis
Vomiting- alkalosis
What are the causes of ketoacidosis?
Diabetic ketoacidosis (most common)
Starvation
Alcoholics
What are the causes of lactic acidosis?
Lack of tissue perfusion:
Shock
Liver failure
Short bowel
What does low pH lead to?
Breathlessness- strong resp drive
What does high pH lead to?
Cramps/tetany- pH affects calcium ionisation and that affects muscle excitability
How can you compensate for changes in pH?
Respiratory compensation (changing breathing rate) occurs fast Metabolic compensation (changing bicarbonate reabsorption and release) occurs slowly
