Physiology of Kidney Acid Base Balance Flashcards
What is the normal pH of arterial blood?
7.4
Protons can be bound to protein or acid, however, in order to contribute to pH, what must proton ions be?
Free
What are some of the sources of protons (H+) in the body?
Respiratory acid
Metabolic acid
What is a respiratory acid?
Carbonic acid is formed from carbon dioxide dissolving in water
What does carbonic acid dissociate into?
Carbonates and protons
Why is increased carbon dioxide production in exercise normally not a problem for the body?
It’s compensated for by increased ventilation
What happens if there is impaired lung function in someone who is exercising?
Carbon dioxide builds up as the body cannot fully release and exhale the carbon dioxide, leading to increased respiratory acid production
Metabolic acids are produced via metabolism. What are the two types of metabolic acid?
Inorganic acids
Organic acids
Give an example of an inorganic metabolic acid.
Sulphuric and phosphoric acid due to sulphur/phosphate containing amino acids
Give an example of an organic metabolic acid.
Fatty acids, lactic acid
What is the major source of alkaline in the body?
Oxidation of organic anion such as citrate
Buffers?
Solutions which minimise changes in pH when proton ions are added or removed
What is the most important extracellular buffer?
Bicarbonate buffer
What is the normal pCO2?
40mmHg or 5.3kPa
Protons do not get removed from the body, so how does it work so that these protons don’t contribute to pH thanks to buffers?
Bicarbonate buffers the protons and the respiratory compensation greatly increases the buffering capacity so that free protons ions are prevented from contributing to pH
->basically, just understand that protons are not removed, just buffered so they can’t contribute to pH
What are the two factors which are important when protecting the pH?
Bicarbonate
Carbon dioxide
How is carbon dioxide regulated?
Respiration
How is bicarbonate regulated?
Directly by the kidney
If there is a disruption to the regulation of the bicarbonate, how is this compensated for?
Compensated by ventilation
List some of the primary intracellular buffers.
Proteins, organic and inorganic phosphates
Haemoglobin in erythrocytes
What can buffering of proton ions by ICF buffers cause changes to?
Can cause changes in plasma electrolytes
What is done as compensation when transporting protons into cells to be buffered intracellularly?
Needs to be co-transport of chloride in red cells or potassium
In acidosis, there is movement of potassium out of the cells. What can this cause?
Hyperkalaemia, leading to depolarisation of excitable tissues, ventricular fibrillation and death
What else provides an additional store for buffer?
Bone carbonate
->very important in chronic acid loads in renal failure as can lead to wasting of bones
How can acidosis lead to electrolyte disturbances?
Has to be co-transport of potassium or chlorine which can cause disruption
How many millimoles of protons a day do we get from the diet?
50-100mmoles
If all these protons were free in TBW, this would dramatically reduce pH. However, as long as two organs are working, pH remains constant. Which two organs?
Lungs and kidneys
What is the purpose of buffering?
To give the kidneys time to excrete the proton loading
How does the kidney regulate bicarbonate?
-Reabsorbing filtered bicarbonate
-Generating new bicarbonate
->both of these processes require active proton secretion from the tubule cells into the lumen
Describe the mechanism in which bicarbonate can be reabsroped.
- Active proton secretion from tubule cells
- Coupled to passive Na reabsorption
- Filtered bicarbonate reacts with secreted protons to form carbonic acid
- Carbon dioxide is freely permeable and can enter the cell
- Within the cell, carbon dioxide is converted into carbonic acid in the presence if carbonic anhydrase which then dissociates to form protons and bicarbonate
Where does the bulk of bicarbonate reabsorption take place?
Proximal tubule
Why is bicarbonate converted to carbon dioxide to transport over the membrane?
Bicarbonate is very large and charged so does not pass easily itself
What is the GFR per day?
180L per day
*GFR= glomerular filtration rate
What in the minimum urine pH?
4.5-5.0
What in the maximum urine pH?
8.0
What is the usual net production of protons per day in humans?
50-100mmoles of protons
What would happen if all these protons (H+) where free proton ions in the urine?
pH of 1…ouch!
Luckily, these protons are buffered in the urine
While several weak acids and bases act as buffers, give some examples of the most common in the body.
Dibasic phosphate
Uric acid
Creatinine
Describe the ‘titratable acidity’.
Buffer quantity and pH is measured to work out the amount of NaOH needed to titrate urine pH back to 7.4 during a 24hr urine sample
How does titratable acidity contribute to active proton excretion?
Produces new bicarbonate to compensate for the loss due to buffering and it actively excretes protons from the body
Titratable acidity compensates for the loss by producing new bicarbonate. What is the source of this new bicarbonate?
Indirectly carbon dioxide…it enters the tubule cells and combines with water to form carbonic acid, which in the presence of carbonic anhydrase dissociates into protons and new bicarbonate
What is the site of formation of titratable acidity?
Distal tubule
Is ammonia water or lipid soluble?
Lipid soluble
So, ammonia is lipid soluble, is ammonium?
No