T4: Acid-Base Flashcards
How much carbon dioxide do we produce a day?
25 mol/day
What level of unmetabolized acid do we maintain?
Where does the acid come from?
40mmol/day
This comes from:
- Glucose and other sugars: incomplete metabolism produces lactate and hydrogen ions.
- Triglycerides - incomplete metabolism produces ketones, triglycerides and fatty acids.
- Amino acid metabolism (ureagenesis) - metabolism of neutral amino acids results in the generation of hydrogen ions.
At what point does a patient become acidaemic?
pH below 7.35
At what point does a patient become alkalaemic?
pH above 7.45
What is pKa?
The inverse log of Ka where Ka is the acid dissociation constant..
What is the Henderson-Hasselbalch equation?
This equation explains how acids and bases contribute to pH and therefore hydrogen ions.
pH = pKa + log[base/acid]
Explain the bicarbonate buffering system.
In physiology bicarbonate can act as a base and carbon dioxide acts as an acid. This is due to the dissociation of ions of carbon dioxide in water. Carbonic acid dissociates into hydrogen ions and carbonic acid. The blood pH depends on the ratio of carbon dioxide and water.
Bicarbonate ions acts as a buffer by mopping up carbon dioxide to produce hydrogen ions. An increase in bicarbonate ions leads to an increase more carbon dioxide. If there is more carbon dioxide, we push the equilibrium to the left increasing hydrogen ion concentration. However it cannot buffer carbon dioxide, as it results in more carbon dioxide. Equilibrium of carbon dioxide therefore requires other non-bicarbonate buffers.
Explain how haemoglobin can be used as a buffering system.
It is a principle non-bicarbonate buffer. It is important for buffering carbon dioxide. It can lead to a:
• Reduction of carbon dioxide in the plasma
• Increasing the amount of bicarbonate in the plasma
• Decreasing the deoxyhaemoglobin
Explain the role of phosphate as a buffering system.
Mono-hydrogen phosphate accepts a hydrogen ion to become dihydrogen phosphate. The concentrations of these ions are too low in plasma to make an appreciable difference. However it is an important buffer in urine where phosphate is present in a higher concentration.
Explain the role of ammonia as a buffering system.
Ammonia
Vast majority of ammonia in the body is already in ammonium (NH4+) form, limiting its buffering capacity, but some sources still claim that NH3 is an important buffer in urine.
More important role of urinary ammonium excretion is providing a route for ammonium disposal that does not result in the generation of H+ (unlike urea synthesis). In the urea cycle there is production of hydrogen ions.
Explain the role of proteins as a buffering system.
Proteins contain weakly acidic and basic groups due to their amino acid composition, and can therefore accept and donate hydrogen ions to some extent. Albumin is a good example. It is a predominant plasma protein and the main protein the buffer compartment- this is as it has a negative charge and so cab mop up hydrogen ions.
In alkalosis some of these hydrogen ions can be released. In acidosis, albumin releases hydrogen ions.
How is the lungs involved in acid-base regulation?
The respiratory control mechanisms are extremely sensitive to the pCO2. In health, the rate of elimination is equal to the rate of production of carbon dioxide.
The relationship between the saturation of haemoglobin and pO2 is a sigmoid. The curve can shift to the right or the left affecting the affinity of haemoglobin for oxygen and so the amount of oxygen released.
The curve shifts to the right when:
• The body temperature increases
• The patient is hypoxic or anaemic due to increased DPG
• The Bohr shift – increases hydrogen ions
This means more oxygen is delivered to the tissues.
How is the kidney involved in acid-base regulation?
Key roles include:
• Excretion of hydrogen ions at the distal tubule.
• Reabsorption of bicarbonate ions at the proximal tubule.
• Regeneration of bicarbonate at the distal tubule.
This creates acidic urine containing almost no bicarbonate.
How is bicarbonate reabsorbed in the kideny?
- Bicarbonate cannot be directly reabsorbed, as luminal membranes are impermeable to it and it relies on the ability of carbon dioxide to diffuse it.
- H2CO3 generated in tubular cell from CO2 and H2O under action of carbonic anhydrase -> H2CO3 formed dissociates into H+ and HCO3-.
- HCO3- formed in the cell then pumped into the plasma (along with Na+ for charge balance).
- H+ ions formed in the cell then secreted into glomerular filtrate in exchange for Na+.
- Secreted H+ ions combine with HCO3- ions in filtrate to form H2CO3 and then CO2. The CO2 diffuses into the tubular cell, providing a substrate for the continued formation of H2CO3.
How is bicarbonate regenerated in the kidney?
- H2CO3 generated from CO2 and H2O under action of carbonic anhydrase -> dissociates into H+ and HCO3-
- H+ actively secreted into glomerular filtrate in exchange for Na+, where H+ ions are excreted as dihydrogen phosphate (H2PO4-). HCO3- and Na+ ions pumped into plasma.
Additionally, ammonia is transported into the urine where it forms NH4+, providing a route for ammonia excretion that does not generate H+.
