Case 3- pH balance Flashcards
Equation for pH
-log10[H+]
Normal body pH
Your pH should be at 7.4, at 6.8 you will be suffering from acidosis and 7.8 you will be suffering from alkalosis. At both these amounts you will die.
What is an acid
A substance that donates a proton
What is a base
A substance that can accept a proton
What are buffers
Weak acids or bases. The presence of buffers effectively neutralises the fall in pH that results from the addition of protons. Buffers do not fully dissociate but are in equilibrium between their ions and their undissociated state.
What happens to a buffer when you add hydrogen
Adding an acid to a weak acid (buffer) will shift the equilibrium to the right to increase the amount of undissociated acid to mop up the protons. Adding an acid to a weak base will do the same as more protonated base is formed to mop up the excess protons. Works in the opposite direction in time.
Major buffer systems of the body
The major buffer system in the body is the (HCO3) bicarbonate system which buffers CO2. Phosphate buffers our urine and intracellular fluid. Haemoglobin buffers blood. Proteins buffer intracellular fluid.
Equation for working out how the concentration of CO2 affects the pH
The amount of CO2 in our blood affects the pH which is balanced by the bicarbonate system.
pH= pK’ + log ([HCO3-] / [dissolved CO2])
Simplified equation for working out the pH from CO2 concentration
pK’ is the negative logorithm of the dissociation constant and has a value of 6.1. At 37 degrees multiplying the partial pressure of CO2 by 0.03 gave the concentration.
pH= 6.1 + log ([HCO3-] / [0.03 x pCO2])
Why is CO2 not a big threat to body pH
Because it can be removed by ventilation. Endogenous acid production (protons produced which are not associated with CO2) are the real threat.
Endogenous acid sources
Meat metabolism, abnormal metabolism and ischaemia. Loss of alkali in stool, acid can be lost in diarrhoea
How does the kidneys control pH (small)
Excretion of H+ and the renal absorption/production of HCO3-
How does the proximal tubule control pH (steps)
1) In the tubular lumen sodium ions are reabsorbed using the Na+/H+ exchanger which push protons into the tubular lumen
2) The protons which are now in the tubular lumen, interact with bicarbonate ions (HCO3- to form carbonic acid (H2CO3).
3) H2CO3 is split into CO2 and water using carbonic anydrase
4) The water and CO2 can be reabsorbed into the proximal tubule cells along with excess CO2 from metabolism. The CO2 can be hydroxylated by carbonic anhydrase type 2 to form bicarbonate ions. Hydrogen is excreted as water
5) The bicarbonate ions are removed from the proximal tubule cells into the peritubular fluid. Either by the beta-1 bicarbonate/sodium transporter or using the bicarbonate/chloride exchanger. The bicarbonate ions are removed into the extracellular fluid where they can form a buffer.
How an increase in protons increase their excretion and bicarbonate reabsorption in the proximal tubule
form a buffer.
When proton conc rises, reabsorption is stimulated as it increases the rate of the sodium hydrogen exchanger. As the more protons you have a in cell, the larger the gradient across the luminal membrane so the exchanger works faster and bicarbonate uptake is increased. The bicarbonate can now continue to work as a buffer
Generation of bicarbonate in the proximal tubule
- In the proximal tubule Glutamine is broken down due to a decrease in pH, to create ammonium ions (NH4+ and α-ketoglutarate.
- NH4+ dissociates into NH3 and H+, ammonium enters the lumen as its permeable. Protons enter via the Na+/H+ exchanger.
- NH3 and H+ combine to form NH4+, can not cross back into the cell because it is impermeable.
- The α-ketoglutarate participates in gluconeogenesis which indirectly creates bicarbonate ions. This moves into the blood stream to replenish our buffer system.
- NH4+ is actively reabsorbed in the thick ascending limb of the loop of Henle. It competes with K+ for transport on the Na:K:2Cl cotransporter.
- The thick ascending limb of the loop of Henle is impermeable to ammonia so ammonia and ammonium move out the basolateral membrane (ammonia passively and ammonium via NHE) and accumulate in the interstitium of the medulla.
- The interstitium ammonium ions disassociate into ammonia and proton ions which can enter the collecting duct
- The ammonia then combines with protons to form ammonium, so it bypasses the cortical section of the nephron so it is less likely to be absorbed into the blood stream. The collecting ducts secretes protons to trap ammonia and secrete it as ammonium.
