Case 3- pH balance Flashcards

1
Q

Equation for pH

A

-log10[H+]

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2
Q

Normal body pH

A

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.

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3
Q

What is an acid

A

A substance that donates a proton

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4
Q

What is a base

A

A substance that can accept a proton

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5
Q

What are buffers

A

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.

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6
Q

What happens to a buffer when you add hydrogen

A

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.

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7
Q

Major buffer systems of the body

A

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.

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8
Q

Equation for working out how the concentration of CO2 affects the pH

A

The amount of CO2 in our blood affects the pH which is balanced by the bicarbonate system.
pH= pK’ + log ([HCO3-] / [dissolved CO2])

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9
Q

Simplified equation for working out the pH from CO2 concentration

A

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])

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10
Q

Why is CO2 not a big threat to body pH

A

Because it can be removed by ventilation. Endogenous acid production (protons produced which are not associated with CO2) are the real threat.

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11
Q

Endogenous acid sources

A

Meat metabolism, abnormal metabolism and ischaemia. Loss of alkali in stool, acid can be lost in diarrhoea

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12
Q

How does the kidneys control pH (small)

A

Excretion of H+ and the renal absorption/production of HCO3-

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13
Q

How does the proximal tubule control pH (steps)

A

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.

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14
Q

How an increase in protons increase their excretion and bicarbonate reabsorption in the proximal tubule

A

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

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15
Q

Generation of bicarbonate in the proximal tubule

A
  • 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.
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16
Q

Generation of bicarbonate ion in titratable acid formation

A

In the PCT. The hydrogen ions secreted in the tubular lumen interacts with HPO4 (2-) to form H2PO4-. It is not reabsorbed along the length of the tubule so there is a net acid loss. Bicarbonate ions can be generated as before with the hydroxylation of CO2 via carbonic anhydrase type 2.

17
Q

Generation of bicarbonate ions in the alpha intercalated cells of the CT and DCT

A

In the apical membrane there are ATPase proton pumps and H+/K+ ATPase pumps which facilitate the loss of protons into the tubular lumen. CO2 can be hydroxylated using carbonic anhydrase type 2 into bicarbonate ions. The HCO3-/Cl- exchanger, on the basolateral membrane removes the bicarbonate ions into the peritubular fluid so they can act as a buffer. Also helps get rid of protons

18
Q

Regulation of bicarbonate ions in the beta intercalated cells of the CT and DCT

A

Respond to high pH and secrete bicarbonate ions into the lumen and extrude H+ into the ECF. The high cellular H+ concentration activates the H+ pump which is also stimulated by low H+ in the tubular fluid. Aldosterone can increase the H+ secreting pump.

19
Q

Respiratory acidosis

A

Retention of CO2, this diffuses across and gets into the plasma. Caused by respiratory problems, hypoventilation, lung death. PaCO2 will go up, pH is reduced and HCO3- is normal. Compensation attempts to restore pH. To do this the kidney will retain the base HCO3- and try to excrete H+ from the body. The pCO2 levels will stay high, the pH approaches a normal value. HCO3- is also raised.

20
Q

Respiratory alkalosis

A

Excessive loss of CO2 generally caused by hyperventilation. Causes a decrease in proton concentration as the equilibrium shift towards the CO2, so the blood becomes more alkali. Initially pCO2 is reduced, pH is raised, and bicarbonate is normal. To compensate there is a net loss of the base bicarbonate from the kidneys. So less protons are removed due to the bicarbonate. At the end pCO2 remains reduced, pH is normalised and HC03- is reduced. In acute respiratory alkalosis there is no metabolic compensation, in chronic there is. In chronic the pH will be lower at a normal 7.4 and the CO2 and bicarbonate levels will be even lower.

21
Q

Metabolic acidosis

A

Caused by many different conditions such as diabetes, heart failure, renal failure and diarrhoea (loss of base in stool). This initially causes HCO3- to be reduced, pH to be reduced and carbon dioxide levels to be normal. Higher H+ concentration. In order to compensate for this you hyperventilate and breathe more heavily to get rid of more CO2. This causes the H+ concentration to drop as the equation shifts to the left the produce more CO2 as it is now being lost. This will restore the pH to normal. The bicarbonate concentration wont change and is still low.

22
Q

Metabolic alkalosis

A

Caused by a net loss of H+ i.e. from vomiting. CO2 levels are normal, pH and bicarbonate are raised because the equation is shifted towards the left. To compensate the lungs, blow out less CO2, hypoventilation. More CO2 will remain in the blood pushing the equation towards the production of H+. PH will decrease to a normal level, the CO2 concentration will increase and bicarbonate levels will remain the same.

23
Q

Equation for how the body handles CO2

A

CO2 + H2O -> H2CO3 -> H+ + HCO3-

H2CO3 (carbonic acid) is formed with carbonic anhydrase

24
Q

How the body handles CO2

A

The equation shifts from one side to another. If we have too much H+, more H2CO3 will be produced which will equilibrate to form more CO2 and water, using carbonic anhydrase. The CO2 can be removed by increased ventilation, this will restore balance to the equation. The opposite is also true