Acid Base Balance Flashcards

Question

What is the Plasma anion gap?

Answer 1

In plasma, as in any other fluid compartment in the body, the number of electrical charges on the cations equals the number of charges on the anions, i.e., electrical neutrality is maintained. However, the routine biochemistry laboratory generally only measures the sodium, potassium, chloride, and bicarbonate ions and, by convention, the difference between the sum of the measured cation concentrations (Na+ + K+) and the sum of the measured anion concentrations (CI- + HCO3−) is called the anion gap

Answer 2

The anion gap (the unmeasured anions) is constituted by the charges of the plasma proteins and amino acids, and the organic acid anions (acetoacetate, β-hydroxybutyrate, etc). The protein concentration remains relatively constant, but the concentrations of other unmeasured anions can vary considerably in disease.

Answer 3

15–20 mmol/L

Answer 4

Its usefulness lies in the fact that metabolic acidosis can be classified as either a high anion gap metabolic acidosis or a normal anion gap metabolic acidosi

Answer 5

urinary anion gap can also be measured and is useful in the diagnosis of renal tubular acidosis.

Answer 6

(a) Bicarbonate buffer (b) Haemoglobin buffer (c) Phosphate buffer (d) Ammonia buffer

Answer 7

1. Na+-H+ exchange leading to H+ secretion 2. Bicarbonate reclamation (reabsorption of filtered HCO3-) 3. Bicarbonate regeneration 4. Excretion of H+ as H2PO4- 5. Renal production of NH3 & excretion of ammonium ion

Answer 8

Bicarbonate system: H2CO3/HCO3- (pK = 6.1) Protein system: protein+/protein- (pK = ??) Haemoglobin system: HHb/Hb- (pK = 7.3) Phosphate system: H2PO4-/HPO42- (pK = 6.8) Ammonia system: NH4+/NH3 (pK = 9.8)

Answer 9

The bicarbonate system is the most important & effective buffer in the body because: ● it accounts for 60-75% of blood buffering capacity (present at higher concentrations than other buffers, with the exception of Hb), ● the lungs can readily dispose of or retain CO2, ● H+ secretion by the kidney depends on it, ● the renal tubules can increase or decrease the rate of reclamation of bicarbonate from the glomerular filtrate, ● it is necessary for efficient buffering by Hb, which provides most of the rest of the blood buffering capacity.

Answer 10

Simple buffer systems, including proteins and Hb, lose their effectiveness when the association of H+ with the base reaches equilibrium with the weak acid.

Answer 11

The only thing which thus limits the effectiveness of the bicarbonate buffer system is the availability of HCO3-. Should the HCO3- concentration drop too much, buffering would cease. Under physiological circumstances however, this is prevented by the fact that the body both conserves existing HCO3- (reclamation), and in the process of excreting H+ also regenerates new HCO3-.

Answer 12

In the bicarbonate buffer system, however, the weak acid carbonic acid can dissociate into H2O and CO2. This process is normally extremely slow, but is accelerated by the enzyme carbonic anhydrase (CA) which is present in the RBC and kidneys. The CO2 which is formed is volatile, and is continually removed by the lungs. This “open” buffering system means that equilibrium is never reached, and continual buffering of H+ is achieved at the expense of continually consuming HCO3-.

Answer 13

CO2 from complete (aerobic) metabolism of fat and carbohydrates, diffuses out of cells into the ECF. In the ECF a small amount combines with water to form carbonic acid, thereby increasing the [H+] and decreasing the pH of the ECF.

Answer 14

In red blood cells (RBC) metabolism is anaerobic and no CO2 is formed. CO2 therefore diffuses into RBC down a concentration gradient. In the RBC the majority of the CO2 combines with water to form carbonic acid, due to the presence of C.A. The carbonic acid dissociates to form H+ and HCO3-, and the H+ are bound by the Hb. Deoxygenated Hb binds H+ more strongly than oxygenated Hb, and in fact the binding of H+ to Hb facilitates the release of oxygen (the Bohr effect). The overall effect of this process is that CO2 is converted to HCO3- in red blood cells. The HCO3- diffuses out of the RBC along a concentration gradient, to be replaced by chloride ions (the chloride shift).

Answer 15

The mechanisms for HCO3- reabsorption and generation are very similar and easily confused, the difference is in net H+ excretion.

Answer 16

In the lungs, the reverse occurs, because of the low partial pressure of CO2 in the alveoli: HCO3- diffuses into the red cells, combines with H+ released when Hb binds oxygen, and is converted into CO2, which diffuses into the alveoli to be excreted. The role of Hb is therefore to transport O2 and by converting CO2 to bicarbonate, to minimise changes in the HCO3- /CO2 ratio between venous and arterial blood, which helps to minimise pH changes.

Answer 17

The glomerular filtrate contains the same concentration of HCO3- as the plasma. The luminal surface of the renal tubular cells is impermeable to HCO3- , and direct reabsorption can thus not occur. Within the renal tubular cells, CO2 combines with water to form carbonic acid, due to the presence of C.A. The carbonic acid dissociates to form H+ and HCO3-. The H+ are secreted into the tubular lumen in exchange for Na+, while the HCO3- pass across the basal border of the cells into the interstitial fluid together with the Na+. In the tubular lumen the H+ combine with the filtered HCO3- , form carbonic acid and then CO2 and water, some of which filters back into the renal tubular cell. The net effect of this process is that filtered HCO3- is reabsorbed, but although there is H+ secretion, there is no net H+ excretion. This process takes place as long as filtered HCO3- is present in the tubular lumen, which is essentially in the proximal renal tubule.

Answer 18

After filtered HCO3- is fully reabsorbed, H+ secretion into the tubular lumen causes a net excretion of H+. This process uses the same mechanism as described above, but requires the presence of urinary buffers, otherwise the H+ gradient created would prevent further H+ secretion. The main urinary buffer is phosphate, which is present predominantly as HPO42-. This combines with secreted H+ to form H2PO4. Another important urinary buffer is ammonia, produced by deamination of glutamine in renal tubular cells. The enzyme responsible, glutaminase, is induced by chronic acidosis, and there is thus an unlimited supply of NH3. The NH3 can diffuse across the renal tubular membrane, but NH4+ cannot. This process takes place as long as there is no more filtered HCO3- present in the tubular lumen.