Week 2 Lectures Flashcards
Source of H+ ions in the body
all acids are produced from METABOLISM
carbonic (volatile) acids=H2CO3–produced from metabolism of carbs and fats; oxidized to CO2 and H2O; exhaled by lungs
non-carbonic (non-volatile) acids=HCl, H2SO4–produced by proteins, sulphates and phosphates; comprises the daily acid load that the kidneys excrete
name the volatile acids
carbonic acids
H2CO3
how are volatile acids removed from the body
exhaled by lungs
Name the non-volatile acids
H2SO4, HCl
how are non-volatile acids removed from the body
comprise the daily acid load that the kidneys excrete
What are the three sources of the body’s buffering system?
- extracellular (major source)
- bone
- intracellular
How do extracellular components participate in the buffering system of the body
HCO3-/CO2 system
plasma proteins
inorganic phosphates
How do intracellular components participate in the buffering system of the body
cellular proteins
phosphates
hemoglobin of RBCs
How does bone participate in the buffering system of the body
bone mineral dissolves to release buffer
especially important during ACIDOSIS
What are the two major mechanisms for compensation during acid base disorders?
- respiratory compensation
2. renal compensation
Describe the mechanism of respiratory compensation during acid base disorders
- in metabolic acidosis–>increase alveolar ventilation to blow off more CO2 (Kusmall breathing = fast and deep breathing characteristic of DIABETIC KETOACIDOSIS)
- in metabolic alkalosis–>decrease alveolar ventilation to retain CO2
describe the mechanism of renal compensation during acid base disorders
- in respiratory acidosis–> increases produced of HCO3-
2. in respiratory alkalosis–> decreases production of HCO3-
Which method (renal or respiratory) acts faster? Which method is more effective at restoring normal values?
renal compensation acts SLOWER but is more effective at restoring normal values than respiratory compensation which works faster
What role does plasma buffering play in acid load?
plasma buffering system uses HCO3- to buffer the H+ load, forming H2CO3 which dissociates into CO2 and H2O–>generated CO2 is expired
What equation describes the relationship between pH, H+, bicarb and pCO2?
henderson hasselbach
henderson hasselbach equation
pH = pKa + log([A-]/[HA])
pH = 6.1 + log10 ([HCO3-]/0.03 X PaCO2])
Describe the renal mechanism for H+ secretion
- H+ is not filtered by the kidney as free ions
- secreted into the lumen at the PT and CD (intercalated cells)
- at the PT uses apical Na+/H+ antiporter
- at the CD uses apical H+ ATPase
- secreted H+ combines with filtered HCO3-, titratable acids (Pi), or ammonium (NH4+)
Describe the renal mechanism for HCO3- reabsorption
-kidney filters 4300 mEq/day of HCO3-
-almost all is reabsorbed at three sites: PT (90%), TAL and CD
-All tubular HCO3- reabsorption is the consequence of H+ secretion
HCO3- + H+ –> H2CO3 –> CO2 and H2O which are both passively reabsorbed
Describe the renal mechanism for HCO3- regeneration (new bicarbonate generated)
- titratable acids–> same process as for HCO3- reabsorption, except that secreted H+ combines with a titratable acid such as a phosphate (HPO4-2)–H+ gets thus trapped in the lumen and secreted as H2PO4—net gain of one HCO3- (unfortunately there is a very limited amount of titratable acids in the body)
- NH4+ secretion–> NH4+ production and excretion increases in response to an increased acid load–under normal circumstances, excretion of NH4+ accounts for less than half of the net acid excreted per day–with an acid load, the kidney can increase NH4+ secretion by about 10X normal to 300-350 mmol/day
How does ammonium act as a buffer in the renal mechanism for NCO3- regeneration
Step 1: ammonium formation in the PT–> 2 NH4+ are generated from each molecule of glutamine in the PT; the same Na+/H+ antiporter is involved, except that NH4+ is transported in place of H+; NH3 can freely reenter the cell while NH4+ is trapped in the lumen
Step 2: ammonium reabsorption and recycling in the TAL/LoH–> reabsorption step at the TAL required to prevent NH4+ from being taken up into the blood and metabolized in the liver to form urea at the cost of 2 HCO3-; NH4+ is recycled into NH3+ and moved to the CD, where urine is more acidic, to reformed NH4+; the same Na+/K+/2Cl- co transporter is involved, except NH4+ is transported in place of K+; free H+ is used in the metabolism of glutamate and alpha-ketoglutarate in the Krebs cycle
Step 3: Ammonium finally acts as a buffer in the CT–> H+ secreted from the aldosterone sensitive ATPase in the CD combines with NH3+ to reform NH4+; NH4+ is trapped in lumen and excreted; net gain in one HCO3- per NH4+ excreted
What defines alkalosis and acidoses?
blood pH >7.4 = alkalosis
What pCO2 defines hyperventilation? Hypoventilation?
- Hyperventilation = pCO2 40 mmHG
What use is the amount of HCO3- in the blood?
gives the status of the primary buffering system of the body
Why must some H+ be secreted with urinary buffers in the case of high acid load?
because the kidney cannot excrete urine more acidic than pH 4.0-4.5
What are titratable acids?
urinary buffers used to excrete excess acid load
How do titratable acids help in acid base balance
- the kidney cannot excrete urine more acidic than pH 4-4.5
- thus, in order to excrete sufficient acid, the kidneys excrete H+ with urinary buffers such as phosphate (Pi) (i.e HPO4-2)–>other urine constituents can also act as buffers (creatinine) but are less important than Pi
- collectively, these urinary buffers are called titratable acids
- however, excretion of H+ as a titratable acid is not sufficient to balance the daily non-volatile acid load–a more important contributor to acid base balance is through the synthesis and excretion of ammonium (NH4+)
How do you calculate the anion gap? (A/G)
Anion Gap = [Na+] + [K+] -([Cl-] + [HCO3-]) = 12+/-2 mEq/L (normal)
Why is K+ often omitted from A/G calculations?
because serum concentrations are normally quite low
why is the anion gap calculated?
to identify the presence of significant unmeasured anions (conjugate bases of a given acid)
when is A/G calculated?
whenever there is a case of metabolic acidosis
- the presence of H+ from an acid will be buffered by HCO3- and therefore deplete the concentration of HCO3-
- some of the H+ will also be buffered by bone minerals and therefore [HCO3-] may decrease by an amount less than the increase in A/G
What are the two types of metabolic acidosis?
- A/G metabolic acidosis
2. Normal A/G metabolic acidosis
What is A/G metabolic acidosis? what does it indicate?
Presents with a change in the anion gap from normal, and indicates the presence of acids
6 causes of A/G metabolic acidosis
“REAL MK”
Renal failure Ethylene glycol Aspirin (ASA) Lactic Acid Methanol Ketoacids
What is normal A/G metabolic acidosis? What does it indicate?
Metabolic acidosis with no change in A/G–>indicates loss of HCO3- (Cl- compensates for loss of HCO3-)
Causes of normal A/G metabolic acidosis
- GI loss from diarrhea
- Renal tubular acidosis–> Type I = impaired HCO3- reabsorption at the PT; Type II - defective H+ secretion at the DT; Type IV = aldosterone deficiency/resistance
- dilution acidosis from rapid ECFV expansion
Symptoms of metabolic alkalosis
- may have confusion or altered mental status
- may have seizures
- may have paresthesias
- may have muscle cramps or tetany
- may have symptoms associated with electrolyte abnormalities
- may have symptoms associated related to specific cause of the disorder (ie drug intake or vomiting)
Signs (labs) indicative of metabolic alkalosis
- high arterial pH
- high HCO3- (primary problem)
- high PaCO2 (compensation)
- often accompanied by low serum Cl- and K+
Signs of metabolic alkalosis on physical exam
- hypoventilation (adaptive)
- confusion
- seizures
- tetany
What 3 factors may be involved in the maintenance of a metabolic alkalosis?
- increased reabsorption of HCO3-
- Hypokalemia
- Chloride depletion
Why might there be increased HCO3- reabsorption?
often due to effective circulating volume depletion
- decrease in GFR leads to activation of the RAAS system
- aldosterone activates H+ ATPase in H+ secreting intercalated cells of the collecting tubule–secreted H+ promotes reabsorption/generation of HCO3-
- aldosterone activates ENaC and Na+/K+ ATPase in principal cells of the collecting tubule–>increased intracellular +ve charge decreases H+ movement into cell
- angiotensin II activates Na+/H+ antiporter in the PT–> Cl- ions follow via pericellular route–> decreases delivery of Cl- ions to HCO3- secreting intercalated cells of the collecting tubule–> decrease in Cl-/HCO3- antiporter activity–>HCO3- retained therefore alkalosis–>secreted H+ promotes reabsorption/generation of HCO3-
How might hypokalemia contribute to causing metabolis alkalosis?
increases HCO3- generation/reabsorption
- transcellular cation exchange of K+ (out of cells) for H+ (into cells) in response to hypokalemia
- increased K+ reabsorption by H+ secreting intercalated cells in the collecting tubule via the K+/H+ antiporter–>secreted H+ promotes reabsorption/generation of HCO3-
How might chloride depletion contribute to causing metabolic alkalosis?
luminal hypochloremia leads to increased distal HCO3- reabsorption
same mechanism as for angiotensin II mediated increase in HCO3- –> decreased delivery of Cl- ions to HCO3- secreting intercalated cells of the CT–>decrease in Cl-/HCO3- antiporter activity–> HCO3- retained in body therefore alkalosis