Acid-Base Balance I Flashcards
What is the average ph of blood?
7.4 (arterial blood is 7.45 and venous blood is 7.35)
What is the pH of the ECF?
Tightly controlled close to 7.4 (40 x 10^-9 micromoles)
What do small changes in pH reflect?
Large changes in H+ concentration
What does an increase in H+ concentration cause?
Reduces pH
What do fluctuations in H+ concentration alter?
Nerve, enzyme and K+ activity
How do changes in [H+] affect the CNS?
Acidosis can lead to depression of the CNS
Alkalosis can lead to over-excitability of the peripheral nervous system and later the CNS
How does metabolic activity affect [H+]?
Causes H+ to be continually added to the body fluid = input must equal output to maintain constant H+
Where is H+ continually added from?
Carbonic acid formation
Inorganic acids produced during breakdown of nutrients
Organic acids resulting from metabolism
What happens to acids when they are in solution?
Strong acids fully dissociate
Weak acids partially dissociate
What is the equilibrium equation for acids?
HA = H+ + A-
What does a buffer system consist of?
A pair of substances = one can yield free H+ as the [H+] decreases, the other can bind free H+ when [H+] increases
What happens to the equilibrium equation when an acid is added to solution?
Equilibrium shifts to the left = protons mopped up by A- leading to formation of more HA, [HA] rises and [A-] falls
Rise in [H+] has been limited by the formation of HA
What happens to the equilibrium equation when a base is added to solution?
Equilibrium shifts to right = base tied up by combining with H+ allowing HA to dissociate, [HA] falls and [A-] rises, rise in pH has been limited by further dissociation of HA
What is the dissociation constant (K) at equilibrium according to the Law of Mass Action?
K = pK + log ([H+][A-]/[HA])
What is the Henderson-Hasselbach equation?
pH = pK + log ([A-]/[HA])
What is the most important physiological buffer?
Co2-HCO3 buffer:
CO2 + H2O = H2CO3 = H+ + HCO3
How is H2CO3 formed?
Formed from CO2 and water, catalysed by carbonic anhydrase
What is the role of the kidneys in the control of [HCO3-]?
Variable reabsorption of filtered HCO3-
Kidneys can add new HCO3- to the blood
Both depend upon H+ secretion into the tubule
What is the rate of filtration of HCO3-?
GFR x [HCO3-]plasma = 4320 mmol/day
What happens to HCO3- in the fluid of the kidneys?
Disappears from tubular fluid and appears in the interstitial fluid
Why is the reabsorption of HCO3- considered unorthodox?
The same HCO3- ion doesn’t cross the epithelium
How is new HCO3- produced by the kidneys?
When [HCO3-] of the tubular fluid is low, secreted H+ combines with the next most plentiful buffer in the filtrate (phosphate) = allows gain of HCO3-
What is the purpose of generating new HCO3-?
To regenerate buffer stores
What is titratable acid?
The amount of H+ excreted as H2PO4-
How is titratable acid measured?
Measured amount of NaOH added to titrate urine pH back to 7.4 = reverses addition of H+ that has occurred as fluid flows along the tubule
What is the maximum amount of titratable acid that can be excreted?
40 mmol/day = 40 mmol of new HCO3- is simultaneously gained by the circulation
What other base besides phosphate can act as a tubular buffer?
Ammonia = acid excreted as NH4+ and new HCO3- is generated
How is NH4+ measured?
Not measured as part of titratable acid = a separate ammonium ion determination is needed
When may NH4+ excretion increase?
May rise from its usual 20 mmol/day to between 500-600 mmol/day during acidosis
What does H+ secretion by the tubule do?
Drives reabsorption of HCO3-
Forms acid phosphate
Forms ammonium ion
What is H+ excretion equal to?
The amount of new HCO3- generated
What is the total H+ secretion into the tubule?
4360 mmol/day
What are the components that make up the total H+ secretion into the tubule?
H+ excretion = 4300 mmol/day
NH4+ excretion = 40 mmol/day
TA excretion = 20 mmol/day
What is the total tubular H+ excretion?
60 mmol/day
What are the components that contribute to total tubular H+ excretion?
TA excretion = 20 mmol/day
NH4+ excretion = 40 mmol/day
What is the vast majority of H+ secretion used for?
HCO3- reabsorption = to prevent acidosis
What does the excretion of NH4+ and TA achieve?
Simultaneously rids the body of acid load and regenerates buffer stores (alkanises body)
