PROBLEM SET 7.2: FLUID AND ELECTROLYTE BALANCE AND ACID-BASE
BALANCE
ANSWER KEY
1. The following were measured in a 24-hour urine collection:
urine
... [Show More] volume = 1.2 L
titratable acid = 33 mmol L-1
urine [HCO3
-
] = 4 mmol L-1
urine [NH4
+
] = 90 mmol L-1
Calculate the amount of HCO3
-
gained or lost from the body through renal mechanisms.
The net amount of HCO3
-
gained or lost from the body is calculated as:
net HCO3
-
= titratable acid + ammonium - HCO3
-
excreted
These are in amounts or rates. In the above example, we deal with amounts. The amount of
titratable acid is its concentration times the urine volume; the same applies to the ammonium and
HCO3
-
. So we have
net HCO3
-
= 1.2 L x 33 mmol L-1 + 1.2 L x 90 mmol L-1 - 1.2 L x 4 mmol L-1
net HCO3
-
= 39.6 mmol + 108 mmol - 4.8 mmol = 142.8 mmol
2. A patient with a history of bronchopulmonary disease was admitted to the hospital and the
following laboratory data were obtained for arterial blood:
Na 143 mM
K 4.8 mM
pH 7.34
PO2 47 mm Hg
Cl 78 mM
HCO3
- 38 mM
PCO2 73 mm Hg
Hemoglobin 17.1 g%
Classify the acid-base status of this patient in terms of acidosis/alkalosis, respiratory or metabolic,
compensated or uncompensated.
Fig. 7.PS2.1 Acid-Base status, problem #2
The acid-base status can be represented by point X on
the pH-HCO3
-
diagram shown. This corresponds to a
metabolic (renal) compensation for a respiratory
acidosis. You could tell that because PCO2 is high and
the blood is acidic; thus it cannot be metabolic acidosis,
because the respiratory system would respond by
hypocapnia. But the decrease in pH is only slight
compared to what would happen if it were just
hypercapnia - there is compensation and the
compensation is brought about by increasing the plasma
[HCO3
-
].
7.PS2.2
3. The concept of free water clearance is sometimes used to assess the kidneys’ ability to handle a
water load. The free water clearance is defined as:
CH2O = QU - [(QU Uosmol)/Posmol]
Here CH2O is the free water clearance, QU is the flow of urine, in mL min-1, Uosmol is the total
concentration of materials in the urine, in osmol/L, and Posmol is the osmolarity of plasma. The term
in brackets is the clearance of osmolarity, and it represents the volume of plasma that contains the
total osmolarity that is excreted.
Calculate the free water clearance for the following conditions [Show Less]