Read in sequence, pH, carbon dioxide and bicarbonate from one composite rower show the metabolic change came first in this SC131 Unit 9 acid base interpretation. Searches like "sc 131 unit 9 assignment example", "sc131 unit 9 sample" and "sc131 unit 9 example" land here.
What a finished SC131 Unit 9 acid base interpretation looks like
Three pages with the values in a small table beside their reference ranges, then the interpretation written as numbered steps. Step one: pH below 7.35, so acidemia. Step two: bicarbonate low and carbon dioxide low, so the primary change must be the one that explains the pH, which is the fall in bicarbonate; a low carbon dioxide would push pH up, not down. Step three: the expected respiratory compensation, calculated with Winter's formula as 1.5 times 14 plus 8, gives 29, within 2 of the measured 30, so the lungs have compensated appropriately. Step four: the source, lactic acid from anaerobic metabolism in working muscle, buffered by bicarbonate. A final paragraph predicts the next hour, lactate cleared, bicarbonate restored, ventilation settling, with the kidneys barely needed.
How a SC131 Unit 9 example is structured
The interpretation opens with the bicarbonate buffer equation, carbon dioxide and water in equilibrium with carbonic acid, hydrogen ions and bicarbonate, because every later step reads movement along it. The Henderson-Hasselbalch relationship follows in words, pH depending on the ratio of bicarbonate to dissolved carbon dioxide, normally about twenty to one. The stepwise reading is the core, and each step states its rule before applying it. A timing section answers the prompt's question directly, explaining that chemoreceptors drive ventilation within minutes while renal compensation takes days, so a lung response already visible means the metabolic change preceded it. A check paragraph recalculates pH from the ratio to confirm the values are internally consistent. The conclusion names the disturbance in one phrase, metabolic acidosis with appropriate respiratory compensation, and ties it back to the rowing effort.
pH read first, always
The interpretation starts from the pH alone, so the primary disturbance is chosen to explain the direction of the pH rather than the most abnormal-looking number.
The primary change identified by direction
Low carbon dioxide cannot cause acidemia, so the fall in bicarbonate must be primary. The step states that logic outright instead of leaving it implied.
Compensation checked against a formula
Winter's formula turns a judgment into arithmetic, showing that the measured carbon dioxide sits where appropriate respiratory compensation would place it.
Speed decides which organ moved
Lungs adjust in minutes and kidneys in days, so visible respiratory compensation after a short effort places the metabolic change first in time.
An internal consistency check
Recomputing pH from the bicarbonate to carbon dioxide ratio confirms the value set hangs together, a step few interpretations include and graders notice.
Where marks go in SC131 Unit 9
A correct label with no route to it is the most common way to lose here, because the unit grades the reasoning path, and a label without steps is indistinguishable from a guess. Choosing the primary disturbance by the most abnormal number rather than by the pH direction comes next. Compensation confused with correction is a steady deduction, the lungs described as fixing the problem when they only limit the pH change. Timing claims that put the kidneys to work within minutes of the effort undermine the answer to the which-organ question itself. Arithmetic slips in the compensation formula, or units omitted on carbon dioxide and bicarbonate, cost precision credit. Clinical treatment beyond the prompt's question draws a deduction, and reference ranges given without a source close the list.
Get a SC131 Unit 9 example written to your instructions
Send the value set supplied with Unit 9, plus its instructions and the SC131 rubric that applies. We read it step by step, pH first, primary change, compensation checked, timing argued, and the first custom interpretation is free, back within 24-48h. Several value sets in one prompt? Include them all.
SC131 Unit 9 questions, answered
How can the values show which organ moved first?
Through speed and completeness. Respiratory compensation begins within minutes because chemoreceptors respond quickly to pH, while renal compensation takes hours to days. If the primary change is metabolic and carbon dioxide has already shifted to compensate, the metabolic change came first. If the primary change is respiratory and bicarbonate has shifted substantially, the disturbance has lasted long enough for the kidneys to act.
What is Winter's formula and do I need it?
It estimates the carbon dioxide level expected when the lungs compensate appropriately for metabolic acidosis: 1.5 times the bicarbonate, plus 8, plus or minus 2. Some SC131 sections teach it and others do not. Where it is taught, using it turns a judgment about compensation into a calculation, which usually strengthens the interpretation and is easy for a grader to verify.
Is exercise really an acid-base disturbance?
Briefly, yes. Intense exercise produces lactic acid faster than it can be cleared, bicarbonate buffers the hydrogen ions, and ventilation rises to blow off carbon dioxide. The values recover within an hour or so in a healthy person. That makes it a useful case for a normal-function course: a real disturbance, a real compensation, and no disease required.