The same 720 mL loss means 8 percent of body weight at 11 months and under 1 percent in an adult, a difference traced to water compartments, turnover and kidney maturity. Searches like "nu 551 unit 7 assignment example", "nu551 unit 7 sample" and "nu551 unit 7 example" land here.
The Same 720 mL: A Fluid Balance Case in an 11-Month-Old and Her Father
[Student Name]
Purdue University Global
NU551: Advanced Physiology and Pathophysiology Across the Lifespan
Unit 7 Case Analysis
[Instructor Name]
[Date]
Both patients are composites and every figure is a worked estimate; no real patient is described.
The Question
When the virus reached the household, the father spent a miserable afternoon and the infant spent the evening in the emergency department. They had lost the same amount of fluid. The question this case answers is why the same volume produced such different illnesses, and the answer lies in the size of the reservoir each loss was drawn from, the speed with which each body normally turns its water over and the tools each has to defend itself. Measured against the body it leaves, a loss of 720 mL is under one percent of the father's weight and eight percent of his daughter's.
The Calculation Block
The estimates below use standard proportions for body water by age (Friis-Hansen, 1961) and the weight-based maintenance method of Holliday and Segar (1957). Each figure is an approximation meant to show proportion, not to guide treatment.
1. Weight: infant [9.0] kg; father [80] kg.
2. Total body water: infant about 65 percent of weight, 5.9 L; father about 60 percent, 48 L.
3. Extracellular volume: infant about 30 percent of weight, 2.7 L; father about 20 percent, 16 L.
4. Fluid lost: 0.72 L for each.
5. Loss as a share of body weight: infant 0.72 of 9.0, or 8.0 percent; father 0.72 of 80, or 0.9 percent.
6. Loss as a share of extracellular volume: infant 0.72 of 2.7, or about 27 percent; father 0.72 of 16, or about 4.5 percent.
7. Daily maintenance water: infant 100 mL for each of the first 10 kg, 900 mL; father 1,500 mL for the first 20 kg plus 20 mL for each kilogram above that, or 2,700 mL.
8. Loss as a share of a usual day's water: infant 720 of 900, 80 percent; father 720 of 2,700, about 27 percent.
What the Numbers Mean
The loss by weight is the figure clinicians use most. A loss of about 8 percent of body weight in an infant falls in the range usually described as moderate to severe dehydration, while a loss under 1 percent in an adult is minimal and may pass without symptoms beyond thirst. Put another way, the father lost the equivalent of a large glass of water from a very large tank; his daughter lost nearly a tenth of herself.
The extracellular figure explains why the infant looks ill so quickly. Losses from vomiting and diarrhea come first from the extracellular fluid, the plasma and interstitial fluid outside cells. The infant has lost more than a quarter of that compartment, which includes the plasma volume that keeps her circulation filled. The father has lost less than a twentieth.
The maintenance figure shows how short the infant's margin is in time. She normally takes in and loses about 900 mL of water in a day. Losing 720 mL on top of normal losses, while vomiting prevents her from drinking, is close to losing an entire day's water. The father has lost about a quarter of a day's water, which a few glasses can replace.
Why the Infant Has Less Margin Before the Illness Starts
The difference is not only a matter of size. Three features of infant physiology narrow the margin before any virus arrives.
A larger share of an infant's body water sits outside cells. At birth, extracellular fluid makes up a much larger proportion of body weight than in adults, and the proportion falls through the first year and beyond as cells and tissues grow (Friis-Hansen, 1961). Because losses from the gut are drawn first from this compartment, the infant's circulating volume is exposed early.
An infant turns her water over quickly. Per kilogram, her metabolic rate is higher than an adult's, and her surface area relative to her weight is larger, so she loses more water through the skin and the lungs each day. Her daily water turnover per kilogram is several times the adult rate, and a skipped feed matters in a way a skipped meal does not for her father.
Her kidney cannot yet save water as well. The infant kidney can concentrate urine to only about half the adult maximum, perhaps 600 to 700 mOsm/kg against around 1,200 in her father, because its loops of Henle are shorter and the concentration of urea in the medulla is lower (Hall & Hall, 2021). She must pass more water to excrete the same amount of solute, so even as she becomes dehydrated, she keeps losing water in urine.
The Father's Defenses
The father's body defends his water in the same ways, only with more capacity. As his plasma becomes slightly more concentrated, osmoreceptors in the hypothalamus trigger thirst and the release of antidiuretic hormone. Antidiuretic hormone inserts water channels into the collecting ducts of his kidney, and his urine becomes concentrated and dark as he conserves water. Crucially, he can respond to thirst himself: he can walk to the kitchen and drink.
The infant cannot. She depends on others to notice her thirst and to offer fluid, and while she is vomiting, she may not keep it down. Her thirst mechanism is intact, but it is useless without a caregiver who recognizes it.
Expected Findings
The percentages predict what a clinician would find. In the infant, a loss of about 8 percent would be expected to produce dry mucous membranes, fewer wet diapers, a sunken anterior fontanelle, reduced tears, tachycardia and a slowed capillary refill, with lethargy or irritability as the loss deepens. In the father, a loss under 1 percent would produce thirst and dark urine and little else; his heart rate and blood pressure would be expected to stay normal.
Electrolytes as Well as Water
Vomiting and diarrhea remove more than water. Vomited gastric fluid carries hydrogen and chloride ions, and its loss tends toward a metabolic alkalosis; diarrheal fluid carries bicarbonate and potassium, and its loss tends toward a metabolic acidosis. In an infant with both, the balance depends on which predominates, but the potassium losses of diarrhea and the bicarbonate losses together commonly leave her acidotic. As her circulating volume falls, her kidney receives less blood and filters less, and poorly perfused tissues produce lactic acid, adding to the acid load. Her faster breathing is partly the body's attempt to lower carbon dioxide and offset that acidosis. The father, with a smaller relative loss and a kidney working at full strength, would be expected to correct any such shift on his own within hours.
Sodium follows the same logic. Most gut losses in viral gastroenteritis are close to isotonic, so serum sodium often stays in the normal range while volume falls. But an infant whose losses exceed her intake of water, or who is given fluids low in salt, can drift toward high or low sodium, and her small total body water makes those shifts larger for each milliliter.
Scope
This case explains why the loss differs, not how to replace it. Rehydration, whether by mouth or by vein, belongs to a later unit.
References
Friis-Hansen, B. (1961). Body water compartments in children: Changes during growth and related changes in body composition. Pediatrics, 28(2), 169-181. https://doi.org/10.1542/peds.28.2.169
Hall, J. E., & Hall, M. E. (2021). Guyton and Hall textbook of medical physiology (14th ed.). Elsevier.
Holliday, M. A., & Segar, W. E. (1957). The maintenance need for water in parenteral fluid therapy. Pediatrics, 19(5), 823-832. https://doi.org/10.1542/peds.19.5.823
How this NU551 Unit 7 example is structured
The case moves from numbers to mechanism to presentation. Every figure appears first in the calculation block and then inside a sentence that says what it means physiologically. The mechanism sections explain why the infant has less margin even before the illness starts: a higher fraction of body water sits outside cells, where losses are drawn first; water turnover per kilogram is several times the adult rate, so skipped feeds bite quickly; and tubular concentrating ability is still maturing. The father's compensation is described in parallel: thirst, antidiuretic hormone and a kidney that can concentrate to around 1,200 mOsm/kg. The presentation section maps expected findings onto the percentages, dry mucosa, fewer wet diapers, a sunken fontanelle and tachycardia in the infant, against thirst and dark urine in the father. Rehydration is excluded by design.
Get an NU551 Unit 7 example written to your instructions
Post the Unit 7 case in the words your NU551 instructor used, including any weights, volumes and ages supplied, together with the rubric. A first case analysis at no cost arrives in 24-48h showing every figure computed, then each difference between the patients traced to a named mechanism rather than to age alone. The paper above is an original model document written by our desk, not a submitted student paper and not an official Purdue University Global document.
NU551 Unit 7 questions, answered
Are the body water percentages exact?
No, and the sample says so. Total body water and extracellular fraction vary with age, body composition and source, so the case uses typical values and cites them. The argument depends on the size of the difference between patients, which holds across reasonable estimates, rather than on any single percentage being precise.
What if the case uses an older adult instead of a child?
The comparison works similarly with different mechanisms. Older adults have lower total body water because of reduced muscle mass, blunted thirst and a kidney that concentrates less well, so an ordinary loss can matter more than in a younger adult. The case would compute the same proportions and explain them through aging changes instead of maturational ones.
Should the case include electrolyte values?
If the prompt supplies them, yes. Sodium in particular helps classify dehydration as isotonic, hypertonic or hypotonic, which changes the physiology of water movement between compartments. The sample works with volumes because its case gave no laboratory values, and it notes where a sodium result would refine the analysis.