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Grandma's TUM-my Trouble

A Case Study in Renal Physiology and Acid-Base Balance



Author:

Ann T. Massey
Nell Hodgson Woodruff School of Nursing
Emory University
ann.massey@emory.edu

Abstract:

An elderly woman living independently with some help from her family is brought to the local emergency room because she is confused and vomiting. While her son suspects a stroke, a quick battery of laboratory tests indicates that her current problems are the result of impaired kidney function, an old-fashioned home remedy for ulcers, and her prescribed blood pressure medication. The combination of patient- and drug-related factors produces an acid-base disorder responsible for her confusion. This disorder further disrupts her kidney function. The case illustrates secretion and reabsorption processes in the kidney tubule which regulate plasma and urinary electrolytes (including calcium) and pH. The role of bicarbonate in maintaining systemic pH is emphasized. Interactions among the renal, respiratory and nervous systems in the regulation of systemic pH are also illustrated. Further, basic principles of pharmacotherapy and issues related to the appropriate use of medications are introduced. The case was developed for use in a physiology or human anatomy and physiology course, but it might be used in undergraduate nursing courses as well (e.g., pharmacology, pathophysiology).

Objectives:
  • Identify the three processes that are involved with urine formation-filtration, reabsorption and secretion-and identify the places in the nephron where they happen.
  • Outline the main mechanisms by which the kidney regulates pH in the body, and discuss the roles of specific transporters for H+ and HCO3- in pH regulation.
  • Discuss the inter-related roles of the nervous system, the respiratory system and the renal system in regulation of pH in the body.
  • Outline the mechanisms by which the kidney regulates plasma electrolyte concentrations, with special emphasis on plasma Ca++, including the pertinent hormones involved.
  • Identify disturbances in plasma electrolytes and pH, given appropriate reference values.
  • Identify possible causes of these disturbances
Keywords: Acid-base disorders; renal function; kidneys; kidney physiology; metabolic alkalosis; calcium regulation; Sippy regimen; bicarbonate
Topical Area: N/A
Educational Level: Undergraduate lower division, Undergraduate upper division, Graduate, Professional (degree program), Clinical education, Continuing education
Formats: PDF
Type/Method: Directed, Discussion, Interrupted
Language: English
Subject Headings: Physiology   Nursing   Biology (General)  
Date Posted: 6/6/2014
Date Modified: 5/20/2016
Copyright: Copyright held by the National Center for Case Study Teaching in Science, University at Buffalo, State University of New York. Please see our usage guidelines, which outline our policy concerning permissible reproduction of this work.

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A very good student pointed out a few things in this case that I'd like some help with. Are Table 2 Electrolytes all serum? Is the Calcium supposed to be Ca++? The ionized Calcium in Table 2 is given in mEq/L, but the Ca++ in Grandma's test (Table 1) is given in mg/dL. How do these units compare? In Table 1, Grandmas's serum Na+ is 128 mg/dL, and in Table 2 Sodium is 135-147 mEq/L. Is the sodium in Table 2 Na+? How do the units compare? It would make more sense to use consistent units, and make it clear which are serum, which are Ca++, etc. Thank you!


Susan Hutchins
Biology
Itasca Community College
Grand Rapids, MN
sue.hutchins@itascacc.edu
4/29/2016

Author’s Reply

“Are Table 2 Electrolytes all serum?”
Yes—the electrolyte measurements are all from serum.

“Is the Calcium supposed to be Ca++?”
In Table 2—No. Since calcium is a divalent ion, it is found both free in the plasma / serum (as an ion, Ca++) and bound to plasma proteins like albumin. Most sources indicate that the percentage of calcium that is free is about 50%, and the percentage that is bound is also about 50%. This means that technically there are three measurements that can be made: free (or ionized, or unbound) calcium; bound calcium; and total calcium (the sum of the free and the bound calcium). There are two entries for calcium in Table 2. Calcium refers to TOTAL calcium, or the total of the free / unbound / ionized calcium and the calcium that is bound to proteins. Calcium, ionized refers to strictly the free / unbound / ionized calcium.

The ionized calcium is the physiologically significant calcium that plays a role in many physiological processes.

“The ionized Calcium in Table 2 is given in mEq/L, but the Ca++ in Grandma’s test (Table 1) is given in mg/dL. How do these units compare?“
Equivalents (Eq) are units that are used when describing concentration of electrolytes or acids / bases in solution. An equivalent represents the number of moles of charges, so to speak, and depends on the valence, or charge of an ion in solution. (The valence charge of the ion is equal to the number of equivalents in one mole of that ion.) Calcium ion has a valence of +2, which means that each mole of Ca++ has 2 equivalents; 1 equivalent of Ca++ is ½ mole of Ca++.

Equivalents are a little archaic, but they are used clinically here in the US to describe ion concentrations in different body fluids, including serum.

You can convert among units of mEq, mmol and mg mathematically with the following equations:
     Concentration (mg / dL) × MW (mmol / mg) × Conversion factor (10dL / L) = mmol / L
     [Concentration (mg / dL)]/MW (mmol/mg) × valence of the ion × Conversion factor (10dL / L) = mEq / L

The calcium measurement in Grandma’s test represents total calcium (see above), and so is usually represented as mg/dL here in the US. In Europe and other countries, it’s more common to see units of mmol/L (SI units). Because this measurement represents total calcium—bound and free—it’s more correct to express this as “Calcium” as opposed to “Ca++.” [This update has been made to the case.]

So, Grandma’s total serum calcium of 15.76 mg/dL could also be expressed as 3.94 mmol/L or 7.88 mEq/L.

“In Table 1, Grandmas’s serum Na+ is 128 mg/dL, and in Table 2 Sodium is 135–147 mEq/L. Is the sodium in Table 2 Na+? How do the units compare?”
This appears to be my error. Sodium will be present only as the ionized form, and traditionally is expressed in units of mEq / L (as is potassium). Grandma’s sodium (Na+) should be 128mEq / L. [This correction has been made to the case.]

“It would make more sense to use consistent units, and make it clear which are serum, which are Ca++, etc.”
Yes, however, there are multiple conventions that are currently in use clinically. Thank you for bringing this to my attention. I hope you got something useful from the case, despite the errors.




Ann T. Massey
Nell Hodgson Woodruff School of Nursing
Emory University
Atlanta, GA
ann.massey@emory.edu
5/20/2016



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