Marathon runners are encouraged to stop at water stations along the course to stay hydrated. However, occasionally a runner drinks too much water and suffers from hyponatremia, or "water intoxication". While water itself is not toxic, drinking too much can upset ion balance in the extracellular space. Based on what you know about ions and nerve signaling, the levels of which important ion are most likely to be disrupted in hyponatremia?

Question

The word hyponatremia derives from the words hypo ("under" or "less") and natrium ("sodium"). The word natrium is the basis for the abbreviation Na for sodium. Hyponatremia is a rare condition, but has become of more concern with the rise in amateur runners participating in marathons. Since they run slower, they spend longer on the course and have more opportunity to drink water and sweat out additional sodium.

A.  hydrogen
B.  potassium
C.  calcium
D.  sodium
E.  oxygen

Clarify Question
· What is the key concept addressed by the question?
· What type of thinking is required?

Gather Content/Choose Answer
· What do you already know about components of the nervous system?  What other information is related to the question?

Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?

Answer 1

D.  sodium


Clarify Question
· What is the key concept addressed by the question?
    o The question is asking you to solve which ion is responsible for hyponatremia.
· What type of thinking is required?
    o This question is asking you to analyze the potential negative consequences of drinking too much water on cellular function.

Gather Content/Choose Answer
· What do you already know about components of the nervous system?  What other information is related to the question?
    o To solve this problem, you’ll need to remember what the relative concentrations of important ions are across the cell membrane.  Most of the cell is composed of hydrogen and oxygen in the form of water or other molecules, but these are not the ions responsible for neural signaling.  Remember that the concentration of sodium is much greater outside the cell than inside, and the opposite is true for potassium.  Since the question states that drinking too much can upset the ion balance in the extracellular space, you can surmise that the extra water will dilute the ions in the extracellular space, decreasing the concentration of ions outside the cell.  Since sodium is usually greatest outside the cell, diluting it would disrupt the gradient which usually results in sodium rushing into the cell, which is the basis of action potentials.  If there is no gradient due to hyponatremia, no action potentials!   Also, diluted extracellular fluid can lead to water coming into your cells (via osmosis) and causing cells to swell.

Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?
    o If you figured out the correct answer, good job!  If not, where did you go wrong?  Did you think it was potassium?  Too little potassium can result in muscle cramps in runners, but the question mentioned the ion balance in the extracellular space and the ion of greatest concentration there is sodium.  While the concentration of calcium ions is also greater outside the cell than inside the cell, it is much smaller than that of sodium.