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 Table of Contents  
MEDICAL EDUCATION TEACHING NOTE
Year : 2018  |  Volume : 3  |  Issue : 1  |  Page : 54-57

Students lose balance over the yin-yang of sodium physiology


Department of Physiology, Faculty of Medicine, University of Malaya, Kuala Lumpur, Malaysia

Date of Submission04-Apr-2018
Date of Acceptance24-Apr-2018
Date of Web Publication19-Jun-2018

Correspondence Address:
Prof. Hwee-Ming Cheng
Department of Physiology, Faculty of Medicine, University of Malaya, 50603 Kuala Lumpur
Malaysia
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/bjhs.bjhs_9_18

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  Abstract 


One of the major areas that students studying physiology stumble over is sodium homeostasis. In particular, there is often an unclear separation in their understanding to differentiate between sodium balance and sodium concentration (NaC) control. Functionally, students need to understand that NaC control involves the osmoreceptors that monitor changes in NaC in the extracellular fluid (ECF) since NaC is the dominant determinant of ECF osmolarity. This teaching note aims to address the common misconception regarding sodium physiology, specifically that ECF NaC is not regulated by the renin-angiotensin-aldosterone (RAAS) family of hormones. The RAAS is instead directed toward total body sodium or sodium balance. Different scenarios of ECF changes that alter sodium balance and/or NaC will be illustrated with a summary of “Tables of Salt.” Hopefully, both students and teachers will find this teaching note useful in their appreciation of the wonderful Yin Yang of sodium physiology.

Keywords: Homeostasis, physiology, sodium


How to cite this article:
Cheng HM, Hoe SZ. Students lose balance over the yin-yang of sodium physiology. BLDE Univ J Health Sci 2018;3:54-7

How to cite this URL:
Cheng HM, Hoe SZ. Students lose balance over the yin-yang of sodium physiology. BLDE Univ J Health Sci [serial online] 2018 [cited 2018 Dec 14];3:54-7. Available from: http://www.bldeujournalhs.in/text.asp?2018/3/1/54/234651




  Changes in Sodium Balance/concentration in Diverse Conditions Top


[Table 1] summarizes five different situations that can affect the extracellular fluid (ECF) sodium concentration (NaC) and/or sodium balance. It is “so dium” essential to distinguish between NaC control and regulation of sodium balance.
Table 1: Changes in extracellular fluid sodium concentration and/or sodium balance in different situations

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The five different scenarios summarized in [Table 1] will help students to appreciate that NaC is not synonymous with sodium balance. Sodium balance refers to the total body sodium (tBS), the cation being predominantly in the ECF.

It is clearly obvious that there is no direct association between changes in ECF NaC and sodium balance. An increased NaC is associated with a negative and a positive sodium balance (exercise and eating salty crisp, respectively). An unchanged ECF NaC (drinking isotonic saline and blood donation) can also be associated with a positive and a negative sodium balance, respectively.[1]

Scenario 1 (drinking water)

When a large volume of water is consumed quickly, the NaC is reduced by ECF dilution. There is a positive water balance. However, the tBS is unchanged.

Scenario 2 (drinking isotonic saline)

When a volume of isotonic saline is drunk, there is a resultant increase in both water and tBS in the body. In this case, there is a positive sodium balance although the ECF NaC is not altered.

Scenario 3 (exercise)

During physical activity, there is both loss of water and salt in the sweat. Exercise leads to both negative water and sodium balance. Since the human sweat is hypotonic, the resultant ECF becomes hyperosmotic. The NaC which is the main determinant of ECF osmolarity is increased.

Scenario 4 (eating salty crisp)

While watching a movie called “The Electrolytes Strike Back”, you wolf down a packet of salty crisp and if you did not drink water, the ECF becomes hypertonic and draws water out of the intracellular space (ICF). There is unchanged water balance in the body. However, both the ECF and the ICF will have an increased osmolarity with an increase in ECF NaC.

Scenario 5 (blood donation)

After a blood donation, the body is depleted of both water and salt. There is both a negative water balance and a negative sodium balance. Isotonic blood is donated and, in contrast to the loss of hypotonic sweat during exercise, the ECF NaC is not affected.

The scenarios 2 (isotonic drink: Positive sodium balance) and 5 (blood donation: Negative sodium balance) highlight the essential link between tBS and ECF/blood volume. Any subsequent compensation to achieve euvolemia will have to normalize both the sodium and the water balance. Note again that the resultant ECF NaC is unchanged in both cases. The homeostatic responses will not be directed to NaC control but to sodium balance regulation.


  Homeostatic Compensations for Changes to Extracellular Fluid Sodium in Diverse Conditions Top


The physiologic responses or homeostatic compensations to the five different scenarios of ECF volume and/or osmolarity changes are summarized in [Table 2].
Table 2: Homeostatic responses to the five different scenarios described in Table 1

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One key concept for students to note is that there are three terms that are synonymous in physiology, namely, osmoregulation, maintenance of water balance, and NaC control of the ECF.[2] This is pictured in the triangle shown in [Figure 1]. This means that any disturbance of water balance will change the ECF NaC. As the concentrations of sodium and its accompanying anions (chloride and bicarbonate) determine the ECF osmolarity, changes in water balance will involve hypothalamic osmosensing and the corresponding secretory responses of posterior pituitary antidiuretic hormone (ADH).[3]
Figure 1: Osmoregulation, control of water balance (H2O), or regulation of extracellular sodium concentration all refer to the same physiological function. Adapted from Physiology Question-Based Learning (page 128) by Cheng HM. Switzerland: Springer International Publishing; 2015 with permission

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A second essential principle to note is that, unless there is a change in the tBS, there will be no need for homeostatic responses via the renin-angiotensin-aldosterone (RAAS) family of hormones. [Table 2], for simplicity, only focuses on the hormonal inputs and omits the role of autonomic sympathetic feedback in ECF volume regulation. Natriuretic peptide hormones are also not shown, but these hormones (cardiac and renal) promote urinary sodium excretion and will be increased during positive sodium balance.

With these two concepts in mind, the hormonal responses will be better appreciated and understood.

Scenario 1 (drinking water)

Drinking water leads only to a positive water balance with unchanged tBS. The rapid compensation will be an inhibition of the osmoreceptor/ADH secretory pathway. Water excretion will be increased to restore the water balance. There is no physiologic need to include the RAAS. To make the latter point, students can be asked “what if RAAS is significantly inhibited since drinking water does increase the ECF volume?” Should that occur, the person who had a normal unchanged sodium balance will end up losing urinary sodium and become sodium depleted!

Scenario 2 (drinking isotonic saline)

When isotonic saline is drunk, there will be both a positive sodium balance and a positive water balance. In this case, both the ADH and the RAAS responses will be suppressed.

Scenario 3 (exercise)

During exercise, there is dehydration with both a negative sodium and negative water balance. The appropriate responses will be a stimulation of both the RAAS and the ADH pathways.

Scenario 4 (eating salty crisp)

When blood is donated, there will be both a negative sodium balance and a negative water balance, even though the ECF NaC is unaffected. The RAAS and the ADH reactions will be activated to restore euvolemia. Students can note that RAAS is triggered in both exercise (scenario 3) and blood donation(scenario 5) although the NaC is increased and unchanged, respectively. This makes the point that NaC control is not the physiologic responsibility of renal renin. Instead, renin is part of the mechanisms for ECF volume or sodium balance regulation.

Scenario 5 (blood donation)

The moviegoer who eats salty crisp will have a more convoluted homeostatic response. We can think about this in a step-wise sequence. Although there is no change in water balance since she/he did not drink, the hypertonic ECF will lead to secretion of ADH in order to normalize the ECF osmolarity. Thus, added to the initial flux of water from the ICF to the ECF, the additional renal water reabsorption stimulated by ADH will produce eventually an expanded ECF (isotonic expansion).

There is then both a positive water balance and a positive sodium balance. The final compensation will be inhibition of both the RAAS and the ADH hormonal responses.


  Yin and Yang of Sodium Physiology Top


In Chinese philosophy of Yin and Yang, opposing forces or influences are involved in the balance of life and health. Yang is described as the positive input and Yin is the counteracting negative factor. Applied to sodium physiology, sodium gaining, conserving or retaining mechanisms (Yang) will be balanced by sodium losing, depleting, and excreting pathways (Yin). The two major weights on each side of sodium balance are depicted in [Figure 2].
Figure 2: Yin and Yang of sodium physiology

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Students should also note that ECF volume sensing (includes both volume receptors and arterial baroreceptors) during hypovolemia stimulates activation of renal sympathetic activity, which conserves sodium. However, during hypervolemia, when ECF and blood volume expand, volume sensing will lead to inhibition of renal sympathetic action to increase urinary sodium excretion. Atrial natriuretic peptide and renal urodilatin secretion will also enhance the sodium excretion.[4]


  Questions to Review and Regain Sodium Balance Top


Questions in [Table 3] can be used to give students time to think through and distinguish between NaC and sodium balance regulation.
Table 3: Review questions for sodium Yin-Yang

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The authors of this teaching note value feedbacks and comments on this Yin-Yang article of sodium physiology.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

1.
Cheng HM, Damayanthi D. Quantitative Based Physiology-Learning (q-PL): Renal System. Selangor: August Publishing; 2010.  Back to cited text no. 1
    
2.
Rose BD, Post TW. Clinical Physiology of Acid-Base and Electrolyte Disorders. 5th ed. New York: McGraw-Hill; 2001.  Back to cited text no. 2
    
3.
Cheng HM. Physiology Question-Based Learning Cardio, Respiratory and Renal Systems. Switzerland: Springer International Publishing; 2015.  Back to cited text no. 3
    
4.
Cheng HM, Durairajanayagam D. Misconceptions highlighted among medical students in the annual international intermedical school physiology quiz. Adv Physiol Educ 2012;36:229-32.  Back to cited text no. 4
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