Disorders of Sodium Metabolism

Question 1

1. Describe the differences between salt and water content in the body.

Answer 1

osmolality equilibrates between body compartments, sodium is restricted to the ECF and water moves to achieve equilibrium

Question 2

2. Explain why the sodium ion determines the extracellular fluid volume.

Answer 2

Na+ stays in the ECF, so Na+ content of the ECF will either cause water to enter or exit this compartment and determine the ECF volume

Question 3

3. Evaluate total body sodium content. What are the signs and symptoms of low sodium?

Answer 3

thirst (angiotensin II)
lightheadedness
palpitations
orthostasis
decreased BP
dry mouth
decreased urine output
dry axilla
low JVP
skin turgor reduced

Question 4

4. Describe sodium transport in the nephron. List the sodium channels located in each section of the nephron.

Answer 4

PCT: Na/H antiporter
Ascending Limb: NKCC channel
DCT: Na/Cl cotransporter
Cortical duct: ENaC (epithelium sodium channel)

Question 5

5. Explain how the body regulates the sodium balance.

Answer 5

volume status is the major mediator of sodium balance

sympathetic system: vasoconstriction , reducing the amount of Na filtered; activates RAAS thru B2 receptors

Renin Angiotensin System: angiotensin II causes vasoconstriction, aldosterone causes reabsorption of salt and reabsorption of water

ANP: vasodilation in response to atrial stretch, can cause increased salt delivery as well as act as a direct inhibitor to sodium reabsorption

pressure natriuresis- increased systemic pressure causes loss of sodium

Question 6

Changes in salt content manifest as changes in _____ volume.

Answer 6

extracellular volume effected by changes in salt content

Question 7

Define effective circulating volume.

Answer 7

effective circulating volume refers to part of the ECF that is in the arterial bed

essentially the pressure perfusing the arterial baroreceptors in the carotid sinus and glomerular afferent arterials that the body uses to monitor volume status

Question 8

Describe why patients with CHF retain water using effective circulating volume.

Answer 8

diminished cardiac output leads to reduced effective circulating volume which causes the system to retain Na+ and water despite appropriate volume (can lead to edema)

note in cirrhosis ECF volume is increased but plasma volume is decreased due to low albumin

Question 9

If you were measuring Na+ and Cl- to determine fluid status, would you test serum concentration or urine concentration?

Answer 9

urine sodium and chloride (modulated by aldosterone) tell you about volume status, unlike serum sodium, they are markers of effective circulating volume

Question 10

Is sunken fontanelle a symptom of sodium gain or loss and why?

Answer 10

sodium gain- expansion of the ECF causes fluid to leave the ICF, causing shrinkage of brain

Question 11

How do you treat volume depletion?

Answer 11

administer isotonic fluids to maintain appropriate intravascular volume then address osmolarity

Question 12

How do you treat volume overload.

Answer 12

remove extra sodium either by decreasing sodium intake or increasing output