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STEP Physiology > Na+ Balance > Flashcards

Na+ Balance Flashcards

1
Q

Revised Henderson eqn

A

pH = 24 x (pCO2/HCO3)but this needs to be converted to [H+]

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2
Q

What is the relationship between pH and [H+]?

A

at pH of 7.4, [H+]= 40change of pH of 0.3= 2x[H+]7.7=207.1=806.8=160

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3
Q

There are no specific “normal” values for urinary concentrations. Why?

A

Plasma constituents of water and numerous solutes are maintained within a very narrow range by the homeostatic mechanisms performed by the kidney. This requires variation in the urinary excretion of water and solutes by the kidney.

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4
Q

Normal plasma concentrations:

A

Arterial blood gas measurements:Sodium 140 meq/l pH 7.40 Potassium 4.0 – 5.0 meq/l CO2 40 mm HgChloride 104 meq/l Bicarbonate 24 meq/lBUN 10 mg/dlCreatinine 1.0 mg/dlCalcium 9.0-10.0 mg/dlPhosphorus 4.0–5.0 mg/dlAlbumin 4.0-4.5 g/dlOsmolality ~290 mosm/kg H2O

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5
Q

TBW represents what percentage of BW?

A

60%2/3 of TBW is intracellular fluid volume (ICFV). ICFV = 0.4 BW1/3 of TBW is extracellular fluid volume (ECFV). ECFV = 0.2 BW3/4 of ECFV is interstitial fluid volume (ISFV). ISFV = 0.15 BW1/4 of ECFV is plasma volume (PV). PV = 0.05 BW

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6
Q

T or F. ICFV osmolality always equals ECFV osmolality

A

T, due to passive movement of H20

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7
Q

What is the major cation in ICFV? ECFV?

A

K+. Na+ is major cation of ECFV since it is virtually restricted to ECFV.

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8
Q

What are the events that follow the ingestion of Na+?

A

-increased TBNa-increased osmolarity of ECFV since Na+ is restricted-leading to increased thirst and H2O intake and AVP secretion

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9
Q

So what is the net result of ingestion of Na+?

A

The net result then is retention of isotonic NaCl and expansion of ECFV with little or no change in ICFV.

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10
Q

How is isotonic volume depletion performed?

A

Isotonic volume depletion occurs when isotonic fluids are lost from the ECFV such is the case in vomiting of gastric contents or diarrhea

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11
Q

What is the result on ICFV of isotonic volume depletion?

A

no change in ICFV since ECFV osmolality is unchanged because the volume lost is isotonic

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12
Q

The composition of ions in gastric and intestinal fluid differ from plasma. How would vomiting (loss of gastric fluid) affect the body?

A

Gastric losses are rich in HCl and would, therefore, be associated with metabolic alkalosis in addition to the ECFV loss.

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13
Q

How would diarrhea (loss of intestinal fluid) affect the body?

A

Intestinal fluid is relatively rich in HCO3- and would result in normal anion gap metabolic acidosis in addition to ECFV loss.

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14
Q

NOTE: Changes in TBNa (up or down) are synonomous with changes in ECFV

A

increased TBNa lead to ECFV expansion and decreased TBNa leads to ECFV depletion

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15
Q

Plasma Na+ concentration (mEq/L) per se tells you nothing about TBNa content or the size of the ECFV. Why?

A

Plasma Na+ concentration is always determined by the ratio of TBNa to TBW (more specifically to ratio of total body cations to total body water, K in ICFV and Na in ECFV)Low plasma Na+ concentration could be associated with low, normal, or expanded ECFV depending on the relative changes in TBNa, TBW, or both. The same would be true for increased Na+ concentration.

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16
Q

A 65 yo women with a weight of 60kg has x L in the extracellular compartment. What is the value of x?

A

10L TBW= 0.5BW in a woman and = 0.6BW in a manand ECF is 1/3 of TBW

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17
Q

A 28 yo women with no other medical problems comes to the ED with nausea and diarrhea. Her lying down BP is 100/70 and on standing drops to 80/60. She is tachycardia with cool extremities. Which set of urine electrolytes would you find in this patient? urine Na mEq/L Uosm A. 5 100B. 75 900C. 5 900 D. 75 100

A

C. Whenever there is extra-renal loss of sodium and the kidneys are normal, the kidneys will work to conserve Na and water

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18
Q

What happens to water balance after ingestion of a water load?

A

Reduction in Posm with low ADH and a dilute urine

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19
Q

Where is most of the plasma volume distributed?

A

80% venous and 20% arterial

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20
Q

A. A 56 yo woman with type 2 diabetes mellitus, HTN. and CAD presents to the ED with chest pain and SOB. Her physical exam is significant for JVD, S3 gallop, bilateral rales (fluid in alveoli) and lower extremity edema. Na 130 mEq/l, K 4.2mEq/L, Cl 89mEq/L, HCO3- 24mEq/L, serum creatinine is 1.5mg/dl, glucose is 120mg/dLWhat will happen? (ECFV= extracellular fluid volume, ECV= effectives circulating volume)

A

Her ECFV is expanded, CO is low, and ECV is reduced Notice here that the total body Na+ is elevated even though serum Na+ is decreased (so this patient is hypoatremic)

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21
Q

Total body Na+ is synonomous with ____.

A

ECFV. Is NOT the same as serum Na+. Can tell it’s elevated by a clinical exam that is similar to CHF (extended EJV, edema, bilateral rales etc.)

22
Q

What are the major constituents of ECF?

A

Na+, Cl-, and HCO3-

23
Q

What happens when the body feels that ECV is low?

A

Afferent sensors such as renin, catecholamine, thirst, and ADH are increased and ANP is decreased leading to Na+ and water retention

24
Q

What are the roles of angio II?

A

-Vasoconstriction-increases PT reabsorption of Na+ (Thus, urine Na+ decreases)

25
Q

What are the roles of aldosterone?

A

-increases distal reabsorption of Na+ Thus, urine Na+ decreases

26
Q

What does ADH result in?

A

distal collecting tube becomes permeable to water for increased reabsorption (making the urine more concentrated- i.e. Uosm will increase)

27
Q

What is ECV?

A

relates to the fullness and tension within the arterial tree Because only 15% of total blood volume is in the arterial compartment, arterial blood volume can be decreased in elation to the holding capacity of the tree.

28
Q

What are some situations where ECV is decreased in the presence of an elevated ECVF?

A

-CHF-liver disease-sepsis-nephrotic syndrome-pregnancy-anaphylaxis

29
Q

Why would liver disease be associated with a decrease in ECV in the presence of an elevated ECVF?

A

Because a lot of the blood is pooled into the splanchnic circulation

30
Q

When volume depletion occurs due to extra renal losses (e.g. diarrhea, burns) with normal renal function, how does the urine present?

A

urine becomes highly concentrated and salt poor

31
Q

When volume depletion occurs due to renal losses (e.g. diuretics), how does the urine present?

A

urine in inappropriate dilute and sometimes rich in salt

32
Q

What are some clinical features of volume depletion?

A

-hypotension-poor skin turgor-cool extremities

33
Q

How does the body react to balance water upon increase in ECF osmolality?

A

-CNS osmoreceptors are activated (after only 2% increase in osmolality) and release ADH release (this is called osmotic ADH release) causing water retention (as well as angio II)

34
Q

How does the body react to balance water upon decrease in effective circulating volume?

A

A decrease in 7-10% of effective circulating volume causes baroreceptor release of ADH (this is called non-osmotic ADH release) and angio II stimulating volume retention

35
Q

What is the eqn. for serum Na+ conc.?

A

Na(TBNa)/water volume (TBW)most causes of hypoatremia are caused by a relative increase in TB water

36
Q

What is hypoatremia defined as?

A

An abnormally low plasma Na+ conc. (less than 135mEq/L)diagnosis can be made from lab values

37
Q

If you see that the patient has hypoatremia, then what should you do next?

A

check osmolality (most will be hypo-osmolar due to the significance of [Na] in the plasma osmolality formula)

38
Q

What things can cause a hyperosmolar hypoatremia?

A

-mannitol is hyper-osmolar so it drags out water from the ICF to cause hypoatremia -hyperglycemia

39
Q

What should you do next once you know a patient has a hypo-osmolar hypoatremia?

A

is the ECVF elevated or lowered

40
Q

A 78yo man is brought into the ED fro decreased appetite and ‘altered mental statue’. On exam he is responsive to strong stimuli, has dry mucous membranes, flat neck veins, sacral decubitus ulcers and appears to be moving all extremities and facial muscles without deficits. Vital T=97.0, BP=80/50, HR=130, RR-14. Na=121, K=4.5, Cl=9.5, HCO3-=32, Glucose=140What are the changes in TBW and TBNa in this patient compared to baseline?

A

TBW and TBNa are both decreased but TBNa is lost more, but ADH has been activated Know TBNa is decreased from clinical signs This is hypovolemic hypoatremia- hypovolemic based on clinical signs and hypoatremic based on labs

41
Q

Describe hypovolemic hypoatremia

A

sodium depletion with reduction in ECF volume and secondary water retention due to non-osmotic stimulation of AVpPrelease Thus, TBW decreases and TBNa decreases even more translating to excess TB water relative to TBNa

42
Q

A 78yo women was brought to the hospital after a fall. Has Hx of HTN for which she took a CCB. A selective SSRI streamline has been prescribed 2 months prior for depression. She reported climbing a flight of stairs when she fell. Before the fall she experienced thirst for a number of days, but did not report other complaints. On exam, she had several cranial bruises, was disoriented to time and place. BP: 120/90, pulse of 80. No apparent sings of extracellular fluid volume contraction (so TBNa+ is normal)Na 125 (low), K 4.2, Creatinine 0.8mg/dL, serum osmolality 255mOsm/kg (low), Urine Na=44 mEq/L, urine Osml=523 mOsm/kg

A

she has hypo-osmolar euvolemic hypoatremiaSo her TBNa+ is normal and TBwater must be elevatedIn this, ADH should shut off and the Uosm should decrease (dilute-less than 100) but here her Uosm is way too high suggesting that she is retaining fluid because she has elevated ADH causing inappropriately concentrated urine

43
Q

Describe euvolemic hypoatremia.

A

water retention due to autonomous or altered regulation of vasopressin release (common cause- SIADH) causing increased TBwater and normal TBNa

44
Q

Causes of euvolemic hypoatremia?

A

-pulmonary disorders, CNS disorders, cancers-vasopressin analouges-hypothyroidism

45
Q

What are some vasopressin analouges?

A

chlorpropamide, clofibrate, narcotics, narcotics, carbemezepine, antipsychotics

46
Q

A 44yo patient with cirrhosis of the liver comes in with SOB. Exam revealed BP 100/70, elevated JVD, ascites, and marked pedal edema. Na-126 Meq/L, K 4.2 mEq/LCl 90, HCO3 28, BUN 20urine Na 10 and urine osmolality is 800mOsm/kgWhy is her serum Na so low?

A

hypo-osmolar hypervolemic hypoatremiaNon-osmotic stimulation of vasopressin release because of reduced ECV

47
Q

Causes of hypervolemic hypoatremia?

A

-CHF-Hepatic cirrhosis-Nephrotic syndrome-Renal disease

48
Q

A 75yo female NH resident is brought to the ED because of altered mental statue. She has stopped drinking after developing diarrhea due to trial gastroenteritis. She is found to communicate poorly. Brief physical exam reveals diminished skin turgor and flat neck veins. BP is 80/60. pulse 120, RR 20. Serum sodium 168, urine Na is 10, and Uosm is 600 mOsm/kgWhats going on?

A

She has external losses of sodium that are exceed by loss of free water Physical exam shows that TBNa is low, and thus the kidneys are conserving the Na and water via ADHThis hypovolemic hypernatremia

49
Q

Does the presence of ADH makes the urine dilute or concentrated?

A

concentrated

50
Q

T or F. Hypernatremia is usually caused by excess water loss rather than sodium gain

A

T. Usually develops in those who cant sense thirst properly or in ADH resistance

51
Q

45 yo WM comes after head injury. She has polyuria (U/O 10L/day). She is confused. BP is normal and she has no edema. Sr. Na is 165 Sr ism 330 Uosm is 70. She is treated with dDAVP nasal spray and within 24 hrs her Sr Osm drops to 295. and urine ism increased to 620. What’s going on here?

A

She has absence of AVPcentral is when you have no ADH, and nephrogenic is when you have ADH but its not working for some reason So this person must have central because treatment worked

STEP Physiology (68 decks)

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