Fluid, Electrolyte, and Acid-Base Balance

Question 1

Starling equation and +/- signs

Answer 1

J_v = K_f [(P_c - P_i) - (pi_c - pi_i)]

If driving force is + then filtration (movement of fluid out into interstitum)

If driving force is - then reabsorption (movement of fluid into capillaries)

P_c + means pressure out of capillaries (favor filtration)

P_i - means pressure out of capillaries

pi_c + means fluid into capillaries

pi_i + means fluid out of capillaries (favor filtration)

Question 2

How would we increase filtration with Starling forces?

Answer 2

Increase P_c or decrease pi_c

Question 3

Edema

Answer 3

When the volume of interstitial fluid exceeds the ability of lymphatics to return it to the circulation

Get edema when (1) increased filtration, or (2) impairment of lymphatic drainage

Question 4

Types of capillaries

Answer 4

Continuous (capillaries of muscle; extreme case is BBB)

Fenestrated (capillaries of kidney, intestine, endocrine gland; special case is glomerulus–specialized for high filtration)

Discontinuous (capillaries of bone marrow, spleen, liver)

Question 5

What determines Pc (pressure inside the capillary)?

Answer 5

Venous pressure, NOT mean arterial pressure! This is because of autoregulation of arteriolar sphincter tone

Question 6

What 3 safety factors minimize potential edema?

Answer 6

1) When filtration increases, lymph flow increases to keep up with it and carry off extra fluid
2) When filtration increases, that dilutes interstitial proteins, which causes water to flow back into capillary (increased difference in oncotic pressures)
3) When filtration increases, Pi increases, which reduces difference in pressure and causes water to flow back into capillary

Note: water does not necessarily flow BACK into capillary as I stated, but it does at least reduce the filtration rate

Question 7

When does increased Pc cause edema?

Answer 7

Remember, Pc increases when VENOUS pressure increases, not when MAP increases!

Increased renal retention of Na+ causes increased plasma volume

Obstruction such as hepatic cirrhosis or deep venous thrombosis

Static exercising muscle can block flow

Question 8

When does decreased oncotic pressure in the capillary cause edema?

Answer 8

Hypoalbuminemia could happen because of leaky glomeruli or decreased albumin synthesis by the liver

Note: have to get really low albumin (2 gm/dL compared to normal 3.7-4.6) in order to see obvious edema

Question 9

When does increased water conductivity (Lp) and/or decreased protein selectivity cause edema?

Answer 9

Causes more protein to leak into interstitum, reducing oncotic pressure

Burns, trauma, inflammation, sepsis and allergic reactions cause this

Question 10

When does lymphatic obstruction cause edema?

Answer 10

When lymphatics obstructed, get fluid filtered into interstitum that is not removed. Also because of stagnation, plasma proteins leave capillary and equilibrate across capillary wall

Cancer itself or biopsy/removal of lymph nodes can cause this.

Question 11

Causes of hypovolemia

Answer 11

Hemorrhage

GI losses (vomiting, diarrhea)

Renal losses (diuresis, adrenal insufficiency, salt wasting nephropathy)

Skin losses (burns, sweat)

“Third-spacing” (internal bleeding, peritonitis)

Question 12

Causes of hypervolemia

Answer 12

Congestive heart failure

Nephrotic syndrome

Cirrhosis

Question 13

Can plasma osmolality of [Na+] tell you about a person’s volume status?

Answer 13

NO!

Weight, CV status (blood pressure, HR, etc) can

Question 14

Hypovolemic hyponatremia

Answer 14

Decreased volume state and decreased [Na+]

Vomiting, diarrhea, hemorrhage, third-spacing

Since you have severe water loss, volume stimuli for ADH release override osmotic stimuli AND this water loss is stimulus for thirst, so you get volume increase without [Na+] increase

When get back to a good volume, Na+ input will volume expand the ECF (using aldosterone?)

Question 15

Hypervolemic hyponatremia

Answer 15

Increased volume and decreased {Na+]

Severely reduced EFFECTIVE circulatory volume: body THINKS there is decreased volume when there’s not (CHF), body has decreased oncotic pressure (hypoalbuminemia in nephrotic syndrome), or get too much vasodilation (cirrhosis)

Volume stimuli for ADH release override osmotic stimuli AND this water loss is stimulus for thirst, so you get volume increase without [Na+] increase

Question 16

Euvolemic hyponatremia

Answer 16

Volume is normal but decreased [Na+]

Syndrome of inappropriate ADH secretion (SIADH) causes increased water reabsorption without Na+ reabsorption so you become hyponatremic but have normal volume (volume stimuli still working to keep volume normal)

SIADH because of ectopic production or hypothalamic production, or potentiation of ADH on kidney

Also could be drugs causing increased ADH (analgesics), compulsive water drinking

Question 17

Primary polydipsia (water intoxication)

Answer 17

Free water intake exceeds rate at which kidney can excrete dilute urine

Schizophrenia, water drinking competitions!

Question 18

Pseudohyponatremia

Answer 18

Person has normal Na balance even though [Na+] is low (<135 mmol/L)

Hyperglycemia causes this by pulling more water out of cells and reducing plasma [Na+] (because glucose is taking up the rest of the “osmole” space in the blood)

Uremia does not have this osmotic effect because urea does not affect water movement (ineffective osmole)

Question 19

Hypernatremia due to non-renal water loss

Answer 19

Sweating, burns, diarrhea

Losing water but main problem is that person is not drinking enough water

1) Inability to access water
2) Altered thirst mechanism (elderly, hyperaldosteronism resets osmostat, reduced osmoreceptor sensitivity, CNS lesions)

Question 20

Hypernatremia due to renal water loss

Answer 20

1) Diabetes insipidus (ADH not signaling, so get lots of dilute, sugar free urine)
2) Interference with urine concentrating mechanisms (osmotic diuresis, loop diuretics, renal failure)

Question 21

Acute hyponatremia

Answer 21

Less than 48 hours duration

No RVD yet, risk of increase in intracranial pressure causing herniation

Seizures, coma

Administer hypertonic saline

Examples: stress causes ADH over-secretion and more water consumption; infants w/diarrhea fed sugar water instead of milk

Question 22

Chronic hyponatremia

Answer 22

Greater than 48 hours duration

Common electrolyte abnormality in hospitalized patients

Do not have symptoms!

Have done RVD

If you over-treat them, will shrink the brain and cause irreversible demyelination

Question 23

Dextrose solutions

Answer 23

Add free water to the body when you have had insensible losses or hypernatremia

Added sugar (glucose) is rapidly metabolized, leaving only water behind

Examples: D5W (~280 mOsm)

Note: D10W (~560 mOsm) is actually hypertonic because glucose sticks around!

Question 24

Saline solutions

Answer 24

Only contain NaCl unless otherwise indicated (3% is 3x normal, normal is isotonic, half normal is half isotonic)

Ringer’s solution: isotonic with physiological conc of KCl, CaCl2, NaCl, lactate (which is converted to bicarb)

Question 25

Where does normal saline go?

Answer 25

ECF only

Question 26

Where does D5W go?

Answer 26

D5W (water!) goes 1/3 to ECF and 2/3 ICF (follows volume distribution of body and distributes freely)

Osmolality decreases

Question 27

Where does half normal saline go?

Answer 27

All the saline part goes to ECF (1/2 of solution) and water part distributes freely (1/6 solution to ECF and 2/6 to ICF).

Overall: 2/3 to ECF and 1/3 to ICF

Osmolality decreases

(Equal parts saline and water)

Question 28

Where does 3% normal saline go?

Answer 28

All (1L) goes into ECF and also brings in 1.4L from ICF to ECF

Osmolality increases

Question 29

Treatment for isotonic hypovolemia

Answer 29

Isotonic saline and D5W or half-normal saline to account for insensible losses

Question 30

Treatment for hypernatremia

Answer 30

Hypotonic fluids (water, D5W) but must take into account chronic vs acute

Question 31

Treatment for hypovolemic hyponatremia

Answer 31

If chronic: isotonic fluids to replace lost volume (turn off excess ADH), then let normal function (aldosterone?) restore osmolality

If acute: hypertonic fluids (cautiously, if necessary), otherwise isotonic

Question 32

Treatment for euvolemic hyponatremia

Answer 32

If not emergent situation, don’t need to do anything

Figure out what is causing hyponatremia and treat that

Question 33

Treatment for hypervolemic hyponatremia

Answer 33

If not emergent situation, don’t need to do anything

Figure out what is causing hyponatremia and treat that (example: CHF–could be made worse by increasing volume further)

Question 34

Normal plasma [Na+]

Answer 34

135 - 145 mmol/L H2O

Question 35

What equation do you use when deciding how much water to add to get to certain [Na+]?

Answer 35

Total body osmoles *proportional to* (TBW)(plasma[Na+])

TBW₁ x Na⁺₁ = TBW₂ x Na⁺₂

Question 36

What equation do you use when deciding how much Na+ to add to get to certain [Na+]?

Answer 36

Extra amount of Na+ to be added = TBW x ([Na+]₂ - [Na+]₁)

Question 37

Why would the kidney retain enough excess salt and water (3 liters!!!) to cause edema?!

Answer 37

1) Volume receptors (arterial side) sense effective circulating volume is low so signal kidney to retain more (CHF)
2) Kidney itself responds abnormally and retains salt and water even when effective circulating volume is normal (nephrotic syndrome)