Fluid imbalances week 2

Question 1

How much water (in ml/day) is gained and lost from the body?

State the approximate amounts and sources of water gains and losses.

Answer 1

Question 2

What are some examples of fluid gains and losses?

What does the clinical consequence of fluid imbalances depend on?

Answer 2

• Fluid balance can be affected by fluid gains and losses.

Examples include

• IV injections

• • isotonic or hypertonic saline
• • lactated Ringer’s solution
• • D5W (5% dextrose in water)
• drinking water
• vomiting
• diarrhea
• excessive sweating

The clinical consequences of fluid imbalances can be very severe and depend on the kinds of fluids lost or gained because their composition varies greatly in pH, [K+], [Na+], and osmolarity.

Question 3

What are the general acid/base affects of fluid loss? (hint: what kind of fluid loss would lead to acid/base affects?)

Answer 3

Many fluids have a pH different than plasma (e.g., vomit, diarrhea). If these fluids are lost, the lost H+ or HCO3 – is replenished from the plasma. Therefore, loss of fluids whose pH is higher or lower compared to plasma creates (metabolic) acidosis or alkalosis, respectively.

Question 4

The main electrolyte effects of fluid losses is on what 2 electrolytes?

Answer 4

Na+ and K+

Question 5

Why can small changes in [K+] have dangerous consequences?

Which K+ balance disorder (hyper- or hypokalemia) is seen more often and why?

Answer 5

The main effect of electrolytes is on Na+ (discussed below) and K+ . [K+]p is small and in generally tightly controlled to be 3.4–4.8 mmol/L. Because [K+] is important in determining membrane potential, small changes can have dangerous effects. Hypokalemia is the most common problem due to K+ loss in fluids. Hyperkalemia is rarely seen in normal persons because of rapid K+ uptake into cells and rapid renal excretion. Hypokalemia leads to widespread functional changes.

Question 6

Generally, how does osmolarity affect fluid balance?

Answer 6

Osmotic gradients between fluid compartments (e.g., extra- and intracellular fluids) cause water to move between the compartments. This has clinical consequences.

Question 7

What is total body water (TBW)?

At what time in life is TBW percentage the highest?

What percentage of body weight is TBW in adult males and females?

Answer 7

The major constituent of the body is water. It acts as a solvent and as a suspending medium for all electrolytes and osmotic substances. It is as essential to and characteristic of living systems as are the organic compounds.

total body water (TBW) = the sum of the water content of all of the body fluids.

Greatest at birth (as much as 83% of body weight in a full term infant) and percentage declines rapidly thereafter.

Adult females: TBW = 45–50% of body weight (use 50% for calculations in this course)

males: TBW = 55–60% of body weight (use 60%)

Question 8

TBW is a constant percentage of ____ ____ ____.

Why are there differences in TBW between individuals and between males and females?

Answer 8

TBW is a constant % of lean body mass. (see attached figure)

Large differences in TBW exist between individuals and between males and females because of

• variable fat content of bodies
• low water content of fat

Question 9

What is the distribution of water (in percentage) between the extracellular fluid (ECF) and the extracellular fluid (ICF)?

Answer 9

The distribution of water between the extracellular fluid compartment (ECF) and the intracellular fluid compartment (ICF) is 40%/60%; there is more water inside the cells than outside the cells.

Question 10

What is the barrier between ECF and ICF? Discuss the water and Na+ permeability of this barrier.

What is the barrier between plasma and interstitial fluid (ISF)?

Answer 10

‪ Barriers between ECF and ICF are the plasma membranes of cells.

• high permeability to water
• much lower permeability to solutes, particularly Na+. Cell walls are essentially impermeable to Na+ because the Na-pump actively removes Na+ from the ICF and transports it out of the cell into the ECF
• as water moves, the volume of ICF and ECF change

The barriers between plasma and ISF are the capillary walls.

Question 11

Generally, how may body fluid volumes be measured?

Answer 11

• Indicator Dilution
• based on concentration (C)= quanitity (Q)/volume (V)
• Administer a known quantity of indicator, wait for equilibration
• Measure concentration of indicator
• Calculate volume of distribution (space)

Tracer characteristics: • non-toxic • only distributes in volume of interest • distributes evenly • distributes rapidly • does not alter existing fluid distribution • is measurable

Question 12

What indicators may be used to measure TBW, ECF, plasma, and blood volume?

Answer 12

Question 13

How is ICF volume calculated?

How may ECF volume be calculated?

Answer 13

ICF volume = TBW – ECF

ISF + lymph volume = ECF – plasma volume

Question 14

Plasma membranes are freely permeable to water. Hence osmotic concentration differences between the ICF and ECF cannot be sustained.

When is an osmotic concentration difference between the ECF and the ICF established?

Answer 14

Plasma membranes are freely permeable to water.

• osmotic concentration differences between the ICF and ECF cannot be sustained
• generally, all body fluids have the same osmolarity

An osmotic concentration difference between the ECF and the ICF is established when cell-impermeable solutes in the ECF is added or taken away.

To illustrate this, consider the addition of urea to the plasma. Urea moves freely into the cells (ICF), so after equilibration the urea concentration in the ICF and ECF is equal and no water moves.

Na+ is effectively cell-impermeable. Any Na+ that moves into the cells is pumped out again. Na+ does not move into cells so water moves out of cells. ECF volume ↑; ICF volume ↓.

see attached figure and slide 16 of notes (for urea movement)

Question 15

The osmolarity of what compartment is measured to estimate osmolar concentration of all body fluids? Why?

Answer 15

In a steady state, all body fluids have the same osmolar concentration. Therefore, one can estimate osmolar concentration of body fluids by measuring the osmolarity of the plasma.

Question 16

What substance in plasma is used to estimate plasma osmolarity? Why?

Answer 16

The osmotic concentration of the plasma may be estimated if you know plasma [Na+]p. Plasma [Na] is a good index of the osmolar concentration of the body fluids because:

• Na+ (with its associated anions) is the largest component of the plasma solute (>90%)
• the concentration of other plasma ions is small and may be assumed to be constant
• the concentration of water in plasma is usually constant, so changes in [Na+]p reflect changes in the osmolar concentration of body water

Question 17

State AND explain the calculation used to estimate plasma osmolarity.

Answer 17

Na+ and its associated anions are about 90% of the plasma solutes. Therefore,

2 × [Na+] = 90% of the plasma osmolar concentration.

The value of the other cations and their associated anions is taken as 10 mOsm/L. (This is not exact, but it is an okay estimate. Some omit this correction completely.) Therefore, the plasma osmolar concentration is

2 × [Na] + 10 mOsm/L. (eq. 1)

However, other solutes may be present in abnormally high concentration. For example, glucose in diabetes mellitus. We can correct for this possibility. The osmolarity due to normal glucose (100 mg/dl approximately) is already in ( eq. 1). The osmolar contribution due to abnormal glucose concentration requires converting the lab measurement into mOsm/L.

To convert [glucose]p given in mg/dl to mOsm/L, divide by the molecular weight of 180 mg/mmol (to convert to mmol/dl) and multiply by 10 (to convert mmol/dl to mmol/L). The contribution of excess glucose to the plasma osmolarity in mOsm/L is:

10 × ([glucose_p ] – [normal glucose]_p) / 180. (eq. 2)

Equations (1) + (2) estimates the total effective osmolar concentration of plasma in mOsm/L.

2 × [Na] + 10 + 10 × ([glucose_p] – [normal glucose_p]) / 180.

Question 18

1 L of D5W (5% dextrose=isoosmotic glucose in water) is administered intravenously (IV) to a patient.

What are 2 issues with giving this type of IV?

What is the final outcome as far as osmolarity is concerned?

Answer 18

1 L liter of D5W (= 5% dextrose = isosmotic glucose in water) is administered intravenously (IV) to a patient. There are two aspects to this problem:

• the glucose will trigger an endocrine response that accelerates glucose uptake, storage (as glycogen) and use by cells.
• once the glucose enters the cells (in minutes) it exerts no net osmotic effect. The problem is equivalent to giving 1L of water.

The administered fluid rapidly distributes within the ECF, diluting the impermeable solutes (most importantly, Na+ ) and creates an osmotic gradient (ECF hypotonic to ICF). Water moves into the cells. This osmotic movement stops when the osmotic gradient is gone.

Final outcome: water enters the total body fluid, equally diluting the fluid in all compartments.

Question 19

What is the outcome (as it pertains to fluid balance and osmolarity) of giving a patient 1 L of isotonic NaCl?

Answer 19

1 L of isosmotic NaCl (= normal saline = 0.9% NaCl) is administered into a patient’s vein.

Cells are functionally impermeable to Na+ so the administered Na+ will remain in the EC compartment along with Cl– . The administered fluid is isosmotic (to cell fluid) so it does not create an osmotic gradient. It simply distributes within the ECF until it is excreted.

Final outcome: only the EC compartment expands, isosmotically.

Question 20

What is the outcome (as it pertains to fluid balance and osmolarity) of giving a patient 1 L of hypertonic saline (NaCl) in an IV?

Answer 20

1 L of hyperosmotic saline (NaCl) is given intravenously.

The NaCl cannot enter the cells. The solution is hyperosmotic with respect to cells. Hence, an osmotic gradient is produced (ECF hypertonic to ICF). This causes water to move out of the cells into the plasma (i.e., from ICF to ECF). The water movement dilutes the ECF and concentrates the IC compartment. Osmosis stops when the osmotic difference between the compartments is gone. Final ECF osmolality is between original value and concentration of hyperosmotic saline.

The final outcome: The volume of the ECF is increased by the volume of fluid infused plus the volume of fluid that moved from the ICF. The volume of IC compartment decreases.

The osmolar concentration of all of the body fluids is increased. Changing the concentration of one body compartment changes the concentration of all compartments.

Question 21

In what types of diseases are increases in isotonic salt and water seen?

What are the signs and symptoms of this due to?

Answer 21

• Excess of isotonic salt and water
• isotonic increase in both Na+ and water
• expands the ECF volume
• seen in cardiac, renal, and endocrine disorders
• results in respiratory and CV changes
• signs & symptoms from elevated Pv: edema, shortness of breath

Question 22

In what cellular compartment does volume decrease when isotonic salt and water are lost?

What circumstances/diseases may cause this type of lfuid loss?

What are the signs and symptoms of this type of fluid loss due to?

Answer 22

This decreases ECF volume. Source of loss may be obvious, like diarrhea, sweating, or exudation from severe burns. However, large fluid volumes may be transferred to the “transcellular compartment” (i.e., the abdominal cavity (ascites), or the gut lumen) or to the ISF (e.g. trauma). This causes a condition equivalent to losing ECF from the body.

Signs and symptoms are largely the result of a decreased blood volume leading to reduced tissue perfusion (CNS, GI, renal). Rapid changes may lead to increased Hct and plasma protein concentration; but slow changes may not.

Question 23

What is the relative osmolarity of the ECF compared to ICF when there is water deficit?

What 2 things may water deficit be due to?

What are the signs and symptoms of water deficit?

Answer 23

This is an increase in concentration of body fluids. It results from

• excess water loss relative to solute (loss of hypotonic fluid). This water is generally lost from the plasma (ECF), so ECF volume decreases and concentration increases; therefore, water shifts from the ICF to ECF. The volume of all compartments is decreased.
• solute gain in excess of water (addition of hypertonic fluid-unreplaced insensible water loss, glucosuria:osmotic diuresis having low salt content), usually into the plasma. The volume and concentration of ECF is increased and there is a shift of water from ICF to ECF.

Signs and symptoms are mainly neurological: restlessness, irritability, ataxia, spasms, seizures, death (respiratory failure).

Question 24

What is the relative osmolarity of the ECF compared to ICF when there is water excess?

What circumstances may cause water excess?

What conditions may develop in water excess?

What are the signs and symptoms of water deficit?

Answer 24

This is a dilution of body fluids, an excess of water relative to solute. It may result from excess water intake (addition of hypotonic fluid), from excess renal water retention (inappropriate ADH secretion–SIADH), or from sweating or GI losses followed by water (only) replacement. Hypokalemia and hyponatremia can result if these ions become diluted enough.

Signs and symptoms are mainly neurological: confusion, disorientation, twitching, seizures, coma, death.

Question 25

Expansion of what compartment is the most important in water excess and why?

Answer 25

Water excess expands all fluid compartments, but expansion of CNS cellular compartment is most important because the cranium is a closed space.

Question 26

The signs and symptoms of water excess or water deficit are mainly due to the effect of water entering (during water excess) or leaving (during water deficit) brain neurons. As [Na+]p (or the concentration of other osmotic substances) changes, an osmolar gradient is created between the blood/brain barrier, causing brain cells to taken water (increase in size) or release water (shrink in size).

Why are rapid changes in ICF volume of CNS cells (either increases or decreases) more harmful than slow changes?

Answer 26

Rapid change in cranial ICF volume can seriously affect CNS function:

• Rapid volume increase compresses cranial blood vessels and reduces blood flow.
• Rapid volume decrease pulls the cranial mass away from the meninges and can tear blood vessels that bridge the space between the meninges and the brain, causing hemorrhage and loss of perfusion.

Slow changes in cranial ICF volume are much less serious because CNS cells have volume regulating mechanisms:

• Slow reduction in ECF osmolarity causes brain cells to extrude electrolytes, thereby reducing the intracellular osmolarity.
• Slow increase in ECF osmolarity causes brain cells to accumulate proteins that counterbalance the elevated extracellular concentration.

Question 27

When body fluids are lost during sweating, vomiting, or diarrhea, the clinical consequences are more than just because of osmotic changes. What other consequences result from fluid loss? (hint: 3)

Answer 27

All fluid loss results in water loss, but other consequences occur because the ionic composition of these fluids is very different.

The water, electrolytes, and H+ /HCO3 – that are lost are replenished from the plasma (and ECF in general). Therefore, depending on the composition of the fluid, any substances lost will deplete these in the plasma and can lead to

• osmotic shifts of water, mainly due to [Na+]p changes • hypokalemia
• metabolic acidosis/alkalosis due to loss of H+ or HCO3-.

Question 28

What are the symptoms of hypokalemia? What are they due to?

Answer 28

• hypokalemia is especially problematic
• symptoms and signs mostly from changes in membrane potential
• disorientation
• psychotic behavior
• weakness
• hypoactive reflexes
• paralytic ileus
• EKG changes
• death by cardiac arrest in diastole

Question 29

Fill in the table.

Answer 29

Question 30

What are the sources of insensible water loss? Explain how/why water is lost from these sources.

Answer 30

Pure water is continuously lost from the body through the evaporative water loss from body surfaces. It does not refer to sweat (i.e. the secretion of the sweat glands). It is the evaporation of pure water from two surfaces:

• skin. Water diffuses through the cutaneous layers to the body surface and vaporizes. The rate of evaporation depends on the skin temperature, the relative humidity and motion of the air. It, therefore, increases in fever and in hot, dry environments. It increases tremendously when the water permeability of skin is increased, as in burns.
• respiratory mucosa. Inspired air is almost always cooler than body temperature and has a relative humidity less than 100% while expired air is fully saturated and is at body temperature. Expired air contains 0.034 ml of water per liter. Hence, a significant amount of solute-free water may be lost. The loss increases during fever and with increased minute ventilation. It could be as great as 1.5 L/day.

Question 31

How much is the obligatory urine volume per day? Why is water lost via this route?

Answer 31

Lastly, recall that even with maximal ADH secretion, the urinary solute excretion does not stop. It is contained in the smallest possible volume of urine at the highest possible concentration, but still contains water. This volume is called obligatory urine volume. It is about 600–800 ml/day. Therefore, even during total fluid deprivation when an individual is totally deprived of food and drink over a long period, water is always lost from insensible water loss and obligatory urine production.