Electrolyte Homeostasis and Fluid Shifting

Question 1

What are the three main fluid compartments?

Answer 1

Total body water is approximately 60% of the body weight and is spread between the three main fluid compartments. The three compartments are the intracellular space, the extracellular space (interstitium), and the intravascular space.

Question 2

How much body water do these three spaces contain?

Answer 2

The intracellular space contains the largest portion of
the total body water (40%). The remaining two spaces each contain 20% of the total body water.

Question 3

How much total body water do infants have and explain why?

Answer 3

Infants have total body water of 70-80% because they have less adipose tissue. During the immediate postnatal time frame, an infant loses about 5% of body weight secondary to a physiologic loss of water. Infants are vulnerable to changes in TBW due to
their higher metabolic rate and the potential for fluid loss through their skin due to their greater
body surface. Diarrhea can result in a significant loss of TBW in infants. Renal controls of fluid and electrolytes are not mature enough to counteract the effects of the loss of TBW, therefore infants are very susceptible to dehydration and it can happen quickly.

Question 4

Why does total body water decrease as we age?

Answer 4

This is due to an increase in body fat, decreased muscle mass, and a decrease in the ability to regulate sodium and water balance. Renal function declines as we age and therefore they do not function as well to maintain TBW. The elderly are very susceptible to dehydration and it may be life-threatening.

Question 5

What is osmolality?

Answer 5

Osmolality is the measure of solute concentration in a solution. In humans, it is basically the concentration of the plasma. The higher the osmolality, the greater the concentration is and therefore the less water is in that compartment. Osmolality is also known as tonicity.

Question 6

What are osmolality ranges?

Answer 6

280-295

Question 7

What is osmotic pressure?

Answer 7

Osmotic Pressure is the amount of pressure or force that is exerted by solute molecules of a given compartment. The greater the solutes (osmolality), the greater the osmotic pressure.

Question 8

What is osmosis?

Answer 8

Osmosis is the movement of water between
compartments from an area of a low concentration of solutes to one that has a high concentration
of solutes. It is a passive force because it does not require energy.

Question 9

What is hydrostatic pressure?

Answer 9

Hydrostatic Pressure is the force within a fluid compartment - the mechanical force of fluid
against the walls of the compartment, i.e. blood pressure. This pressure is a pushing force and
pushes fluid outside the compartment.

Question 10

What is oncotic pressure?

Answer 10

Oncotic pressure (colloid osmotic pressure) is the force that helps to keep water/fluid within a compartment. It contributes to osmotic pressure and is exerted by plasma proteins (albumin).

Question 11

What is the purpose of a hypertonic IV solution?

Answer 11

A hypertonic IV solution, when administered, will increase the solute concentration in the intravascular space. The intravascular space will become more concentrated and will experience an increased osmolality. Because water flows via osmosis, water will then flow into the intravascular space from the extracellular space. This leaves the extracellular space more concentrated (higher concentration of solutes), and water will then move from the intracellular
space to the extracellular space, which will cause the cell to shrink.

Question 12

What is the purpose of a hypotonic solution?

Answer 12

Administration of a hypotonic intravenous solution will cause the intravascular space to become
more dilute (less solute concentration and a lower osmolality). Based on the concept of osmosis,
water will move from the intravascular space to the extracellular space. This will cause a dilution
of the extracellular space causing water to then shift into the intracellular space, consequently
causing the cell to swell.

Question 13

What is EABV?

Answer 13

This is the amount of blood within the arterial space that effectively perfuses organs and tissues. Volume
changes in the extracellular fluid compartment will cause changes in the EABV in the same direction. Decreased ECF volume will cause a decrease in EABV and vice versa.

Question 14

What are the hormones that control fluid balance?

Answer 14

Antidiuretic hormone, aldosterone, Renin angiotensin aldosterone system, and natriuretic hormone

Question 15

What does antidiuretic hormone do for fluid balance?

Answer 15

Antidiuretic Hormone is secreted by the pituitary gland in response to a water deficit, sodium
excess, or a low BP. It causes the kidneys to reabsorb water thus increasing plasma volume.

Question 16

What does aldosterone do for fluid balance?

Answer 16

Aldosterone is a hormone secreted from the adrenal cortex and is responsible for increased renal
sodium resorption and potassium excretion. Where sodium goes water follows. This is the basis
of osmosis

Question 17

What does RAAS do for fluid balance?

Answer 17

Renin-angiotensin aldosterone system (RAAS) is activated by a low blood volume. A low blood
volume triggers the release of renin. Renin converts angiotensinogen (a plasma protein) to
angiotensin I. ACE converts angiotensin I to angiotensin II. Angiotensin II causes arterial
vasoconstriction and stimulates the release of aldosterone. Aldosterone then stimulates renal Na
reabsorption and potassium excretion. Where sodium goes water follows via osmosis, and water
is retained. The person produces less urine and the blood volume increases.

Question 18

What do natriuretic peptides do for fluid balance?

Answer 18

Natriuretic Hormones are hormones that are released from the atria or ventricle of the heart.
They work opposite of the RAAS to decrease blood volume. They promote urinary excretion of
sodium and water thus decreasing blood volume. Examples of these hormones are atrial
natriuretic peptides (ANP) and B-type natriuretic peptides (BNP).

Question 19

How does weight relate to fluid balance?

Answer 19

One liter of water weighs 2.2 lbs (1kg), so monitoring weight is a very good way to monitor water balance. This is done frequently for heart failure, dialysis patients, and liver failure patients. The patient’s weight is monitored closely, and an increased weight is often one of the first indicators that an individual is retaining fluid.

Question 20

What are the signs and symptoms of fluid volume deficit?

Answer 20

S/S include poor skin turgor, dry mucous
membranes, sunken eyes, sunken fontanels (in babies), less urinary output, and fatigue.

Question 21

What are the signs and symptoms of fluid volume excess?

Answer 21

S/S include edema, tightness of the skin, puffiness of the eyes, and rales upon auscultation.

Question 22

What is the pathophysiology of edema?

Answer 22

Edema is an accumulation of fluid within the interstitial space. This is usually influenced by increased hydrostatic pressure, lowered plasma oncotic pressure, increased capillary membrane permeability, and lymphatic channel obstruction.

Question 23

What are etiologies found from venous obstruction?

Answer 23

Thrombophlebitis, hepatic obstruction, tight clothing, and prolonged standing are common causes of venous obstruction.

Question 24

What are etiologies found in decreased oncotic pressure?

Answer 24

Liver disease and protein malnutrition will result in a decreased oncotic pressure resulting in fluid moving into the interstitial space (decreased production of plasma proteins). Glomerular disease, trauma victims, hemorrhage, and burns will also decrease oncotic pressure due to protein wasting that occurs.

Question 25

How is sodium homeostasis regulated?

Answer 25

Sodium homeostasis is tightly regulated by the secretion of antidiuretic hormone and the kidneys’ ability to respond to ADH. When plasma osmolality rises above 290- 295 mOsm/kg (mmol/kg), the thirst center in the hypothalamus is stimulated and individuals drink water. Sodium balance is also maintained by RAAS and the kidneys. Sodium balance is maintained by the renal tubular system which is responsible for the absorption and excretion of sodium.

Question 26

Where is potassium located?

Answer 26

Potassium is mainly an intracellular ion. Approximately 98% of the body’s potassium is
located intracellularly. Most of the ingested potassium is absorbed in the GI tract. Internal potassium balance refers to the distribution of potassium on either side of the cell membrane

Question 27

What causes potassium out of the cell?

Answer 27

Insulin deficiency, aldosterone deficiency, acidosis, and strenuous exercise shift potassium to the extracellular space.

Question 28

How are osmolality and potassium related?

Answer 28

Hyperosmolar states will cause potassium to
shift out of the cells. This mechanism is dependent on osmosis. When the intravascular space
has a high osmolality, water will flow from the intracellular space into the intravascular space.
This leaves the inside of the cell with an increase in the concentration of potassium. This then
triggers the potassium to diffuse across the cell membrane out of the cell into the extracellular
space and causes hyperkalemia.