Exam 4: Fluid Balance

What factors affect body water content?

Age, sex, lean/fat mass

What percentage of water is in adipose tissue and skeletal muscle?

Adipose tissue – ~20%, Skeletal muscle – ~75%.

What are fluid compartments in the body?

Areas separated by selectively permeable membranes with different chemical compositions.

What percentage of body water is intracellular fluid (ICF)?

65%.

What percentage of body water is interstitial fluid (IF)?

25%.

What percentage of body water is plasma/lymph?

8%.

What are electrolytes?

Substances that dissociate into ions in water (e.g., Na+, Cl-, K+, HPO42-).

What are nonelectrolytes?

Substances that do not dissociate into ions, like urea and glucose.

What contributes more to osmosis: electrolytes or nonelectrolytes?

Electrolytes, as they have a greater osmotic draw.

How does water move in relation to solutes?

Water moves more freely than solutes, and changes in solute concentration will induce movement of water.

What are the two types of water gains?

Preformed water (ingested) and metabolic water (cellular metabolism).

What are the two types of water losses?

Sensible water loss (urine, feces, sweat) and insensible water loss (expired breath, cutaneous transpiration).

What activates the hypothalamic thirst center?

Osmoreceptors detecting ECF osmolarity, decreased blood volume/pressure (via baroreceptors), dry mouth, and increased blood osmolarity reducing salivation.

How is water absorbed and regulated in the body?

Water is absorbed from the small intestine and helps restore blood osmolarity/volume with a 30+ minute delay. This is a short-term response.

What triggers the inhibition of the hypothalamic thirst center?

Cooling and moistening of the mouth, and distension of the stomach and small intestine.

What is the role of the hypothalamic thirst center in overhydration?

It prevents overhydration by rapidly inhibiting thirst when needed.

What is obligatory water loss?

Insensible loss plus at least ~500 mL/day of sensible loss.

What factors affect additional water loss?

Intake, climate, and activity level.

How do kidneys regulate water loss?

Kidneys modulate the rate of water loss by adjusting Na+ and H2O reabsorption, concentrating urine.

When is excess water excreted?

Excess water is excreted ~30+ minutes after intake.

How does low ADH affect urine and blood volume?

Low ADH results in dilute urine and reduced blood volume/pressure.

How does high ADH affect urine and blood volume?

High ADH results in a small volume of concentrated urine.

How do blood volume changes influence ADH secretion?

Large changes in blood volume impact ADH secretion via baroreceptors or the RAA mechanism. ADH also acts as a vasoconstrictor.

What happens during dehydration in terms of renin and thirst?

Dehydration increases blood osmolarity and reduces blood pressure, stimulating osmoreceptors, reducing salivation, and creating a sense of thirst.

What is the process of rehydration?

Ingested water rehydrates the blood, distends the stomach and intestines, and inhibits thirst in the short term.

What is fluid deficiency?

When intake is less than output, leading to dehydration and negative water balance, causing increased blood osmolarity and electrolyte imbalances.

Why are infants more vulnerable to fluid deficiency?

Infants are more vulnerable due to their smaller body size and faster metabolism.

What is volume depletion (hypovolemia)?

A loss of proportionate amounts of water and electrolytes, leading to circulatory shock due to reduced blood volume.

What is fluid excess?

Fluid excess includes conditions like hypotonichydration (water intoxication) and volume excess, both causing electrolyte imbalances.

What is hypotonic hydration?

Overhydration from extreme fluid intake, renal insufficiency, or ADH abnormalities, causing dilution of ECF and electrolyte imbalances.

How is hypotonic hydration treated?

By administering hypertonic saline IV to reverse the osmotic gradient.

What happens in volume excess?

Both Na+ and water are retained, leading to swelling and imbalances.

What is fluid sequestration?

Buildup of fluid in a particular location.

What is edema?

Abnormal accumulation of fluid in interstitial spaces.

What is pleural effusion?

Accumulation of fluid in the pleural cavity.

What are other examples of fluid sequestration?

Internal hemorrhage and anaphylactic shock.

How do kidneys respond to fluid imbalances?

Kidneys compensate very well for fluid excesses, but not for deficiencies

What happens when fluid intake is too low?

Hypovolemia can occur, leading to dangerous drops in blood volume.

What happens when fluid intake is too high?

Water diuresis occurs, where excess water is excreted.