Lecture 5 – Fluid Compartments and Balance

1) How does age affect fluid percentage in the body? How does body composition affect fluid levels?

• Age decreases fluid percentage; infants 75%, elderly 45%. More muscle means higher fluid; more adipose means lower fluid.

2) Why do we care about how much fluid someone has in their body?

• People with lower body fluid are more likely to have fluid and electrolyte imbalances.

3) In what fluid compartment is most of the water in our body found?  Which electrolytes are in a higher concentration within this compartment?

• Most water is in the intracellular fluid (ICF) with more K⁺, Mg²⁺, phosphate, and (-) charged proteins.

4) What are the two compartments within the extracellular fluid? Which of these two compartments has more fluid? What is the only difference between these two compartments when it comes to solutes?

• Two compartments are interstitial fluid and plasma; interstitial fluid has more fluid, plasma has more proteins.

5) What three organ systems regulate fluid balance in the body? What three factors are measured to do this?

• Endocrine, urinary, and cardiovascular systems regulate fluid balance by measuring blood volume, pressure, and osmolarity.

6) Fluid balance is not only the balance between fluid intake and output but also …..

• Fluid balance is also the normal distribution of water and solutes between compartments.

7) When we drink water, how does this affect the osmolarity of the blood? What fluid movement between compartments occurs because of this?

• Drinking water decreases blood osmolarity, causing water to move into interstitial fluid and then into cells.

8) What happens to blood osmolarity when someone becomes dehydrated or consumes large amount of sugar? What fluid movement between compartments occurs because of this?

• Dehydration or excess sugar/salt increases blood osmolarity, causing water to move out of cells into blood.

9) What are the two forms of fluid intake, and which one does most of our fluid intake come from thus is regulated?

Metabolic water (~200 mL/day) and preformed water (~2300 mL/day); preformed water provides most intake and is regulated.

10) What three things can stimulate the thirst center, and which two hormones are involved?

Decreased salivary secretions, increased blood osmolarity, and decreased blood pressure stimulate thirst; hormones are ADH and angiotensin II.

11) Distension of the stomach does what to the thirst center?

• Distension of the stomach inhibits the thirst center.

12) Which form of fluid loss occurs regardless of our hydration status? What types of water loss occur in this way?

• Obligatory fluid loss occurs regardless of hydration from breathing, sweating, transpiration, feces, and 500 mL urine.

13) Controlled water loss (called ______________________ fluid output) is through the regulation of ……

Facultative fluid output is controlled by regulating the amount of urine produced.

14) Which four hormones regulate facultative fluid output? Which ones decrease fluid output? Which one increases fluid output?

Hormones: angiotensin II, ADH, aldosterone, ANP. First three decrease urine output; ANP increases it.

15) What two things can stimulate the release of Renin?

• Low blood pressure and low NaCl in macula densa stimulate renin release.

16) A refresher question from cardiovascular unit: Explain the conversion of angiotensinogen into angiotensin II. (include organs and enzymes needed as well)

• Liver makes angiotensinogen, renin from kidneys converts it to angiotensin I, ACE from lungs converts to angiotensin II.

17) What are the effects of angiotensin II function? (don’t just say an increase in blood volume/pressure)

• Angiotensin II causes vasoconstriction, decreases urine output, stimulates thirst, ADH, and aldosterone release.

18) What three things can stimulate the release of ADH?

• Low blood pressure, low blood volume, and high osmolarity stimulate ADH release.

19) What are the effects of ADH function?

• ADH causes thirst, water reabsorption by adding aquaporins, and at high levels vasoconstriction.

20) What three things can stimulate the release of aldosterone?

• Angiotensin II, low Na⁺, and high K⁺ stimulate aldosterone.

21) What are the effects of aldosterone function?

• Aldosterone increases Na⁺ reabsorption, water reabsorption, and K⁺ secretion in kidneys.

22) What stimulates the release of ANP?

• Increased atrial stretch from high blood volume and pressure releases ANP.

23) What are the effects of ANP function?

• ANP causes vasodilation, increases urine output, inhibits renin, ADH, and aldosterone, and raises GFR.

24) Of these four hormones, which one would be released if someone ate a sugary or salty snack?

• ADH is released after eating a salty or sugary snack.

25) Fluid imbalances occur because of what two things? 

• Fluid imbalances occur when intake ≠ output or when fluids are abnormally distributed.

26) What are the two factors used to determine the type of fluid imbalance a person might have?

• Determined by whether it’s a fluid excess or deficit and whether it changes osmolarity.

27) Which two fluid imbalances do NOT cause fluid movement between compartments?

• Volume depletion and volume excess do not cause fluid movement between compartments.

28) What are some of the causes of volume depletion? What could cause volume excess?

• Volume depletion from hemorrhage, burns, vomiting, diarrhea; volume excess from kidney failure, excess Na⁺, aldosterone, or heart failure.

29) Which two fluid imbalances DO cause fluid movement between compartments?

• Dehydration and hypotonic hydration cause fluid movement between compartments.

30) When someone is dehydrated, which compartment does the fluid move to? What could cause dehydration?

• In dehydration, fluid moves from cells into blood; caused by sweating, diabetes, alcohol, low ADH, low water intake, cold.

31) When someone has water intoxication, which compartment does the fluid move to? What could cause hypotonic hydration?

In water intoxication, fluid moves from blood into cells; caused by ADH hypersecretion or drinking plain water after sweating.

32) Edema is what type of fluid imbalance? Which compartment contains more fluid than it should when someone has edema?

• Edema is a type of fluid sequestration imbalance. In edema, there is excess fluid in the interstitial fluid compartment around the cells.

33) What could cause edema?

• Edema can be caused by changes in cardiovascular system function, blood composition, or lymphatic vessel function, which alter net filtration pressure (NFP) in systemic capillaries, causing more fluid to leave capillaries and remain in the interstitial space.

34) What are other examples of fluid sequestration?

Other examples of fluid sequestration include internal hemorrhaging, pericardial effusion, and pleural effusion.