Fluid & Electrolytes

How much body water in infants?

73%

Adult Males?

60%

Adault females:

50%

What does this decrease to in old age?

45%

Total body water in an average person? (L) How is it divided?

40L

• Intracellular fluid 40% 25L

• Extracellular fluid 20% 15L

1. Interstitial fluid 80% of ECF 12L

2. Plasma 20% of ECF 3L

Water is a _____

Universal solvent, lots of things can dissolve in water and become solutes

What is classified as a nonelectrolyte? What do they form?

• Does not dissociate in water: e.g., glucose, lipids, creatine, and urea

• No charged particles created

What is an electrolyte? What do they form? What do they do better than nonelectrolytes?

• Dissociates in water. E.g., NaCl dissociates into Na+ and Cl-.

• Charged particles created

• Inorganic salts, acids, bases, some proteins

• Greater osmotic power than nonelectrolytes, causes fluid to shift more?

ICF, levels of Na+, Cl-, K+, and proteins (and HPO4 2-)

• Low Na+ and Cl-

• High amounts of K+

• High amounts of HPO4 2-

• More soluble proteins than in plasma

ECF, IF, levels of Na+, Cl-, K+, and proteins

• High levels of Na+

• High levels of Cl-

• Low levels of protein

• Negligible levels of K+

ECF, Plasma, levels of Na+, Cl-, K+, and proteins

• Low levels of cl-

• high levels of protein

• negligible levels of Na+ and Cl-

What forces actually move fluid between layers of the body?

• Osmotic (oncotic) pressure

• Hydrostatic pressure

• The fluid moves along osmotic gradients, from areas of low solute concentration to areas of high solute concentration

If ECF osmolarity is high?

Water leaves the cell

If ECF osmolarity is low?

Water enters the cell

How does this movement pattern differ between the IF and ICF across the cell membrane?

• Two-way osmotic flow of water

• Ions move selectively; nutrients, wastes, gases, unidirectionally.

How much water should you drink a day, and why?

• 2.5L

• This is the amount of water you lose on average per day

How much water is lost through fecees? %

4%

How much water is lost through sweat?

8%

How much water is lost via insensible loss via skin and lungs?

28%

How much water is lost through the urine?

60%

What is normal osmolality in the body?

280-330 mOsm

What does a rise in osmolality cause?

• Stimulate thurst

• Release ADH

What does a decrease in osmolality cause?

• Inhibit thrist

• ADH inhibition

What governs the thirst mechanism?

The hypothalamus

What stimulates the hypothalamus thirst center?

• Osmoreceptors detect ECF osmolality; activated by:

• Plasma osmolality of 1-2%

• Dry mouth

• Decreased blood volume or pressure

• Angiotensin 2 or baroreceptor input

Detailed pathway Plasma Volume —> Thirst

1. Blood plasma volume decreases.

2. Blood pressure drops

3. Granular cells in the kidney are no longer being stretched

4. Granular cells in the kidney release renin

5. Renin activates angiotensin 2

6. Angiotensin 2 activates our hypothalamic thirst center

7. We drink enough water to increase our plasma volume back to normal

Yay this pathway works :)

ECG osmolarity —> Thirst

1. ECF osmolarity increases

2. Osmoreceptors in the hypothalamus realizes we dont have enough fluid

3. Our mouth becomes dry as less saliva is produced to try and retain fluid

4. We drink water

5. However, once our mouth and throat are moistened, this reflex can stop early

This pathway sometimes doesn’t work fully :(

What does ADH do?

• Antiduretic hormone leads to the reabsorption of water at the kidneys

• This is good because our blood plasma volume will return to normal

• However we will have scant (Concentrated) urine

What stimulates ADH release?

• Large changes in blood volume or pressure

• Renin stimulated ADH as well

What is dehydration? What could cause it?

• When we are losing more water than we are taking in

• Could be caused by hemorrhage, severe burns, prolonged vomiting, profuse sweating, water deprivation, etc..

Symptoms of dehydration?

• “Cottony” oral mucosa

• Thirst

• Dry flushed skin

• oliguria

What is the internal consequence?

ECF osmotic pressure rises, so cells shrink

Hypotonic hydration

• This is water intoxication

• ECF osmolality goes waaaaaaaay down

Internal consequence?

• ECF becomes less concentrated

• Osmotic pressure falls

• Cells swell

What are Edemas and what can cause them?

• Accumulating fluid between the cells in the IF

• Too much IF increases distance between BV and tissue, compresses BV and tissue which also hurts

• Caused by capilaries leaking too much fluid into the interstitial space