osmotic pressure

what are the three main compartments of body fluid distribution?

-plasma (~10%)

-interstitial fluid (~35%)

-intracellular fluid (~55%)

through what structures can water and small molecules pass freely between plasma and interstitial fluid?

-capillary membranes

-through or between endothelial cells

What allows water and small uncharged molecules to move between interstitial and intracellular fluid?

cell membrane

what is the cell membrane?

acts as a semi-permeable barrier that allows water and some mall uncharged molecules to pass freely

How does the body lose water?

-urine

-feces

-breathing

-sweat

-milk

what is osmotic pressure?

the amount of pressure needed to be applied to a solution to prevent water influx

What determines osmotic pressure in biological fluids?

sodium, potassium, chloride, glucose (or lactose), and urea

What unit measures osmotic pressure?

milliOsmolar (mOsm)

How is osmotic pressure measured?

-By freezing point depression

-higher osmolality lowers the freezing point

in a system where the inside has higher osmotic pressure, what happens to water movement?

Water moves into the compartment with higher osmotic pressure, increasing its volume

in a system where the inside has lower osmotic pressure, what happens to water movement?

Water moves out of the compartment, decreasing its volume

What type of membrane allows complete equilibration of water?

phospholipid bilayer

Why do polar molecules that contribute to osmotic pressure require transport proteins?

they cannot freely pass through the hydrophobic phospholipid bilayer

main classes of molecules contributing to osmotic pressure?

-ions → Na+, K+, Cl+

-sugars → glucose, lactose

-nitrogenous compounds → urea, amino acids

In the mammary gland, where can water freely diffuse?

Between interstitial fluid, blood, and the cytosol of mammary epithelial cells, fibroblasts, and endothelial cells

What is the approximate osmotic pressure of most body fluids?

~300 mOsm

What extracellular fluid compartment can vary dramatically in osmotic pressure?

Urine, helps regulate overall osmotic balance

Compare ion composition between blood plasma and cytosol.

-Plasma: high in Na⁺ and Cl⁻

-Cytosol: high in K⁺ and amino acids

-Both total ~300 mOsm

Why must milk osmotic pressure equal that of other body fluids?

prevent water movement across membranes that would disrupt milk composition and volume

What is the primary regulator of milk osmotic pressure in species with high lactose content?

Lactose (4–8% of milk content)

In animals with low milk lactose content, what regulates osmotic pressure instead?

Ions such as K⁺, Na⁺, and Cl⁻

What happens if lactose (342 g/mol) in milk is replaced by glucose (180 g/mol)?

To maintain equal osmotic pressure, milk would need only ~2.5% glucose instead of 5% lactose, halving sugar energy content

Why is lactose important for milk osmotic balance?

allows for high caloric sugar content without increasing osmotic pressure

How does water enter milk?

follows osmotic gradients created primarily by lactose and ions

Why do secretory vesicles in mammary cells look “empty”?

water follows lactose into the vesicles after secretion, diluting their contents

Summarize osmotic balance across body fluids

Water moves freely between compartments to ensure equal osmolality (~300 mOsm) across plasma, interstitial fluid, cytosol, and milk