L24 - Cell Membrane Physiology Osmolarity, Tonicity

What is Osmosis?

Passive movement of water toward the higher solute concentration across a semipermeable membrane

What is Osmotic Pressure?

The pressure required to stop osmotic flow

• Depends on the number of solute particles

What determines the direction of Osmosis?

Differences in particle concentration across a semipermeable membrane

Does Osmosis require ATP?

No

• Osmosis is passive transport

What does Osmolarity measure?

• Total number of dissolved particles per liter of solution
  → Units = mOsm/L

Why is Osmolarity different from Molarity?

• Molarity: counts formula concentration/L

• Osmolarity: counts the total particles after dissociation

What is the Osmolarity of 9 mM NaCl?

18 mOsm/L

• NaCl → Na⁺ + Cl⁻ = 2 particles x 9 mM

Does Osmolarity depend on the type of solute?

No

• It depends mainly on the total number of dissolved particles

Why can temperature & pressure affect Osmolarity?

They can change solution volume, and osmolarity is measured per liter

What does Osmolality measure?

• Number of osmoles per kilogram of solution.
  → Units = mOsm/kg

What is the key difference between Osmolarity and Osmolality?

• Osmolarity = particles/L

• Osmolality = particles/kg

Which is more accurate: Osmolarity or Osmolality? Why?

Osmolality, because mass does not change with temperature or pressure

Which is more practical clinically?: Osmolarity or Osmolality? Why?

Osmolarity, because measuring liquid volume is more convenient

How can plasma osmolality be measured?

Osmometer

What are the Body's 2 Major Fluid Compartments?

1. Intracellular fluid (ICF)

2. Extracellular fluid (ECF)

What makes up the ICF?

Fluid inside cells

What makes up the ECF?

Plasma + interstitial fluid

Which ions are HIGH in the ECF?

• Na⁺ HIGH + Cl⁻ HIGH

• K⁺ low

Which ion is HIGH in the ICF?

• K⁺ HIGH

• Na⁺ and Cl⁻ low

What maintains the Na⁺/K⁺ gradient between ICF and ECF?

Na⁺/K⁺-ATPase (sodium-potassium pump)

What electrical charge is maintained inside the cell?

Negative intracellular charge

What happens if ECF Osmolality INCREASES?

Water moves ICF → ECF

• Cell dehydrates/shrinks

What happens if ECF Osmolality DECREASES?

Water moves ECF → ICF

• Cell swells

What normal ECF osmolality is emphasized in the lecture?

About 290 mOsm/kg H₂O

• At homeostasis, ICF is also ~290

What is Hydrostatic Pressure?

Pressure that PUSHES fluid OUT of capillaries → interstitial space

What is Oncotic Pressure?

Osmotic pressure from plasma proteins that PULLS fluid INTO blood vessels

Which plasma protein is most important for Oncotic Pressure?

Albumin

What happens at the Arterial End of a capillary?

HYDROSTATIC > oncotic

• Fluid pushed OUT of capillary

What happens toward the Venous End?

• Hydrostatic pressure falls

• Oncotic pressure draws fluid back INTO the capillary

How does Hydrostatic Pressure change across a capillary?

Decreases from arterial → venous end

How does Oncotic Pressure behave across the capillary?

Remains relatively constant and is largely determined by plasma albumin

Hydrostatic vs. Oncotic?

• Hydrostatic = PUSH OUT

• Oncotic = PULL

What is the key difference between Osmolality and Tonicity?

• Osmolality: counts all solutes

• Tonicity: counts only effective, non-permeable solutes

What is an Effective Osmole?

A solute that cannot freely cross the membrane and therefore causes sustained water movement

What is an example(s) of an Ineffective Osmole?

Urea, because it can freely cross the ICF/ECF barrier.

What is an example(s) of an Effective Osmole?

• Na⁺

• Glucose,

• K⁺,

• Cl⁻

What does Tonicity determine about cells?

Whether cells:

• Gain water

• Lose water,

• Maintain their volume.

What does an Isotonic solution do to cells?

No net water shift → cells maintain normal size.

What does a Hypertonic solution do to cells?

Water moves out of cells → cells shrink

What does a Hypotonic solution do to cells?

Water moves into cells → cells swell

Why is Tonicity important when giving fluids?

It determines water shifts and whether cells shrink, swell, or remain stable

Which tonicity was selected for blood loss in the lecture example?

Isotonic normal saline, to avoid causing cells to shrink or swell

How can kidney damage cause proteinuria/albuminuria?

Glomerular/tubular damage allows albumin to be lost into urine

How does albumin loss affect plasma Oncotic Pressure?

↓ Albumin → ↓ oncotic pressure → ↓ ability to pull water back into capillaries

Explain the pathway to nephrotic edema in CKD.

1. Kidney damage

2. Albumin loss in urine

3. ↓ Plasma albumin

4. ↓ Oncotic pressure

5. ↓ Fluid return to capillaries

6. Water remains in tissues

7. Edema

Hydrostatic vs. Oncotic Pressure

• Hydrostatic pressure → PUSHES water out

• Oncotic pressure (albumin) → PULLS water back in

Why does low albumin promote edema?

Blood loses part of its ability to pull water back into capillaries

What diagnostic finding can indicate albumin loss?

Proteinuria/albuminuria

What is Exocrine Pancreatic Insufficiency (EPI)?

Decreased production of digestive enzymes by the pancreas

What clinical signs of EPI are emphasized?

• Polyphagia

• Weight loss

• Large-volume loose stools

Why do undigested nutrients remain in the intestine with EPI?

There aren't enough pancreatic digestive enzymes to properly digest fats, proteins, and carbohydrates

Why does EPI cause Osmotic Diarrhea?

1. Undigested nutrients = ↑ solutes in intestinal lumen

2. Water is drawn into lumen

3. Diarrhea

How can Osmotic Diarrhea contribute to Dehydration?

Water is pulled into and lost through the intestinal lumen

What is the complete EPI → osmotic diarrhea pathway?

EPI is about having lots of solute particles in the intestinal lumen

1. ↓ Pancreatic digestive enzymes

2. Food isn't properly digested

3. Undigested nutrients stay inside the intestinal lumen

4. ↑ Solute concentration in the lumen

5. Water is drawn into the lumen

6. More water stays with the fecal material

7. = Osmotic Diarrhea

What common principle connects CKD edema and EPI diarrhea?

Changes in solutes/proteins alter where water moves between body compartments

Osmolarity vs. Osmolality SUMMARY

	Osmolarity	Osmolality
Measures	# particles per liter	# particles per kg
Units	mOsm/L	mOsm/kg
Based on	Volume	Mass
Affected by temp/pressure?	Yes	No
More accurate	No	Yes
More practical	Yes	No
Memory trick	OsmolaRity → LiteR	Osmolality → kg

ICF vs. ECF SUMMARY

K⁺ = Keeps inside
Na⁺ + Cl⁻ = outside in ECF

The Na⁺/K⁺ gradient is maintained by Na⁺/K⁺ active transport 

	ICF	ECF
Location	Inside cells	Outside cells
Includes	Intracellular fluid	Plasma + interstitial fluid
Na⁺	↓ LOW	↑ HIGH
K⁺	↑ HIGH	↓ LOW
Cl⁻	↓ LOW	↑ HIGH

ECF Osmolality & Water Movement SUMMARY

Water moves toward the side with MORE solute particles

ECF	Water moves	Cell response
↑ ECF osmolality	ICF → ECF	Cell dehydrates
Normal	Balanced	Normal cell
↓ ECF osmolality	ECF → ICF	Cell swells

Hydrostatic vs. Oncotic Pressure SUMMARY

	Hydrostatic	Oncotic
Main action	PUSHES water	PULLS water
Direction	OUT of capillary	INTO capillary
Main contributor	Blood pressure	Albumin
Arterial → venous	↓ Decreases	Relatively constant
Key Concept	PUSH OUT	PULL IN

Arterial end 

• Hydrostatic > Oncotic 

• Blood → Interstitial space 

Venous end 

• Oncotic > Hydrostatic 

• Interstitial space → Blood

Osmolality vs. Tonicity SUMMARY

Osmolality = How many particles are there?

Tonicity = What will those particles do to the cell? 

	Osmolality	Tonicity
Counts	ALL solute particles	Only effective osmoles
Permeable solutes count?	Yes	No
Non-permeable solutes count?	Yes	Yes
Example permeable solute	Urea	Urea doesn't count
Key Concept	Total particles	Effect on cell volume

Tonicity & Cell Size SUMMARY

Solution	Example	Osmolality	Tonicity
Hypotonic	~250 mOsm/L	ECF → ICF	Swells
Isotonic	~290 mOsm/L	No net shift	Normal
Hypertonic	~330 mOsm/L	ICF → ECF	Shrinks

Clinical Applications SUMMARY

CKD: Hydrostatic vs. Oncotic Pressure

Kidney damage → albuminuria → ↓ blood albumin → ↓ oncotic pressure → ↓ water return to capillaries → water remains in tissues → EDEMA

EPI: Osmosis / Osmolarity

↓ digestive enzymes → undigested fats/proteins/carbs remain in intestinal lumen → ↑ luminal solutes → water drawn into lumen → water accumulates in lumen → OSMOTIC DIARRHEA

	CKD → Nephrotic Edema	EPI → Osmotic Diarrhea
Initial problem	Kidney/glomerular damage	↓ Pancreatic enzymes
What accumulates/is lost?	Albumin lost in urine	Undigested nutrients in intestinal lumen
Consequence	↓ Plasma albumin	↑ Solutes in lumen
Water movement problem	↓ Water pulled into capillaries	Water drawn into intestinal lumen
Result	Edema	Diarrhea + dehydration