Fluid Distribution and Membrane Transport in Humans

Intracellular Fluid (ICF)

25 L of fluid within cells in humans.

Extracellular Fluid (ECF)

15 L of fluid outside cells in humans.

Interstitial Fluid (IF)

12 L of fluid surrounding cells.

Phospholipid Bilayer

Membrane structure with hydrophilic heads and hydrophobic tails.

Passive Diffusion

Movement from high to low concentration without energy.

Hydrophilic Solutes

Cannot pass lipid bilayers directly, use channels.

Hydrophobic Solutes

Easily diffuse through lipid bilayers.

Membrane Flux Equation

The amount of fluid (like water) that passes through a membrane's surface area per unit time. 

Driving Force

Concentration gradient driving solute movement.

Membrane Permeability

Ease of substance crossing the membrane.

Active Transport

Moves substances against concentration gradient using energy.

Primary Active Transport

Carrier protein also acts as ATPase.

Secondary Active Transport

Uses cotransport driven by another gradient without ATPase activity.

Osmolarity

Concentration of solutes in a solution. It’s the total concentration of solute particles per liter — includes all solutes (penetrating and non-penetrating).

Tonicity

The effect of non-penetrating solutes on cell volume.

Osmoregulator

Maintains stable osmotic concentration despite stress.

Osmoconformer

Body fluid concentration matches environmental osmotic concentration.

Euryhaline

Survives wide range of osmotic conditions.

Stenohaline

Tolerates limited osmotic conditions.

Water Budget

Balance of water and salts in organisms.

Transcellular Transport

Movement through epithelial cells.

Paracellular Transport

Movement between adjacent epithelial cells.

Transport Epithelial Cells

Specialized directional transport cells for solute and water transport

Seen in kidneys, gills, intestines.

Channel Proteins

Create water-filled pores for solute passage.

Carrier Proteins

Transport molecules without forming open channels.

Symport Carriers

Transport multiple substrates in the same direction.

Antiport Carriers

Transport substrates in opposite directions.

Kangaroo Rat Water Balance

Relies on metabolic water, minimal external water.

Human Water Balance

Majority from drinking, some from food.

Glucose Transport

Glucose enters via SGLT (symport with Na⁺) and exits through GLUT (facilitated diffusion).