Lecture 2: Osmosis, Tonicity, and Pressures

Osmosis is the movement of _____ across a semipermeable membrane _______ lower solute concentration ________ higher solute concentration.

water

from

toward

Osmosis is not simply diffusion of water. It occurs because of _________.

a pressure difference

What creates the osmotic pressure differences that drive osmosis?

solute concentration differences

A cell with a higher solute concentration will have a [greater/lesser] osmotic pressure, which will [pull/push] water.

greater

pull

osmotic vs hydrostatic pressure

osmotic pressure: created by a difference in solute concentration across a semipermeable membrane (pulling force created by non-permeable solutes)

hydrostatic pressure: pressure exerted by a stationary fluid at equilibrium (causes water/fluid to move or be pushed)

In capillaries, osmotic pressure is generated by plasma proteins, and is called _________.

oncotic pressure

Under normal physiological conditions, the balance of hydrostatic and oncotic pressures results in net filtration from ________ into tissues. Excess fluid is returned to circulation via the ________ system.

capillaries

lymphatic

osmolar concentrations

• express osmotic strength of solutions-

  • urine, plasma, NaCl

• clinical importance: urine specific gravity, dehydration (increased blood osmolarity)

osmolarity vs osmolality

• osmolarity

  • concentration of osmotically active particles per liter of solution (osm/L or mOsm/L)

• osmolality

  • concentration of osmotically active particles expressed as osmoles/kg water

used interchangeably very often

osmolarity vs tonicity

osmolarity

• measures total concentration of dissolved particles in a solution

• includes all solutes, whether they can cross the membrane or not

• determines potential for osmotic pressure

• “How many solute particles are present?”

tonicity

• describes how a solution affects cell volume

• depends only on solutes that cannot easily cross the cell membrane

• predicts whether a cell will shrink, remain the same, or swell

• “What will happen to the cell?”

isotonic

cell stays the same size

hypertonic

water leaves cells, cell shrinks

hypotonic

water enters cells, cell swells

Plasma oncotic pressure limits filtration by opposing __________ and helps maintain intravascular fluid volume.

capillary hydrostatic pressure (Pcap)