Lecture 2 – Body Fluid Compartments & Water Transport

  • describe the relationship between cells, tissues, organs, and organ systems

  • describe the location and composition of the various fluid compartments in the body

  • define homeostasis and explain how it relates to ECF

  • gain an understanding of how changes in the composition of ISF can influence cell volume

Define homeostasis

The dynamic process of maintaining a stable internal environment depsite external changes

Relationship to interstital fluid (ISF)

ISF is the internal environment. It bathes cells directly, so its composition must be tightly controlled for cell survival

Locations & compositions of fluid compartments

Body fluid is divided into ICF and ECF. ECF is further split into plasma, interstitial fluid, and transcellular fluid. Each has a unique ionic composition

Changes in fluid composition & cell volume

Changes in ECF osmolality cause water to move into or out of cells, leading to swelling or shrinking

  

Fluid Compartments: Location & Composition 

  • Total Body Water: ~60% of body weight in men / ~50% body weight in women

  • Major Divisions:

    • Intracellular Fluid: Fluid inside cells (2/3 of TBW)

    • Extracellular Fluid: Fluid outside cells (1/3 of TBW)

 

  • Interstitial fluid: fluid that directly bathes the cells (3/4 of ECF)

  • Plasma: fluid portion of blood (1/4 of ECF)

  • Transcellular fluid: specialised fluids (this is only a small portion of the ECF) 

Ionic Composition:

  • ECF (High Na+, Low K+): [Na+] = 142 mM      [K+] = 4.4 mM       : this is good for signalling and acting as a reservoir

  • ICF (Low Na+, High K+): [Na+] = 15 mM      [K+] = 120m mM       : this is good for setting membrane potential and enzymatic function

 

Maintaining Gradients: Na+/K+ ATPase 

  • Pump: a protein in the cell membrane that uses energy to move ions against their gradients

  • Action: pumps 3 Na+ out of the cell and 2 K+ into the cell per cycle

  • Why it's important: it creates and maintains the ion concentration gradients. Without it, the gradients would dissipate, and cells would swell and die. It's responsible for:

  • Setting the membrane potential: the unequal charge distribution creates a voltage across the membrane

  • Generating electrical activity: in nerves and muscles, this potential is ised to create signals    

  • Driving nutrient uptake: the Na+ gradient is used to co-transport nutrients into the cell

  • Cell volume regulation: it prevents the cell from swelling by pumping out more ions than it brings in

 

 Cell Volume Regulation: Osmolality & Tonicity

What determines cell volume?

  • Number of osmotically active particles inside the cell

  • Osmolality of the ECF

 

Important Terminology

Osmole

A unit of measurement for the number of solute particles
1 osmole = 1 mole of dissolved particles

Osmolality

Number of osmoles per kilogram of water

Human body fluids are ~290 mOsm/kg

Osmolarity

Number of osmoles per liter of solution

mOsm/L

Tonicity

The effect a solution has on cell volume.
It depends on the concentration of non-penetrating solutes

 

Tonicity & Cell Volume

  • Isotonic solution: same concentration of non-penetrating solutes as the no net water movement; cell volume stays the same

  • Hypotonic solution: lower concentration of non-penetrating solutes than the cell water moves into the cell; cell swells (may lyse)

  • Hypertonic solution: higher concentration of non-penetrating solutes than the cell water moves out of the cell; cell shrinks (crenate)

** PERMEANT SOLUTES

  • Solutes like urea can cross the cell membrane freely

  • They'll initally cause water movement, but because they can enter the cell and equilibrate,
    they have no long-term effect on cell volume