A View from the Back of the Envelope top
Your enormous surface area...
Area
Area per volume

The idea: You've an enormous surface (~400 m2, with skin merely ~2 m2). With it, you defuse, food and oxygen, with your environment. Just like all other cellular life. But your surface is easily forgotten, tucked away inside you as lungs and intestine. Here is a reminder...

Lungs as sheet

skin as box

Intestines as sheet

Person is ~1.5 m tall.
Lungs and intestine sheets are paper-thin.

Person is ~1.5 m tall and ~0.08 m3.
Skin is ~2 m2, so person as a box is ~1 m2 × 0.1 m thick.
Lung is ~100 m2 and ~0.01 m3, so lung as a sheet is (~7 m)2 × ~10-4 m thick.
Intestine is ~300 m2 and ~0.01 m3, so intestine as a sheet is (~12 m)2 x ~10-4 m thick.

Links

The Digestive System
The Respiratory System

A View from the Back of the Envelope
Comments encouraged. - Mitchell N Charity <mcharity@lcs.mit.edu>
Notes:
   Sources:
     Skin area: 1.8 , about 2 , nearly 2 , 1-2 , [italicized links are broken]
     Lung area: 70 to 100 , 90 , 140 , 100 , 80 , 80 , 70-80 , 70-80 , 160 , 70
     Digestive area: 300 , 300 , 250 , 10 
Doables:
  Explain volume&area preserving calc.
  Thicken stick-figure - too scrawny.
  Area-vol-density typical of cell life.
  Alveoli level transport mainly diffusive?
    Diffusion vs exchange, vol, rates, by hierarchy.
  Consider a spherical human.
  Clarify re active transport?
  Ocean story - sea, atmos, cells, of self and food.
  Keyword: digestive system area, intestinal area, human surface area
  Get trustworthy numbers.  Discuss variance.
  What age/size dependence?
  Select extra pictures?
  Better link for lungs.  skin
  Discuss packing.

History:
  2003-Feb-03  Fixed links - 1 repaired, 1 replaced, 11 flagged.
  2001-Oct-05  Corrected typo - thanks to a reader.
  1999.Jul.16  Changed "Lungs" to "Lungs as sheet", etc, to clarify.
  1999.Apr.09  Noted paper-thickness of sheets (thanks to a reader).
  1999.Mar.21  Created page.
  1999.Jan.10  Did numbers.