Slide 1
A large surface inside a tube
The small intestine absorbs most nutrients from food. Folds and tiny projections give its lining much more surface than a smooth tube.
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The organ has three successive regions: duodenum, jejunum and ileum. Their functions overlap, but they do not contribute equally to every digestive and absorptive task.
Slide 2
Projections on projections
Villi rise from the intestinal lining, and microvilli project from individual absorptive cells. These are two different scales of surface expansion.
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Each villus contains blood vessels and a small lymph vessel called a lacteal. Absorbed material therefore has nearby transport routes instead of accumulating indefinitely inside the lining.
Slide 3
Breaking down and taking up
Digestive enzymes break food into smaller molecules. Absorptive cells then move suitable products across the boundary into the body.
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Pancreatic juice and bile enter the early small intestine. Digestion consequently depends on secretions from outside the tube as well as the work performed by the intestinal wall itself.
Slide 4
Movement has two jobs
Intestinal movements mix contents with digestive fluids and carry them onward. Mixing increases contact with the absorptive surface.
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Peristalsis produces traveling waves that move contents along the tract. Local mixing movements need not propel a meal a long distance, so motion can serve exposure as well as transport.
Slide 5
The next section still matters
The large intestine recovers water from the remaining contents. Material left after digestion and absorption becomes part of stool.
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Microbes in the colon act on substances that escaped earlier digestion. A meal therefore passes through several environments, each contributing something different rather than merely repeating the stomach’s work.
Sources
- The Small and Large Intestines
OpenStax, Rice University
- Your Digestive System & How it Works
NIDDK, NIH