HUMAN BIOLOGY

DiGestioN aND NUtritioN 199

What roles do cheWing and sWalloWing play

in food digestion?

• Chewing and swallowing are the first steps of food processing in
  the GI tract.


• Chewing breaks food up mechanically. Enzymes in saliva begin
  the chemical digestion of starches.


• Swallowed food passes down the esophagus, through the
  lower esophageal sphincter, and into the stomach.

taKe-Home message

into the pharynx (fare-inks), the throat. This passageway
connects with the windpipe, or trachea (Figure 11.6), which
leads to the lungs. It also connects with the esophagus,
which leads to the stomach. Mucus secreted by the mem-
brane lining the pharynx and esophagus lubricates the
bolus, helping move food on its way.

swallowing has voluntary
and involuntary phases

Swallowing food might seem simple, but it involves a
sequence of events (Figure 11.6). Swallowing begins when
voluntary skeletal muscle contractions push a bolus into
the pharynx (Figure 11.6A), stimulating sensory recep-
tors in the pharynx wall. The receptors trigger a reflex in
which involuntary muscle contractions keep food from
moving up into your nose and down into the trachea.
As this reflex occurs, the vocal cords are stretched tight
across the entrance to the larynx (your “voice box”). Then,
the flaplike epiglottis is pressed down over the vocal
cords as a secondary seal. For a moment, breathing stops

as food moves into the esophagus,

so you normally don’t choke when

you swallow. When swallowed food

reaches the lower esophagus, it

passes through a sphincter into the

stomach (Figure 11.6B and 11.6C).

Waves of muscle contractions called

peristalsis (pare-ih-stal-sis) help

push the food bolus along.

muscles

relaxed

muscles relaxed

food bolus

Circular

muscles

contract,

squeezing

bolus toward

the stomach.

Lower esophageal

sphincter opens

and food enters

stomach.

stomach

VOLUNTARY

PHASE

INVOLUNTARY

PHASE

hard

palate

food

bolus

Contracted

muscles close

off esophagus.

trachea

(airway) open

Larynx rises;

trachea closes, and

muscle contractions

squeeze food into

esophagus.

epiglottis

A

BC

Figure 11.6 Animated!

Swallowing and peristalsis

move food toward the stomach.

a Contractions of the tongue push

the food bolus into the pharynx.

Next, the vocal cords seal off the

larynx, and the epiglottis bends

downward, helping to keep the

trachea closed. Contractions of

throat muscles then squeeze the

food bolus into the esophagus.

B, c Finally, peristalsis in the

esophagus moves the bolus

through a sphincter, and food

enters the stomach.

(© Cengage Learning)

esophagus Passageway

from the pharynx to the

stomach.

peristalsis Rhythmic

smooth muscle contractions

that propel food through the

GI tract.

pharynx The throat.

© iStockphoto.com/tomazl

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