2. PHYSIOLOGY OF THE DIGESTIVE SYSTEM
• Specifically, the digestive system
takes in food (ingests it), breaks
it down physically and chemically
into nutrient molecules (digests
it), and absorbs the nutrients
into the bloodstream, then, it
rids the body of indigestible
remains (defecates).
4. Food Ingestion and Breakdown
• Once food is placed in the mouth, both mechanical and chemical digestion begin.
• Physical breakdown. First, the food is physically broken down into smaller
particles by chewing.
• Chemical breakdown. Then, as the food is mixed with saliva, salivary amylase
begins the chemical digestion of starch, breaking it down into maltose.
• Stimulation of saliva. When food enters the mouth, much larger amounts of
saliva pour out; however, the simple pressure of anything put into the mouth and
chewed will also stimulate the release of saliva.
• Passageways. The pharynx and the esophagus have no digestive function; they
simply provide passageways to carry food to the next processing site, the
stomach.
5. Food Swallowing and Propulsion
• Deglutition. Deglutition, or swallowing, is a complex process that involves the
coordinated activity of several structures
1.Buccal phase of deglutition. The first phase (voluntary) occurs in the mouth;
once the food has been chewed and well mixed with saliva, the bolus is forced into
the pharynx by the tongue.
2.Pharyngeal-esophageal phase. The second phase (involuntary) transports food
through the pharynx and esophagus.
• Food routes. All routes that the food may take, except the desired route, are
blocked off; the tongue blocks off the mouth; the soft palate closes off the nasal
passages; the larynx rises so that its opening is covered by the flaplike epiglottis.
• Stomach entrance. Once food reaches the distal end of the esophagus, it presses
against the cardioesophageal sphincter, causing it to open, and food enters the
stomach.
7. Food Breakdown
• Gastric juice & Gastrin. The presence of food and a rising pH in the
stomach stimulate the stomach cells to release the hormone gastrin, which
stimulate the stomach glands to produce the protein-digesting enzymes
(pepsinogen), mucus, and hydrochloric acid.
• Pepsinogen
• Rennin. Rennin, the second protein-digesting enzyme produced by the
stomach, works primarily on milk protein.
• Food entry. As food enters and fills the stomach, its wall begins to stretch.
• Stomach wall activation. Then the three muscle layers of the stomach wall
become active; they compress the food, breaking it apart physically, all the
while continuously mixing the food with the enzyme-containing gastric
juice so that the semifluid chyme is formed.
8. Food Propulsion
• Peristalsis. Once the food has been well mixed, peristalsis begins in
the upper half of the stomach, and the contractions increase in force
as the food approaches the pyloric valve.
• Pyloric passage. The pylorus of the stomach, which holds about 30 ml
of chyme. The pyloric sphincter barely opens, each contraction of the
stomach muscle release 3 ml or less of chyme into the small intestine.
• Enterogastric reflex. When the duodenum is filled with chyme and its
wall is stretched, the enterogastric reflex occurs. This reflex stops the
gastric activity and slows the emptying of the stomach by inhibiting
the vagus nerves and tightening the pyloric sphincter.
10. Food Breakdown and Absorption
• Digestion. Food reaching the small intestine is only partially digested;
carbohydrate and protein digestion has begun, but no fats have been
digested up to this point.
• Brush border enzymes. The microvilli of small intestine cells bears a few
important enzymes, the so-called brush border enzymes, that break
down double sugars into simple sugars and complete protein digestion.
• Chyme stimulation. When chyme enters the small intestine, it
stimulates the mucosa cells to produce several hormones; two of these
are secretin and cholecystokinin which influence the release of
pancreatic juice and bile.
11. • Pancreatic juice. Food entering the small intestine is flooded with
enzyme-rich pancreatic juice, as well as bile from the liver. Pancreatic
juice contains enzymes that, along with brush border enzymes,
complete the digestion of starch, carry out about half of the protein
digestion, and are responsible for fat digestion.
• Absorption. Absorption of water and of the end products of digestion
occurs all along the length of the small intestine by the process
of active transport. Lipids or fats are absorbed passively by the
process of diffusion.
• Debris. At the end of the ileum, all that remains are some water,
indigestible food materials, and large amounts of bacteria; this debris
enters the large intestine through the ileocecal valve.
14. Food Breakdown and Absorption
• Metabolism. The “resident” bacteria that live in its lumen metabolize some
of the remaining nutrients, releasing gases (methane and hydrogen sulfide)
that contribute to the odor of feces.
• Flatus. About 50 ml of gas (flatus) is produced each day, much more when
certain carbohydrate-rich foods are eaten.
• Absorption. Absorption by the large intestine is limited to the absorption of
vitamin K, some B vitamins, some ions, and most of the remaining water.
• Feces. Feces, the more or less solid product delivered to the rectum,
contains undigested food residues, mucus, millions of bacteria, and just
enough water to allow their smooth passage.
15. Propulsion of the Residue and Defecation
• Haustral contractions. The movements most seen in the colon are
haustral contractions, slow segmenting movements lasting about one
minute that occur every 30 minutes or so.
• Mass movements. Mass movements are long, slow-moving, but
powerful contractile waves that move over large areas of the colon
three or four times daily and force the contents toward the rectum.
• Rectum. The rectum is generally empty, but when feces are forced
into it by mass movements and its wall is stretched, the defecation
reflex is initiated.
16. • Defecation reflex. The defecation reflex is a sacral region reflex that
causes the walls of the sigmoid colon and the rectum to contract and
anal sphincters to relax.
• Impulses. As the feces is forced into the anal canal, messages reach
the brain giving us time to make a decision as to whether the external
voluntary sphincter should remain open or be constricted to stop
passage of feces.
• Relaxation. Within a few seconds, the reflex contractions end and
rectal walls relax; with the next mass movement, the defecation reflex
is initiated again.
