digestive system. gastrointestinal (gi) tract gi or alimentary canal – continuous, coiled, hollow...
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Digestive System
Gastrointestinal (GI) Tract
• GI or alimentary canal – continuous, coiled, hollow tube that winds through ventral body cavity from mouth to anus– Entire GI is ~30 feet in cadaver, but shorter
in living person due to muscle contractions– Includes mouth, pharynx, esophagus,
stomach, small intestine, large intestine, and ends at the anus
– Other accessory organs include salivary glands, pancreas, liver, gall bladder
Anatomy of Digestive System
Functions of Digestive System
1. Ingestion – putting food into mouth
2. Propulsion – moving food from one organ to the next
3. Mechanical digestion – breaks down food physically into smaller pieces
4. Chemical digestion – large molecules are broken down into their building blocks by enzymes
5. Absorption – transport of digested end products from the GI tract to the blood or lymph
6. Defecation – elimination of indigestible food that leaves through the anus in the form of feces
Activities of Mouth, Pharynx, and Esophagus
• Mouth– Chewing begins mechanical breakdown
of food• Pharynx – Propels food into esophagus by
peristalsis (involuntary smooth muscle contractions)
• Epiglottis covers windpipe (trachea) to prevent food from moving down into lungs
• Esophagus– Propels food into stomach via peristalsis
Activities of Stomach
• Stomach – a storage tank– C-shaped sac with sphincters at each
end• Cardiac or esophageal sphincter
prevents food from going back into esophagus • Pyloric sphincter opens to allow
food to move into the small intestine– Physical digestion occurs as stomach
grinds and churns food– Chyme - partially digested food that
leaves the stomach
–Chemical digestion occurs with enzymes• Gastric glands – in stomach lining; release gastric juice–Gastric juice – includes digestive enzymes, intrinsic factor, hydrochloric acid (HCl), and mucus»Intrinsic factor helps absorb vitamin B12
• Pepsin – enzyme that breaks down protein (activated by HCl)
Activities of the Small Intestine
• A tube that is 8 – 18 feet long and twisted into coils
• The major digestive organ• Three subdivisions–Duodenum (5% of total length)– Jejunum (40% of total length)– Ileum (almost 60% of total length)
• Nearly all food absorption occurs in the small intestine!
• Duodenum–Pancreatic ducts dump
pancreatic juice into duodenum–Bile duct (joins with pancreatic
ducts) dumps bile from liver into duodenum
• Jejunum and ileum - primary function is absorption
• Intestinal villi tiny projections of inner wall of small intestine; greatly increase surface area for absorption–Each villus contains blood
capillaries and a lacteal into which foodstuffs are absorbed
• Ileocecal sphincter – opens to let chyme pass into large intestine
Activities of the Large Intestine
• Larger in diameter than the small intestine, but not nearly as long (~5 feet)
• Major function is to dry out indigestible food residues by absorbing water and then eliminate them as feces
• Subdivisions: cecum, appendix, colon, rectum, and anal canal
• Cecum – first part of large intestine; sac-like with worm-like appendage called appendix
• Colon – parts in order are: ascending (travels up), transverse (travels across), descending (travels down), and then sigmoid (S-shaped)
• Rectum – stores feces and leads to anal canal
• Anus – ending of GI; voluntary sphincter opens to release feces
Activities of Accessory Organs
• Teeth – breakdown, grind, and tear food into smaller pieces
• Salivary glands release saliva; contain mucus to soften food and salivary amylase, which begins carbohydrate digestion– Parotid gland – largest, lie anterior and
somewhat inferior to each ear– Sublingual gland – smallest; lie on floor of
mouth inferior to tongue– Submanibular gland – lie in floor of mouth
on inside surface of lower jaw
• Pancreas – soft, pink, triangular gland; secretes pancreatic juice–Pancreatic duct – tube that carries pancreatic juice to duodenum
– Pancreatic juice contains the following: • bicarbonate - neutralizes acidic
chyme coming from stomach • amylase - enzyme that breaks down
carbohydrates • lipase - enzyme that breaks down
lipids • trypsin, chymotrypsin, and carboxypeptidase - enzymes that breakdown proteins• nuclease – enzyme that breaks down
nucleic acids
• Liver – largest gland in the body; digestive function is to produce bile– Bile – leaves liver through the hepatic duct
and enters duodenum through bile duct• Yellowish-green liquid containing bile salts,
bile pigments (bilirubin), cholesterol, electrolytes, and lipids (no enzymes)• Only the bile salts and lipids help with
digestion• Bile salts emulsify fats by physically
breaking large fat globules into smaller ones
• Gall bladder – small, green sac under liver that stores bile
Homeostatic Imbalances• Gall stones – if bile is stored in gall
bladder too long or too much water is removed, the cholesterol in it crystallizes forming gall stones– Can cause blockage of hepatic or bile
ducts and bile begins is released into bloodstream instead• Bile in blood and eventually tissues
causes jaundice • Hepatitis – inflammation of liver; often
cause by virus (also called hepatitis)• Cirrhosis – chronic inflammation of liver;
can be from drinking too much alcohol
• Heartburn – when cardiac sphincter does not close tightly and allows acidic stomach juices to enter esophagus– Hiatal hernia – superior part of stomach
protrudes slightly above the diaphragm; which weakens cardiac sphincter
• Peptic ulcers – erosion of stomach lining (open sore in membrane), which exposes the lining to acid; often caused by some bacteria (Helicobacteri pylori)
• Pancreatitis – inflammation of pancreas caused by pancreatic enzymes breaking down the pancreatic tissue itself
• Diverticulitis – diverticula are formed by the inner layer of the intestine protruding through the large intestine wall; causes inflammation and pain– Caused by not eating enough bulk (fiber);
colon narrows and puts more pressure on walls
• Diarrhea – large intestine does not absorb enough water from feces; loss of electrolytes and fluids can cause severe dehydration
• Constipation – too much water is removed from feces; can be caused by low fiber diets
• Impacted teeth – teeth that remain embedded in jawbone; can cause pressure and pain; often occurs with wisdom teeth
• Appendicitis – infected appendix; bacteria can accumulate because its twisted
• Hemorrhoids – enlarged and inflamed branches of the rectal vein in the anal canal that cause itching, pain, and sometimes bright red bleeding
• Vomiting – complex reflex that empties stomach through esophagus, pharynx, and mouth (vomiting center in medulla oblongata)