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 nutrients 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)