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Chapter 15 Digestion

Introduction A. Digestion refers to the mechanical and

chemical breakdown of foods so that nutrients can be absorbed by cells.

B. The digestive system carries out the process of digestion.

C. The digestive system consists of the alimentary canal, leading from mouth to anus, and several accessory organs whose secretions aid the processes of digestion.

Fig15.01

ALIMENTARY CANAL

Anus

ACCESSORY ORGANS Salivary glands!Secrete saliva, which contains!enzymes that initiate breakdown!of carbohydrates

Mouth!Mechanical breakdown!of food; begins chemical!digestion of carbohydrates Pharynx!Connects mouth with!esophagus.

Esophagus!Peristalsis pushes!food to stomach

Stomach!Secretes acid and!enzymes. Mixes food!with secretions to!begin enzymatic!digestion of proteins Small intestine!Mixes food with bile!and pancreatic juice.!Final enzymatic breakdown!of food molecules;!main site of!nutrient absorption Large intestine!Absorbs water and!electrolytes to form feces Rectum!Regulates elimination!of feces

Pancreas!Produces and secretes!pancreatic juice, containing!digestive enzymes and!bicarbonate ions,!into small intestine

Gallbladder!Stores bile and!introduces it into!small intestine

Liver!Produces bile, which!emulsifies fat

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B. Structure of the Wall 1.The wall of the alimentary canal

consists of the same four layers throughout its length, with only slight variations according to the functions of specific sections of the canal.

a. Mucosa (epithelium) b. Submucosa (connective tissue) c. Muscularis (smooth muscle) d. Serosa (epithelium)

Fig15.03

Intestinal gland Lymph nodule!

Villi Lacteal Epithelium

Mucosa

Serosa

Mesentery

Mucosa

Serosa

Nucleus

Artery Vein

Circular fold

Submucosa Microvilli Goblet cell

Simple columnar!epithelium

Circular muscle Longitudinal muscle

Nerve!plexuses

Mucous gland in submucosa Circular muscle Longitudinal muscle Muscular!

layer

C. Movements of the Tube 1. The motor functions of the

alimentary canal are of two types- mixing movements and propelling movements.

2. Mixing movements occur when smooth muscles contract rhythmically in small sections of the tube.

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3. Propelling movements include a wavelike motion called peristalsis, which is caused by contraction behind a mass of food as relaxation allows the mass to enter the next segment of the tube.

Fig15.04

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(c)

(a)

Movement of contents (b)

Digesting material

Wave of!contraction

Mouth A. The mouth is the first portion of the

alimentary canal; it functions to receive food and begins mechanical digestion by chewing or mastication.

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Fig15.06

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Frontal sinus

Nasal cavity

Hard palate

Hyoid bone

Esophagus

Laryngopharynx

Epiglottis Lingual tonsil Oropharynx Palatine tonsil Uvula Oral cavity Nasopharynx Soft palate Opening of auditory tube Pharyngeal tonsil Sphenoidal sinus

Lip

Larynx

Vestibule

Tongue Tooth

Trachea

E. Teeth 1. Two sets of teeth develop in sockets

within the alveolar processes of the maxillary and mandibular bones.

2. The 20 primary teeth are shed in the order they appeared and are replaced by 32 secondary teeth.

3. Through the actions of chewing, teeth break food into smaller pieces, beginning mechanical digestion.

Fig15.08

Incisors

Canine (cuspid)

Canine (cuspid)

Premolars!(bicuspids)!

Premolars!(bicuspids)!

Molars

Incisors

Molars

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Salivary Glands A. The salivary glands secrete saliva, which

moistens and dissolves food particles, binds them together, allows tasting, helps to cleanse the mouth and teeth, and begins carbohydrate digestion.

Sublingual gland

Parotid!gland!

Mandible (cut)

Submandibular!duct!

Masseter!muscle

Submandibular!gland

Tongue

Fig15.10

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Pharynx and Esophagus A.  The pharynx is a cavity lying behind the

mouth, and the esophagus is a muscular tube leading to the stomach.

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D. Esophagus 1. The esophagus is a straight,

collapsible passageway leading to the stomach, through a diaphragm opening (esophageal hiatus).

2. Mucous glands are scattered throughout the submucosa of the esophagus and produce mucus to moisten and lubricate the inner lining of the tube.

3. The lower esophageal sphincter helps to prevent regurgitation of the stomach contents into the esophagus.

Stomach A.  The stomach is a J-shaped muscular organ

that receives and mixes food with digestive juices, and propels food to the small intestine.

B.  The stomach produces gastric juice consisting of enzymes, acid (HCl), and mucous. It also produces the hormone gastrin which helps regulate digestion (that is released into the blood, not the stomach)

Fig15.11

Lower esophageal!sphincter!

Fundus

Body

Greater!curvature!

Lesser!curvature!

Gastric folds (rugae) Pyloric!antrum!

Pyloric!canal!Pylorus

Pyloric opening

Duodenum Pyloric!sphincter!

Cardia

Esophagus

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F. Mixing and Emptying Actions 1. Following a meal, mixing actions of

the stomach turn the food into chyme and pass it toward the pyloric region using peristaltic waves.

2. The rate at which the stomach empties depends on the fluidity of the chyme and the type of food.

3. As chyme fills the duodenum, accessory organs—the pancreas, liver, and gallbladder—add their secretions.

Pancreas A. The pancreas has an exocrine function of

producing pancreatic juice with many enzymes that aid digestion.

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Fig15.14

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Right hepatic duct Left hepatic duct Cystic duct Common hepatic duct Bile duct

Bile duct

Gallbladder

Head of pancreas

Duodenum

Major duodenal papilla

Intestinal lumen

Pancreatic duct

Pyloric sphincter

Minor duodenal!papilla

Major duodenal!papilla

Hepatopancreatic!sphincter

Pancreatic!duct

Tail of pancreas

Hepatopancreatic!ampulla Hepatopancreatic!

sphincter

Liver A. The reddish-brown liver, located in the

upper right quadrant of the abdominal cavity, is the body’s largest internal organ.

Fig15.16

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Right lobe Left lobe

Right lobe Cystic duct Left lobe

Hepatic duct

Bile duct

Caudate lobe (b) (a)

Inferior vena cava Coronary ligament

Gallbladder

Round!ligament Gallbladder

Inferior vena cava

Quadrate lobe

Hepatic artery Hepatic portal!vein

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Liver Functions A. The liver carries on many diverse

functions for the body. B. The liver is responsible for many

metabolic activities, such as the metabolism of carbohydrates, lipids, and proteins.

C. The liver also stores glycogen, vitamins A, D, and B12, iron, and blood.

4. The liver filters the blood, removing damaged red blood cells and foreign substances, and removes toxins.

5. The liver's role in digestion is to secrete bile which helps digest fats.

Gallbladder A. The gallbladder is a pear-shaped

sac lying on the interior surface of B. A sphincter muscle controls the

release of bile from the common bile duct.

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Small Intestine A. The lengthy small intestine receives

secretions from the pancreas and liver, completes digestion of the nutrients in chyme, absorbs the products of digestion, and transports the remaining residues to the large intestine.

B. Parts of the Small Intestine 1. The small intestine consists of the

duodenum, jejunum, and ileum. 2. The duodenum is the shortest and

most fixed portion of the small intestine; the rest is mobile and lies free in the peritoneal cavity.

3.The small intestine is suspended from the posterior abdominal wall by a double-layered fold of peritoneum called mesentery.

Fig15.21

Stomach

Jejunum

Duodenum

Ascending colon

Mesentery Appendix

Cecum

Ileum

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C. Absorption in the Small Intestine 1. The small intestine is the major site

of absorption within the alimentary canal.

2. The small intestine carries on segmentation and peristaltic waves.

Large Intestine A. The large intestine absorbs water and

electrolytes and forms and stores feces.

Fig15.27

Anal canal

Anus

Sigmoid colon

Haustra

Descending colon

Tenia coli

Rectum

Appendix

Cecum

Orifice of appendix

Ileocecal sphincter

Ascending colon

Serous layer Mucous membrane

Transverse colon

Muscular layer

Ileum

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B. Parts of the Large Intestine 1. The large intestine consists of the

cecum (pouch at the beginning of the large intestine with the appendix projecting downward from it.), colon (ascending, transverse, descending, and sigmoid regions), the rectum, and the anal canal.

2. The anal canal opens to the outside as the anus; it is guarded by an involuntary internal anal sphincter and a voluntary external anal sphincter muscle.

C. Functions of the Large Intestine 1. The large intestine does not digest

or absorb nutrients, but it does secrete mucus.

2. The large intestine absorbs electrolytes and water.

3. The large intestine contains important bacteria (intestinal flora) which synthesize vitamins and use cellulose.

D. Movements of the Large Intestine 1. The movements of the large

intestine are similar to those of the small intestine.

2. Peristaltic waves (mass movements) happen only two or three times during the day.

3. Defecation is stimulated by a defecation reflex that forces feces into the rectum where they can be expelled.

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E. Feces 1. Feces are composed of undigested

material, water, electrolytes, mucus, and bacteria.

2. Both the color of feces and its odor is due to the action of bacteria.