chapter 23

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1

Digestive Process

1. Ingestion

2. Propulsion

3.Mechanical breakdown

4. Digestion

5. Absorption

6. Defecation

2

Propulsion include

1. Swallowing(voluntary)

2. Peristalsis( involves alternating waves of contraction and relaxationof muscles in the organ walls)

3

Digestive system organs fall into two main groups

1. The alimentary canal, or gastrointestinal (GI) tract (the mouth, pharynx, esophagus, stomach, small intestine, and large intestine.)

2. Accessory digestive organs aid digestion physically and produce secretions that break down foodstuff in the GI tract; the organs involved are the teeth, tongue, gallbladder, salivary glands, liver, and pancreas.

4

Peretoneum

of the abdominopelvic cavity its the most extensive of these membranes

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Visceral peritoneum

covers the external surface of the most digestive organs

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parietal peritoneum

lines the body wall

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mesentry

is a double layer of peritoneum that extends to the digestive organs from the body wall. It allows blood vessels, lymphatics, and nerves to reach the digestive organs, and holds the organs in place as well as stores fat

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1.Ingestion

2.Propulsion

3.Mechanical breakdown

4.Digestion

5. Absorption

6. Defecation

1.is the simple act of putting food into the mouth

2. moves food through the alimentary canal and includes both swallowing and peristalsis

3. is the physical process of preparing the food for chemical digestion and involves chewing, mixing, churning, and segmentation

4. is a series of catabolic steps in which complex food molecules are broken down to their chemical building blocks by enzymes

5. is the passage of digested end products from the lumen of the GI tract through the mucosal cells into the blood or lymph
6.eliminates indigestible substances from the body via the anus as feces

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Retroperitoneal organs

organs not suspended by a mesentery. include most of the pancreas and parts of the small and large intestine

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Define the splanchnic circulation

serves the digestive system and includes those arteries that branch off the abdominal aorta to serve the digestive organs and the hepatic portal circulation

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Layers of the alimentary canal

1. Mucosa is the innermost, moist, epithelial membrane that lines the entire digestive tract. It secretes mucus, digestive enzymes, and hormones; absorbs digestive end products into the blood; and protects against infectious disease

2.the Submucosa is a moderately dense connective tissue layer containing blood and lymphatic vessels, lymphoid follicles, and nerve fibers.

3.Muscularis externa typically consists of smooth muscle and is responsible for peristalsis and segmentation.

4. Serosa, the protective outer layer of the intraperiton

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major function of the mucosa

- secrete mucus, digestive enzymes, and hormones

- absorb the end products of digestion into blood

- protect against infectious disease

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the alimentary canal is constructed from either stratified squamous or simple columnar epithelium

the mucosa

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Intrinsic nerve plexuses found in

the walls of the alimentary canal : the submucosal and meyenteric nerve plexuses

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Basic function of mouth, lips and cheeks, tongue

-Mouth is a stratified squamous epithelial mucosa-lined cavity with boundaries of the lips, cheeks, palate, and tongue

-The lips and cheeks have a core of skeletal muscle covered externally by skin that helps to keep food between the teeth when we chew and plays a small role in speech.

-The tongue is made of interlacing bundles of skeletal muscle and is used to reposition food when chewing, mix food with saliva, initiate swallowing, and help form consonants for speech.

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Function of saliva

- cleanes the mouth

-dissolves food chemical so they can be tasted

-moistens food and helps compact it into a bolus

- contains enzymes that begin the chemical breakdown of starchy foods

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Most saliva produced by the

- the major or extrinsic salivary glands(outside oral cavity)

-minor or intrinsic salivary glands

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The salivary glands are composed of two types of secretory cells

1. serous cell produce a watery secretion containing enzymes, ions, and tyni bit of mucin

2. mucous cell produce mucus, a stringly, viscous solution

19

Describe the mechanisms of chewing and swallowing.

•Mastication, or chewing, begins the mechanical breakdown of food and mixes the food with saliva (p. 863).

•Deglutition, or swallowing, is a complicated process that involves two major phases (p. 863; Fig. 23.13).

1.The buccal phase is voluntary and occurs in the mouth where the bolus is forced into the oropharynx.

2.The pharyngeal-esophageal phase is involuntary and occurs when food is squeezed through the pharynx and into the esophagus.

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Chyme

(in stomach)a temporally "storage tank" where chemical breakdown of proteins begins and food is converted to a creamy paste

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types of the gland cells

Mucous neck cells, found in the upper, or “neck,” regions of the glands, produce a thin, soluble mucus

Parietal cells,found mainly in the middle region of the glands, simultaneously secrete hydrochloric acid (HCl) and intrinsic factor. Although the parietal cells appear spherical when viewed with a light microscope, they actually have three prongs that bear dense microvilli (they look like fuzzy pitchforks!). This structure provides a huge surface area for secreting H and Cl into the stomach lumen. HCl makes the stomach contents extremely acidic (pH 1.5–3.5), a condition necessary for activation and optimal activity of pepsin. The acidity also helps in food digestion by denaturing proteins and breaking down cell walls of plant foods, and is harsh enough to kill many of the bacteria ingested with foods. Intrinsic factor is a glycoprotein required for vitamin B12 absorption in the small intestine.

Chief cells, occurs mainly in the basal regions of the gastric glands.he chief cells produce pepsinogen, the inactive form of the protein-digesting enzyme pepsin. Chief cells also secrete insignificant amounts of lipases (fat-digesting enzymes).

Enteroendocrine cells - (“gut endocrine”), typically located deep in the gastric glands, release a variety of chemical messengers directly into the interstitial fluid of the lamina propria.

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intrinsic factor

is required for intestinal absorption of vitamin B12, needed to produce erythrocytes

23

Explain how gastric secretion and stomach motility are regulated.

Neural and hormonal mechanisms

•Stimulatory and inhibitory events occur in three phases:
1.Reflex phase (cephalic): few minutes prior to food entry: smell/taste
2.Gastric phase: 3–4 hours after food enters the stomach
3.Intestinal phase: brief stimulatory effect as chyme enters the duodenum, followed by inhibitory effects (enterogastric reflex and enterogastrones): not too much now!

24

Define and account for the alkaline tide.

As H is pumped from the cell and HCO3 (bicarbonate ion) accumulates within the cell,HCO3 is ejected through the basal cell membrane into the capillary blood. As a result, blood draining from the stomach is more alkaline than the blood serving it. This phenomenon is called the alkaline tide.

25

Identify and describe structural modifications of the wall of the small intestine that enhance the digestive process.

•the mucousa and submucousa are modified to reflect the intestines functions in the digestive pathway

•epithelium= simple columnar absorptive cells bound by tight junctions (responsible for nutrient and electrolyte absorption)

intestinal crypts= secretory cells the secrete intestinal juice (serve as a carrier fluid for absorbing nutrients from chyme)

26

Differentiate between the roles of the various cell types of the intestinal mucosa.

simple columnar absorptive cells=nutrient and electrolyte absorption

•crypt epithelial cells= secretory cells the secrete intestinal juice

•specialized cell types: absorptive cells, goblet cells, enteroendocrine cells, paneth cells

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Bile

is a fat emulsifier. Bile does not usually enter the small intestine until the gallbladder contracts when stimulated by cholecystokinin.

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Bilirubin

a waste product of the heme of hemoglobin formed during the breakdown of worn-out erythrocytes

29

The gallblader

stores and concentrates bile that is not needed immediately for digestion.

30

State the role of pancreatic juice in digestion.

•Pancreatic juice consists mainly of water and contains enzymes that break down all categories of foodstuffs and electrolytes.

•Secretion of pancreatic juice is regulated by local hormones and the parasympathetic nervous system.

31

Describe how entry of pancreatic juice into the small intestine is regulated.

Pancreatic juice is regulated by neural stimuli and, more importantly, hormones (cholecystokinin and secretin).

When no digestion is occurring, the hepatopancreatic sphincter (guarding the entry of bile and pancreatic juice into the duodenum) is closed and the released bile backs up the cystic duct into the gallbladder, where it is stored until needed

32

List the major functions of the large intestine.

The large intestine absorbs water from indigestible food residues and eliminates the latter as feces

33

Describe the regulation of defecation.
Feces forced into the rectum by mass movements stretch the rectal wall and initiate the defecation reflex.

This spinal cord–mediated parasympathetic reflex causes the sigmoid colon and the rectum to contract, and the internal anal sphincter to relax. As feces are forced into the anal canal, messages reach the brain allowing us to decide whether the external (voluntary) anal sphincter should be opened or remain constricted to stop feces passage temporarily. If defecation is delayed reflex contractions end within a few seconds, and the rectal walls relax.With the next mass movement, the defecation reflex is initiated again—and so on, until the person chooses to defecate or the urge to defecate becomes unavoidable.

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card image

List the enzymes involved in chemical digestion; name the foodstuffs on which they act.

•salivary amylase - Chemical digestion of carbohydrates begins in the mouth, where salivary amylase breaks large polysaccharides into smaller fragments.

•Pepsin - secreted by the chief cells, begins the chemical digestion of proteins in the stomach.

•Rennin - is produced in infants and breaks down milk proteins.

•Pancreatic enzymes - such as trypsin and chymotrypsin, further break down proteins in the small intestine.

•The brush border enzymes - carboxypeptidase, aminopeptidase, and dipeptidase work on freeing single amino acids in the small intestine.

•Lipases are secreted by the pancreas and are the enzymes that digest fats after they have been pretreated with bile.

35

List the end products of protein, fat, carbohydrate, and nucleic acid digestion.

•Protein
oAmino acids are absorbed by cotransport with sodium ions.
oSome dipeptides and tripeptides are absorbed via cotransport with H+ and hydrolyzed to amino acids within the cells.
oAmino acids leave the epithelial cells by facilitated diffusion, enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein.

•Fat
oFatty acids and monoglycerides enter the intestinal cells via diffusion.
oFatty acids and monoglycerides are recombined to form triglycerides and then combined with other lipids and proteins within the cells, and the resulting chylomicrons are extruded by exocytosis.
oThe chylomicrons enter the lacteals of the villi and are transported to the systemic circulation via the lymph in the thoracic duct.
oSome short-chain fatty acids are absorbed, move into the capillary blood in the villi by diffusion, and are transported to the liver via the hepatic portal vein.

•Carbohydrate
oMonosaccharides (simple sugars), the monomers of carbohydrates, are absorbed immediately
oGlucose and galactose are absorbed via cotransport with sodium ions.
oFructose passes via facilitated diffusion.
oAll monosaccharides leave the epithelial cells via facilitated diffusion, enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein.

•Nucleic Acid
oUnits enter intestinal cells by active transport via membrane carriers.
oUnits are absorbed into capillary blood in the villi and transported to the liver via the hepatic portal vein.

36

Describe the process of absorption of breakdown products of foodstuffs that occurs in the small intestine.

1.Large fat globules are emulsified (physically broken up into smaller fat droplets) by bile salts in the duodenum.

2.Digestion of fat by the pancreatic enzyme lipase yields free fatty acids and monoglycerides. These then associate with bile salts to form micelles which “ferry” them to the intestinal mucosa.

3.Fatty acids and monoglycerides leave micelles and diffuse into epithelial cells. There they are recombined and packaged with other lipoid substances and proteins to form chylomicrons.

4.Chylomicrons are extruded from the epithelial cells by exocytosis. The chylomicrons enter lacteals. They are carried away from the intestine by lymph.

37

Degestion

is a catabolic process that breaks down large food molecules to monomers

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Monosaccharides( simple sugar)

glucose

fructose

galactose

39

Describe embryonic development of the digestive system

1.The epithelial lining of the developing alimentary canal forms from the endoderm with the rest of the wall arising from the mesoderm.

2.The anteriormost endoderm touches the depressed area of the surface ectoderm where the membranes fuse to form the oral membrane and ultimately the mouth.

3.The end of the hindgut fuses with an ectodermal depression, called the proctodeum, to form the cloacal membrane and ultimately the anus.

4.By week 8 the alimentary canal is a continuous tube stretching from the mouth to the anus.

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Describe abnormalities of the gastrointestinal tract at different stages of life

GI tract motility declines, digestive juice production decreases, absorption is less efficient, and peristalsis slows, resulting in less frequent bowel movements and often constipation.

•Diverticulosis, fecal incontinence, and cancer of the GI tract are fairly common problems in the elderly.

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Micelles

are collections of fatty elements clustered together with bile salts in such a way that the polar (hydrophilic) ends of the molecules face the water and the nonpolar portions form the core

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Chylomicrons

triglycerides are combined with lecithin and other phospholipids and cholesterol, and coated with a “skin” of proteins to form water-soluble lipoprotein droplets