Gastrointestinal Anatomy

KAAP 310

Alimentary Canal and Accessory Organs

• Alimentary Canal:– Mouth (oral cavity)– Pharynx– Esophagus– Stomach– Small intestine– Large intestine– Anus

• Accessory Organs*:– Tongue– Salivary Glands– Liver– Pancreas– Gall bladder

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Mouth

• Oral (buccal) cavity– Bounded by lips, cheeks, palate, and tongue – Lined with stratified squamous epithelium

• Functions– Ingestion– Mechanical digestion– Chemical digestion– Propulsion

Oral Cavity, Pharynx, & Esophagus

Oropharynx

LaryngopharynxHyoid Bone

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Pharynx • Throat, passes air and food via sequential

contraction of muscles

Esophagus• Carries food from throat to stomach,

collapsed when empty• Diaphragm and esophageal sphincter (and

gravity) keep food in stomach

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Figure 23.13 Deglutition (swallowing).Bolus of food

Tongue

Pharynx

Epiglottis

Glottis

Trachea

During the buccal phase, the upper esophageal sphincter is contracted. The tongue presses against the hard palate, forcing the food bolus into the oropharynx.

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Uvula

Bolus

Epiglottis

Esophagus

The pharyngeal-esophageal phase begins as the uvula and larynx rise to prevent food from entering respiratory passageways. The tongue blocks off the mouth. The upper esophageal sphincter relaxes, allowing food to enter the esophagus.

The constrictor muscles of the pharynx contract, forcing food into the esophagus inferiorly. The upper esophageal sphincter contracts (closes) after food enters.

Peristalsis moves food through the esophagus to the stomach.

The gastroesophageal sphincter surrounding the cardial oriface opens, and food enters the stomach.

Relaxed muscles

Circular musclescontract

Bolus of food

Longitudinal musclescontract

Gastroesophagealsphincter closed

Relaxedmuscles

Circular muscles contract

Gastroesophagealsphincter opens

Upperesophagealsphincter

Bolus

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4

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Stomach

Slide 1

Stomach

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Stomach

• Mechanical breakdown• Denaturation of proteins by HCl• Enzymatic digestion of proteins by pepsin (and milk

protein by rennin in infants)• Delivers chyme to small intestine• Lipid-soluble alcohol and aspirin absorbed into blood• Secretion of intrinsic factor for vitamin B12 absorption

• Only stomach function essential for life• B12 needed mature red blood cells

Small Intestine

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Small Intestine

• Major organ of digestion and absorption• 2-4 m long; from pyloric sphincter to illeocecal

valve • Subdivisions

– Duodenum (retroperitoneal)– Jejunum (attached posteriorly by mesentery)– Ileum (attached posteriorly by mesentery)

Duodenum

• Curves around head of pancreas; shortest part – 25 cm

• Bile duct (from liver) and main pancreatic duct (from pancreas)

Jejunum & Illeum

• Jejunum– Extends from duodenum to ileum– About 2.5 m long

• Ileum– Joins large intestine at illeocecal valve– About 3.6 m long

Digestion in Small Intestines

• Chyme from stomach contains– Partially digested carbohydrates and proteins – Undigested fats

• 3–6 hours in small intestine– Most water absorbed– ~ All nutrients absorbed

• Small intestine, like stomach, no role in ingestion or defecation

Large Intestine

• Cecum – first part of large intestine• Appendix – masses of lymphoid tissue

– Part of MALT of immune system– Bacterial storehouse recolonizes gut when

necessary– Twisted enteric bacteria accumulate and

multiply

Large Intestine

• Retroperitoneal except for transverse and sigmoid regions

• Ascending colon (right side – to level of right kidney) right colic (hepatic) flexure

• Transverse colon left colic (splenic) flexure

• Descending colon (left side) • Sigmoid colon in pelvis rectum

Large Intestine

Anus

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Digestion in Large Intestine

• Residue remains in large intestine 12–24 hours• No food breakdown except by enteric bacteria• Vitamins (made by bacterial flora), water, and

electrolytes (especially Na+ and Cl–) reclaimed• Major functions - propulsion of feces to anus;

defecation• Colon not essential for life

Accessory Organs

Liver

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Accessory Organs

• Pancreas– Endocrine function

• Pancreatic islets secrete insulin and glucagon

– Exocrine function• Acini (clusters of secretory cells) secrete pancreatic juice

– To duodenum via main pancreatic duct

• Liver– Many functions; only digestive function bile production

• Bile – fat emulsifier

• Gallbladder– Chief function bile storage

• Spleen

Rectum and Anus

• Rectum– Three rectal valves stop feces from being passed

with gas (flatus)• Anal canal

– Last segment of large intestine– Opens to body exterior at anus

• Sphincters– Internal anal sphincter—smooth muscle– External anal sphincter—skeletal muscle

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Figure 23.29b Gross anatomy of the large intestine.

Rectal valveRectumHemorrhoidalveinsLevator ani muscle

Anal canal

External analsphincterInternal analsphincterAnal columns

Pectinate lineAnal sinuses

Anus

Digestive Processes

• Six essential activities1. Ingestion2. Propulsion3. Mechanical breakdown4. Digestion5. Absorption6. Defecation

Digestive ProcessesIngestion• Bringing food in via the mouth

Propulsion• Swallowing – voluntary• Peristalsis – involuntary contraction and relaxation of muscles in organ walls

Mechanical breakdown• Chewing, mixing food with saliva, churning food in stomach, and segmentation – rhythmic local

constrictions of the small intestine

Digestion• Enzymes break down complex food molecules to their chemical building blocks

Absorption• Passage of digested end products from the lumen of the GI tract into the blood or lymph

Defecation• Elimination of indigestible substances from the body via the anus

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Figure 23.2 Gastrointestinal tract activities.

Ingestion

Mechanicalbreakdown

Digestion

Propulsion

Absorption

Defecation

Food

PharynxEsophagus• Chewing (mouth)

• Swallowing (oropharynx)• Peristalsis (esophagus, stomach, small intestine, large intestine)

Stomach

Lymphvessel

Small intestineLargeintestine

Bloodvessel

Mainly H2OFeces

Anus

• Churning (stomach)• Segmentation (small intestine)

Functions of Gastrointestinal OrgansMouth

– Ingestion, propulsion, mechanical breakdown, digestion

Pharynx & Esophagus– Propulsion

Stomach– Propulsion, mechanical breakdown, digestion, absorption

Small Intestine & associated accessory organs (liver, gallbladder, pancreas)– Propulsion, mechanical breakdown, digestion, absorption

Large Intestine– Digestion, absorption, propulsion, defecation

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Figure 23.3 Peristalsis and segmentation.

Frommouth

Peristalsis: Adjacent segments of alimentary tract organs alternately contract and relax, moving food along the tract distally.

Segmentation: Nonadjacent segments of alimentary tract organs alternately contract and relax, moving food forward then backward.Food mixing and slow food propulsion occur.