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ANATOMY

The Digestive System - Purpose

• Every cell in the body needs nutrition. This includes not only oxygen, but also other nutrients that must be ingested.

• Foods are composed of larger molecules not readily absorbed. They are broken down into smaller pieces.

The Digestive System - Purpose

• The digestive system breaks down foods (carbohydrates, fats and proteins) into molecules that are absorbed by the small intestine & carried to the liver by the blood stream for further processing.

• After processing by the liver, these molecules are distributed to the body by the cardiovascular system.

• Digestion is the conversion of complicated foodstuffs to their simplest forms: six-carbon sugars, amino acids, and fatty acids.

Overview of the Digestive System

• Digestive organs are divided into two groups:

– Alimentary canal • Mouth, pharynx, and esophagus• Stomach, small intestine, and large intestine

– Accessory digestive organs• Teeth and tongue• Gallbladder, salivary glands, liver, and pancreas

Overview of the Digestive System

Digestive Processes

Digestive Processes

• Ingestion – occurs in the mouth

• Propulsion – movement of food

– Peristalsis – major means of propulsion

Peristalsis

• Peristalsis is the rhythmic contraction of smooth muscle (circular and longitudinal) to propel contents through the digestive tract from the mouth to the anus.

Digestive Processes

• Ingestion – occurs in the mouth

• Propulsion – movement of food

– Peristalsis – major means of propulsion

• Mechanical digestion – prepares food for chemical digestion

– Chewing, churning of food in stomach, segmentation

• Segmentation is rhythmic local constrictions of intestine

Digestive Processes

• Chemical digestion – – complex molecules broken down to chemical components

• Mouth• Stomach• Small intestine

• Absorption – – transport of digested nutrients

• Defecation – – elimination of indigestible substances as feces

Histology of the Alimentary Canal Wall

• Usually four layers from esophagus to rectum:

– The mucosa – innermost layer, containing small glands and lymphatic nodules

• Epithelium - epithelium is a tissue composed of a layer of cells.

• Lamina propria - is a thin layer of loose connective tissue which lies beneath the epithelium.

• Muscularis mucosae – is a thin layer of smooth muscle located outside the lamina propria.

Histology of the Alimentary Canal Wall

– The submucosa – external to the mucosa

• Thin layer of connective tissue containing blood and lymphatic vessels, nerve fibers

• Strongest layer of the large intestine due to collagen fibers

– The muscularis externa – external to the submucosa

• Two layers

– Circular smooth muscle – inner layer

– Longitudinal smooth muscle – outer layer

– The serosa – the outermost layer

• Is the visceral peritoneum

• Absent in the Esophagus

• Absent in most of Rectum

The Peritoneal Cavity and Peritoneum

• Peritoneum – a serous membrane that forms a sac within the abdomen:

– Parietal peritoneum – lines the body wall– Visceral peritoneum – surrounds digestive organs

• Peritoneal cavity –

– The peritoneal cavity is a potential space between the parietal peritoneum and visceral peritoneum.

The Peritoneal Cavity and Peritoneum

• Peritoneal organs– Lie within the peritoneal

cavity– Digestive organs that keep

their mesentery

• Retroperitoneal organs – Lie behind the parietal

peritoneum

The Peritoneal Cavity and Peritoneum

• Mesentery – a double layer of peritoneum– Holds organs in place– Provides a route for

circulatory vessels (arteries, veins, and lymphatics) and nerves

– Provides sites for fat storage

Mesenteries

• Superficial view of abdominal organs

Mesenteries

• Lesser omentum attaches to lesser curvature of stomach

Mesenteries

• Greater omentum – a “fatty apron” of peritoneum– Portion connecting

stomach with transverse colon is the gastrocolic ligament

– Remaining portion attaches to anterior surface of transverse colon and hangs down anterior to rest of bowel

• (Picture shows greater omentum and transverse colon reflected)

Mesenteries – Small Bowel

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Mesenteries

• (Sagittal section through the abdominopelvic cavity)

• Mesenteries attach to the posterior abdominal wall.

• Mesocolons are mesenteries of the large intestine.

Figure 22.10d

Mesenteries - Mesos of the Colon

• The appendix, part of the cecum, the transverse colon and the sigmoid colon present with a mesentery or meso, and they are easily moved out of the way during surgery.

• The ascending and the descending colon do not have a mesentery or meso but are attached to the posterior abdominal wall by a reflection of their serosal covering from which they must be separated during surgery.

• The mesorectum is actually a misnomer as the attachment of the rectum is not covered by peritoneum, but the name persists due to common usage.

Anatomy – Abdominal Organs

The Stomach

Figure 22.18a

Pyloric Sphincter

The Small Intestine

• Longest portion of the alimentary canal

– ~ 4-7 m (20 ft.) long– Site of most enzymatic digestion and absorption

• Three subdivisions

– Duodenum - ~ 25 cm (10 in.) long – “12 fingerbreadths)– Jejunum - ~ 1-2 m (3-6.5 ft.) long– Ileum – 2-4 m (6.5-13 ft.) long

The Duodenum

• Receives gastric contents via the pylorus

• Receives digestive enzymes and bile

• Main pancreatic duct and common bile duct enter duodenum– Sphincters control entry of bile and pancreatic juices

Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

The Duodenum and Related Organs

The Lower GI Tract

The Large Intestine

• The large intestine is comprised of:

– The colon—• That portion distal to the ileum and proximal to the rectum• Approximately 150 cm (5 feet) in length• Larger diameter proximally

– The rectum—• That portion distal to the colon and proximal to the anus• Approximately 12-15 cm (5-6 inches) in length

– The anal canal—• That portion distal to the rectum, terminating at the anal verge• Approximately 4 cm (1 ½ inches) long

Characteristics of the Colon

• The colon has 3 unique characteristics:

– The haustra—• A haustrum is a dilatation or sacculation of the colon.• These are multiple (haustra is plural).

– The tenia coli—• These are three longitudinal bands of thickened smooth muscle

(muscularis externa) traversing the entire length.• All 3 originate jointly at the base of the vermiform appendix.• They gradually disappear at the rectosigmoid by diverging

uniformly.– The appendices epiploicae—

• These are small pouches of fat along the tenia coli.

The Large Intestine

Characteristics of the Colon

Appendices Epiploicae

Haustra

Tenia Coli

Characteristics of the Rectum

• The rectum is an “S” shaped reservoir found in the pelvis.

• Most of the rectum has only 3 tissue layers—mucosa, submucosa, and muscularis—since all but the upper anterior rectum is below the visceral peritoneal covering (extra- and infra-peritoneal) and, therefore, has no serosa.

• The adipose tissue contained within the fascia propria posterior to the rectum is (incorrectly) referred to as the mesorectum.

• The tenia coli completely surround the rectum, having merged in the rectosigmoid from 3 distinct columns in the colon.

• For surgical purposes, the rectum is divided into proximal, middle, and distal rectum.

• The rectal curvatures result in inner mucosal infoldings, known as Valves of Houston, which can make traversing the rectum in retrograde fashion difficult during endoscopy or surgery.

Characteristics of the Rectum

Characteristics of the Rectum

Characteristics of the Anal Canal

• The anal canal begins at the level of the levator ani muscles.

• The anal canal is surrounded by strong muscles and represents an anteroposterior slit due to their tonic contraction.

• There are 2 “tubes” of muscle—inner is smooth, and outer is skeletal.

• This short segment of GI tract is essential to fecal continence.

• It is also prone to many diseases which can lead to major symptoms.

• The midportion of the anal canal contains an undulating demarcation known as the dentate line.

• The anal verge is the distal external-most portion of the GI tract.

Characteristics of the Anal Canal

Anal Verge

Vasculature of Lower GI Tract

Vasculature of Lower GI Tract

Blood Supply to the Colon--SMA

• The Superior Mesenteric Artery (SMA):– Supplies blood to the

duodenum, pancreas, jejunum, ileum, and the right and transverse colon.

• The ileocolic artery provides blood supply to the ileocolic region, cecum and appendix.

• The right colic artery provides blood supply to the ascending colon and the hepatic flexure.

• The middle-colic (mid-colic) artery provides blood supply to the transverse colon and the hepatic and splenic flexures.

Blood Supply to the Colon--IMA

• The Inferior Mesenteric Artery (IMA)– Supplies blood to the left colon,

rectosigmoid and the superior 2/3 of the rectum.

– The left colic artery supplies blood to the descending colon.

– The sigmoidal arteries traverse the mesosigmoid and supply blood to the sigmoid colon.

• After supplying the sigmoidal arteries, the inferior mesenteric artery becomes the superior rectal artery.

• This artery divides to left and right superior rectal arteries, and supplies the upper rectum.

Blood Supply to the Colon—The Marginal Artery

• The branches of the superior and inferior mesenteric arteries split and, with the exception of the rectal arteries, create a long artery that runs along the inner margin of the colon and is called the Marginal Artery of Drummond.

Blood Supply to the Colon—The Arc of Riolan

• The Arc of Riolan is present in a small number of people and is a secondary arcade which may help to provide collateral blood supply.

• It is important to observe the high number of vascular arcades that are formed within the colon vasculature which allow for creative reconstructive intestinal surgery.

Arc of Riolan

Blood Supply to the Rectum

• The superior 2/3 of the rectum is supplied by the left and right branches of the superior rectal artery (arising from the lower portion of the inferior mesenteric artery).

Blood Supply to the Rectum

• The middle rectal arteries (when present) and inferior rectal arteries supply the lower portion of the rectum, the anal canal and the anal sphincter.

The middle and inferior rectal arteries arise from the internal iliac arteries, which is important especially in cancer surgery as the proximal blood supply would have been divided.

Due to inconsistencies in the blood supply to the lower rectum, it is often better to anastomose lower rather than higher as long as there is no tension.

Lymphatic Drainage of the Colon and Rectum

• The lymphatic system is a system of microscopic open-ended tubules and filter nodules whose function is to recuperate extravasated fluid and materials and redirect them to the cardiovascular system.

• The lymphatic system also plays a roll in the absorption of fats in the digestive system.

• It plays a crucial role in cancer as it allows for the passage of cancer cells from one organ to another through its system.

• In the colon and rectum, the lymphatic system is linked closely with the vascular system.

• Resection of major feeding arteries in cancer surgery will lead to resection of the regional lymph nodes, allowing for better staging of the tumors.

Lymphatic Drainage of the Colon and Rectum

Lymphatic Drainage in Cancer

• Plays crucial role in cancer as it allows for spread of cancer cells from one organ to another through the lymphatic system.

• Colonic lymphatics follow arteries and veins.

• Resection of vessels will result in resection of lymphatics in cancer surgery and allow for additional staging of a tumor.

Anatomy and Pathology

• With an understanding of normal anatomy,

• We are in the position to understand Pathology.