Digestive System

Alimentary Canal (GI Tract)

• tube extending from the mouth to the anus

  • mouth

  • esophagus

  • stomach

  • small and large intestines

  • rectum

  • anal canal

Accessory Digestive Organs (External to GI Tract)

• teeth/tongue

• salivary glands

• liver and gall bladder

• pancreas

Peritoneal Cavity

• between the visceral and parietal peritoneum

Visceral Peritoneum

• lines organs

Parietal Peritoneum

• lines the body wall

Mesentery

• double layer of peritoneum

• hold everything in place

  • organs

  • nerves

  • blood vessels

Intraperitoneal Organs

• surrounded by peritoneum

• attached to body wall by mesenteries

Secondary Retroperitoneal Organs

• behind the peritoneal cavity

• lose their mesenteries

Falciform Ligament

• supports the liver

• anterior of liver to the anterior abdominal wall and diaphragm

Lesser Omentum

• supports the stomach

• liver to the superior part of the stomach

Dorsal Mesentery

• connects organs to the dorsal wall

Ventral Mesentery

• Falciform Ligament

• Lesser Omentum

Dorsal Mesentery

• greater omentum

• transverse mesocolon

• sigmoid mesocolon

Greater Omentum

• connects the stomach to the posterior abdominal wall

Transverse Mesocolon

• connects the transverse colon to the posterior wall

Sigmoid Mesocolon

• connects the sigmoid colon to the posterior pelvic wall

Retroperitoneal Organs

• not covered by peritoneum

  • pancreas

  • duodenum

  • ascending and descending colon

  • rectum

Digestive Process

1. ingestion

2. propulsion

3. mechanical breakdown

4. digestion

5. absorption

6. defecation

Sex/Gender and Digestive Disorders

• females have a greater risk of developing most digestive disorders

  • IBS, dyspepsia, celiac disease, constipation, etc.

• women are less likely to discuss digestive issues and feel more shame about these diseases

• women’s pain is less likely to be taken seriously by physicians

  • misdiagnosed as related to menstrual cycle

Anatomy of Alimentary Canal

• mucosa

• submucosa

• muscularis externa

• serosa

Circular Muscle Layer

• muscle runs around the tube in a circle

  • contraction = squeezing the tube (gets smaller)

Longitudinal Layer

• muscle runs along the length of the tube

  • contraction = tube gets shorter

Lumen

• central opening

Mucosa Associated Lymphoid Tissue

• within the Lamina Propria

Serosa and Submucosa

• contain nerves, arteries, veins and lymphatic vessels

Intermediate Filaments

• in smooth muscle

• pull on dense bodies for contraction

Variscosities

• in smooth muscle

• bulges in nerve fibres release neurotransmitters

  • muscle contraction is regional (not individual cells)

Enteric Nervous System

• “brain” within the alimentary canal

• can function independently — but has interactions with the central nervous system via the vagus nerve

  • more information travels up from the digestive system to the brain than from the brain to the digestive sysem (80-90%)

• myenteric nerve plexus

• submucosa nerve plexus

Myenteric Nerve Plexus

• muscularis externa

Submucosa Nerve Plexus

• submucosa

• innervates glands

Enamel

• hardest part of the body — harder than bone

  • can wear down over time and can’t regrow because it’s inorganic

Dentin

• second protective layer (after enamel)

  • not as hard

  • absorbs bite impact to not crack enamel

Root Canal Procedure

• enamel and dentin wear down and pulp is exposed and gets infected

  • pulp is removed and reproduced with synthetic pulp — dentin and enamel are also replaced with synthetic material

Rugae of Mucosa

• give space for stomach to stretch out with food

Pyloric Sphinctor

• smooth muscle — closed when food is in the stomach

  • open to let out small portions of food at a time

    • it takes 1-2 hours to completely empty the stomach

Cell Types in the Stomach

• mucous cells

• mucous neck cells

• parietal cells

• chief cells

• enteroendocrine cells

Mucous Cells

• secrete bicarbonate mucous on epithelial surface

  • act as a buffer against stomach acid to prevent burns in stomach wall (ulcer)

Mucous Neck Cells

• occur in the neck of gastic gland

• secrete mucous of unknown function

Parietal Cells

• produce HCl

Chief Cells

• secrete pepsinogen

  • inactive enzyme (activated by HCl)

Enteroendocrine Cells

• hormone producing cells throughout the alimentary canal

  • secrete hormones in blood vessels in the Lamina Propria

Small Intestine

• longest segment of the alimentary canal

• most of the digestion happens here

• most of the absorption happens here

• uses peristalsis to move chyme (partially digested food bolus) along the canal

3 Subdivisions of the Small Intestine

• duodenum

• jejunum

• ileum

Duodenum

• receives digestive enzymes and chemicals from the accessory digestive organs

  • pancreas

  • gall bladder

  • liver

• all digestive enzymes from accessory organs enter through the hepatopancreatic ampulla through the major duodenal papilla

• sphinctors control how much of each enzyme enters the small intestine

  • more fatty foods require more bile

Absorption in the Small Inestine

• facilitated by very large surface area

  • long length

  • circular folds

  • villi

  • microvilli

Circular Folds

• force chyme to spiral

  • to ensure that all chyme comes into contact with walls for absorption

Villi

• contain blood and lymphatic vessels, absorptive cells

Microvilli

• on the surface of epithelial cells, enzymes to complete final digestion

Lacteals

• absorb digested foods

• go into general circulation before reaching the liver (hepatic portal system) unlike other digested material

• bypasses “first pass metabolism”

  • doesn’t pass through the liver

    • could be dangerous if toxic materals that dissolve in fats are ingested

Small Intestine Cells

• absorptive enterocytes

• goblet cells

• enteroendocrine cells

Absorptive Enterocytes

• absorb digested material

Goblet Cells

• secrete lubricating mucus

Enteroendocrine Cells

• secrete hormones for bile and enzyme release (to increase/decrease production)

Appendix

• stores good bacteria

Teniae Coli

• smooth muscle

  • contracts to compact waste and help with peristalsis

Epiploic Appendages

• fat globules

  • unknown function

Large Intestine Cells

• colonocytes

• goblet cells (abundant)

Colonocytes

• absorptive cells

  • water and electrolytes

External Anal Sphinctor

• under conscious control

Internal Anal Sphinctor

• under parasympathetic control (unconscious)

Control of Defecation

1. feces move into and distend the rectum, stimulating stretch receptors there. The receptors transmit signals along afferent fibers to spinal cord neurons

2. a spinal reflex is initiated in which parasympathetic motor (efferent) fibers stimulate contraction of the rectal walls and relaxation of the internal anal sphinctor

3. if it is convenient to defecate, voluntary motor neurons are inhibited, allowing the external anal sphinctor to relax so that feces may pass

Liver

• filters the blood

  • hepatic portal vein — digestive blood flow

• produces bile for lipid digestion

  • bile stored in gallbladder

• produces glucose, fats, amino acids

• makes blood proteins

Bare Area

• part of liver not surrounded by peritoneum

  • direct attachment to diaphragm

Blood Flow Through the Liver

• blood moves from the outside to the center

Bile Flow Through the Liver

• bile moves from the center to the outside

Gallstones

• cause is not entirely known

  • most likely from excess cholesterol and/or bilirubin

• painful

  • block bile — stop breakdown of fats