A&P: Lecture 15

The Digestive System 2

Overview of the GI System

The Small Intestine

• Duodenum

  • First segment of small intestine

  • ~ 10” long

  • Receives chyme from stomach and digestive secretions from pancreas and liver

  • duodenum needs to bring chyme pH from 1.5 - 7.0 using pancreatic buffers (Example: Bicarbonate)

  • Neutralizes acid before it can damage the absorptive surfaces of the small intestine

The Small Intestine

• Jejunum

  • Middle segment of small intestine

  • ~ 8.2’ long

  • The location of most:

    • Chemical digestion

    • Nutrient absorption

The Small Intestine

• Ileum

  • Final segment of small intestine

  • ~ 11.5’ long

  • Ends at the ileocecal sphincter

    • Controls flow of material from ileum into large intestine

Histology of Small Intestine

• Plicae circulares

  • Transverse folds in intestinal lining

  • Permanent features; do not disappear when intestines fill

  • Slow the passage of chyme (food) through the intestines

  • Increases surface area of absorption

• 2200ft² of surface area

  • Great for absorbing nutrients

Histology of Small Intestine

• Intestinal villi

  • Fingerlike projections in mucosa of small intestine

• Increases surface area

• Covered by simple columnar epithelium

• Covered with microvilli

Histology of Small Intestine

• Intestinal glands

  • Mucous cells between columnar epithelial cells

  • Eject mucins onto intestinal surfaces

• Crypts of Lieberkühn

  • Openings from intestinal glands to intestinal lumen, located at bases of villi

  • Entrances for brush border enzymes

The Intestinal Wall

The Intestinal Wall

The Intestinal Wall

The Small Intestine

• Brush Border Enzymes

  • Integral membrane proteins

  • On surfaces of intestinal microvilli

  • Secreted from intestinal glands

  • Break down materials in contact with brush border

• Enteroendocrine cells

  • Produce intestinal hormones

  • Ex: gastrin, cholecystokinin, and secretin

The Small Intestine

• Glands of the Duodenum

  • aka: Submucosal or Brunner’s glands

  • Produce large quantities of mucus when chyme arrives from stomach

• Intestinal Secretions

  • Moisten chyme

  • Assist in buffering acids

  • ~1.8 Liters/day enter intestinal lumen

  • Keep digestive enzymes and products of digestion in solution

Peptic Ulcers

• Esophageal ulcers – in esophagus

• Gastric ulcers – inside stomach

• Duodenal ulcers – in duodenum

Peptic Ulcers

Helicobacter pylori bacteria

• Causes Ulcers

• 2/3 of people globally have H. pylori

  • Some people can handle it and some can’t

Gross Anatomy of the Pancreas

The Pancreas

• Functions of the Pancreas

  1. Endocrine cells:

     • Secrete insulin and glucagon into bloodstream

  2. Exocrine cells:

     • Secrete pancreatic juice ~1000 mL (1 qt) pancreatic juice per day

     • Controlled by hormones from duodenum

     • Contain pancreatic enzymes

Pancreatic Enzymes

1. Pancreatic alpha-amylase

   • A carbohydrase

   • Breaks down starches

   • Similar to salivary amylase

2. Pancreatic lipase

   • Breaks down complex lipids

3. Pancreatic Nucleases

   • Break down nucleic acids

4. Proteolytic enzymes

   • Break down certain proteins

   • Proteases break large protein complexes

   • Peptidases break small peptides into amino acids

   • Account for ~70% of all pancreatic enzyme production

   • Secreted as inactive proenzymes, and are activated after reaching small intestine

The Liver

• Largest visceral tissue mass ~3+ lbs.

  • Performs essential (1) metabolic and (2) hematological regulation as well as (3) bile production

• Anatomy of the liver:

  • Wrapped in tough fibrous capsule

  • Covered by visceral peritoneum

  • Divided into lobes

The Liver

• Hepatic blood volume

  • 1/3 is Arterial blood

    • From hepatic artery

• 2/3 is Venous blood from hepatic portal vein, originating at the:

  • Stomach

  • Spleen

  • Pancreas

  • Small & Large intestines

Histological Organization of the Liver

• Each lobe of the liver is divided:

  • Into ~ 100,000 liver lobules

  • The functional units of the liver

  • ~ 1 mm diameter each

• Each Lobule

  • Hexagonal in cross section

  • With 6 portal areas (hepatic triads)

    • One at each corner of lobule

Histological Organization of the Liver

• Each portal area of a lobule contains 3 structures

  • Branch of: hepatic portal vein, hepatic artery proper, and bile duct

Histological Organization of the Liver

Lobules of the liver

Liver Histology

Liver Histology

• Hepatocytes (hepato = liver, cyto = cells)

  • Adjust circulating levels of nutrients

    • Through selective absorption and secretion

  • Arranged like wheel spokes

  • As blood flows through sinusoids, the hepatocytes:

    • Absorb solutes from plasma

    • Secrete materials such as plasma proteins

Liver Histology

• Kupffer cells aka: stellate reticuloendothelial cells

  • Located in sinusoidal lining

  • Line blood vessels

  • Macrophages that play a critical role in maintaining liver functions

  • Immune cells that protect liver from bacterial infections

  • Respond to liver injury by producing cytokines that recruit other immune cells (Monocytes)

The Liver

• Main categorical functions:

  1. Metabolic regulation

  2. Hematological regulation

  3. Bile production

Metabolic Regulation of Liver

1. Nutrient Metabolism

   • Carbohydrate metabolism

   • Lipid metabolism

   • Amino acid metabolism

2. Nutrient Storage

   • Vitamin storage

   • Mineral storage

Metabolic Regulation of Liver

3. Waste Product Removal

   • Wastes (not utilized by the liver) are converted and carried out by bile to small intestine

   • … Or carried by blood to kidneys

   • Digested proteins → Ammonia → Urea

4. Drug Inactivation

   • Removes and breaks down circulating drugs

     • Ex: Acetaminophen (Tylenol)

   • Limits duration of their effects

   • Consider Rx drug dosages

https://www.nagwa.com/en/videos/535135361617/

Hematological Regulation of Liver

• Composition of Circulating Blood

  • All blood leaving absorptive surfaces of digestive tract

    • Enters hepatic portal system

    • Flows into the liver

  • Liver cells extract nutrients (or toxins) from blood

    • Before they reach systemic circulation

Hematological Regulation of Liver

• Composition of Circulating Blood

  • Liver removes and stores excess nutrients

    • Corrects nutrient deficiencies by mobilizing stored reserves or performing synthetic activities

• Largest blood reservoir in the body

  • Receives 25-30% of cardiac output

Hematological Regulation of Liver

• Functions of Hematological Regulation:

  • Removal of toxins

  • Synthesis of plasma proteins

  • Removal of circulating hormones

  • Phagocytosis and antigen presentation

  • Removal of antibodies

Bile Production in the Liver

• Functions of Bile

• Liver breaks down hemoglobin (Heme) to make bile

• Mechanical processing in stomach creates large lipid droplets

• Dietary lipids are not water soluble

• Pancreatic lipase is not lipid soluble

  • Interacts only at surface of lipid droplet

• Bile salts break droplets apart (emulsification)

  • Emulsification helps break down fats

  • Increases surface area exposure to enzymatic attack

  • Creates tiny emulsion droplets coated with bile salts

The Gallbladder

• Functions of the Gallbladder

  • Stores bile prior to excretion

  • Secretes, but only under stimulation of hormone cholecystokinin (CCK)

  • Without CCK

    • Hepatopancreatic sphincter remains closed

    • Bile exiting liver in common hepatic duct cannot flow through common bile duct into duodenum

    • So, the bile enters cystic duct and is stored in gallbladder

Coordination of Secretion and Absorption

• Neural mechanisms coordinate digestion (1 of 2)

  • Parasympathetic innervation

    • Prepares digestive tract for activity

    • Secretes a lot of mucus

  • Sympathetic innervation

    • Inhibits digestive tract activity

  • Coordinate movement of chyme along digestive tract

Coordination of Secretion and Absorption

• Hormonal mechanisms coordinate digestion (2 of 2)

  • Hormones of duodenal enteroendocrine cells include:

    • Secretin

    • Cholecystokinin (CCK)

    • Gastrin

    • Enterocrinin

    • Gastric inhibitory peptide (GIP)

    • Vasoactive intestinal peptide (VIP)

Coordination of Secretion and Absorption

1. Secretin

   • Released when chyme arrives in duodenum

   • Increases secretion of bile (liver) and buffers (pancreas)

   • First Hormone ever described

   • Discovered by Earnest Starling

2. Cholecystokinin (CCK)

   • Secreted in duodenum

     • When chyme contains lipids and partially digested proteins

   • Accelerates pancreatic production and secretion of digestive enzymes

   • Relaxes hepatopancreatic sphincter and gallbladder

     • Ejecting bile and pancreatic juice into duodenum

Coordination of Secretion and Absorption

3. Gastrin

   • Secreted in duodenum upon exposure to undigested proteins

   • Stimulates acid and enzyme production

4. Enterocrinin

   • Released when chyme enters duodenum

   • Stimulates mucin production

5. Gastric Inhibitory Peptide (GIP)

   • Secreted when fats and carbohydrates enter duodenum

6. Vasoactive Intestinal Peptide (VIP)

   • Stimulates secretion of intestinal glands

Coordination of Secretion and Absorption

• Intestinal Absorption

  • 90% of absorption happens in the small intestines.

    • by the time chyme gets into the Large Intestine its about all waste.

  • Takes ~ 5 hours for materials to pass from duodenum to end of ileum

  • Movements of the mucosa increases absorptive effectiveness

    • Stir and mix intestinal contents

    • Constantly change environment around epithelial cells

The Large Intestine

• The colon extends from end of ileum to anus

  • Frames small intestine ~ 5’ long and 3” wide

• Functions to:

  • Reabsorb water

  • Compact intestinal contents into feces

  • Absorb important vitamins produced by bacteria

    • Vitamins we make in our digestive system are made in the Large intestines.

  • Store fecal material prior to defecation

• Diarrhea = food moves too quickly through large Intestine

• Constipation = food moves too slowly through large Intestine

The Large Intestine

• The Cecum

  • Receives material arriving from the ileum

• The Colon

  • Larger diameter and thinner walls than small intestine

    • Forms a series of pouches

    • Haustra permit expansion and elongation of colon

The Large Intestine

• Rectum

  • Forms last 6” of digestive tract

  • Expandable organ for temporary storage of feces

  • Movement of fecal material into rectum triggers urge to defecate

• Anus

  • Internal anal sphincter

    • Smooth muscle, not under voluntary control

  • External anal sphincter

    • Skeletal muscle, under voluntary control

Histology of Large Intestine

• Large intestine tissue lacks villi

  • Does not produce enzymes because nutrient absorption is nearly complete

  • Presence of distinctive intestinal glands:

    • Deeper than glands of small intestine

    • Abundance of mucous cells

      • Provides lubrication for fecal material

Mucosa and Glands of the Colon

The Large Intestine

• Three Vitamins Produced in the Large Intestine

  1. Vitamin K (fat soluble)

     • Required by liver for synthesizing 4 clotting factors

     • K for klotting factors

  2. Biotin (water soluble)

     • Important in glucose metabolism

     • Sweet Hair (Hlucose metabolism and biotin)

  3. Pantothenic acid, aka B5 (water soluble)

     • Required in manufacture of steroid hormones and some neurotransmitters

The Large Intestine

• Movements of the Large Intestine

  • Gastroileal reflex

    • Moves materials into cecum while eating

  • Movement from cecum to transverse colon is slow

    • Allowing hours for water absorption

  • Peristaltic waves move material along length of colon

  • Stimulus is distension of stomach and duodenum; relayed over intestinal nerve plexuses

  • Distension of the rectal wall triggers defecation reflex

The Large Intestine

• Elimination of Feces

  • Requires relaxation of internal and external anal sphincters

  • Reflexes open internal sphincter, close external sphincter

  • Opening external sphincter requires conscious effort

Digestive System Review (1 of 3)

• Oral cavity

  • Mechanical processing with teeth and tongue

• Salivary Glands

  • Saliva lubricates food; contains enzymes to break down carbohydrates

• Pharynx

  • Musculature propels food toward esophagus

• Esophagus

  • Transportation of food to stomach

Digestive System Review (2 of 3)

• Stomach

  • Chemical digestion – acids and enzymes

  • Mechanical digestion – muscular contractions

• Liver

  • Generation of bile, nutrient storage, metabolic and hematological functions

• Gallbladder

  • Storage and secretion of bile for lipid digestion

Digestive System Review (3 of 3)

• Pancreas

  • Buffers and digestive enzymes secreted by exocrine cells

  • Hormones secreted by endocrine cells

• Small intestine

  • Enzymatic digestion and absorption of water, organic substrates, vitamins and ions

• Large intestine

  • Dehydration and compaction of indigestible material

The enzymatic breakdown of large molecules into their basic building blocks is called

chemical digestion

The outer layer of the digestive tract is known as the

serosa

Double sheets of peritoneum that provide support and stability for the organs of the peritoneal cavity are the

mesenteries

A branch of the hepatic portal vein, hepatic artery proper, and branch of the bile duct form

a portal triad

Label the digestive system structures in the following figure.

(a) oral cavity (mouth);

(b) liver;

(c) gallbladder;

(d) pancreas;

(e) large intestine;

(f) salivary glands;

(g) pharynx;

(h) esophagus;

(i) stomach;

(j) small intestine;

(k) anus

Label the four layers of the digestive tract in the following figure

(a) mucosa;

(b) submucosa;

(c) muscular layer;

(d) serosa

Most of the digestive tract is lined by epithelium.

simple columnar

Regional movements that occur in the small intestine and function to churn and fragment the digestive material are called

segmentation

Bile release from the gallbladder into the duodenum occurs only under the stimulation of

cholecystokinin

Label the three segments of the small intestine in the following figure.

(a) duodenum;

(b) jejunum;

(c) ileum

The major function(s) of the large intestine is (are)

(a) reabsorption of water and compaction of feces,

(b) absorption of vitamins liberated by bacterial action,

(c) storage of fecal material prior to defecation

Vitamins generated by bacteria in the colon are

vitamin K, biotin, and pantothenic acid

The final enzymatic steps in the digestive process are accomplished by

brush border enzymes of the intestinal microvilli

What are the six main processes of the digestive system?

The six main processes of the digestive system are

(1) ingestion

(2) mechanical digestion and propulsion

(3) chemical digestion

(4) secretion

(5) absorption

(6) defecation

Name and describe the layers of the digestive tract, proceeding from the innermost layer nearest the lumen to the outermost layer.

Layers of the digestive tract from the innermost layer nearest the lumen to the outermost layer are

(1) the mucosa: the epithelial layer that performs chemical digestion and absorption of nutrients;

(2) the submucosa: the connective tissue layer containing lymphatic and blood vessels and the submucosal nerve plexus;

(3) the muscular layer: the smooth muscle layer containing the myenteric nerve plexus;

(4) the serosa: the outermost layer, epithelium and connective tissue that forms the visceral peritoneum (or connective tissue that forms the adventitia).

What three basic mechanisms regulate the activities of the digestive tract?

Activities of the digestive tract are regulated by local, neural, and hormonal mechanisms.

What are the three phases of swallowing, and how are they controlled?

The three phases of swallowing—the buccal, pharyngeal, and esophageal phases—are controlled by the swallowing center of the medulla oblongata by the trigeminal and glossopharyngeal cranial nerves. The motor commands originating at the swallowing center are distributed by cranial nerves V, IX, X, and XII. Along the esophagus, primary peristaltic contractions are coordinated by afferent and efferent fibers within the glossopharyngeal and vagus cranial nerves, but secondary peristaltic contractions occur in the absence of CNS instructions.

What are the primary digestive functions of the pancreas, liver, and gallbladder?

The pancreas provides digestive enzymes, plus bicarbonate ions that elevate the pH of the chyme. The liver produces bile and is also the primary organ involved in regulating the composition of circulating blood. The gallbladder stores and releases bile, which contains additional buffers and bile salts that aid in the digestion and absorption of lipids

Which hormones produced by duodenal enteroendocrine cells effectively coordinate digestive functions?

The hormones include the following: enterocrinin, which stimulates the submucosal glands of the duodenum; secretin, which stimulates the pancreas and liver to increase the secretion of water and bicarbonate ions; cholecystokinin (CCK), which causes an increase in the release of pancreatic secretions and bile into the duodenum, inhibits gastric activity, and appears to have CNS effects that reduce the sensation of hunger; gastric inhibitory peptide (GIP), which stimulates insulin release at pancreatic islets and the activity of the duodenal submucosal glands; vasoactive intestinal peptide (VIP), which stimulates the secretion of intestinal glands, dilates regional capillaries, and inhibits acid production in the stomach; gastrin, which is secreted by G cells in the duodenum when they are exposed to large quantities of incompletely digested proteins; and, in small quantities, motilin, which stimulates intestinal contractions; villikinin, which promotes the movement of villi and associated lymph flow; and somatostatin, which inhibits gastric secretion.

What are the three primary functions of the large intestine?

The large intestine reabsorbs water and compacts the intestinal contents into feces, absorbs important vitamins liberated by bacterial action, and stores fecal material prior to defecation.

What two positive feedback loops are involved in the defecation reflex?

Positive feedback loops in the defecation reflex involve

(1) stretch receptors in the rectal walls, which promote a series of peristaltic contractions in the colon and rectum, moving feces toward the anus in the intrinsic myenteric defecation reflex; and (2) the sacral parasympathetic defecation reflex, also activated by the stretch receptors, which stimulates peristalsis by motor commands.

During defecation

(a) stretch receptors in the rectal wall initiate a series of peristaltic contractions in the colon and rectum,

(b) stretch receptors in the rectal wall activate parasympathetic centers in the sacral region of the spinal cord

Increased parasympathetic stimulation of the intestine would result in

none of these:

(a) decreased motility,

(b) decreased secretion,

(c) decreased sensitivity of local reflexes,

(d) decreased segmentation

A drop in pH below 4.5 in the duodenum stimulates the secretion of

secretin

Through which layers of a molar would an oral surgeon drill to perform a root canal (removal of the alveolar nerve in a severely damaged tooth)?

A root canal involves drilling through the enamel and the dentin.

How is the stomach protected from digestion?

The stomach is protected from digestion by mucous secretions of its epithelial lining and by neural and hormonal control over the times and rates of acid secretion.

How does each of the three phases of gastric secretion promote and facilitate gastric control?

(1) The cephalic phase of gastric secretion begins with the sight or thought of food. Directed by the CNS, this phase prepares the stomach to receive food. (2) The gastric phase begins with the arrival of food in the stomach; this phase is initiated by distension of the stomach, an increase in the pH of the gastric contents, and the presence of undigested materials in the stomach. (3) The intestinal phase begins when chyme starts to enter the small intestine. This phase controls the rate of gastric emptying and ensures that the secretory, digestive, and absorptive functions of the small intestine can proceed reasonably efficiently.

Nutritionists have found that after a heavy meal, the pH of blood increases slightly, especially in the veins that carry blood away from the stomach. What causes this increase in blood pH?

After a heavy meal, bicarbonate ions pass from the parietal cells of the stomach into the interstitial fluid. The diffusion of bicarbonate ions from the interstitial fluid into the bloodstream increases blood pH. This sudden influx of bicarbonate ions into the bloodstream has been called the alkaline tide.

Some people with gallstones develop pancreatitis. How could this occur?

If a gallstone is small enough, it can pass through the bile duct and block the pancreatic duct. Enzymes from the pancreas then cannot reach the small intestine. As the enzymes accumulate, they irritate the duct and ultimately the exocrine pancreas, producing pancreatitis.

Harry is suffering from an obstruction in his colon. He notices that when he urinates, the color of his urine is much darker than normal, and he wonders if there is any relationship between the color of his urine and his intestinal obstruction. What would you tell him?

The darker color of his urine is probably due to increased amounts of the pigment urobilin, which gives urine its normal yellow color. Urobilin is derived from urobilinogen, which is formed in the large intestine by the action of intestinal bacteria on bile pigments. In an intestinal obstruction, the bile pigments cannot be eliminated by their normal route, so a larger-than-normal amount diffuses into the bloodstream, where the kidneys eliminate it.

A condition known as lactose intolerance is characterized by painful abdominal cramping, gas, and diarrhea. The cause of the problem is an inability to digest the milk sugar lactose. How would this cause the observed signs and symptoms?

If an individual cannot digest lactose, this sugar passes into the large intestine in an undigested form. The presence of extra sugar in the chyme increases its osmolarity, so less water is reabsorbed by the intestinal mucosa. The bacteria that inhabit the large intestine can metabolize the lactose, and in the process they produce large amounts of carbon dioxide. This gas overstretches the intestine, which stimulates local reflexes that increase peristalsis. The combination of more fluid contents and increased peristalsis causes diarrhea.The overexpansion of the intestine by gas, which is directly related to increased gas production by the bacteria, causes the severe pain and abdominal cramping.

Recently, more people have turned to surgery to help them lose weight. One form of weight control surgery involves stapling a portion of the stomach shut, creating a smaller volume. How would such a surgery result in weight loss?

The primary effect of such surgeries would be a reduction in the volume of food (and thus in the amount of calories) consumed because the person feels full after eating a small amount. This can result in significant weight loss.