Chapter 17 Digestive System

1
Chapter 17 Digestive System
2
Digestive System

• Digestion is the mechanical and chemical
  breakdown of food into a form that the bodys
  cells can absorb.
• The digestive system has two sets of organs
• The Alimentary Canal the organs in this set are
  make up the tube which food uses to pass thru
  the body as it is absorbed.
• Accessory Organs these structures help to break
  food down as it passes thru the tube.

3
Digestive System

• The alimentary canal is a muscular tube made up
  of 4 layers
• The Mucosa is formed from primarily of epithelium
  that is invested with goblet cells. This inner
  lining of this layer has many tiny folds on it in
  order to increase the overall absorptive surface
  area. This is the layer of the tract that
  actually absorbs and secretes (absorbs food,
  secretes digestive enzymes).
• The Submucosa in made up primarily of loose
  connective tissue, blood vessels, glands, and
  nerves.

4
Digestive System

• Muscular Layer this layer is made up of 2
  layers of smooth muscle, and provides movement
  for the tube. The inner layer of muscle is
  circular while the outer layer is longitudinal.
• The Serosa is the outer most layer of the tube
  and is primarily composed of serous epithelium
  and some connective tissue. The serous layer
  secretes serous fluid.

5
Digestive System

• Segmentation is a movement of the tube which
  helps to mix food with the digestive secretions.
  
• Peristalsis is a contraction of the tube which
  causes food to move forward thru the tube. It
  begins when food expands the tube, and as the
  contractions occur the segment ahead of the tube
  relaxes to allow advancement of the food bolus.
• The tube has both sympathetic and parasympathetic
  innervation. An increase in parasympathetic
  nerve flow is usually responsible for increasing
  peristalsis and digestive secretions.

6
Digestive System

• Organs of the Digestive System
• The Mouth is the first item in line in the
  alimentary canal. Mechanical digestion begins
  here with mastication (chewing).
• The Tongue is an accessory organ of digestion.
  It functions to position food in the mouth. It
  is held to the floor of the mouth by the lingual
  frenulum. The top of it is covered with
  structures called papillae which help to handle
  food and provide the sense of taste. The tongue
  also helps in the formation of speech.

7
Digestive System

• The Palate forms the roof of the oral cavity. It
  consists of a hard (bony) anterior portion and a
  soft posterior portion. A cone shaped projection
  called the uvula hangs from the posterior most
  aspect of the palate.
• The Teeth are the hardest structures in the body.
  They aid digestion by way of mastication. There
  are two sets of teeth in humans, deciduous and
  permanent. Children will typically have 20 teeth
  that are gradually replaced by the adult teeth.
  Adults typically have 32 teeth.

8
(No Transcript)
9
Digestive System

• Teeth are arranged in this order
• Central Incisor
• Lateral Incisor
• Cuspid
• Bicuspid (Pre-molar)
• Molar
• Each type of tooth is designed to do a specific
  job. Incisors snip food, Cuspids and Bicuspids
  grip and tear, and Molars grind.
• Teeth break food into smaller pieces. This
  increases the overall surface area of the food
  allowing the digestive enzymes to act more
  efficiently on the food for the process of
  chemical digestion.

10
Digestive System

• Each tooth is made of a few different layers.
  Enamel covers the top of the tooth . Below that
  is the dentin. Each tooth has a cavity called
  the pulp cavity or root canal. This cavity is
  home to a connective tissue pulp as well as blood
  vessels and nerves.
• Below the Gingiva (gums) the tooth is surrounded
  by a substance called cementum and held in place
  by a Peridontal Ligament.

11
Digestive System

• The Salivary Glands secrete the fluid Saliva
  which helps to moisten food and begin the
  chemical break down of some nutrients. Saliva
  helps to cleanse the mouth and teeth, and helps
  to enhance taste .
• The pH of saliva is between 6.5 and 7.5. Saliva
  also contains a digestive enzyme called Salivary
  Amylase, which is responsible for beginning the
  break down of starchy foods. The major salivary
  glands are the Submandibular, the Sublingual, and
  the Parotid.

12
Digestive System

• The Pharynx is divided into 3 separate regions
• Nasopharynx
• Oropharynx
• Laryngopharynx
• The pharynx is connects the mouth to the
  Esophagus. Neither of these organs digest food,
  but are important passage ways to the Stomach.

13
Digestive System

• The swallowing mechanism (Deglutition) is divided
  into a voluntary phase and an involuntary phase.
  The first phase is voluntary and occurs when food
  is chewed and mixed with saliva.
• The involuntary phase occurs when the tongue
  pushes food to the back of the pharynx and the
  swallowing reflex is triggered. During this time
  the Epiglottis closes the trachea and allows food
  to enter the esophagus.
• The swallowing reflex momentarily inhibits
  breathing. When food enters the esophagus it is
  moved down the tube toward the stomach by the
  action of Peristalsis.

14
Digestive System

• The Esophagus is a straight, collapsible tube
  that leads from the pharynx thru the thoracic
  cavity to the stomach. It penetrates the
  diaphragm thru an opening called the Esophageal
  Hiatus.
• The esophagus terminates at the Cardiac Sphincter
  just above the stomach.

15
Digestive System

• The Stomach is a curved pouch-like organ that is
  located in the upper left quadrant of the
  abdomen. The inner lining is marked by thick
  folds called rugae. The stomach secretes Gastric
  Juice that has a very low pH and contains an
  enzyme called Pepsinogen that initiates the
  digestion of proteins.
• Where most of the organs of digestion have two
  layers of smooth muscle to help with peristalsis,
  the stomach has three layers. This helps the
  stomach to churn and mix food in the process of
  mechanical digestion.

16
Digestive System
17
Digestive System

• There are three types of secretory cells
  associated with gastric glands in the stomach.
• Mucous cells secrete mucous
• Chief cells secrete digestive enzymes
• Parietal cells produce hydrochloric acid
• Pepsinogen is an inactive form of the enzyme
  pepsin. Pepsinogen is activated into Pepsin by
  HCl secreted by the parietal cells. It begins
  the break down of most dietary protein.
• Mucous stops the action of the both HCl and
  Pepsin and thus keeps the stomach from
  auto-digesting itself.

18
Digestive System

• The stomach is divided into 4 regions. The
  cardiac region is located lust inside the cardiac
  sphincter. The fundus is the superior most
  portion located above the cardiac sphincter. The
  body makes up the bulk of the organ in the
  middle. The pyloric region is the inferior most
  area, just above the Pyloric Sphincter.
• The pyloric sphincter retains food in the stomach
  until it is released into the Small Intestines.

19
Digestive System
20
Digestive System

• Intrinsic Factor is another component of gastric
  juice. It is critical for the absorption of
  Vitamin B12.
• Please note that very little food is actually
  absorbed in the stomach. The stomach will absorb
  some water and many electrolytes, but most other
  nutrients are absorbed else where.
• Vomiting is a reflex where the gears of the
  digestive system work in reverse motion. This
  reflex ultimately results in the emptying of the
  contents of the stomach.

21
Digestive System

• The Pancreas is located primarily in the upper
  left quadrant of the abdomen, and has both
  endocrine and exocrine functions. In addition to
  insulin and glucagon secretion, it secretes many
  digestive enzymes. These secretions join bile
  secreted by the liver and enter the small
  intestines thru the Ampulla of Vater
  (Hepatopancreatic Ampulla).
• The pancreas has a head, a body and a tail.

22
Digestive System
23
Digestive System

• Pancreatic Juice contains enzymes that digest
  carbs, fats, proteins and nucleic acids.
• Pancreatic Amylase finishes the break down of
  carbohydrates into disaccharides.
• Pancreatic Lipase breaks down triglyceride
  molecules into fatty acids and monoglycerides.
• The Proteases are Trypsin, Chymotrypsin, and
  Carboxypeptidase. These enzymes reduce proteins
  to amino acid chains.
• Nucleases are enzymes that break down nucleic
  acids into nucleotides.
• Pancreatic juice also contains a high bicarbonate
  concentration. This gives it an alkaline pH
  which is favorable for the actions of the above
  enzymes, as well as neutralizing most of the acid
  in from the stomach.

24
Digestive System

• The Liver is the largest of all internal organs.
  It is located mostly in the upper right quadrant
  of the abdomen. The liver is enclosed by a
  fibrous capsule and divided into a right and left
  lobe. Two minor lobes also exist. They are the
  caudate lobe and the quadrate lobe.
• Blood digestive tract, carried in the hepatic
  portal vein brings newly absorbed nutrients into
  the sinusoids of the liver. Here blood is
  cleansed of impurities and microbes by Kupffer
  Cells (phagocytes).

25
Digestive System
26
Digestive System

• The liver performs several important functions.
  It plays a key role in carbohydrate metabolism by
  helping to maintain blood glucose levels. It
  aids in lipid and protein metabolism. It forms
  urea by deaminating amino acids. It synthesizes
  clotting factors for the blood and helps the
  spleen in breaking down damaged and old RBCs.
  It also synthesizes Bile.

27
Digestive System

• Bile is a yellow-green liquid that is
  continuously secreted by the liver. It is then
  stored in the Gall Bladder until needed.
• The Gall Bladder is found on the inferior side of
  the liver and stores bile until it is called for
  by the body. It concentrates the bile by
  dehydration and keeps it in this form until
  release. Under certain conditions this liquid
  can form a crystal and an accumulation of these
  crystals is referred to as a gall stone. This
  process is referred to as Choleolithiasis.
  Choleolithiasis can lead to Choleocystitis, which
  can lead to a medical emergency.

28
Digestive System

• Bile salts aid digestive enzymes by the
  emulsification of fats. In this process fat
  droplets are sequestered from each other,
  increasing the overall surface area and
  maximizing enzymatic action of the lipases.

29
Digestive System

• The Small Intestine is a tubular organ that
  extends from the pyloric sphincter to the Large
  Intestines. This organ is divided into three
  parts
• The Duodenum is the proximal most portion and
  receives secretions from both the pancreas and
  the liver.
• The Jejunum is usually larger in diameter than
  the ileum and more vascular
• The Ileum is the distal portion of this organ and
  is usually less active than the jejunum.
• The Mesentery are a double fold of peritoneum
  that suspend most of the small intestine within
  the abdominal cavity. The mesentery supports
  blood vessels and nerves that supply the
  intestinal wall.

30
Digestive System

• The interior lining of the intestinal wall are
  covered by numerous Villi. These structures
  project into the lumen of the intestine and
  greatly absorb the absorptive capacity of this
  organ.
• Each villus is made up of a layer of epithelium
  and a core of connective tissue containing blood
  capillaries, nerve fibers, and a lymphatic
  Lacteal. At their free surface the epithelial
  cells have many fine extensions called Microvilli
  that form what is known as the Brush Boarder
  that further enhances absorption. The
  capillaries and lacteals carry absorbed nutrients
  into general circulation.

31
Digestive System

• The small intestines also have an exocrine
  function. Mucous is secreted in varying amounts.
  Also, the following enzymes are secreted
• Peptidases which break down protein.
• Sucrase, maltase, and lactase which split
  disaccharides into monosaccharides.
• Intestinal lipase which breaks down fats.

32
Digestive System

• The small intestine is the main organ of
  absorption. It is incredibly efficient in this
  process. Carbs, once broken down into
  monosaccharides, are absorbed into a villus and
  enter the blood capillaries.
• Proteins are broken down into smaller amino acid
  chains and absorbed by a villus into the blood
  capillaries.
• Most fats are absorbed by lymphatic lacteals and
  ultimately make their way to the cysterni chyli,
  then on to general circulation.
• Electrolytes are moved by active transport, and
  water is generally moved by osmosis.

33
Digestive System

• Generally, very little in the way of actual
  nutrients reaches the end of the digestive tract.
  
• If the small intestine begins to push chyme thru
  too fast, nutrients will not be as well absorbed.
  This includes water, and the result is a watery
  stool called Diarrhea. Prolonged diarrhea can
  cause dehydration and electrolyte imbalance.
• The small intestine ends where it joins the large
  intestine at the Ileocecal Valve (another
  sphincter muscle).

34
Digestive System

• The Large Intestine is so named because of its
  diameter. This organ absorbs water,
  electrolytes, and remnants of the digestive
  secretions remaining in the left over waste
  product that was once food. It also forms and
  stores Feces.
• The colon has a few parts
• Ascending portion
• The Cecum is the pouch like end of the ascending
  portion
• The Veriform Appendix is the terminal portion of
  the cecum
• Transverse portion
• Descending portion
• The Sigmoid Colon is the final portion of the
  descending portion

35
Digestive System

• The Anal Canal is made up two sets of sphincter
  muscles. The internal anal sphincter is made of
  smooth muscle and is under voluntary control.
  The external anal sphincter is made up of
  skeletal muscle and is under voluntary control.
• The large intestine has little or no digestive
  function, other than to eliminate an unneeded
  waste product. Its epithelial lining has many
  goblet cells in it and thus mucus is the only
  significant secretion of this organ.

36
Digestive System

• Many bacteria inhibit the large intestine and
  make up the Intestinal Flora. Many of these
  bacteria synthesize vitamins such as K and B12
  which are then absorbed by the mucosa.
• Movement thru the large intestine are caused by
  peristalsis, but these movements are usually less
  prevalent here than in the small intestine.
• Feces are composed of materials that were not
  digested (such as cellulose), or not absorbed,
  some water and electrolytes, mucus, and bacteria.