By: Jocelyn Pearce

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The digestive system

• By Jocelyn Pearce

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The Mouth

• Inside the mouth, food is mixed with saliva.
  Humans have three pairs of salivary glands that
  do this. The parotid, submandibular, and
  sublingual glands.
• The nervous system controls the secretions
• Although our mouth is constantly secreting
  saliva, it increases when food enters the mouth
• When food is ready to be swallowed, the tongue
  pushes it to the back of the mouth

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The mouth cont.

• Swallowing occurs when the soft palate moves up
  and pushes against the pharynx
• Pressure on the pharynx stimulates a reflex,
  where the larynx is raised, pushing the glottis
  against the epiglottis.
• All of this directs the food into the esophagus,
  making sure it does not enter the nasal passage
  or respiratory tract

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The esophagus

• After being swallowed, food enters the esophagus,
  a muscular tube connecting the pharynx to the
  stomach
• Food is moved down the esophagus by rhythmic
  waves of muscle contractions called peristalsis
• Sphincters in the stomach let food in but
  prevents it from exiting

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The stomach

• It is like a sack that can fold up when empty and
  expand when filled
• Contains an extra layer of smooth muscle that
  mixes food with gastric juices (an acidic
  substance secreted by the walls of the stomach)
• Two kinds of secretory glands, parietal and chief
  cells
• Parietal cells secrete hydrochloric acid
• Chief cells secrete pepsinogen

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The Stomach cont.

• The pepsinogen a weak protein-digesting enzyme
  that is activated by the HCLs low pH
• When activated they cleave each other producing a
  more active protein- digesting enzyme called
  pepsin
• This prevents the chief cells from eating
  themselves
• Pepsin makes food proteins into shorter
  polypeptides that are digested in the small
  intestines
• The parietal cells also secrete intrinsic factor,
  a polypeptide needed for the intestines to absorb
  vitamin B12

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The Stomach Cont.

• The mixture of food and gastric juice is called
  chyme
• The acidic solution on the stomach kill most of
  the bacteria that we ingest in food
• The bacteria not killed grows and multiplies in
  the intestines
• Chyme leaves through the pyloric sphincter to
  travel to the small intestine

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The Small Intestine

• Neural and hormonal signals are used as
  communications between the stomach and the small
  intestines
• Limited room in the intestines and the digestion
  takes a long time
• Approx. 4.5 meters long in living person
• The first 25 cm is the duodenum
• The rest is divided into the jejunum and the
  ileum

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The Small Intestine cont.

• The duodenum receives chyme from the stomach
• It also takes in digestive enzymes and
  bicarbonate from the pancreas, and bile from the
  liver and gallbladder
• These juices help to digest food into smaller
  molecules
• This happens primarily in the duodenum and jejunum

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The small intestine cont.

• The inner wall of the small intestine is covered
  with villi
• The villi are covered in microvilli
• The villi and microvilli increase the surface
  area of the intestine
• They also contain digestive enzymes which help
  with digestion of disaccharides like lactose and
  sucrose

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Absorption in the small intestines

• When we digest proteins and carbohydrates, amino
  acids and monosaccharides are left
• They are transported across the microvilli and
  then across the membrane, into the blood
  capillaries within the villi
• These products are then taken into the liver by
  way of the hepatic portal vein
• Fats are absorbed in a different way
• Fats are broken down into fatty acids which are
  absorbed into the epithelial cells and then are
  reassembled into fats. The fats then combine
  with proteins called chylomicrons and then
  absorbed into the lymphatic capillaries

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The Pancreas

• A large gland near the connection of the stomach
  and small intestine
• Pancreatic fluid is secreted into the duodenum
  through the pancreatic duct
• This fluid contains many enzymes that digest
  proteins, starches, etc.
• Enter duodenum as inactive zymogens and activate
  when they come in contact with the microvilli

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The Pancreas cont.

• Pancreatic fluid contains bicarbonate
  neutralizing the HCL from the stomach. Produced
  from acini
• The pancreas also secretes hormones into the the
  blood that control the levels of glucose and
  other nutrients

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The Liver

• Secretes bile made up of bile pigments and bile
  salts that are delivered to the duodenum during
  digestion
• Bile pigments do not help in digestion! They are
  just waste products and in the end are eliminated
  with feces
• Bile salts help digest fat by making fat into
  smaller droplets
• After bile is produced in the liver it is stored
  in the gallbladder

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The Gallbladder

• When fatty foods arrive, in the duodenum, they
  trigger trigger neural reflexes
• The reflexes cause the gallbladder to contract,
  injecting bile into the duodenum

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The large intestine

• Called large because of its larger diameter, not
  length (which is shorter than the small
  intestine)
• Makes up the last meter of the digestive tract
• No digestion occurs here
• Primary function is to concentrate waste
  materials, although it does absorb some water,
  sodium, and vitamin K

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The large intestine cont.

• Undigested material is compacted and stored here
• Many bacteria live and reproduce here and are
  incorporated into feces
• Compacted feces pass from the large intestines
  into the rectum, driven by peristaltic
  contractions
• From the rectum, the feces then exit through the
  anus

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Why is it necessary?

• This system is necessary because it is our way of
  breaking down foods and getting the nutrients out
  of them. If we didnt have the digestive system
  then we would be in trouble. There would be no
  way to really eat, therefore we would starve and
  also die from the lack of nutrients. We would
  have to inject ourselves with nutrients or
  something like that.

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Interdependency

• Many parts of this system are controlled by
  neural and hormonal signals, for example the
  movement of fluids from the stomach to the small
  intestine
• Also, the secretions by the salivary glands are
  controlled by the nervous system
• Works with the endocrine and nervous systems

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Evolutionary development

• Single-celled organisms digest their food
  intracellularly while other organisms digest
  their food extracellularly in digestive
  cavities
• The most primitive digestive tract is in
  nematodes, a tubular gut lined with epithelial
  membrane.
• Earthworms have specialized systems for the
  ingestion, storage, fragmentation, digestion, and
  absorption of foods
• Vertebrates have similar specialization, more
  complex

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Diseases/Disorders

• Gastric Ulcers- overproduction of gastric acid.
  The acid eats a hole through the lining of the
  stomach
• Gallstones- formed from a hardened precipitate of
  cholesterol. These block the bile duct.
• Herpes esophagitis is a viral infection of the
  esophagus, caused by the herpes simplex virus