The Digestive System

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The Digestive System
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The Hierarchy in Biology

• Large organisms have many cells and are more
  complex, and therefore they must specialize, to
  do a more specific function
• Cells that have similar functions are grouped
  together to form tissues
• There are four main tissues that make up the
  human body connective tissue, epithelial tissue,
  nervous tissue, and muscle tissue
• Atom lt molecule lt organelle lt cell lt tissue lt
  organ lt organ system

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Building a Digestive System

• An organ consists of several types of tissues
  that come together and coordinate to perform one
  overall function.
• Examples of organs within the digestive system
  are the stomach, liver and small intestine.
• An organ system is a group of several organs that
  work together to perform a vital body function.
• Bodies are made of several organ systems which
  work together in order to maintain the internal
  environment (homeostasis)

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Digestive System
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What does the digestive system look like?

• Also called the gastrointestinal tract of the
  alimentary canal.
• Looks like a tube that is open at both ends.
• Muscular tube from mouth to the anus
• The inner surface, called the lumen is continuous
  with the outer surface of the body.
• It also includes some accessory organs salivary
  glands, pancreas, liver, and gall bladder.
• Begins with the oral cavity and goes to the
  mouth, pharynx esophagus, stomach, small
  intestine, large intestine, and finally the anus.

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What are the structures of the digestive system?

• The inner layer of the small intestine is folded
  into ridges and has many small finger-like
  projections called villi that help to increase
  the surface area.
• Each villus is covered in even smaller,
  microscopic microvilli.
• Each villus has a network of tiny blood vessels
  called capillaries where all nutrients except
  fats are absorbed into the bloodstream
• Each villus also has a lacteal vessels which is
  where digested fats are absorbed and they travel
  through the lymphatic system and then into the
  bloodstream.

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Layers of tissue making up the lumen
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The Steps in Digestion

• The main function of the digestive system is to
  change the food we eat into chemical forms that
  our body can use.
• We start by ingesting our foods.
• Two types of digestion mechanical and chemical.
• Mechanical when food masses are bitten (using
  our incisors) torn, shredded (using our canine
  teeth), ground (using our molars), shaken (in
  stomach)
• Chemical happens when food is in small pieces
  (high surface area). Food is mixed with chemical
  juices from digestive glands and then enzymes act
  on the broken-down food

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The Steps in Digestion continued

• Next, we absorb our food. Usually done by the
  small intestine. The large intestine generally
  only absorbs water and vitamins.
• Elimination is what happens through the rectum
  and anus.
• The nervous and endocrine systems help the
  digestive system by providing impulses and
  hormones that target and stimulate the digestive
  organs and glands.

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The Organs of the Digestive TractThe Oral Cavity

• Takes in the food and dismantles it
• Lips, tongue, teeth and jaw muscles.
• The tongue manipulates and moves the food around
  in the mouth.
• Saliva contains an enzyme salivary amylase,
  which begins the chemical digestion of
  carbohydrates.
• At the end, right before swallowing, the food is
  called a bolus
• Passes through the pharynx, over the epiglottis
  (trap door that prevents food going into the
  trachea), into the esophagus.

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The Esophagus

• Muscular tube connecting the pharynx and the
  stomach
• Ring of smooth muscle just before the stomach
  called the lower esophageal sphincter or cardiac
  sphincter (when constricted, it prevents a reflux
  of food going back up the esophagus)
• Peristalsis is a series of coordinated muscular
  contractions that help move the food through the
  tract not by gravity. Is a coordinated action
  between circular and longitudinal muscle layers
  and makes a wave of constriction

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The Stomach

• J-shaped and stretchable that can hold about 1.5L
  of food
• The pyloric sphincter controls the passage of
  food in the stomach to the small intestine
• The stomachs muscular walls turn and squeeze
  bolus
• Thick layers of smooth muscle and the many folds
  in the stomach (called rugae) allow the stomach
  to expand
• Gastric glands in the stomach lining contain
  cells that secrete hydrochloric acid (can
  sterilize the upper digestive tract and destroys
  invading microbes) and other cells that secret
  pepsinogen that changes into pepsin (an enzyme
  that breaks down proteins)

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The Stomach

• Mucus is secreted which lines the stomach,
  forming a protective coating against the
  corrosive effects of hydrochloric acid.
• Another enzyme called rennin is found in children
  that helps to slow down the breakdown of milk, so
  they get more absorption of nutrients.
• At the end, the stomach changes the bolus into a
  paste called chyme and sends it to the small
  intestine through the pyloric sphincter.
• No absorption happens in the stomach
• Ulcers are common disorders that can happen when
  the hydrochloric acid creates a hole in the
  lining of the stomach caused by excessive
  hydrochloric acid secretions.
• Some ulcers may be caused by the bacterial
  infection Helicobacter pylori. This causes a
  loss of the mucosal lining as well, but can be
  cured with antibiotics.

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The Small Intestine

• Major site of digestion and absorption
• 3 sections duodenum, jejunum, ileum
• chyme that enters is further mechanically
  digested through segmentation movements
• Intestinal juices are from the intestinal glands
  as well as the accessory glands pancreas and
  liver

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The Small Intestine

• The ileocaecal valve separates the small
  intestine from the large intestine.
• Small intestine is smaller in diameter, but is
  actually a lot longer than the large intestine.
• Absorption used both active and passive transport
  in order to get through the walls of the small
  intestine.
• The lining has finger-like extensions called
  villi.

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The Large Intestine

• 4 parts caecum, colon, rectum, and anus
• Divided into ascending, transverse, descending,
  and sigmoid regions
• Absorbs water, salts, some vitamins
• Jutting out from the caecum is the appendix
  (function is debatable probably a part of the
  lymphoid system

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• Also assembles some vitamins because of the
  micro-organisms that are present.
• Wastes are kept here as the body prepares for
  elimination
• Defecation is controlled by two anal sphincters
  and usually occurs once or twice a day in humans.
  Feces are about 75 water and 25 solid.
• Constipation is a common problem of the digestive
  tract. Laxatives and fibres can help cure it.

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Digestive Accessory Organs
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Salivary Glands

• Secrete saliva
• Two types of glands
• Parotids secrete watery fluid that contains
  salivary amylase (that breaks down starch)
• Buccal secrete mucus

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Liver and Gall Bladder

• Liver is second largest organ in the body (after
  skin)
• Liver makes the bile (bile salts, bile acids,
  cholesterol, phospholipids, fatty acids, water)
• Two large lobes
• In a recess under the right lobe is the
  gallbladder (muscular sac that stores bile)

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• When fats enter the duodenum, endocrine cells
  release a hormone CCK into the blood. CCK
  causes the gallbladder to contract and sends bile
  through the bile duct into the duodenum.
• Liver regulates metabolism (all blood leaving the
  intestines flows through liver first so that it
  can take out any toxins)
• Toxins are decomposed by catalase enzyme
• The body recognizes alcohol as a toxin and
  therefore tries to eliminate it from the body.
• After a long time of alcohol abuse, it can cause
  cirrhosis of the liver. Cirrhosis is when the
  tissue is scarred and hinders blood flow and
  disrupts the homeostasis.

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The Pancreas

• Lies behind the stomach and extends from the
  duodenum
• Finger-shaped and about 15cm long.
• Chyme leaving the stomach is acidic, and must be
  neutralized.
• Once some acid is released into the duodenum, a
  hormone called secretin is released and tells the
  pancreas (through the blood stream) to release
  bicarbonate ions that will neutralize and make it
  slightly basic pH of 9.
• The basic environment causes the pepsin (from
  stomach) to become inactive.
• Pancreas also secretes lipases (break down
  lipids), carbohydrases (digest sugars and
  starches) and proteases (proteins).

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Pancreas continued

• An example of homeostasis also occurs in the
  pancreas. The regulation of blood sugar levels
  is controlled by Negative Feedback which can be
  broken into 3 steps
• A variable is identified that has changed in the
  body, and is either falling below or rising above
  its normal range or set point.
• Receptors detect the change and signal other
  parts of the body to respond in order to restore
  the steady state.
• Organs receive the signal and respond.
• We do not want our blood sugar to go lower, or
  higher than 4-6 mmol/L (set point). After a
  meal, we have a higher blood glucose level, which
  is detected by receptor cells. These receptor
  cells then stimulate the release of insulin from
  the beta cells in the pancreas. Insulin helps
  the body cells take in the glucose and the levels
  decrease in the blood.
• If we fall below our set point for blood sugar
  levels, the hormone glucagon from the alpha cells
  in the pancreas stimulate reactions for the liver
  to release energy (glycogen).

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Human Digestive Enzymes

• Enzymes are proteins that speed up reactions.
• Produced by two types of glands those with ducts
  (pancreas and salivary) and those without
  (stomach and small intestine)
• Carbohydrate digestion begins in the mouth with
  salivary amylase (breaks down starch into
  disaccharides) and then finishes in the duodenum.
• Pepsin breaks down proteins into shorter amino
  acid chains in the stomach

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Digestive enzymes continued

• Pancreatic enzymes work in a basic environment
  (hence the bicarbonate ions)
• Pancreatic Juice is a mixture of several enzymes
  proteases, trypsin (completes protein digestion
  by breaking into single amino acids), pancreatic
  amylase, lipases, nucleases

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Bile

• Has 2 functions
• Emulsifies fat by breaking down into small
  globules
• Bile salts help the absorption of lipids from the
  intestine.
• Secreted by the gallbladder
• Gallstones caused when large amounts of water are
  absorbed from the bile, leaving behind solids.

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Endoscopy

• Tiny camera with a light that is inserted through
  the mouth to look at the upper GI tract.
• Can also have a colonoscopy, which is inserted
  through the anus and looks at the lower GI tract.