BIOM 3200 Lecture Notes - Lecture 12: Gastric Acid, Small Intestine, Large Intestine
BIOM3200 – The Digestive System
The Digestive System
• The digestive system is made up of the digestive tract (a series of hollow organs joined in a long,
twisting tube from the mouth to the anus) and other accessory organs that help the body break
down and absorb food
• Organs that make up the digestive tract include:
o Mouth
o Pharynx
o Esophagus
o Stomach
o Small intestine
o Large intestine (colon)
o Rectum
o Anus
• These hollow organs are lines with a mucosa that plays an important roole in digesting and
absorbing food
• The digestive tract also contains layers of smooth muscle that contract rhythmically to help break
down food and move it along the tract
• Three of the accessory organs (salivary glands, liver and pancreas) produce digestible juices that
reach the digestive system through small tubes (=ducts)
o The gallbladder does not release its products continuously, but stores the liver’s digestive
juices until they are needed in the intestine
• Parts of the nervous and circulatory systems also play major roles in the digestive sysste,
regulating gut motility as well as secretions from the accessory glands
The Physiological Role of the Digestive System
• Digestion = breaking down food substances so the body can take them up and use and use them
to build/repair tissues, nourish cells and provide energy
• Digestion involves mixing food with juices, moving it through the digestive tract and breaking
down large molecules of smaller molecules that can be absorbed across the intestinal mucosa
• Digestion begins in the mouth, when you chew and swallow, and is completed in the small
intestine, with the large intestine mainly being involved in the recovery of water and inos
Movement of Food Through the System
• Ingestion Propulsion Mechanical Digestion (stomach) chemical digestion (stomach and
intestine) absorption (intestines) defecation
• Large, hollow organs of the digestive tract contain a layer of smooth muscle
o The movement of the walls of the digestive tract propels food and liquid through the
system and can mix the contents within each organ
o Food moves from one organ to the next through a muscle action = peristalsis
▪ Peristalsis looks like an ocean wave travelling through the muscle
o The muscle of the organ contracts to create a narrowing and then propels the narrowed
portion slowly down the length of the organ
o These waves push the food and fluid in front of them through each hollow organ
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o They also mix the food with the secretions of the digestive tract (=segmental
movements), so that maximal use is made of everything that we swallow
• Figure 18.13: Segmentation of the Small Intestine
o Simultaneous contractions of numerous segments of the intestine help mix the chyme
with digestive enzymes and mucus
• The first major muscle movement occurs when food/liquid is swallowed
o Although swallowing is initiated voluntarily, once the swallow begins, it becomes
involuntary and proceeds under the control of the autonomic nerves
• Swallowed food is pushed into the esophagus, which connects the throat above with the stomach
below
o The larynx and epiglottis close transiently, to shut off the larynx and trachea to make sure
the airways are not blocked by food travelling down the esophagus
o At the junction of the esophagus and stomach, there is a ring-like muscle (=esophageal
sphincter) closing the passage between the two organs
o As food approaches the closed sphincter, the sphincter relaxes and allows food to pass
through to the stomach
• The stomach is a highly muscular organ, with three layer of muscle (longitudinal, circular and
oblique)
o It has three main functions:
▪ Stores the swallowed food and liquid
• The muscle of the upper part of the stomach relaxes to accept large
volumes of swallowed material
▪ Mixes the food, liquid and digestive juice produced by the stomach
• Lower part of the stomach mixes these materials by muscle action
▪ Empties its contents slowly into the small intestine
• Occurs in a controlled way that will not overwhelm the digestive system
o Several cofactors affect emptying of the stomach including:
▪ Type of food
• Carbohydrates spend the least amount of time in the stomach
• Fats stay in the stomach longer than proteins
▪ Degree of muscle action
o As the food dissolves into the juices from the pancreas, liver and intestine, the contents of
the intestine are mixed and pushed forward to allow further digestion
▪ The time in transit for each meal is determined by a complex interplay between
the hormonal and neural control mechanisms affecting gastric and intestinal
motility
• The digestive nutrients are absorbed through the intestinal walls and transported throughout the
body
o Waste products of this process include undigested parts of the food (fiber) and older cells
that have been shed from the mucosa
o These materials are pushed into the colon, where they remain until the feces are expelled
by a bowel movement
Production of Digestive Juices
• Salivary glands in the mouth secrete ptyalin (alpha-amylase) in the saliva which begin to digest
the starchsugars
• Digestive glands in the stomach lining produce stomach acid and pepsin (digests proteins)
o Thick mucous layer coats mucosa of stomach and helps keep the acidic digestive juices
from dissolving the tissue of the stomach
o The stomach mucosa is able to resist the juice, allow food and tissues cannot
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