Download Chapter 3: Digestion, Absorption and Transport

Chapter 3: Digestion, Absorption and Transport
I. Digestion
(A) Anatomy: GI (Gastrointestinal) Tract is a “tube within a tube”. It’s contents can be
considered external to the body. Digestion is followed by absorption and assimilation. Digestion is
the breakdown of food, both mechanical and chemical. Absorption is into body fluids, either blood
or lymph. Assimilation is into the body’s cells.
-1- mouth: the site of mechanical digestion, *bolus formation, taste (sweet, salty, sour, bitter,
umami & water receptors), minimal chemical digestion (starch). The bolus is a discrete mass of
food and/ or drink necessary for swallowing.
-2- pharynx (throat): shared with the respiratory tract. The epiglottis seals the glottis (tracheal
opening) during swallowing.
-3- esophagus: is the tube connecting the pharynx with the stomach.
-4- stomach: the bolus becomes chyme, a soupy, acidic mixture. Mechanical digestion is
accomplished by churning, which is active muscular contractions of the stomach wall. There is
limited chemical digestion of protein. Some absorption of substances such as water, alcohol,
certain drugs. Transit time in stomach generally between 2-6 hours. Time is longer with increased
volume, and amounts of protein and fat.
-5- small intestine: three sections – duodenum, jejunum and ileum, measure approximately ten
feet long. Surrounded by blood capillaries, lymph lacteals (tiny vessels, and nerves. Receives
secretions from the gallbladder and pancreas via the common bile duct.
-6- large intestine: extends from ileocecal valve leading into the cecum (with appendix
attached), leads into colon (ascending, transverse, descending, sigmoid portions) then rectum,
opens into anus, with two sphincters. The external anal sphincter is voluntary. Site of absorption
of water, vitamins and minerals. E. coli bacteria provide Vitamins Bs and K.
(B) Muscular Action
-1- peristalsis: wave-like contractions, throughout GI tract.
-2- churning: intensified by extra layer of muscle tissue in stomach wall.
-3- segmentation: “back and forth” motion increases surface area and contact time between food
and secretions in small intestines.
-4- sphincters: regulate passage within tubes. Close when contracted. Most are involuntary.
(C) Secretions
From salivary glands, stomach, pancreas, liver to gallbladder, and small intestine. Includes
enzymes: specific catalysts promoting hydrolysis: the breakdown of chemicals by adding water,
-1- saliva: lubricates, protects (antimicrobial), forms bolus, digests starch into sugars).
-2- gastric juice: contains water, and enzyme to digest protein, HCl, intrinsic factor for Vitamin
B12 absorption. Low pH is antimicrobial and promotes initial protein digestion. Stomach walls
protected with ample mucous and rapid cell replacement.
-3- pancreatic and intestinal secretions: bicarbonate from pancreas buffers acidity; enzymes from
both digest all food chemicals.
-4- bile: produced in liver, stored, concentrated and released from the gallbladder.
-5- protective factors: resident bacteria, mostly E. coli, competitively exclude others, produce
vitamins. Various secretions are protective- e.g. saliva, mucous, HCl. Immune cells (WBCs) are
also present.
(D) Final Stage
Elimination of undigested material, water, salts and secretions.
II. Absorption
Most from the small intestine, which has a surface area of a tennis court. Transport via simple and
facilitated diffusion and active transport.
(A) Anatomy Small intestine lined with villi and microvilli to enhance surface area.
Deeper crypts with tubular glands throughout. Goblet cells secrete mucous.
(B) Intestinal Cells Membranes have transport proteins which act as pumps for digested
food chemicals. Requires energy in the form of ATP molecules.
II. Circulatory Systems
(A) Vascular System – the bloodstream.
A closed circuit of vessels surrounds the heart:
Heart to arteries to arterioles to capillaries (the exchange vessels) to venules to veins, back to
the heart.
The systemic circuit between the heart and the rest of the body to nourish all tissues.
The pulmonary circuit between the heart and lungs to oxygenate the blood.
The coronary circuit nourishes the heart muscle.
Portal Systems: passage of “dirty’ blood from one organ to another for special purposes. e.g.
hepatic portal system between the small intestine and the liver, Ingested toxins received by liver
first for detoxification.
(B) Lymphatic System
One-way, open-ended system of branching tubes. Lymph flows upward, drains into veins in neck.
Transports lipids from small intestine into blood.
IV. Regulation of Digestive Processes
(A) Neurological
(B) Hormonal: The release of hormones is customized to the meal.
-1- Gastrin from the stomach increases stomach activity.
-2- Secretin from the small intestine stimulates the pancreas.
-3- CCK from the small intestine stimulates the gallbladder.
-4- GIP from the small intestine inhibits the stomach.
V. Pathologies
(A) Choking Treatment is the Heimlich Maneuver
(B) Vomiting – reverse peristalsis.
(C) Diarrhea – rapid transit time through colon due to irritation.
(D) Constipation – slow transit time. May result in hemorrhoids (distended veins). Linked
to low fiber diet.
(E) Diverticulitis inherited. Diverticula (pouches on colon) become infected. Aggravated
with low fiber diet.
(F) Gas due to increased bacterial activity.
(G) Gastroesophageal reflux (Heartburn) - increased cancer risk.
(H) Ulcers – Duodenal (most) vs. gastric (more severe). Cause may be bacterial, treatable
with antibiotics.