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CHAPTER NINE: THE DIGESTIVE SYSTEM
I. Introduction to the Digestive System A. Overview of Digestion Digestion is the breaking down of food we eat into molecules small enough to be absorbed into the blood and used by our cells. Breaking down food can be done in two ways:
Mechanical digestion is the physical breakdown and mixing of food by physical
structures of digestion, for example: teeth. Chemical digestion is the chemical breakdown by enzyme action of large
carbohydrate, lipid, and protein molecules into smaller chemical building blocks
that are usable by body cells.
The digestive system is made up of organs responsible for this process. Most of the organs involved in the digestive system form a long tube (from mouth to anus), which is called the gastrointestinal tract or GI tract, or alimentary canal. As food travels through the GI tract, five actions of digestion take place:
Ingestion: taking food into the mouth.
Peristalsis: movement (propulsion) of food along the digestive tract.
Digestion: breakdown of food by mechanical and chemical processes
Absorption: passage of digested food components into blood circulation
Defecation: elimination of indigestible and left over food material
B. Overview of Digestive Organs The structures of the digestive system are organized into two categories: organs of the alimentary canal, and accessory digestive organs.
1. The organs of the alimentary canal (GI tract) form a continuous muscular digestive
tube about 25 - 30 feet long in a cadaver. The length in a living person is not as long
due to muscle tone. The organs of the GI tract are:
Mouth Pharynx (throat) Esophagus
Stomach Small intestine Large intestine Rectum Anus
All of these GI organs are hollow, allowing food to pass through them. The open area
that the food travels through is called the lumen. We will explore each of these organs
in headings below.
2. The accessory digestive organs contribute to digestion but do not have food move
through them structurally. The accessory organs are:
Teeth Tongue Salivary glands Gastric glands Pancreas Liver Gall bladder
The function of each of these accessory digestive organs will be discussed below.
C. Histology and Structure of the GI Tract The GI tract has 4 basic layers. From superficial to deep they are:
Mucosa - innermost layer - lines the lumen and consists of mucous membrane
made of mucus-secreting epithelial cells. It acts to help absorb nutrients, secrete enzymes, and provide protection to inner layers from acidic gastric juices and pathogens.
Submucosa - is a connective tissue binding the mucosa to the other layers; it
contains many blood vessels and lymphatic vessels to absorb nutrients into circulation.
Muscularis - consists of two layers of smooth muscle that work together to push
food through the GI tract. The inner circular muscle fibers stop food from moving backwards by constricting the width of the lumen. In some areas this circular layer is thickened and forms sphincters. Longitudinal muscle fibers push food forward and further along the tract. It is the muscularis that accomplishes peristalsis, the squeezing of food through the GI tract by sequential contraction of muscles along
the tube. In the lower pharynx and upper esophagus the muscularis is partly skeletal muscle for voluntary swallowing.
Serosa - or visceral peritoneum - is the outermost layer of the GI tract and the
inner layer of a serous membrane surrounding the digestive organs in the peritoneal cavity. It secretes serous fluid (peritoneal fluid) for lubrication and allows organs to move within the peritoneal cavity without friction (such as when you are jogging or jumping up and down).
The peritoneum is the largest serous membrane in the body. Its inner layer (visceral peritoneum) attaches to abdominal organs. The outer layer (parietal peritoneum) attaches to surrounding structures and walls of the peritoneal cavity. The peritoneum holds the organs in place within the cavity and prevents friction between organs. There are folds in the peritoneum called mesenteries that contain lots of adipose (fat) tissue for protection, cushioning, insulation and fat storage.
The term retroperitoneal refers to organs that lie posterior to the peritoneum, such as the kidneys.
D. Regulation of the Digestive System The digestive system is regulated by the autonomic nervous system and endocrine system. Regulation by the parasympathetic division of the ANS:
The rest and digest response is influenced by the feeding and satiety centers in
the hypothalamus that regulate food consumption. Via CN IX (glossopharyngeal nerves) and CN VII (facial nerves), salivary glands
are regulated. Via CN X (vagus nerves), peristalsis is stimulated as well as various exocrine
glands of the GI tract. Via sacral autonomic nerves, the muscular walls of the rectum and anus are
innervated for peristalsis and elimination of feces.
Regulation of the sympathetic division of the ANS:
The fight or flight response inhibits peristalsis and exocrine gland secretions Regulation by the endocrine system:
Hormones of the endocrine system affect the release of gastric juices, production
and release of pancreatic enzymes, and secretion of bile.
Hormones of the gastrointestinal tract include gastrin, secretin, and
cholecystokinin
II. The Chemistry of Digestion The basic nutrients in the food we eat are proteins, carbohydrates and fats; we also eat food for minerals and vitamins, as well as water. The first three must undergo chemical digestion to be absorbed while the latter don't need to be chemically broken down, just freed from food and gotten to fine particles through mechanical digestion.
A. Proteins Proteins can't be absorbed until they are broken into amino acids. There are many different bonds between amino acids so that they need a lot of enzyme action to break them down. Proteins are long chains of amino acids which must be broken into short chains called peptides. Peptides must be broken into their constituent amino acids before they are absorbed into blood capillaries through the walls of the small intestine. The type of digestive enzyme that breaks down proteins is called a protease. Examples of protease include pepsin, trypsin, and carboxypeptidase.
In summary: proteins peptides amino acids into blood capillaries.
Protein digestion begins in the stomach and is completed in the small intestine.
B. Carbohydrates Carbohydrates must be broken into monosaccharides to be absorbed. Carbohydrates are generally in the form of starch (from plants) and glycogen (from animals) when ingested. Starches are polysaccharides, very big molecules made of many small, simple sugar molecules strung together. Starches must be broken down into simple sugars to be absorbed.
Digestive enzymes will break down polysaccharides into dextrins (10 sugar subunits) and then into disaccharides (2 sugar compounds) and finally into monosaccharides (a single sugar unit) which can be absorbed through the intestinal wall into blood capillaries. The difference between sugar and starches is the length of the chain. When ingesting monosaccharides directly from food, they will be directly and quickly absorbed into the blood.
The digestive enzyme type that breaks down starch is called an amylase. The digestive enzyme type that breaks down disaccharides into monosaccharides is called a disaccharidase.
In summary: carbohydrate polysaccharides dextrins disaccharides monosaccharides into blood capillaries
Starch digestion begins in the mouth, is halted in the stomach, and resumes in the small intestine.
The following chart lists the various monosaccharides and disaccharides. Monosaccharides are: Disaccharides are:
glucose fructose galactose
maltose sucrose lactose
Pathology note: A person who is lactose intolerant does not make the enzyme lactase, which is necessary to break down the disaccharide lactose into the absorbable monosaccharide galactose and glucose. Some bacteria in the large intestine can digest the lactose and in the process produce copious amounts of gas as a waste product; this leads to abdominal bloating, discomfort, and sometimes diarrhea. It may also lead to overgrowth of bacteria in the small intestine and large intestine.
C. Fats (Lipids) Most dietary fats (lipids) are in the form of triglycerides, which are formed from three fatty acids bound to a glycerol. But depending on how many fatty acids are attached to the glycerol, products of triglyceride digestion include monoglycerides, diglycerides, and free fatty acids.
The type of digestive enzyme that breaks down lipids is called a lipase.
When fats are broken down, the resulting components are glycerol, free fatty acids, and monoglycerides (a glycerol with only 1 fatty acid attached). All of these components are then absorbed into lacteals (specialized lymphatic capillaries in the intestinal wall). In summary: lipids (triglycerides) glycerol, monoglycerides, fatty acids into lacteals (lymph capillaries)
Fat digestion begins in the mouth, but most of it occurs in the small intestine.
Note: Amino acids and monosaccharides are absorbed into the blood, but fats are absorbed into the lacteals.
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