Content Summary:
The digestive system's primary functions are digestion and absorption of ingested food molecules to provide energy for cell respiration and tissue rebuilding. In addition to digestion and absorption, this system provides motility, secretion, storage, elimination, and an immune barrier for the body. It is divided into the tubular gastrointestinal(GI) tract (alimentary canal) and the accessory digestive organs. The GI tract has a length of approximately 9 m (30ft). The organs within the GI tract are the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine. The accessory organs are the teeth, tongue, salivary glands, liver, gallbladder and pancreas.

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Summary #1
The GI tract has four layers (tunics) that have different functions. They are:
  1. Mucosa
    1. Lines the lumen of the GI tract
    2. Consists of simple columnar epithelium
    3. Absorbs and secretes
  2. Submucosa
    1. Thick and highly vascular
    2. Consists of connective tissue
    3. Serves the mucosa
    4. Contains glands and nerve plexuses
      1. The submucosal plexus (Meissner's plexus) provides a nerve supply to the small and large intestine muscularis mucosae
  3. Muscularis
    1. Consists of an inner circular and outer longitudinal layers of smooth muscle
    2. Responsible for segmental contractions and peristaltic movement through the GI tract
  4. Serosa
    1. Outer wall of the GI tract
    2. Serves to bind and protect
    3. Consists of areolar connective tissue covered with a simple squamous epithelium layer


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Summary #2
An ulcer is a lesion of the skin or mucous membrane that is inflamed, necrotic and sloughs damaged tissue. Peptic Ulcers can occur when the mucous membranes of the stomach and duodenum become irritated and even erode due to an excess HCl. Zollinger-Ellison syndrome is the name for ulcers of the duodenum produced by excessive gastric acid secretion due to high levels of gastrin. In Zollinger-Ellison syndrome, a pancreatic tumor secretes gastrin and is considered a rare condition. Scientists have found that a bacterium called Helicobacter pylori, which is found in the GI tract of half the world's population, causes peptic ulcers. When inflammation continues, histamine is released and causes more acid secretion and turns into a condition known as acute gastritis. People who have gastric and peptic ulcers should refrain from any substance that increases acid secretion. These include coffee and alcohol. Antacids may help relieve symptoms and in Helicobacter pylori activity, antibiotics may help.


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Summary #3
There are three phases of gastric secretion. They are:
  1. Cephalic --controlled by the brain via the vagus nerves. Chief cells secrete pepsinogen and NT stimulate the secretion of HCl by the parietal cells. This phase lasts approximately 30 minutes from the start of a meal.
  2. Gastric--gastric secretion is stimulated by distension of the stomach due to chyme and also the chemical nature of chyme. This phase is regulated by a positive feedback system and a negative feedback mechanism.
  3. Intestinal --regulated by chyme entering the small intestine. The presence of fat within the chyme stimulates hormonal inhibition of gastric function by enterogastrones. Several hormones have this affect such as glucose-dependent insulinotropic peptide (GIP), somatostatin, cholecystokinin (CCK) and glucaagon-like peptide-1 (GLP-1)

Application:
As an OTA, it will be important for me to understand the workings of the digestive system in order to be sensitive to issues that may be facing my clients. They may be dealing with these issues on a secondary basis and it will be important for me to be empathetic.

On a personal note, I have had family members who have dealt with gall stones and have had their gall bladders removed. Gall stones can be very painful and cause much distress. I enjoy reading the clinical applications within the text as they usually have information that can be related to everyday life. Gall stones was one of those.

Essential Questions:
1. Two organs of the digestive system, their structure contribution to function are:
  1. Esophagus
    1. Muscular tube approximately 10 inches in length (25 cm)
      1. Upper 1/3 contains skeletal muscle
      2. Middle 1/3 contains skeletal and smooth muscle
      3. Lower 1/3 (terminal) contains smooth muscle
    2. The structure allows food to be pushed down into the stomach
    3. Food moves from the oral cavity through the esophagus by peristalsis through skeletal and smooth muscle contractions
    4. Lumen in the terminal portion is narrowed because circular muscles thicken, and is called the lower esophageal sphincter, and prevents stomach contacts from coming back up into the esophagus

  1. Stomach
    1. J- shaped and distensible
    2. Food enters the top (cardiac region) and is moved downward through the phyloric region towards the small intestine
    3. Upper region (fundus) and lower region (pyloric)
    4. Pyloric region widens--atrum and phyloric spincter
    5. Stores food, initiates the digestion of proteins, gastric juices kill bacteria and chyme is formed
    6. The structure allows food to move downward and into the small intestine
2. Carbohydrates, lipids and proteins
  1. Carbohydrates
    1. Digested in the mouth and duodenum
    2. Enzymes needed-salivary amylase, pancreatic amylase that releases oligosaccharides, maltose and maltriose
    3. Moved across the epithelial cell membrane by secondary active transport, and glucose may move passively by facilitative diffusion
    4. Used for energy
  2. Lipids
    1. Digested in the duodenum
    2. Enzymes needed-pancreatic lipase and phospholipase A
    3. Free fatty acids, monoglycerides and lysolecithin leave micelles, pass through the microvilli and enter the intestinal epithelial cells. They resynthesize triglycerides and phopholipids. Triglycerides, phospholipids and cholesterol combine with protein to form chylomicrons, which are secreted into the central lacteals
    4. Used for energy by skeletal muscles, by adipose tissue for synthesis of stored fat, cholesterol serves as a parent molecule for steroid hormones, prostaglandins regulate blood vessel diameter, ovulation, uterine contractions, inflammation reactions and blood clotting
  3. Proteins
    1. Digestion begins in the stomach, with most occurring in the duodenum and jejunum
    2. Enzymes needed--trypsin, chymotrypsin, elastase, carboxypeptidase, and aminopeptidase
    3. Free amino acids are absorbed by cotransport with Na+ into the epithelial cells, secreted into blood capillaries. Dipeptides and tripeptides enter epithelial cells by secondary active transport, hydrolyzed and secreted into the blood
    4. Used in the structure of different tissues, enzymes and antibodies are proteins and serve as receptors and carriers for specific regulator molecules

References: Human Physiology by Stuart Ira Fox andTaber's Cyclopedic Medical Dictionary