Chapter 23 - The Digestive System Flashcards
General Anatomy of the Digestive Tract
A long tube to which a series of accessory glands are attached.
Regions of the Digestive Tract
There are 5 regions:
Mouth, esophagus, stomach, small intestine, large intestine.
Layers of the Digestive Tract
Tunica Mucosa
Tunica Submucosa
Tunica Muscularis
Tunica Serosa
Tunica Mucosa
The membrane that lines the lumen. It contains stratified squamous epithelium and around the stomach changes to simple columnar epithelium. It has some areolar tissue and smooth muscle. Functions in secretions, absorption of food products into the blood, and protection against infections.
Tunica Submucosa
A thick layer of either dense or areolar connective tissue. Has a vascular network, blood vessels, lymphatics, nerves, and glands. Function to carry away absorbed nutrients.
Tunica Muscularis
A double layer or smooth muscle. The inner layer has fibers arranged in circles. The outer layer’s fibers are arranged longitudinally. The circular layer sometimes forms a valve, a sphincter, to propel and mix food.
Tunica Serosa
Connective tissue, called the visceral peritoneum. In the esophagus, it’s called adventitia.
Intrinsic Nerve Plexuses
Nerve complexes found between the tunics.
Accessory Glands
The wall of the tract contains a large number of glands, there are three glands that lie outside the tract and dump their secretions into the tract via ducts.
Salivary Glands
3 pairs of glands. Produce saliva which enters the mouth. The parotids, the submandibular, and the sublinguals.
Parotid Salivary Glands
Largest of the pairs, located beneath the ears. If they become inflamed, it’s called the mumps.
Submandibular Salivary Glands
Lie medial to the angle of the mandible
Sublingual Salivary Glands
On the floor of the mouth, beneath the tongue. Has 2 kinds of secretions: serous, or watery, and mucus secretions which are viscous due to mucin.
Pancreas
Large gland is located beneath the stomach. It produces pancreatic juice which contains many digestive enzymes, by the acini cells.
Liver
The largest organ in the body, it’s divided into left and right lobes. It has cords and cells that radiate outward from the center. It has a capillary network of sinusoids. It has a falciform ligament that separates the right and left lobes.
Bile Production
The hepatocytes in the liver secrete bile into the hepatic duct.
Function of the liver
-production of albumin and other plasma proteins
-manufactures prothrombin and other blood clotting factors
-regulates carbs, proteins, and lipid metabolism
-synthesizes and stores glycogen
-produces waste products like urea and ketone bodies
-stores iron, vitamin A,D,K and E.
-detoxifies most materials toxic to body cells
plays a role in body defense
Segmentation
Stationary contractions along the digestive tract. They mix the food
Peristalsis
Wave-like contractions to move material down the tract
Control of Flow
The directional movement of materials throughout the tract is regulated by a series of valves that are often referred to as sphincters.
Mouth and Pharynx
The mouth and oral cavity are the first parts of the tract. They extend from the lips to the oropharynx. The digestive activities include:
- analysis/ingestion
- mastication
- salivation
Analysis/Ingestion
Analyze the food before it’s swallowed
Mastication
Chewing, breaking down into smaller sizes to provide a greater surface area for enzymes to work on.
Tongue
Has glands that secrete an enzyme, lingual lipase, which breaks down lipids. Also has taste buds and papillae. Has intrinsic muscles which allow it to change it’s shape, and extrinsic muscles which change the position of the tongue. It’s held to the floor of the mouth via the lingual frenulum.
Salivation
While chewing, saliva is produced.
Composition of Saliva
About 97% water. Has mucin, electrolytes, and salivary amylase.
Swallowing
The food bolus is forced into the pharynx by the tongue. Series of reflexes close off other opening into pharynx. Esophagus opens, food bolus is forced in, peristaltic waves move the bolus down the esophagus to the stomach.
Esophagus
A muscular tube that goe from the pharynx to the stomach. About 25cm long. The upper part has skeletal muscle which transitions into smooth muscle. The skeletal muscle helps in swallowing. Where the esophagus meets the stomach is called the cardiac orifice or z-line, and the epithelium changes from squamous to columnar.
Rugae
Large, long folds that are present when the stomach is empty.
Functions of the Stomach
- Stores food and slowly releases it to the intestine
- Mixes food with the stomach secretions, resulting in liquefaction.
- Protein digestion begins in the stomach.
- Production of intrinsic factor, a glycoprotein which must be present for the absorption of vitamin B12.
Gastric Secretions
The stomach produces several different substances which are collectively termed gastric juice. The epithelium that lines the stomach is a simple columnar one made up entirely of goblet cells that secrete an alkaline mucus that coats and protects the stomach from digestion. Gastric juice is produced by the gastric glands.
Mucus Neck Cells
Found at the top of the glands, they secrete acidic mucus.
Parietal Cells
Produce HCl and intrinsic factor.
HCl
Speed up the breakdown of protein by activating pepsinogen. Provides an appropriate pH for pepsin, which is what breaks down proteins. Kills microorganisms, and stimulates the release of pancreatic juice.
Chief (Zygomatic) Cells
Produce pepsinogen
Enteroendocrine Cells
Produce hormones
Rennin
An enzyme secreted by gastric glands in infants. It works on milk protein, casein.
Control of Gastric Secretions
The stomach produces up to three liters of gastric juice per day. Secretion is under both neuronal and hormonal control.
Phases of Secretion
Cephalic. Gastric. Intestinal.
Cephalic Phase
This is stimulated by vagal activity to the stomach. The trigger can be the sight, smell, or taste of food.
Gastric Phase
The trigger for this is distention of the stomach, increase in gastric pH, and the presence of proteins in the stomach.
Intestinal Phase
This occurs when acid chyme from the stomach begins to distend the duodenum.
Functions of the Small Intestine
Digestion - this is where the bulk of digestion takes place. Absorption - this is where the majority of absorption takes place. Include organic nutrients, vitamins, minerals, and water.
Duodenum
The first 25 cm of the small intestine. It also receives secretions from the liver and the pancreas. It’s thick, non-moveable, c-shaped, and comparatively short. It has Brunner’s glands in the submucosa which produce an alkaline mucus.
Jejunum
The middle part, 2.5m long, majority of absorption happens here.
Ileum
The last part, 3.5m long. Has Peyer’s patches, which protect the blood stream from microorganisms.
Digestive Juices
The small intestine receives three major digestive juices. Collectively these three contain all the enzymes and substances which are necessary for the breakdown and absorption of all food categories.
Pancreatic Juice
Composed of digestive enzymes; proteases, lipases, amylases, carbohydrases; water, and bicarbonate which has a pH of 8 because it neutralizes the HCl that comes from the stomach.
Secretin
Stimulates the release of bicarbonate
Cholecystokinin
Stimulates the release of enzymes, mainly responds to proteins and fats.
Carb Breakdown
Salivary amylase in the moth works on starch. Brush border enzymes like maltase, sucrase, and lactase.
Protein Breakdown
Have to be cleaved by trypsin and chymotrypsin which are secreted by the pancreas.
Lipid Breakdown
The small intestine is the sole site of lipid digestion by lipases.
Nucleic Acid Breakdown
DNA and RNA are hydrolyzed by pancreatic nucleases.
Bile
Bile is released into the duodenum via the common bile duct, or is stored in the gallbladder. The bile duct and the pancreatic duct together form the hepatopancreatic ampulla.
Composition of Bile
The principle components of liver bile are water, electrolytes, and bile salts. Also, cholesterol, phospholipids, bilirubin, and biliverdin.
Function of Liver Bile
Emulsifies fats, helps in absorption of fat soluble vitamins, aids in calcium absorption, and eliminates excess hemoglobin and cholesterol.
Gall Bladder
A sac-like organ whose major function it is to store bile
Obstructive Jaundice
Bile is a major route for cholesterol excretion. If your bile salts or lecithin are inadequate, cholesterol starts to crystallize, forming gallstones which obstruct the flow of bile. A yellow pigment accumulates in the blood and shows on the skin.
Intestinal Juice
This juice is produced by the cells of the mucosae, crypts of Lieberkuhn, and the Brunner’s glands which are located in the submucosa of the duodenum.
Absorption
Occurs across the epithelial cells that line the mucosa of the small intestine. Mostly in the jejunum where the plicae and the microvilli are best developed.
Intestinal Villi
Microscopic finger-like projections that cover the mucosa, they increase the surface area.
Peyer’s Patches
Nodules of lymphoid tissue which protect the small intestine against bacteria from the large intestine.
Absorption of Carbs
Absorbed across the villus by protein carriers
Absorption of Proteins
Coupled with sodium and broken down into amino acids and taken across the membrane
Absorption of Fatty Acids
Associated with micelles, they then enter the epithelium by diffusion. They will then be resynthesized into triglycerides which combine with proteins to form chylomicrons and then migrate to the lymph.
Absorption of Pentose Sugars
Moves across by active transport
Absorption of Vitamins
Usually are by passive diffusion along with dietary fats, B12 binds to the intrinsic factor.
Anatomy of the Large Intestine
About 1.5m long, extends from the ileocecal valve to the anus. It has 6 different regions. The cecum, the ascending colon, the transverse colon, the descending colon, the sigmoid colon, and the rectum.
Function of the Large Intestine
Reabsorption of water, synthesis of vitamins, and elimination of wastes.
Movements of the Large Intestine
Sluggish peristalsis - the slow and wave-like movements taking 18-24 hrs to move through.
Mass peristalsis - the rapid and forceful waves, called haustral coiling.
Vomiting
A complex movement, a reflex coordinated by the vomiting center on the medulla. It’s a reverse peristalsis.
Diarrhea
Fecal matter becomes very watery. Due to over secretion of fluids, usually by an intestinal infection or food poisoning.
Constipation
A reduced mobility of the large intestine. Could be caused by the colon reabsorbing too much water, diet, lack of exercise, or a nervous upset.
Peptic Ulcer
An erosion of the mucosa of the stomach or small intestine by acid and digestive enzymes. Erosion may continue into the blood vessels causing a bleeding ulcer.
Hepatitis
Inflammation of the liver, usually due to a virus
Cirrhosis of the Liver
A chronic inflammation, due to alcoholism, severe hepatitis, etc. It shrinks and forms scar tissue on the liver.
Effects of Aging
There is a decrease in secretion and a loss of motility with age. Digestion takes longer and food remains in the tract for extended periods.
