Digestion



Unit Overview:In this section you will learn about all the different parts that make up the digestive system. Also that the liver gets rid of bile through the hepatic duct system. In an adult the liver is the largest internal organ. The liver has the ability to filter out hormones and drugs. Later on you will learn about how most carbohydrates are ingested as starch.



1. Introduction to the Digestive System
The Digestive System is made up of may different organs.
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  • Oral Cavity- Teeth, Tongue, Salivary Glands
    • The lips and cheeks are involved in mastication and
      speech.
    • The tongue is involved in speech, taste, mastication, and swallowing.
    • There are 32 permanent teeth, including incisors, canines, premolars, and molars. Each tooth consists of a crown, neck, and root.
    • The roof of the oral cavity is divided into the hard and soft palates.
    • Salivary glands produce serous and mucous secretions. The three pairs of large salivary glands are the parotid, submandibular, and sublingual glands.

  • Pharynx, Esophagus
    • The pharynx consists of the nasopharynx, oropharynx, and laryngopharynx
    • The esophagus connects the pharynx to the stomach. The upper and lower esophageal sphincters regulate movement.

  • Stomach
    • The stomach has a cardiac opening from the esophagus and a pyloric opening into the duodenum.
    • The wall of the stomach consists of three muscle layers: longitudinal, circular, and oblique.
    • Gastric glands produce mucus, hydrochloric acid, pepsin, gastrin, and intrinsic factor.




  • Small and Large Intestine
    • The small intestine is divided into the duodenum, jejunum, and ileum.
    • Circular folds, villi, and microvilli greatly increase the surface area of the intestinal lining.
    • Goblet cells and duodenal glands produce mucus.
    • The cecum forms a blind sac at the junction of the small and large intestines. The appendix is a blind sac off the cecum.
    • The colon consists of ascending, transverse, descending, and sigmoid portions.
    • The large intestine contains mucus-producing crypts.
    • The rectum is a straight tube that ends at the anal canal.

Other accessory organs include the Liver, Gallbladder, and Pancreas.
  • Liver

    • The liver receives blood from the hepatic artery and the hepatic portal vein.
    • Bile leaves the liver through the hepatic duct system. The right and left hepatic ducts join to form the common hepatic duct. The cystic duct joins the common hepatic duct to form the common bile duct. The common bile duct joins the pancreatic duct and empties into the duodenum.
    • The liver is divided into lobules with portal triads at the corners. Portal triads contain branches of the hepatic portal vein, hepatic artery, and hepatic duct.
    • Hepatic cords, formed by hepatocytes, form the substance of each lobule. A bile canaliculus, between the cells of each cord, joins the hepatic duct system.
    • Branches of the hepatic artery and hepatic portal vein empty into hepatic sinusoids, which empty into a central vein in the center of each lobe. The central veins empty into hepatic veins, which exit the liver.
  • Pancreas
    • The pancreas is an endocrine and an exocrine gland. Its endocrine function is to control blood nutrient levels. Its exocrine function is to produce bicarbonate ions and digestive enzymes.
  • Gallbladder
    • The gallbladder stores and concentrates the bile. It releases bile through the cystic duct and common bile duct to the duodenum.

The GI tract is approximately 30 ft long from mouth to anus.


The digestion of food molecules involves the hydrolysis of these molecules into their subunits.
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- The digestion of food occurs in the lumen of the GI tract and is catalyzed by specific enzymes.
- The digestion products are absorbed through the intestinal mucosa and enter the blood or lymph.

The layers (tunics) of the GI tract are, from the inside outward, mucosa, submucosa, muscularis, and serosa.
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- The mucosa consists of a simple columnar epithelium, a layer of connective tissue called the lamina propria, and a thin layer of smooth muscle called the muscularis mucosa.
- The submucosa is composed of connective tissue; the muscularis consists of layers of smooth muscles; and the serosa is connective tissue covered by the visceral peritoneum.
- The submucosa contains the submucosal plexus, and the muscularis contains the myenteric plexus of autonomic nerves.

Regulation of GI Tract

-The GI Tract is stimulated by the sympathetic and parasympathetic parts of the Autonomic Nervous System.
-Autonomic regulation is extrinsic to the GI Tract is superimposed on intrinsic modes of regulation, because the GI Tract has intrinsic sensory neurons.
    • This helps in local regulation of the digestive tract by the enteric nervous system.

The Digestive System is regulated extrinsically by the Autonomic Nervous System and Endocrine system and intrinsically by the Enteric Nervous System and some paracrine regulators.

From Mouth to Stomach

Peristaltic waves of contraction push food through the lower esophageal sphincter into the stomach.
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The stomach consists of a cardia, fundus, body, and pyloris (antrum). The pylorus with the pyloric sphincter.
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- The lining of the stomach is thrown into folds, or rugae, and the mucosal surface forms gastric pits that lead into gastric glands.

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- The parietal cells of the gastric glands secrete HCl; the chief cells secrete pepsinogen.
- In the acidic environment of gastric juice, pepsinogen is converted into the active protein- digesting enzyme called pepsin.

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- Some digestion of protein occurs in the stomach, but the most important function of the stomach is the secretion of intrinsic factor, which is needed for the absorption of vitamin B12 in the intestine.

2. Liver, Gallbladder, and Pancreas 18.5
In an adult the liver is the largest internal organ in the body located directly below the diaphragm. The liver has the ability to rejuvenate itself when injured.The blood that is put back into the circulation of blood goes directly to the liver to have substances filtered out of it before going through the rest of the body. They take out the certain compounds and drain them into the intestines to go along with the bile. Some of these compound may end up in the small intestines which bring them back to the liver again. Because the liver receives venous blood from the intestine it has a lot of functions, more than another in the body. The liver takes out about 250-1500 ml of bile per day. Urobilinogen is what gives urine the amber color after it is filtered by the kidneys. When the liver makes bile acids from cholesterol, it is how there is a breakdown of cholesterol in the body. The liver is able to remove hormones, drugs, and other active molecules that are in the blood. Drugs like steroid drugs are increased in water solubility by certain enzymes in the liver, which makes it easier to bring them into the kidneys to be brought into the urine. The liver has the ability to help regulate the blood glucose concentration by either adding or taking away glucose from the blood. Globulines, which are produced by the liver, transport cholesterol, triglycerided, steroids, thyroid hormones, and inhibit blood clotting.

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The gallbladder is a sac-like organ that is attached to the bottom surface of the liver. The gallbladder is there for storage of bile from the liver. The bile takes up a yellow-green color containing bili salts, cholesterol, bilirubin, and other compounds. Behind the stomach is where the pancreas is located. It consists of both exocrine and endocrine functions. The endocrine function is to secretes insulin and glucagon into the blood. On the other hand the exocrine function is to secrete the pancreatic juice. Pancreatic juice contains about twenty different enzymes. First you have amylase that can digest starch, then you have trypsin that can digest proteins, and lastly you have lipase which has the ability to digest triglycerides.

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3. Digestion and Absorption of Carbohydrates, Lipids, and Proteins

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Digestion and Absorption of Carbohydrates
Most carbohydrates are ingested as starch. The digestion of starch begins in the mouth with the action of salivary amalyse. The digestive action of salivary amylase stops some time after the swallowed bolus enters the stomach because this enzyme is inactivated at the low pH of gastric juice.
Digestion of starch mainly occurs in the duodenum as a result of the action of pancreatic amylase. This enzyme cleaves the straight chains of starch to produce the disaccharide maltose and the trisaccharide maltriose. Maltose, maltriose, and oligosaccharides are hydrolyzed to their monosaccharides by brush border enzymes. These are located on the microvilli of the epithelial cells in the small intestine.

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Digestion and Absorption of Lipids

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Fat digestion occurs at the surface of the droplets through the enzymatic action of pancreatic lipase. The emulsification of fat aids digestion because the smaller and more numerous emulsification droplets present a greater surface area than the unemulsified fat droplets. Through hydrolysis, lipase removes two of the three fatty acids from each triglyceride molecule. The free fatty acids, monoglycerides, and lysolecithin derived from the digested lipids are more polar than the undigested lipids.






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Digestion and Absorption of Proteins

Digestion of protein begins in the stomach with the action of pepsin. Some amino acids are liberated in the stomach, but major products of pepsin digestion are short-chain polypeptides. Most protein digestion occurs in the the duodenum and jejunum. Trypsin, chymotrypsin, and elastase cleave peptide bonds in the interior of the polypeptide chains. These enzymes are grouped together as endopeptidases. Exopeptidases are enzymes that remove amino acids from the ends of polypeptide chains. The free amino acids are absorbed by contransport with Na+ into the epithelial cells and secreted into blood capillaries.

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Essential Questions:
-Choose two organs of the digestive system. Explain how the structure of that organ contributes to the function of that organ.
The liver is there to help filter the blood. It takes out the chemicals that the body does not need in the blood keeping the blood clean and healthy for the rest of the places that the blood goes to from the liver until it gets back to it.
The gallbladder is there for storage. It holds bile until it is needed.

-The three molecules, carbohydrates, proteins, and lipids, are important molecules in food that need to be physically and chemically digested by the digestive system. For each nutrient (carbohydrates, proteins, and lipids):
  • Describe where the nutrient is digested in the digestive system
  • Describe the enzymes that help to hydrolyze the molecules
  • Describe how the nutrient is absorbed
  • Describe how the body uses the molecule (nutrient)
You can use a chart to organize this answer
Molecules
Location nutrient is digested
Enzymes that help hydrolyze molecules
How nutrient is absorbed
How body uses molecule
Carbohydrates
Begins in the mouth but mainly in the duodenum
Pancreatic Alpha-Amylase; Lactase hydrolyzes the disaccharide lactose
Monosaccharides
After broken into simple sugars, enzymes have their own function; lactose is the primary carbohydrate in milk; enzymes break down things into simpler forms
Proteins
Stomach
Trypsin, chymotrypsin, elastase, and acrboxypeptidase
Amino acids which is secreted into the blood stream
Each enzyme has a different specialty. Trypsin breaks peptid bonds in amino acids
Lipids
Small intestine
Pancreatic lipase and bile salts
Free fatty acids enter epithelial cells and are resynthesized into triglycerides and phospholipids
Chylomicrons are formed and are transported in the blood and combined with apolipoprotein which allows them to bind to receptors on capillaries in muscle and fat



How does this apply to PTA:
This applies to us because if the person has a hiatal hernia we would have to be more careful not to put them on their stomach and also to try to keep their head elevated a little more. It would also be important for us to know the functions of all of these organs because for example lets say that they cut themselves accidentally and its not clotting very good or not at all, we will automatically know that the liver is what helps with the clotting factor.


1. Fox, Stuart I. "Human Physiology." New York: McGraw-Hill, 2011. Print.
2. http://highered.mcgraw-hill.com/sites/0073378119/student_view0/chapter18/textbook_images.html
3.Seeley R. ,Stephens T., Tate T. "Essential of Anatomy and Physiology third edition." New York: McGraw-Hill, 1998. Print.