THE DIGESTIVE SYSTEM

Here is a video about digestion. I thought it was great!!



Which organ is the most important organ in the body? Most people would say the heart or the brain, completely overlooking the gastrointestinal tract (GI tract). Though definitely not the most attractive organs in the body, they are certainly among the most important. The 30+ foot long tube that goes from the mouth to the anus is responsible for the many different body functions which will be reviewed in this chapter. The GI tract is imperative for our well being and our life-long health. A non-functioning or poorly functioning GI tract can be the source of many chronic health problems that can interfere with your quality of life. In many instances the death of a person begins in the intestines.
The old saying "you are what you eat" perhaps would be more accurate if worded "you are what you absorb and digest". Here we will be looking at the importance of these two functions of the digestive system: absorption and digestion.
The Gastrointestinal System is responsible for the breakdown and absorption of various foods and liquids needed to sustain life. Many different organs have essential roles in the digestion of food, from the mechanical disrupting of the teeth to the creation of bile (an emulsifier) by the liver. Bile production of the liver plays an important role in digestion: from being stored and concentrated in the gallbladder during fasting stages to being discharged to the small intestine.

Mastication
Digestion begins in the mouth. A brain reflex triggers the flow of saliva when we see or even think about food. Saliva moistens the food while the teeth chew it up and make it easier to swallow. Amylase, which is the digestive enzyme found in saliva, starts to break down starch into simpler sugars before the food even leaves the mouth. The nervous pathway involved in salivary excretion requires stimulation of receptors in the mouth, sensory impulses to the brain stem, and parasympathetic impulses to salivary glands.
Swallowing your food happens when the muscles in your tongue and mouth move the food into your pharynx. The pharynx, which is the passageway for food and air, is about five inches (5") long. A small flap of skin called the epiglottis closes over the pharynx to prevent food from entering the trachea and thus choking. For swallowing to happen correctly a combination of 25 muscles must all work together at the same time. Salivary glands also produce an estimated three liters of saliva per day.

Anatomy of the Stomach

Stomach

The stomach a thick walled organ that lies between the esophagus and the first part of the small intestine (the duodenum). It is on the left side of the abdominal cavity; the fundus of the stomach lying against the diaphragm. Lying beneath the stomach is the pancreas. The greater omentum hangs from the greater curvature.
A mucous membrane lines the stomach which contains glands (with chief cells) that secrete gastric juices, up to three quarts of this digestive fluid is produced daily. The gastric glands begin secreting before food enters the stomach due to the parasympathetic impulses of the vagus nerve, making the stomach also a storage vat for that acid.
The secretion of gastric juices occurs in three phases: cephalic, gastric, and intestinal. The cephalic phase is activated by the smell and taste of food and swallowing. The gastric phase is activated by the chemical effects of food and the distension of the stomach. The intestinal phase blocks the effect of the cephalic and gastric phases. Gastric juice also contains an enzyme named pepsin, which digests proteins, hydrochloric acid and mucus. Hydrochloric acid causes the stomach to maintain a pH of about 2, which helps kill off bacteria that comes into the digestive system via food.
The gastric juice is highly acidic with a pH of 1-3. It may cause or compound damage to the stomach wall or its layer of mucus, causing a peptic ulcer. On the inside of the stomach there are folds of skin called the gastric rugae. Gastric rugae make the stomach very extendable, especially after a very big meal.

Single celled organisms and sponges digest their food intercellularly.

Other animals digest their food extracellularly within a digestive cavity.

**Specialization is when the digestive tract has a separate mouth and anus. This allows for the transport of food in one direction.

**In a flatworm the digestive tract only has one opening.

Functions of the Digestive System

1. Motility- The movement of food through the digestive tract.

• Ingestion: taking food into the mouth
• Mastication: chewing the food and mixing it with saliva. The enzyme amylase that starts the breakdown of carbohydrates.
• Deglutition: swallowing the food
• Peristalsis: the rhythmic, wavelike contractions that move the food through the gastrointestinal tract

2. Secretion- This includes both the exocrine and endocrine secretions.

• Exocrine secretions: Water, hydrochloric acid, bicarbonate and other digestive enzymes are secreted into the lumen of the gastrointestinal tract. The stomach alone, for example, secretes 2-3 liters of gastric juice a day.
• Endocrine secretions: The stomach and small intestine secrete a number of hormones that help to regulate the digestive system.

3. Digestion- Which refers to the breakdown of food molecules into their smaller subunits, which can be absorbed.

4. Absorption- This refers to the passage of the digested end products into the blood and lymph.

5. Storage and elimination- This refers to the temporary storages and the subsequent elimination of indigestible food molecules.

Liver

The various functions of the liver are carried out by the liver cells or hepatocytes:

• The liver produces and excretes bile requires for dissolving fats. Some of the bile drains directly into the duodenum, and some is stored in the gallbladder
• The liver performs several roles in carbohydrate metabolism
• Gluconeogenesis (the formation of glucose from certain amino acids, lactate or glycerol)
• Glycogenolysis (the formation of glucose from glycogen)
• Glycogenesis (the formation of glycogen from glucose)
• The breakdown of insulin and other hormones
• The liver is responsible for the mainstay of protein metabolism
• The liver also performs several roles in lipid metabolism
• Cholesterol synthesis
• The production of triglycerides (fats)
• The liver produces coagulation factors I (fibrinogen), II (prothrombin), V, VII, IX, X and XI, as well as protein C, Protein S and antithrombin
• The liver breaks down hemoglobin, creating metabolites that are added to bile as pigment
• The liver breaks down toxic substances and most medicinal products in a process called drug metabolism. This sometimes results in toxication, when the metabolite is more toxic than its precursor.
• The liver converts ammonia to urea
• The liver stores a multitude of substances, including glucose in the form of glycogen, vitamin B12, iron, and copper
• In the first trimester fetus, the liver is the main site of red blood cell production. By the 32nd weeks of gestation, the bone marrow has almost completely taken over that task
• The liver is responsible for immunological effects the reticuloendothelial system if the liver contains many immunologically active cells, acting as a 'sieve' for antigens carried to it via the portal system

Gallbladder

The gallbladder is a pear shaped organ that stores about 50 ml of bile (or "gall") until the body needs it for digestion. The gallbladder is about 7-10cm long in humans and is dark green in appearance due to its contents (bile), not its tissue. It is connected to the liver and the duodenum by biliary tract.
The gallbladder is connected to the main bile duct through the gallbladder duct (cystic duct). The main biliary tract runs from the liver to the duodenum, and the cystic duct is effectively a "cul de sac", serving as entrance and exit to the gallbladder. The surface marking of the gallbladder is the intersection of the midclavicular line (MCL) and the trans pyloric plane, at the tip of the ninth rib. The blood supply is by the cystic artery and vein, which runs parallel to the cystic duct. The cystic artery is highly variable, and this is of clinical relevance since it must be clipped and cut during a cholecystectomy.
The gallbladder has a epithelial lining characterized by recesses called Aschoff's recesses, which are pouches inside the lining. Under epithelium there is a layer of connective tissue, followed by a muscular wall that contracts in response to cholecystokinin, a peptide hormone by the duodenum.
The gallbladder stores bile, which is released when food containing fat enters the digestive tract, stimulating the secretion of cholecystokinin (CCK). The bile emulsifies fats and neutralizes acids in partly digested food. After being stored in the gallbladder, the bile becomes more concentrated than when it left the liver, increasing its potency and intensifying its effect in fats.

Pancreas

The pancreas is located posterior to the stomach and in close association with the duodenum.
In humans, the pancreas is a 6-10 inch elongated organ in the abdomen located retro peritoneal. It is often described as having three regions: a head, body and tail. The pancreatic head abuts the second part of the duodenum while the tail extends towards the spleen. The pancreatic duct runs the length of the pancreas and empties into the second part of the duodenum at the ampulla of Vater. The common bile duct commonly joins the pancreatic duct at or near this point.
The pancreas is supplied arterially by the pancreaticoduodenal arteries, themselves branches of the superior mesenteric artery of the hepatic artery (branch of celiac trunk from the abdominal aorta). The superior mesenteric artery provides the inferior pancreaticoduodenal arteries while the gastroduodenal artery (one of the terminal branches of the hepatic artery) provides the superior pancreaticoduodenal artery. Venous drainage is via the pancreatic duodenal veins which end up in the portal vein. The splenic vein passed posterior to the pancreas but is said to not drain the pancreas itself. The portal vein is formed by the union of the superior mesenteric vein and splenic vein posterior to the body of the pancreas. In some people (as many as 40%) the inferior mesenteric vein also joins with the splenic vein behind the pancreas, in others it simply joins with the superior mesenteric vein instead.
The function of the pancreas is to produce enzymes that break down all categories of digestible foods (exocrine pancreas) and secrete hormones that affect carbohydrates metabolism (endocrine pancreas).
Pancreatic juice contains bicarbonate and a wide variety of digestive enzymes. The enzymes include amylase, which digests starch, trypsin which digests protein, lipase which digests triglycerides.

Disorders of the Pancreas, Liver and Gallbladder

Disorders of the pancreas, liver, and gallbladder affect the ability to produce enzymes and acids that aid in digestion. examples of these disorders are.

Cystic Fibrosis

• Cystic fibrosis is a chronic, inherited illness where the production of abnormally thick mucous blocks the duct or passageways in the pancreas and prevents the digestive fluids from entering the intestines, making it difficult for the person with the disorder to digest protein and fats which cause important nutrients to pass through without being digested. People with this disorder take supplements and digestive enzymes to help manage their digestive problems.

Hepatitis

• Hepatitis is a viral condition that inflames a person's liver which can cause it to lose it's ability to function. Viral hepatitis, like hepatitis A, B, and C, is extremely contagious. Hepatitis A, which is a mild form of hepatitis, can be treated at home, but more serious cases that involve liver damage, might require hospitalization.

Cholecystitis

• Acute or chronic inflammation if the gallbladder causes abdominal pain. 90% of cases of acute cholecystitis are caused by the presence of gallstones. The actual inflammation is due to secondary infection with bacteria of an obstructed gallbladder, with the obstruction caused by the gallstones. Gallbladder conditions are very rare in kids and teenagers but can occur when the kid or teenager has sickle cell anemia or in kids being treated with long term medications.

Cholestasis

• Cholestasis is the blockage in the supply of bile into the digestive tract. It can be "intrahepatic" (the obstruction is in the liver) or "extrahepatic" (outside the liver). It can lead to jaundice, and is identified by the presence of elevated bilirubin level that is mainly conjugated.

Biliary colic

• This is when a gallstone blocks either the common bile duct or the duct leading into it from the gallbladder. This condition causes severe pain in the right upper abdomen and sometimes through to the upper back. It is described by many doctors as the most severe pain in existence, between childbirth and a heart attack. Other symptoms are nausea and vomiting and diarrhea, bleeding caused by continual vomiting, and dehydration caused by the nausea and diarrhea. Another more serious complication is total blockage of the bile duct which leads to jaundice, which if it is not corrected naturally or by surgical procedure can be fatal as it causes liver damage. The only long term solution is the removal of the gallbladder.

Small Intestine
The small intestine is the site where most of the chemical and mechanical digestion is carried out. Tiny projections called villi line the small intestine which absorbs digested food into the capillaries. Most of the food absorption takes place in the jejunum and the ileum.
The functions of a small intestine is, the digestion of proteins into peptides and amino acids principally occurs in the stomach but some also occurs in the small intestine. Peptides are degraded into amino acids; lipids (fats) are degraded into fatty acids and glycerol; and carbohydrates are degraded into simple sugars.

The three main sections of the small intestine is The Duodenum, The Jejunum, The Ileum.

Layers of the GI Tract
The GI tract is composed of four layers or also know as Tunics. Each layer has different tissues and functions. From the inside out they are called: mucosa, submucosa, muscularis, and serosa.

Mucosa: The mucosa is the absorptive and secretory layer. It is composed of simple epithelium cells and a thin connective tissue. There are specialized goblet cells that secrete mucus throughout the GI tract located within the mucosa. On the mucosa layer there are Villi and Micro Villi.

Submucosa: The submucosa is relatively thick, highly vascular, and serves the mucosa. The absorbed elements that pass through the mucosa are picked up from the blood vessels of the submucosa. The submucosa also has glands and nerve plexuses.

Muscularis: The muscularis is responsible for segmental contractions and peristaltic movement in the GI tract. The muscularis is composed of two layers of muscle: an inner circular and outer longitudinal layer of smooth muscle. These muscles cause food to move and churn with digestive enzymes down the GI tract.

Serosa: The last layer is a protective layer. It is composed of avascular connective tissue and simple squamous epithelium. It secretes lubricating serous fluid. This is the visible layer on the outside of the organs.




Questions

1. The digestion of protein involves all of these except:
a. pepsin from the stomach
b. chemical digestion
c. increasing surface area
d. mechanical digestion

2. The capillary networks and lacteals in the villi of the small intestine are important for:
a. absorption of nutrients
b. chemical digestion
c. increasing surface area
d. mechanical digestion

3. A high fiber diet reduce your risk of:
a. diabetes
b. diverticulitis
c. colon cancer
d. irritable bowel syndrome
e. all of the above

4. The pyloric region of the stomach secretes all of the following EXCEPT:
a. G cells
b. Chief cells
c. Goblet cells
d. Parietal cells
e. all of these are secreted in the pylorus

5. Large amounts of undigested lactose in the intestine causes which of the following:
a. diarrhea
b. cramps
c. constipation
d. brain freeze
e. a and b only

6. The other day as I sat on the toilet my rectal valves opened. As this happened, I eliminated what?
a. cellulose
b. undigested material
c. water
d. bacteria
e. all of the above

7. What is absorbed by osmosis following the absorption of minerals?
a. water
b. sodium
c. vitamin B12
d. all of the above


8.The hepatic portal takes_____to the liver that is absorbed through the small intestine.
a. amino acids
b. pancreatic enzyme
c. blood
d. proteins

9. Cholera causes diarrhea due to a chemical called?
a. celiac sprue
b. enterotoxin
c. vibrio cholera
d. bile canaliculus

10. What types of contractions occur in small intestine?
a. segmentation
b. peristalsis
c. gastroesophageal
d. both b and c
e. both a and b

11. There is a general assumption that everything we eat is absorbed through the intestines. That is not entirely true. What are the only items that are absorbed through the stomach?
a. ibuprofen and lactose
b. alcohol and aspirin
c. acetaminophen and potassium
d. breast milk and baking soda
e. water and vitamin B-12

12. Ulcers are caused by:
a. eating large amounts of spicy foods
b. stress
c. H-pylori
d. both a & b
e. all of the above

13. At the first stages of digestion, food particles are mixed with saliva, which contains the enzyme salivary amylase. What is salivary amylase used for?
a. to neutralize hydrochloric acid that enters the esophagus from the stomach
b. to break down proteins in the duodenum
c. to assist the bile as an extra surfactant
d. to catalyse the partial digestion of starch prior to deglutition
e. to sent a message to the enteric nervous system to commence digestion

14. What is the ph level of gastric juices?
a. 1-3
b. 3-6
c. 2-4
d. 8-10
e. 11-13

15. Mechanical digestion includes all of the following except:
a. chewing
b. conversion of starch to maltose
c. emulsification of fat by bile salts
d. contractions of the stomach

16. The functions of saliva include all of these except:
a. taste
b. swallowing
c. starch digestion
d. protein digestion

17. In the gastric mucosa, the parietal cells secrete:
a. hydrochloric acid
b. pepsin
c. pepsinogen
d. complete gastric juice

18. After eating my mothers home made burritos, I explained to her that when the burrito reaches my stomach secretion of my gastric juice will be stimulated by ________ which is a hormone.
a. epinephrine
b. secretin
c. gastrin
d. cholecystokinin

19. By the process of transmination, the liver synthesizes the:
a. enzymes for detoxification
b. non-essential amino acids
c. essential amino acids
d. clotting factors

20. While I was searching for the Frill-necked Lizard I felt the urge to defecate. The stimulus for the defecation reflex is:
a. contraction of the rectum
b. contraction of the internal anal sphincter
c. stretching of the rectum
d. relaxation of the internal anal sphincter

21. The cells of the liver that phagocytize pathogens are:
a. macrophages
b. Kupffer Cells
c. both a and b are correct
d. neither a nor b is correct

22. The liver is able to detoxify potentially harmful substances by means of the synthesis of specific :
a. plasma proteins
b. lipoproteins
c. steroids
d. enzymes

23. What do sodium, fatty acids, and vitamin A have in common?
a. they are not enzymatically modified prior to absorption into the bloodstream
b. they cross the apical membranes of enterocytes by simple diffusion
c. they are transported into the blood capillaries in the villi
d. they are all hydrophilic
e. all of the above