The Digestive System
Functions
Enzymatic hydrolysis- the breakdown of food by the chemical action of enzymes
Absorption- movement of small, diffusible molecules into the bloodstream so they can enter cells
Mechanical breakdown of food- where food is moved around by the tongue and broken up into smaller pieces by the teeth
Layers
Submucosa- contains blood vessels, nerve fibers, lymphatic vessels, and lymph nodes
Muscularis externa- helps move food along the digestive tract and helps mix digestive contents into smaller pieces
Mucosa-absorption of digestive foods, and protection against foreign invaders
Serosa- made of up connective tissue and binds to organs to the surrounding tissues
Mouth
Salivary glands- secrete saliva. One of the first enzymes that starts the breakdown of food is produced here.
3 salivary glands- parotids, submandibulars, sublinguals
First area where food enters.
Teeth- consists of 3 layers (crown, dentin, pulp). The teeth are responsible for cutting and tearing food.
Esophagus
Moves food from the mouth to the stomach
It moves food by peristalsis, which is a series of wave-like contractions
Stomach
Fundus- dome-shaped region between the cardiac region and the body
Body-forms the central and largest region of the stomach
Cardiac region- first narrow part of stomach where food enters from the cardiac sphincter
Pylorus- narrow portion at the lower end of the stomach and contains the pyloric sphincter. This sphincter regulates passage from the stomach to small intestine.
The stomach breaks down food through chemical and mechanical action.
Small Intestine
Stomach Gastric Juice
Parietal cells- produces intrinsic factor and hydrochloric acid. Intrinsic factor binds to B12 in the stomach allowing B12 to be absorbed by the small intestine
Chief cells- they produce the enzymes pepsinogen and gastric lipase. Pepsinogen is activated by pepsin by the action of HCl which then breaks down proteins. Gastric lipase breaks down fats.
Mucous cells- mucus is needed to coat the stomach to protect it from the high acid content and enzymes
G cells- release the hormone gastrin which increases the activity of stomach acid.
Gastric juice is made up of mucus, hydrolytic enzymes, and hydrochloric acid (HCl)
Jejunum- section where food moves at a slower pace so it can undergo absorption
Ileum- last section of small intestine where more absorption occurs.
Duodenum- receives the acid chyme from the stomach. Most of the enzymatic action of food is done here.
Villi- fingerlike projections on the inside of the intestine
Microvilli- furthur projections along the villi which gives even more surface area for absorption
Plicae circularis- deep folds of the mucosal and submucosal layers that force the liquid chyme to spiral through the intestine
The main function of the small intestine is absorption
Large Intestine
Cecum-first sac like structure from the small intestine and has the vermiform appendix hanging off of it. The large intestine contains bacteria that are good for us. The bacteria produces Vitamin B12 and Vitamin K.
The colon circles the small intestine and starts with the ascending colon on the right side. Then goes through the splenic flexure to the descending colon, then sigmoid colon, then rectum, then anus.
Once food has reached the large intestine it is mostly unabsorbable, useless material as most of the nutrients have been taken out in the small intestine.
Accessory Organs
Gallbladder- secretes bile into a duct. It is stored until stimulated by a hormone to release it into the duodenum.
Pancreas- secretes pancreatic juices that contain the majority of the enzymes used in digestion.
Liver- ability to regenerate dead or damaged tissues.Cells in liver produce bile. Bile salts break down fats we eat into the size needed for absorption.
All of these accessory organs release their fluids into the duodenum through a series of ducts.
Peritoneum and Mesenteries
Mesentery- connective tissue that holds the small intestine in numerous folds in the abdomen so they will fit.
Mesocolon- helps hold the large intestine in place.
Lesser Omentum- this layer helps hold the stomach and the duodenum below the liver.
Falciform ligament- helps to attach the liver to the diaphragm and the umbilicus.
Greater Omentum- this layer of fat and connective tissue lies over the entire abdominal cavity.
The peritoneum lines the entire abdominal cavity which is made up of two layers: the parietal peritoneum and the visceral peritoneum.
Pathway through the Digestive Tract
Mixing and Peristalsis: alimentary canal contains circular and longitudinal muscle layer. Muscles move nutrients along digestive tract
The esophagus: Peristalsis occurs here
Esophageal phase: involuntary. Peristalsis moves food to lower esophageal sphincter. When food gets down to lower end of esophagus, sphincter relaxes & food enters stomach
The stomach: Both types of motility occur here
Pharyngeal phase: involuntary. Nerve receptors trigger the uvula and soft palate to lift up closing off the nasopharynx. Upper esophageal sphincter is relaxed so food can move into the esophagus.
The small intestine: Both types of motility occur here
Voluntary phase: where you have the conscious control and push the food to the back of the throat or oropharynx
The large intestine: Peristalsis occurs here
Deglutition is the process of swallowing which consists of 3 phases:
Defecation: when fecal matter reaches the rectum, stretching signals nervous stimulation for the defecation reflex
Digestion
Mechanical: Physical breakdown of foods and occurs in the mouth and the stomach.
Chemical: Uses chemicals to break down large macromolecules into smaller molecules for absorption. Begins in the mouth with saliva, continues to the stomach with HCl and enzymes, then most of the chemical digestion occurs in the small intestines.
Mouth- Amylase: carbohydrates are the foods that this enzyme works on
Stomach- Gastric Juice
Pepsin/Pepsinogen: breaks down proteins
Gastric Lipase: breaks down small amounts of fat
Pancreas- Pancreatic Juice
Pancreatic amylase: breaks down carbohydrates
Pancreatic lipase: breaks down fats
Trypsinogen/trypsin: breaks down proteins that made it through the stomach
Liver- Bile Salts: responsible for emulsification of the acid chyme
Gallbladder- Concentrated Bile: during storage of bile, it concentrates it
Small Intestine
Sucrases: break down sucrose into monosaccharides
Lactases: break down lactose into monosaccharides
Peptidases: break peptides down into individual amino acids
Maltases: break down maltose into monosaccharides
Hormones
Cholecystokinin: produced by the enteroendocrine cells. Stimulates the pancreas to secrete enzymes into the duodenum and stimulates the gallbladder to empty
Secretin: produced by enteroendocrine cells. Stimulates the pancreas to release digestive juice that is rich in bicarbonate
Gastrin: hormone produced in the stomach by G cells. Stimulates acid secretion which helps to move things along