Anatomy and Physiology of the Digestive System
Let’s learn about the digestive system.
The digestive system consists of a long tube called the alimentary canal. The canal extends
from the mouth to the anus and is about 30 feet long.
The tube consists of 4 layers. There is an inner mucous secreting mucosal layer, a submucosa
that contains blood and lymphatic vessels, a muscular layer that contains 2 layers of
smooth muscle a layer that runs along the length of the tube as well as a layer that
encircles the tube. The outer serosa or serous layer secretes a slimy serous fluid.
The alimentary canal begins in the mouth. Teeth break down food into smaller pieces
in a process called chewing or mastication. Types of teeth include molars, premolars,
canines and incisors. Children have 20 deciduous or temporary teeth
while adults have 32 teeth. The uvula which is part of the soft palate
rises when swallowing. We can also see the palatine tonsils.
The salivary glands secrete mucous and salivary amylase which breaks down carbohydrates.
There are 3 pair of salivary glands. The parotids, sublinguals and submandibular glands.
The tonsils are masses of lymphoid tissue that contain white blood cells that help to
fight off pathogens. There are the phyaryngeals or adenoids, the
palatines and the lingual tonsils. The pharynx consists of 3 parts. The upper
part is called the nasopharynx, the middle section is called the oropharynx and the lower
section is the laryngopharynx. The pharynx contains constrictor muscles that function
in swallowing. The esophagus is a muscular tube containing
smooth muscle. The lower esophageal sphincter is at the bottom of the esophagus. The sphincter
opens during swallowing to let food into the stomach. An peristaltic wave progagates through
the espophagus during swallowing. The stomach consists of various parts. The
first portion is called the cardiac region, here is the fundus, the large body, and the
pylorus. The inside of the stomach consists of folds
or rugae. The rugae help to mix the food. The stomach also contains 2 curves here is
the greater curvature. Here is the lesser curvature.
The inner lining of the stomach, the Gastric mucosa contains gastric pits or tubelike openings. Gastric glands secrete substances into gastric
pits. The cells of the stomach include Parietal
cells which secrete hydrochloric acid Chief cells which secrete pepsinogen and gastric
lipase, Mucous secreting cells that secrete an alkaline
mucous and Endocrine cells secrete intrinsic factor
Pepsinogen combines with hydrochloric acid to produce pepsin which digests proteins.
Food mixes with the secretions of the stomach (called gastric juice) to form a pasty substance
called chime. The pyloric sphincter relaxes to let chime
out of the stomach and into the first portion of the small intestine called the duodenum.
The small intestine is built for absorption with a large surface area. Some special structures
include plicae circulares which are circular folds that increase the surface area and vili
which are tiny fingerlike projections containing blood vessels and lacteals which are part
of the lymphatic system. The pancreas also secretes substances into
the duodenum via the pancreatic duct. These include a pancreatic amylase for carbohydrate
digestion, a pancreatic lipase for fat digestion, trypsin, chymotrypsin, and carboxypepsidase
for protein digestion. Bicarbonate ions are also secreted by the pancreas to neutralize
the acidic chime from the stomach. The digestive enzymes work better under more alkaline conditions.
Bile is secreted into the duodenum via the common bile duct. The common bile duct ends
in a small sphincter called the hepatopancreatic sphincter.
Bile is made by the liver and stored in the gallbladder.
Fatty chyme entering duodenum triggers the release of cholecystokinin (CCK) which promotes
the secretion of bile. Bile works to emulsify fats.
Chyme then moves to the jejunum and ileum parts of the small intestine. Here most of
digestion and absorption is finished. Some intestinal enzymes include Intestinal
Nucleases, and Maltase The large intestine begins at the cecum. There
is a valve between the ileum and cecum called the ileocecal valve.
The Ileocecal valve keeps the contents from large intestine from moving back into small
intestine The cecum contains the appendix.
Appendix–may have no digestive function but functions in immunity and may act as a reservoir
for bacteria The large intestine has a different structure
than the small intestine. It contains large pouches called haustra and a long band of
smooth muscle called the taenea coli. The major parts of the large intestine include
the ascending colon, transverse colon, descending colon, sigmoid colon, rectum and anus.
The function of the Large intestine included absorbing water and vitamins (from bacterial
flora). Feces exits the large intestine when it reaches
the rectum and triggers the defecation reflex. The greater omentum is a large peritoneal
membrane in the abdominal region. It attaches to the greater curvature of the stomach.
The greater omentum is a site of fat storage, immunity, and helps to fight off infections
We hope you have learned something about the digestive system and..see you next time.