Chapter 7.
Digestive System
The purpose of digestion is to alter the foodstuff by chemical action and to convert them into simple forms that can be absorbed into the blood and used by the tissues of the body according to the body’s needs.
The process of digestion takes place in the alimentary canal and is aided by the salivary glands found in the mouth, the liver (in the right upper abdominal cavity), and the pancreas (in the left upper abdominal cavity).
Foods are dealt with in four stages: ingestion (which is taking the food into the mouth, chewing the food and swallowing), digestion, absorption, and excretion.
Ingestion or the taking in of food, is a function of the mouth, teeth, and tongue. The pharynx and esophagus are concerned with swallowing.
Digestion starts in the mouth with saliva, but is carried out mainly in the stomach which is as a J shaped bag, a large muscle sac which contains HCL and can hold about 0.5 gallons .
The liver is an accessory digestive organ that produces bile, an alkaline compound which helps the breakdown of fat. Bile aids in digestion via the emulsification of lipids. The gallbladder, a small pouch that sits just under the liver, stores bile produced by the liver which is afterwards moved to the small intestine to complete digestion.
The pancreas is a gland, having both an endocrine and a digestive exocrine function. As an endocrine gland, it functions mostly to regulate blood sugar levels, secreting the hormones insulin, glucagon, somatostatin, and pancreatic polypeptide.
As a part of the digestive system, it secretes pancreatic juice into the duodenum through the pancreatic duct. This juice contains bicarbonate (which neutralizes acid entering the duodenum from the stomach), and digestive enzymes, which break down carbohydrates, proteins, and fats in food entering the duodenum from the stomach.
Absorption can take place in any part of the alimentary canal, but most food is absorbed into the small intestine (which can be 16 – 20 ft. long). The small intestine is an organ in the gastrointestinal tract where most of the end absorption of nutrients and minerals from food takes place. It lies between the stomach and large intestine, and receives bile and pancreatic juice through the pancreatic duct to aid in digestion.
The small intestine has three distinct regions – the duodenum, jejunum, and ileum. The duodenum, the shortest, is where preparation for absorption through small finger-like protrusions called villi begins.
The jejunum is specialized for the absorption through its lining by enterocytes: small nutrient particles which have been previously digested by enzymes in the duodenum.
The main function of the ileum is to absorb vitamin B12, bile salts, and whatever products of digestion were not absorbed by the jejunum.
The large intestine (about 5 feet long) absorbs water and vitamins, and is responsible for preparing the residue for excretion in the form of feces. Most sources define the large intestine as the combination of the cecum, colon, and rectum. The colon is the largest portion of the large intestine.
Enzymes are mixed in from glands in the digestive system. The enzymes have the power to act on the food and converted into a simple form that can be absorbed and used by the body.
Excretion is the process of eliminating the remains (undigested food) through the rectum and anus.
Please watch the following videos:
- Meet the gastrointestinal tract! | Gastrointestinal system physiology | NCLEX-RN | Khan Academy (about 5 minutes)
- Stomach | Gastrointestinal system physiology | NCLEX-RN | Khan Academy (about 6 minutes)
- Liver | Gastrointestinal system physiology | NCLEX-RN | Khan Academy (about 7 minutes)
- Colon, rectum, and anus | Gastrointestinal system physiology | NCLEX-RN | Khan Academy (about 10 minutes)
Back to Introduction Ch. 6 – Respiratory System Ch. 8 – Nervous System