Gastrointestinal tract or the Digestive System
Lecture I
The gastrointestinal or digestive tract, also referred to as the GI tract or the alimentary canal or the gut, is the system of organs within multicellular animals which takes in food, digests it to extract energy and nutrients, and expels the remaining waste.
The GI tract differs substantially from animal to animal. For instance, some animals have multi-chambered stomachs but humans only have one stomach, so much better.
BASIC ANATOMY
In a normal human adult male, the GI tract is approximately 7½ meters long (25 feet) and consists of the following components:
Upper gastrointestinal tract
Mouth (buccal cavity; includes salivary glands, mucosa, teeth and tongue)
Pharynx
Esophagus and cardia
Stomach, which includes the antrum and pylorus and pyloric sphincter
Lower gastrointestinal tract
Bowel or intestine:
Small intestine, which has three parts:
Duodenum
Jejunum
Ileum
Large intestine, which has three parts:
Cecum (the vermiform appendix is attached to the cecum).
Colon (ascending colon, transverse colon, descending colon and sigmoid flexure)
Rectum
Anus
Related organs
The liver secretes bile into the small intestine via the biliary system, employing the gallbladder as a reservoir. The pancreas secretes an isosmotic fluid containing bicarbonate and several enzymes, including trypsin, chymotrypsin, lipase, and pancreatic amylase, as well as nucleolytic enzymes, into the small intestine. Both these secretory organs aid in digestion.
PHYSIOLOGY
Digestion and excretion
Food, after being mostly mechanically broken down in the mouth by the teeth and tongue, and slightly chemically broken down by the saliva, passes through the esophagus by means of peristalsis to the stomach, where the process of breakdown continues, mostly mechanical, as relatively large parts of food (now called "bolus") are minimized into smaller portions, and slight amounts of chemical processing takes place, especially on protein, by the enzymes present in the stomach. It then passes to the small intestine where further breakdown occurs, by enzymes and with the aid of bacteria, and the useful particles are absorbed into the bloodstream. The remaining particles pass through the large intestine and are ultimately expelled as feces.
Digestion is regulated both hormonally and by the autonomic nervous system:
The major hormones that control the functions of the digestive system are produced and released by cells in the mucosa of the stomach and small intestine. These hormones, such as secretin, gastrin and cholecystokinin, are released into the blood by the digestive tract and stimulate digestive juices and cause organ movement.
The two arms of the autonomic nervous system both influence the digestive process; parasympathetic nerves stimulate secretions and peristalsis while the sympathetic influence is more inhibitory.
Specialization of organs
Four organs are subject to specialization in the kingdom Animalia.
¨ The first organ is the tongue, which is only present in the phylum Chordata.
¨ The second organ is the esophagus. The crop is an enlargement of the esophagus in birds, insects and other invertebrates that is used to store food temporarily.
¨ The third organ is the stomach. In addition to a glandular stomach (proventriculus), birds have a muscular "stomach" called the ventriculus or "gizzard." The gizzard is used to mechanically grind up food.
¨ The fourth organ is the large intestine. An outpouching of the large intestine called the cecum is present in non-ruminant herbivores such as rabbits. It aids in digestion of plant material such as cellulose.
Immune function
The gastrointestinal tract is also a prominent part of the immune system. The low pH (ranging from 1 to 4) of the stomach kills many microorganisms that enter it. Similarly, mucus (containing IgA antibodies) neutralizes many of these microorganisms. Other factors in the GI tract help with immune function as well, including enzyme in the saliva and bile enhancing intestinal bacteria serve to prevent the overgrowth of potentially harmful bacteria in the gut.
The whole digestive system comprises of:
Mouth - Pharynx - Crop - Esophagus - Stomach - Pancreas - Gallbladder - Liver - Small intestine (duodenum, jejunum, ileum) - Colon - Cecum - Rectum – Anus
MOUTH
The mouth, also known as the buccal cavity or the oral cavity, is the opening through which an animal or human takes in food.
The human mouth is covered by an upper and lower lip. They play an important role in speech, facial expression, kissing, drinking (especially with a straw), and smoking. Infants are born with a sucking reflex, by which they instinctively know to suck for nourishment using their lips and jaw. Lips are often adorned with lipstick or lip gloss. The philtrum is the vertical groove in the upper lip, formed where the nasomedial and maxillary processes meet during embryo development. When these processes fail to fuse fully, a hare lip and/or Cleft palate can result.
According to etiquette the mouth is kept closed, especially when chewing.
ESOPHAGUS
The esophagus (also spelled oesophagus/esophagus), or gullet is the muscular tube in vertebrates through which ingested food passes from the mouth area to the stomach. Food is passed through the esophagus by using the process of peristalsis. Specifically, in mammals, it connects the pharynx, which is the body cavity that is common to the digestive system and respiratory system behind the mouth (buccal cavity), with the stomach, where the second stage of digestion is initiated (the first stage of digestion is in the mouth, with teeth and tongue masticating food and mixing it with saliva).
The esophagus is lined with mucous membrane, and is more deeply lined with muscle that acts with peristaltic action to move swallowed food down to the stomach.
The junction between the esophagus and the stomach is not actually considered a valve, although it is sometimes called the cardiac valve, cardia or cardias, but is actually more of a stricture. Many people experience acid reflux, where stomach acid gets pushed up into the esophagus, causing a burning sensation, commonly termed heartburn. Extended exposure to heartburn may erode the lining of the esophagus, leading to a potentially cancerous condition called Barrett's Esophagus.
Some people also experience a sensation known as globus esophagus, where it feels as if a ball is lodged in the lower part of the esophagus.
The word "esophagus" is the result of the "o" being dropped from the typographic œ (oe) in "œsophagus".
Esophageal diseases and conditions:
The following are diseases and conditions that affect the esophagus:
Achalasia
Bleeding varices
Chagas disease
Caustic injury to the esophagus
Esophageal cancer
Esophageal web
Esophageal speech
Esophageal spasm
Esophageal stricture
Esophagitis
GERD
Mallory-Weiss syndrome
Neurogenic dysphasia
Plummer-Vinson syndrome
Schatzki's ring
Zenker's diverticulum