Digestive System_lecture I

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Gastrointestinal tract or the Digestive System

Lecture I

The gastrointestinal or digestive tract, also referred to as the GI tract or the alimentarycanal 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 7 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 cardiaStomach, which includes the antrum and pylorus and pyloric sphincter

Lower gastrointestinal tract

Bowel or intestine:

Small intestine, which has three parts:

DuodenumJejunum

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


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Related organs

The liver secretes bile into the small intestine via the biliary system, employing thegallbladder 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 secretoryorgans aid in digestion.

PHYSIOLOGY

Digestion and excretion

Food, after being mostly mechanically broken down in the mouth by the teeth andtongue, 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 onprotein, 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 usefulparticles 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, suchas 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.


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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 potentiallyharmful 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 throughwhich 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 fornourishment 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.


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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, inmammals, 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 inthe 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 upinto the esophagus, causing a burning sensation, commonly termed heartburn. Extended

exposure to heartburn may erode the lining of the esophagus, leading to a potentiallycancerous condition called Barrett's Esophagus.

Some people also experience a sensation known as globus esophagus, where it feels as ifa ball is lodged in the lower part of the esophagus.

The word "esophagus" is the result of the "o" being dropped from the typographic (oe)

in "sophagus".

Esophageal diseases and conditions:

The following are diseases and conditions that affect the esophagus:

Achalasia

Bleeding varicesChagas disease

Caustic injury to the esophagus

Esophageal cancer

Esophageal webEsophageal speech

Esophageal spasm

Esophageal strictureEsophagitis

GERD

Mallory-Weiss syndromeNeurogenic dysphasia

Plummer-Vinson syndrome

Schatzki's ring

Zenker's diverticulum


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