Second Entry

For today's biology Journal, I will be talking about the human's digestive system in depth. For many of us if we remember, we have already got to know about the human's digestive system in primary school. However, I will be going further and more in depth.

The above picture shows the Human Digestive Tract.

OVERVIEW

The digestive system is made up of a series of hollow organs joined in a long, twisting tube from the mouth to the anus together with the other organs that help the body break down and absorb food (see figure).

Inside these hollow organs is a lining called the mucosa. The mucosa contains tiny glands that produce juices to help digest food. The digestive tract also contains a layer of smooth muscle that helps break down food and move it along the tract.

Two “solid” digestive organs, the liver and the pancreas, produce digestive juices that reach the intestine through small tubes called ducts. The gallbladder stores the liver’s digestive juices until they are needed in the intestine. Parts of the nervous and circulatory systems also play major roles in the digestive system.

PARTS AND FUNCTIONS IN THE DIGESTIVE SYSTEM

Mouth and Tongue

The mouth is the first portion of the alimentary canal that receives food and begins digestion by mechanically breaking up the solid food particles into smaller pieces and mixing them with saliva. During chewing, the tongue moves food about and manipulates it into a mass called a bolus. The bolus is pushed back into the throat and is forced through the opening to the oesophagus

Pharynx

Pharynx describes the part of the throat that begins from behind the nose to the beginning of the voice box and the oesophagus. The nasal airway and the food passage share the same pharyngeal passage to conduct air and food. After passing the pharynx, the airway goes into the lungs while food goes into the esophagus. It blocks food from entering the windpipe preventing us to choke.

Oesophagus

The oesophagus is a muscular tube in the chest that connects the mouth and throat to the stomach. Muscular contractions and relaxations will enable food to propel forward. When the food reaches the sphincter, the sphincter will open up and allow food to pass it and into the ctomach where further digestion take place.

Stomach

In most mammals, the stomach is a hollow, muscular organ of the digestive system, between the throat and the small intestine. It is involved in the second phase of digestion, following chewing. The stomach churns food before it moves on to the rest of the body.

The stomach is composed of five layers. Starting from the inside and working our way out, the innermost layer is called the mucosa. Stomach acid and digestive juices are made in the mucosa layer. The next layer is called the submucosa. The submucosa is surrounded by the muscularis, a layer of muscle that moves and mixes the stomach contents. The next two layers, the subserosa and the serosa are the wrapping for the stomach. The serosa is the outermost layer of the stomach.

Liver

The liver is an organ in the upper abdomen that aids in digestion and removes waste products and worn-out cells from the blood. The liver is the largest solid organ in the body. The liver has a multitude of important and complex functions. Some of these functions are to:

1. Metabolize and store carbohydrates, which are used as the source for the sugar (glucose) in blood that red blood cells and the brain use.

2. Synthesize, store, and process (metabolize) fats, including fatty acids (used for energy) and cholesterol.

3. Detoxify, by metabolizing and/or secreting, drugs, alcohol, and environmental toxins.

Pancreas

A fish-shaped spongy grayish-pink organ about 15 cm long that stretches across the back of the abdomen, behind the stomach. The head of the pancreas is on the right side of the abdomen and is connected to the duodenum (the first section of the small intestine). The narrow end of the pancreas, called the tail, extends to the left side of the body.

The pancreas makes pancreatic juices and hormones, including insulin. The pancreatic juices are enzymes that help digest food in the small intestine

As pancreatic juices are made, they flow into the main pancreatic duct. This duct joins the common bile duct, which connects the pancreas to the liver and the gallbladder.

Gallbladder

The function of the gallbladder is to store bile and concentrate. Bile is a digestive liquid continually secreted by the liver. The bile emulsifies fats and neutralizes acids in partly digested food. A muscular valve in the common bile duct opens, and the bile flows from the gallbladder into the cystic duct, along the common bile duct, and into the duodenum (part of the small intestine).

Duodenum

The duodenum is a short portion of the small intestine connecting the rest of the intestine to the stomach. Though the duodenum is such a tiny fraction of the small intestine, it is the site of most of the breakdown of the food passing through it. The duodenum is lined with Brunner's glands, which secrete an alkaline mucus that supports the intestinal enzymes and aids in the absorption of nutrients.

IIeum

The ileum is a very long part of the small intestine. After protein digestion in the stomach and starch digestion in the mouth, the ileum releases many enzymes to digest the remaining products of starch hydrolyis (which are sugars).

Jejunum

The jejunum is the second and largest section of the small intestine. The surface area of the lining of the jejunum is greatly increased by many small, finger-like outgrowths called villi. This makes it easier for digested material to be absorbed, which is the main function of the jejunum.

Large Intestines

The large intestine is about 1.5 m long and consists of the caecum, appendix, colon and rectum - which are distributed in the abdominal cavity.

The large intestine does the following:

1. reabsorbs water and maintains the fluid balance of the body

2. absorbs certain vitamins

3. processes undigested material (fibre)

4. stores waste before it is eliminated

Caecum

The caecum is the first part of the large intestine. Shaped like a small pouch and located in the right lower abdomen, it is the connection between the small intestine and the colon.

The caecum accepts and stores processed material from the small intestine and moves it towards the colon.

Appendix

The appendix is a small projection emerging from the caecum.

In human beings, the appendix has no known function and is thought to be a remnant from a previous time in human evolution.

Colon

Shaped like an inverted 'U', the colon is the longest part of the large intestine.

Within the colon, the mixture of fibre, small amounts of water, and vitamins, etc, mixes with mucus and with bacteria that live in the large intestine - and starts to form faeces.
As faeces travels through the colon, the lining of the colon absorbs most of the water and some vitamins and minerals.

Rectum

The rectum is the final part of the large intestine. It is where stool (faeces) is stored before being passed as a bowel motion.

Last but not least, here's a video clip for your enjoyment!

Links/Resources:

http://images.main.uab.edu/healthsys/ei_0132.gif
http://digestive.niddk.nih.gov/ddiseases/pubs/yrdd/
http://en.wikipedia.org/wiki/Mouth
http://weblogs.newsday.com/sports/watchdog/blog/mouth-diseases.jpg
http://www.sghhealth4u.com.sg/Health4U/otolaryngology/the_throat.htm
http://jeremyboelter.com/images/GERD3small.gif
http://en.wikipedia.org/wiki/Stomach
http://www.mamashealth.com/organs/stomach.asp
http://www.medterms.com/script/main/art.asp?articlekey=4179
http://www.mamashealth.com/organs/gallbladder.asp
http://www.wisegeek.com/what-is-the-duodenum.htm
http://wiki.answers.com/Q/What_is_the_function_of_the_iieum
http://www.daviddarling.info/encyclopedia/J/jejunum.html
http://www.gesa.org.au/digestive-system/large_intestine.cfm
http://www.youtube.com/watch?v=1nFwO-9iU5Y
Proteins, Enzymes, Genes The Interplay of Chemistry and Biology by Joseph S. Fruton