The human digestive system is a complex series of organs and glands that processes food. In order to use the food we eat, our body has to break the food down into smaller molecules that it can process; it also has to excrete waste.
Most of the digestive organs (like the stomach and intestines) are tube-like and contain the food as it makes its way through the body. The digestive system is essentially a long, twisting tube that runs from the mouth to the anus, plus a few other organs (like the liver and pancreas) that produce or store digestive chemicals.
The Digestive Process:The start of the process - the mouth: The digestive process begins in the mouth. Food is partly broken down by the process of chewing and by the chemical action of salivary enzymes (these enzymes are produced by the salivary glands and break down starches into smaller molecules).
On the way to the stomach: the esophagus - After being chewed and swallowed, the food enters the esophagus. The esophagus is a long tube that runs from the mouth to the stomach. It uses rhythmic, wave-like muscle movements (called peristalsis) to force food from the throat into the stomach. This muscle movement gives us the ability to eat or drink even when we're upside-down.
In the stomach - The stomach is a large, sack-like organ that churns the food and bathes it in a very strong acid (gastric acid). Food in the stomach that is partly digested and mixed with stomach acids is called chyme.
In the small intestine - After being in the stomach, food enters the duodenum, the first part of the small intestine. It then enters the jejunum and then the ileum (the final part of the small intestine). In the small intestine, bile (produced in the liver and stored in the gall bladder), pancreatic enzymes, and other digestive enzymes produced by the inner wall of the small intestine help in the breakdown of food.
In the large intestine - After passing through the small intestine, food passes into the large intestine. In the large intestine, some of the water and electrolytes (chemicals like sodium) are removed from the food. Many microbes (bacteria like Bacteroides, Lactobacillus acidophilus, Escherichia coli, and Klebsiella) in the large intestine help in the digestion process. The first part of the large intestine is called the cecum (the appendix is connected to the cecum). Food then travels upward in the ascending colon. The food travels across the abdomen in the transverse colon, goes back down the other side of the body in the descending colon, and then through the sigmoid colon.
The end of the process - Solid waste is then stored in the rectum until it is excreted via the anus.
Digestive System Glossary:anus - the opening at the end of the digestive system from which feces (waste) exits the body.appendix - a small sac located on the cecum.ascending colon - the part of the large intestine that run upwards; it is located after the cecum.bile - a digestive chemical that is produced in the liver, stored in the gall bladder, and secreted into the small intestine.cecum - the first part of the large intestine; the appendix is connected to the cecum.chyme - food in the stomach that is partly digested and mixed with stomach acids. Chyme goes on to the small intestine for further digestion.descending colon - the part of the large intestine that run downwards after the transverse colon and before the sigmoid colon.duodenum - the first part of the small intestine; it is C-shaped and runs from the stomach to the jejunum.epiglottis - the flap at the back of the tongue that keeps chewed food from going down the windpipe to the lungs. When you swallow, the epiglottis automatically closes. When you breathe, the epiglottis opens so that air can go in and out of the windpipe.esophagus - the long tube between the mouth and the stomach. It uses rhythmic muscle movements (called peristalsis) to force food from the throat into the stomach.gall bladder - a small, sac-like organ located by the duodenum. It stores and releases bile (a digestive chemical which is produced in the liver) into the small intestine.ileum - the last part of the small intestine before the large intestine begins.jejunum - the long, coiled mid-section of the small intestine; it is between the duodenum and the ileum.liver - a large organ located above and in front of the stomach. It filters toxins from the blood, and makes bile (which breaks down fats) and some blood proteins.mouth - the first part of the digestive system, where food enters the body. Chewing and salivary enzymes in the mouth are the beginning of the digestive process (breaking down the food).pancreas - an enzyme-producing gland located below the stomach and above the intestines. Enzymes from the pancreas help in the digestion of carbohydrates, fats and proteins in the small intestine.peristalsis - rhythmic muscle movements that force food in the esophagus from the throat into the stomach. Peristalsis is involuntary - you cannot control it. It is also what allows you to eat and drink while upside-down.rectum - the lower part of the large intestine, where feces are stored before they are excreted.salivary glands - glands located in the mouth that produce saliva. Saliva contains enzymes that break down carbohydrates (starch) into smaller molecules.sigmoid colon - the part of the large intestine between the descending colon and the rectum.stomach - a sack-like, muscular organ that is attached to the esophagus. Both chemical and mechanical digestion takes place in the stomach. When food enters the stomach, it is churned in a bath of acids and enzymes.transverse colon - the part of the large intestine that runs horizontally across the abdomen.Related Pages:
Digestive System (simple version)Label the digestive system.Answers
Digestive SystemLabel the digestive system.Answers
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Reproduction in Animals: Sexual Reproduction

By Regina B, About.com Guide

See More About:

Human Ovum(Egg) and Sperm

As discussed in part one of this series about asexual reproduction, reproduction is a marvelous culmination of individual transcendence.

Individual organisms come and go, but, to a certain extent, organisms "transcend" time through reproducing offspring.

Now let's take a look at sexual reproduction.

Sexual Reproduction

In sexual reproduction, two individuals produce offspring that have genetic characteristics from both parents. Sexual reproduction introduces new gene combinations in a population.

Gametes

In animals, sexual reproduction encompasses the fusion of two distinct gametes to form a zygote. Gametes are produced by a type of cell division called meiosis.

The gametes are haploid (containing only one set of chromosomes) while the zygote is diploid (containing two sets of chromosomes).

In most cases, the male gamete, called the spermatozoan, is relatively motile and usually has a flagellum. On the other hand, the female gamete, called the ovum, is nonmotile and relatively large in comparison to the male gamete.

Types of Fertilization

There are two mechanisms by which fertilization can take place.

The first is external (the eggs are fertilized outside of the body); the second is internal (the eggs are fertilized within the female reproductive tract).

Patterns and Cycles

Reproduction is not a continuous activity and is subject to certain patterns and cycles. Oftentimes these patterns and cycles may be linked to environmental conditions which allow organisms to reproduce effectively.

For example, many animals have estrous cycles that occur during certain parts of the year so that offspring can typically be born under favorable conditions.

Likewise, these cycles and patterns can be controlled by hormonal cues as well as other seasonal cues like rainfall.

All of these cycles and patterns allow organisms to manage the relative expenditure of energy for reproduction and maximize the chances of survival for the resulting offspring.

Share Your Opinions

Have questions about sexual reproduction? Come on over to the Biology Forum and share your thoughts, opinions, and feelings.

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