Digestive and Excretory Systems



Overview and Homeostasis

Nervous System and Drugs

Skeletal, Muscular and Integumentary

Circulatory and Respiratory

Digestive and Excretory

Endocrine and Reproductive

Immune System and Disease


Vocabulary: carbohydrates, proteins, small intestine, villi, kidneys, nephrons, dialysis

Digestive System
The digestive system, also known as the alimentary canal, consists of many organs. The job of the digestive system is to break down our food into small particles so they are able to diffuse into our cells easily.

Your mouth is the first site of digestion. The food enters here and your teeth help to grind the food into a paste. The paste is made when saliva mixes with the food with the help of the tongue. The saliva contains an enzyme that starts to break down carbohydrates into smaller sugars.

The esophagus is a tube that leads from the mouth to the stomach. The movement of food through the esophagus is caused by contractions of the smooth muscle that lines the esophagus. These muscle contractions are known as peristalsis.

Once the food reaches the stomach, the muscles of the stomach churn the food, mixing it with stomach acid. The acidity of the stomach helps to activate an enzyme in the stomach that breaks down proteins into amino acids.

When the food leaves the stomach, it enters the small intestine. In the small intestine, enzymes from the pancreas and liver/gall bladder are released and mix with the food to help break it down even further. Lipids are broken into fatty acids and glycerol, carbohydrates are broken down into sugars, and proteins are broken down into amino acids. Now that the food is broken down into particles small enough to diffuse, they are able to enter the bloodstream to circulate around the body to the cells and tissues that need them. The small intestine is lined with tiny villi that help with the absorption of nutrients by adding surface area. The capillaries of the villi allow for the diffusion of the nutrients into the bloodstream.

The food passes to the large intestine after it leaves the small intestine. In the large intestine, many vitamins and minerals are absorbed, as well as water. The waste becomes more solid at this point, and the remains pass through the rectum and anus as feces.

Excretory System
The excretory system is a way for humans to get rid of chemical wastes, which plays a role in maintaining homeostasis. Many of the chemicals we rid from our bodies can be toxic if they are not removed. Some of the wastes we produce in our cells are carbon dioxide, urea, and salts.

The liver, while part of the digestive system, also plays a role in the excretory system. Excess amino acids are often found in the bloodstream from the foods that we eat. The liver converts excess amino acids in our bloodstream into other compounds the body can use. Urea, a waste product, is produced as a result. The urea then gets removed from the bloodstream by the kidneys.

The kidneys help to maintain homeostasis by filtering wastes out of the blood. They also remove excess water from the bloodstream, which maintains the blood's volume and blood pressure. The pH of the blood is kept in check when urea is removed from the body, as well.

Humans have two kidneys located in the lower back. A tube called a ureter leaves each kidney and they connect to the urinary bladder, where urine collects. The urine is held in the bladder until it is ready to be released from the body. The urethra is the pathway that urine takes from the bladder to the outside of the body.

The kidneys have millions of filtering units called nephrons. Each nephron has arteries, veins and capillaries connected to them. As the blood passes through the nephron, water, urea, amino acids, and other materials are filtered out of the blood. Most of these materials make their way back into the bloodstream, but the excess materials form urine and eventually find their way out of the body.

Since many materials are filtered out of the body by the kidneys and released in urine, testing urine has proved useful in seeing if substances like drugs have been in a person's system recently. Also, finding certain substances in the urine, like glucose, can indicate a problem with the body like diabetes.

As long as a person has at least one kidney, their body can maintain homeostasis. If a person has no fully functioning kidney, they may need to have their blood filtered by dialysis to remove the wastes that could build up in the body. Kidney dialysis can be done with a machine, but it is expensive and time consuming.



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