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The digestive and excretory systems review

Pojęcia kluczowe

Digestive systemThe body system that converts food into energy and nutrients to fuel the body
Chemical digestionThe breaking down of food using chemical agents, such as enzymes and bile
Mechanical digestionThe breaking down of food by physical means, such as chewing
AbsorptionThe process by which nutrients pass through the walls of the digestive system into the blood
Excretory systemThe body system that removes metabolic wastes from the body
ExcretionThe process of removing wastes and excess water from the body

The digestive system

The human digestive system breaks food down into small molecules that can be used by cells in the body.
The major components of the digestive system.
Image from OpenStax, CC BY 4.0
Digestion begins when food enters the mouth (oral cavity). Both mechanical and chemical digestion occur in the mouth. Teeth grind and break up food (mechanical), while an enzyme in saliva called amylase begins to break down carbohydrates (chemical).
After it is swallowed, the chewed food (now called a bolus) moves down the esophagus. The esophagus acts as a connection between the mouth and the stomach, but no digestion occurs here.
The bolus then reaches the stomach, where more mechanical and chemical digestion take place. The muscles in the stomach walls churn the bolus (mechanical), allowing it to mix with digestive enzymes and gastric acids (chemical). This process converts the bolus into a liquid called chyme.
Digestion continues in the stomach for several hours. During this time, an enzyme called pepsin breaks down most of the protein in the food.
Image detailing the structures of the digestive system
Image from OpenStax, CC BY 4.0
The chyme is slowly transported into the small intestine, where most chemical digestion takes place. Bile, which is made in the liver, is released from the gallbladder to help digest fats. In addition, enzymes from the pancreas and intestinal walls combine with the chyme to start the final part of digestion.
Most nutrient absorption occurs in the small intestine. Nutrients are absorbed through its walls into the circulatory system and by the time the chyme exits the small intestine, only water and undigestible substances are left behind.
The chyme then enters the large intestine. Here, water is removed and bacteria break down some undigestible materials, producing important compounds (such as vitamin K). The concentrated waste material that remains is called feces, which is passed into the rectum and eliminated from the body through the anus.

Accessory organs

Accessory organs help with digestion but are not part of the digestive tract. These include:
  • Salivary glands: moisten food and begin chemical digestion of starches.
  • Liver: creates bile for fat digestion, detoxifies blood, processes absorbed vitamins
  • Gallbladder: stores bile produced by the liver
  • Pancreas: secretes pancreatic juices to help digestion of proteins and carbohydrates

The excretory system

The excretory system removes metabolic wastes from the body.
The major organs of excretion are the kidneys, a pair of bean-shaped organs located below the liver. The kidneys filter blood and regulate water balance in the body.
There are several other organs that are also involved in excretion, including:
  • the skin, which removes excess water and salt via sweat,
  • the lungs, which exhale carbon dioxide, and
  • the liver, which breaks down toxic substances in the blood and convert nitrogenous waste into urea

Urinary tract

The urinary tract is a major part of the excretory system. It filters wastes and water from the blood, and eliminates them from the body.
The urinary system (kidneys, ureter, bladder, urethra)
Image from Wikimedia, Public domain
The kidneys produce a waste product called urine using special functional units called nephrons. The urine is then excreted from the body. This process takes place in three steps:
  1. Filtration: Blood enters a nephron, which filters out impurities.
  2. Reabsorption: The impurities move through tubules, while the rest of the blood is reabsorbed through capillary walls into the blood.
  3. Excretion: Urine is transported from the kidneys through the ureters and into the urinary bladder. It remains stored in the bladder until it is released through the urethra.

Często spotykane błędy i nieporozumienia

  • Digestion does not begin in the stomach. While some digestion occurs in the stomach, the process actually begins in the mouth, where chewing and salivary amylase act on the food.
  • The digestive system does not produce urine. Some people think that the digestive system has two outlets - one for feces and one for urine. However, urine is a product of the excretory system, not the digestive system.
  • The small intestine is actually longer than the large intestine. In fact, at approximately 20 feet in length, the small intestine is nearly four times as long as the large intestine (5 feet long)! However, the intestines are named for their diameters, not their lengths. The large intestine has a diameter of about 3 inches compared to the small intestine, with a diameter of about 1 inch.

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