14. Our body needs to remove the wastes that build up from cell activities and from digestion. If these wastes are not removed, then our cells can stop working and we can get very sick. The organs of our excretory system help to release wastes from our body. The excretory system consists of a pair of kidney, a pair of ureters, a urinary bladder and a urethra. Each kidney is made up of nearly one million complex tubular structures called nephrons. The formation of urine involves various processes that takes place in the different parts of the nephron. Each nephron consists of a cup-shaped upper end called Bowman's capsule containing a bunch of capillaries called glomerulus. Bowman's capsule leads to tubular structure - proximal convoluted tubule, loop of Henle and distal convoluted tubule which ultimately joins the collecting tubule. (A) The following substances are the excretory products in animals. Choose the least toxic form.
Urea
Uric acid
monia
Co2
Answers
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.
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 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)
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:
Filtration: Blood enters a nephron, which filters out impurities.
Reabsorption: The impurities move through tubules, while the rest of the blood is reabsorbed through capillary walls into the blood.
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.
Common mistakes and misconceptions
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.
Uric acid is the least toxic form.
- The excretion of uric acid requires less amount of water as it is the least toxic form. It is the excretory product of birds and reptiles. Hence, these animals are called uricotelic animals.
- The excretion of ammonia requires maximum amount of water as it is the most toxic form. It is the excretory product of aquatic animals such as bony fishes and some amphibians. Hence, these animals are called ammonotelic animals.