Human Digestive System | Urinary System | How does it work?


Proud Tips | What’s your digestive system and urinary system? Read about the human digestive system and urinary system structure and function. How does it work? in this article, I explain it.

What is Digestion?

main parts of the human digestive systemWhen you put food into your mouth, it begins a journey through a set of organs inside your body that work together to break down the food so your body can use it. This system is called the digestive system. The journey takes more than a day. Along the way, the food is broken down into small bits, or molecules, and substances that your body needs are extracted from it. This process of breaking down foods into nutrients that the cells of the body can absorb is called digestion.


The substances your body gets from digestion are called nutrients. Some nutrients are needed for the body to grow and repair itself. Some are needed for parts of the body, such as the brain, to work. Some provide the energy needed for walking, talking, and all the millions of chemical reactions that happen in the body. Energy is even needed to digest the food itself!

Digestive System Parts

The digestive system starts at the mouth, where food goes in and ends at the anus, where waste comes out. It is made up of a long tube called the alimentary canal, which is also called the digestive tract, or gut. Various organs and glands, such as the liver and salivary glands, are also part of the digestive system. As food passes through the alimentary canal, the food is broken down so that the useful substances it contains can be absorbed into the blood. Solid waste is passed out of the body as feces.

What is the Urinary System?

#urinary systemThe digestive system gets rid of the waste parts of the food that you eat. The urinary system is a separate system, but it also gets rid of waste—waste substances from the blood. The waste combines with water from the blood and makes urine. The urinary system is in the abdomen. It is made up of two kidneys that filter the blood; a pair of tubes through which urine passes from the kidneys to the urinary bladder; and a tube that carries the urine out of the body.

Teeth and Tongue

Teeth & tongueThe digestive system starts in the mouth. Teeth and saliva begin the digestive process before swallowing food. The teeth break pieces of food into smaller bits The food is further broken down in the stomach and small intestine. Saliva begins to act chemically on the food, making it easier to swallow. The mouth is the only part of the digestive system that you control. The rest works completely automatically.

Teeth Types

mouth to stomachAdults have a total of 32 teeth, 16 each fixed firmly in the upper and lower jaws by long roots. There are eight incisors and four canines at the front of the mouth. These sharp front teeth are used to tear and bite the mass on the food.

From Mouth to the Stomach

A bolus is a lump of chewed food mixed with saliva that is ready to be swallowed. When you swallow a bolus, muscles of the pharynx (a part of the throat) push the bolus into the next part of the alimentary canal, called the esophagus. This tube links the mouth to the stomach. The esophagus is also called the gullet.


Movements of the tongue push a bolus to the very back of the throat. Then the process of swallowing begins. The base of the tongue and muscles in the top of the throat push the bolus down into the esophagus. Once you start to swallow, all the muscles do the job automatically. It is important that food goes into the esophagus and not anywhere else. During swallowing the soft palate at the top of the back of the mouth closes to keep food from going into the nasal passages. And a flap called the epiglottis closes to keep food from going down the trachea (windpipe) into the lungs.

Down the Esophagus

The esophagus is lined with muscles that move food down into the stomach. The muscles automatically contract and relax, one after the other, forcing the bolus downward. This process is called peristalsis. It happens all along the alimentary canal. The esophagus is about 10 inches (25 centimeters) long. Food passes through it from mouth to stomach in about nine seconds. In the lining of the esophagus are glands that produce mucus. This lubricates the esophagus, allowing food to slide down more easily. At the bottom of the esophagus is a muscular valve called a sphincter. It opens to let food into the stomach and then closes again.


Because food moves downward through the digestive system, you might think it is moved by gravity. In fact, it is moved by the actions of muscles in the walls of the alimentary canal. The muscles make the walls ripple, pushing the food along as if it were riding a wave. This process is called peristalsis. It happens from the top of the esophagus to the end of the large intestine.

Gut Structure

The walls of the alimentary canal are similar all the way along. The outer layer protects it from damage. This layer is also slippery so that, for example, coils of intestines can safely lie against each other. The inner layer—the mucosa—is the gut’s lining. Between the inner and outer layers are two layers of muscle. In the inner muscle layer, muscle fibers go around the tube. In the outer muscle layer, muscle fibers go along the tube. The mucosa contains blood vessels that supply blood to the gut, and nerves that control the muscles.

Muscle Movements

#Muscle movementsWhen a bolus enters a part of the alimentary canal, that part of the tube stretches. Nerves detect this stretching and begin to make the muscles work. Muscles behind the bolus contract, squeezing the tube and pushing the bolus along. Then the muscles relax again. The moving bolus activates the next section of the muscles, and the process is repeated. The result is a wave of muscle contractions or tightenings, that move the bolus along. Peristalsis stops in a section of the alimentary canal when there is no food there to push along.

Digestive valves

There are several structures called sphincters in the alimentary canal. They are rings of muscle that work like valves. They close off sections of the canal from each other, keeping food from moving the wrong way.

The Stomach


The stomach lining magnified 700 times. The lumps are individual cells of the lining.

When the food arrives at the bottom of the esophagus, it passes into the stomach. The stomach is made of strong muscle and is shaped like a bag or sack. It expands so that a meal of food can fit into it. An adult’s stomach can hold a little over one quart (0.95 liters). The stomach’s job is to act as a storage place for food and to start digesting the food with chemicals. After a meal, some food spends three to five hours in the stomach.

Stomach Muscles

The stomach has four regions. Food from the esophagus goes into the top region, called the cardia. Then it gradually moves into the lower region, called the pylorus. Strong bands of muscles in the stomach walls move to churn the food. The stomach walls stretch to contain food until it is ready to be slowly released into the intestines.

Stomach acid

The stomach lining is full of tiny glands. These make liquids that are released into the stomach and combine into a fluid called gastric juice. The churning and squeezing of the stomach mix the juice with the food. Gastric juice is a mixture of different chemicals, some of which make hydrochloric acid. It helps to break up the food but also kills many harmful microorganisms, tiny living organisms such as bacteria, that the food may contain.

Chemical Workers

Other chemicals in the gastric juice begin breaking down some of the chemicals in the food. A chemical called pepsinogen in the juice combines with the stomach acid to make another chemical called pepsin.

This breaks up chemicals in the food called proteins so the body can use them. Gastric juice also contains a chemical called intrinsic factor, which allows the body to use one of the Vitamins, known as B12, found in food. A mixture of partially digested food and gastric juice is called chyme. The stomach works automatically. It is controlled by nerves and chemicals called hormones. It releases chemicals and starts churning only when you smell and taste food and when nerves detect food in your mouth.

The Small Intestine

small intestineThe small intestine is the longest part of the digestive system. It is about 22 feet (7 meters) long but only about 1 inch (2.5 cm) in diameter. It is called the small intestine because it is narrow compared to the wider large intestine. The small intestine is coiled into the abdomen. In the small intestine, nutrients are absorbed into the blood. But more digestion happens in the duodenum first.

The Duodenum

The duodenum is the first part of the small intestine. The duodenum is about 10 inches (25 centimeters) long. Small amounts of chyme from the stomach are squirted into the duodenum through a valve called the pyloric sphincter. More digestive fluids are added to the chyme in the duodenum.

These fluids are bile, which comes from a small pouch-shaped organ called the gallbladder, and pancreatic juice, which comes from the pancreas. Bile breaks up fat into small droplets so that chemicals can break them up more easily. Pancreatic juice contains a mixture of digestive chemicals that break up proteins, carbohydrates, and fats. These chemicals can not work in the acid that comes from the stomach, so pancreatic juice also contains alkalis, substances that neutralize, or stop the effect of acid.

Absorbing Nutrients

Such nutrients as sugars are made by digestion and absorbed into the blood and the lymphatic system through the lining of the small intestine. The surface of the lining is covered with millions of fingerlike tufts called villi. These give the lining a surface area larger than that of a tennis court. This huge surface area helps the body absorb more nutrients more quickly. The villi wave around to stir the contents of the intestine, which helps the nutrients to be absorbed. Food passes through the small intestine in about four or five hours.

The Pancreas

The pancreas is a gland that plays an important part in digestion. The pancreas produces pancreatic juice, which flows into the duodenum. The pancreas is similar in shape to a carrot and about 6 to 8 inches (15 to 20 centimeters) long. The wide end is next to the duodenum. The pancreas also has another job: it controls the amount of sugar in the blood.

A magnified cross-section of the pancreas. Around the outside are cells that make pancreatic juice. The lighter area in the center is an islet of Langerhans that produces insulin.

Making juice

Pancreatic juice is made inside the pancreas by bunches of cells called acini. The juice flows along narrow tubes that connect to the pancreatic duct. This duct joins the alimentary canal in the duodenum.
Pancreatic juice is a mixture of different digestive chemicals. The chemicals trypsin and chymotrypsin break up proteins, amylase breaks up starches, and lipase breaks up fats. Pancreatic juice also contains alkalis that neutralize stomach acid. The chemicals in pancreatic juice are powerful. They start to work only when they mix with the alkalis in the duodenum—otherwise, they would digest the pancreas itself.

The large intestine

large intestineThe large intestine is the last part of the digestive tract. The large intestine is much shorter and wider than the small intestine. Watery material from the small intestine enters the large intestine. The large intestine absorbs water and small amounts of minerals from this watery material. The water and minerals pass into the blood. This leaves a firm material called feces,  which is passed out of the body.

Large intestine structure

The large intestine is made up of three main sections. They are the cecum, the colon, and the rectum. The cecum is short and wide. It joins the small and large intestines together. The colon is divided into four sections. They are the ascending colon, transverse colon, descending colon, and sigmoid colon. An adult’s colon is about 5 feet (1.5 meters) long, and about 2.5 inches (6.3 centimeters) wide. The rectum is about 5 inches (12.5 centimeters) long.

Water and nutrients

A slushy mixture of water and undigested material enters the large intestine from the small intestine through a valve.  By now, the small intestine has removed most nutrients from the food, but there are still some useful minerals left. These pass into the blood through the walls of the large intestine. Most of the water in the mixture also is absorbed. About 4 pints (2 liters) enter the large intestine every day, and about 3.6 pints (1.8 liters) of it are absorbed.

After the minerals and water have been removed, the material left over is called feces.  About two-thirds of it is water and about one-third is a solid material, made up mostly of undigested plant fiber,  bacteria, bile, mucus,  and old cells from the lining of the alimentary canal. Feces are stored in the rectum until you go to the toilet when they are released through the anus. Always thoroughly wash your hands with soap and water after going to.-the toilet.

Bacteria in the gut

Harmless bacteria live in every part of your alimentary canal, from your mouth to your large intestine. Millions of bacteria live in the large intestine and are contained in feces. Many are friendly bacteria that help with digestion.

  • An X-ray of the first part of the large intestine. The fingerlike object on the left is the appendix.


The pancreas also makes a chemical called insulin. This is a hormone that controls the use by the body of sugar and other food for energy. Insulin is made in groups of cells called islets of Langerhans. There are about a million of them spread all through the pancreas. Insulin is not released into the pancreatic duct but directly into the blood that flows through the pancreas.

Digestion Chemicals

All the food you eat is made up of chemicals. The tiny particles that form these chemicals are called molecules. Molecules are made up of atoms joined together. Some molecules in food contain a few atoms, but most contain dozens or hundreds of thousands. Your body can not absorb most of these molecules because they are too large. The digestive system must break down the molecules into smaller molecules that the body can absorb.

A diabetic person using a blood sugar testing device. The results will tell her whether she needs an insulin injection.

A magnified view of a stomach ulcer. An ulcer forms when the stomach lining itself is digested because mucus does not protect it.

Digestive Enzymes

Enzymes for foodBreaking up molecules is a chemical change. It is done by chemicals called digestive enzymes. These work in the mouth, the stomach, and the small intestine, where they mix with food. Some enzymes are made by cells in the lining of the alimentary canal, and some are made by glands connected to the alimentary canal.

Enzymes For Food Types

Different enzymes are designed to break down different chemicals in the foods you eat. The three main large molecules that need to be broken down are starches, fats (also called lipids), and proteins. Starch molecules are broken up by enzymes called ptyalin, contained in saliva, and amylase, contained in pancreatic juice. Ptyalin breaks the starch molecules into a simple sugar called maltose. Amylase breaks the maltose into even simpler molecules of a sugar called glucose.

Fats have molecules made up of two chemicals called fatty acids and glycerol. Fats are broken first into small pieces by the chemicals in bile. These chemicals work like detergent. They pull fats apart. Then the molecules of fat are broken up into fatty acids and glycerol by an enzyme in pancreatic juice called lipase. Proteins are made up of chemicals called amino acids that are joined together in long chains. Enzymes in the stomach called pepsin, and in the small intestine, called trypsin, break the bonds of these chains to turn proteins into amino acids.

The liver

#liverThe liver is not part of the alimentary canal, but it is a vital part of the digestive system. The liver is like a chemical-processing factory. It takes chemicals that were absorbed in the small intestine and turns them into chemicals that the body can use and store for future use. The liver also makes bile, and it cleans the blood.

Shape and structure

The liver is almost the shape of a triangle. It is about 8 inches (20 centimeters) long and weighs about 3 pounds (1.4 kilograms). It has four parts, called lobes. Most livers contain up to a hundred thousand lobules. A lobule is made up of hundreds of liver cells situated around a central blood vessel that carries blood to and from the cells.
Blood reaches the liver through two blood vessels.

The portal vein brings nutrient-rich blood straight from the intestines. The hepatic artery brings oxygen-rich blood from the heart. These two blood vessels divide into thousands of tiny blood vessels that pump blood into spaces in the liver called sinusoids. Liver cells take food and oxygen from the blood flowing through the sinusoids. Blood vessels from the lobules join up to form the hepatic vein, which carries blood away from the liver. The liver processes about 3 pints (1.4 liters) of blood every minute.

A thin slice of liver seen through a microscope. It shows the center of a lobule. The central hole is a blood vessel.

Digestive function

Liver cells process food molecules that come from the intestines. These cells break down some molecules into simpler ones and create nutrients from them. The liver stores some of the nutrients and puts others into the blood to be transported to other parts of the body. For example, the liver turns glucose into glycogen and stores it. Later the glycogen can be turned back into glucose and used for energy.

Cleaning the blood

As well as being part of the digestive system, the liver is a vital part of the circulatory system. This system is made up of the blood, the heart, and blood vessels. The blood carries nutrients and oxygen to all the cells in the body and carries carbon dioxide and other waste chemicals away from them. The liver removes some waste chemicals from the blood and also removes harmful chemicals called toxins.

Waste and toxins

Some of the chemicals that the liver removes from the blood are toxic substances from outside the body. These include poisons, alcohol, drugs, and pollutants breathed from the atmosphere, such as radon and sulfur dioxide. Other harmful chemicals are by-products of the body’s own processes. The liver breaks them down into less harmful chemicals. This is done by hundreds of specialized enzymes. The less harmful chemicals are put back into the blood or the bile to be removed from the body.

Your liver breaks down harmful chemicals that you breathe in from the atmosphere, such as those pumped out by chemical factories.

Waste ammonia

Ammonia is an example of a waste product from the body’s cells. It is made when proteins are broken down into, or built up from, amino acids. Ammonia is a toxic chemical that would cause harm if the body did not get rid of it. Ammonia from the cells goes into the blood. When it reaches the liver, the liver combines it with carbon dioxide, which is another waste product from cells.
This makes a less harmful chemical called urea, which the liver puts back into the blood. Urea is removed from the blood by the kidneys.

Waste routes

The less harmful chemicals made by the liver leave the body by different routes. Some chemicals that go back into the blood, such as urea, leave in urine made by the kidneys. Some go to the lungs and are breathed out. Some come out through the skin in sweat. Waste chemicals that go into bile leave the body as part of feces.

A breath test measures the level of alcohol in the blood. The liver breaks down alcohol, lowering the level.

The kidneys

kidneys functionThe kidneys are the first part of the urinary system. The urinary system has two main jobs. It removes waste chemicals from the body, and it removes water if there is too much of it in the body. The kidneys remove waste and water from the blood using tiny structures that work as filters. The other parts of the urinary system drain the excess water and waste from the body. You have one kidney on each side of your body. They look like huge purplish-brown beans. Each one is about 4.5 inches (11.4 centimeters) long.

Kidney function

Blood flows to the kidneys through the renal artery and then flows away again through the renal vein. The kidneys filter waste chemicals from the plasma (the watery part of the blood) as it passes through them. A kidney has three layers. From inside to outside, these are the pelvis, the medulla, and the cortex. The cortex of each kidney contains more than a million tiny blood­ filtering structures called nephrons.

Blood filters

In each nephron is a tiny bunch of blood vessels inside a case called a Bowman’s capsule. There most of the water and other chemicals are filtered from the blood. They flow into a long, twisting tube that leads away from the capsule. The tube is surrounded by blood vessels that lead to the renal vein. Useful chemicals and nearly all the water in the tube pass through the walls of the tube and go back into blood vessels, which join up to form the renal vein. Waste chemicals stay in the tube. Tubes from all the nephrons go into the medulla.

They join together at the pelvis of the kidney and then narrow to form a single tube called a ureter. The water and waste materials are called urine. An adult produces about 1 to 2 quarts (0.95 to 1.9 liters) of urine per day.

A magnified cross-section of part of a kidney. The largo white shape is a Bowman’s capsule. Tiny blood vessels arc bundled inside it.

The urinary tract

Urine from the kidneys flows out of the body along with a system of tubes called the urinary tract. The track is made up of two ureters (one from each kidney), the bladder, and the urethra. The ureters carry urine to the bladder, the bladder stores the urine, and the urethra carries it from the bladder out of the body.


The ureters are tubes about 12 inches (30 centimeters) long. They have muscles in their walls that squeeze urine along to the bladder in a way similar to peristalsis in the alimentary canal. The ureters go from the kidneys, down the back of the abdomen,  and into the top of the bladder. Tiny folds at the bottom of the ureters help keep urine from going back up the ureters to the kidneys.

A cross-section of a ureter viewed through a microscope. You can see the central channel through which urine flows. The outer two layers are muscle.

Bladder and urethra

The urinary bladder is a bag-shaped organ made up of muscle fibers that store urine. When the bladder is empty, it is small and wrinkled. It expands as urine trickles in from the ureters. It can hold more than a pint (half a liter) of urine. The urethra is a tube that leads from the bottom of the bladder to the outside of the body.

Going to the toilet

When your bladder is about half full, you begin to feel that you want to go to the toilet. When you do go, you relax a ring of muscles called the urethral sphincter that surrounds the opening from the bladder to the urethra. Then, the muscles in the bladder automatically squeeze urine out through the urethra. Children learn to control their bladders when they are about two or three. Always thoroughly wash your hands with soap and water after going to the toilet.

Eat Natural Stay healthy: Natural Food Energy

A specialized X-ray shows fluid draining from the kidneys into the ureters. The red patch on the left is a kidney stone.


How long is the human digestive system?
The alimentary canal is about 30 feet (9 meters) long. The small intestine is the longest part. It measures about 22 feet (7 meters) and is coiled up to fit inside the body. Some parts of the alimentary canal are wider than others. The stomach is the widest part, and the small intestine is the narrowest.
What are teeth made of?
Teeth are made up of several layers. The outer, visible part is white enamel. Enamel is the hardest material in the body. Under the enamel is a layer called dentin. Dentin is harder than bone. In the middle is pulp, which contains blood vessels and nerves.
What makes me choke?
You choke when food fails to go into your esophagus and instead gets stuck in your trachea, the tube that carries air to your lungs. Your body makes you cough violently to try to get the food out. First aid for a person choking on food includes thrusting upward against the middle of the person’s abdomen. This method is known as the Heimlich maneuver.
Can I swallow if I am upside down?
Yes, you can. The muscles in the esophagus create very strong waves to push the food along. They can push food along the esophagus from the mouth to the stomach, even against gravity.
What protects the inside of my stomach from the burning action of the acid in it?
The hydrochloric acid in your stomach is a powerful chemical. If you put some of it in a test tube with a piece of metal, such as iron, it would eat away the metal. The lining of your stomach makes and coats itself with a substance called mucus that protects it from the acid.
What are gallstones?
The gallbladder is a small bag-shaped organ that stores bile until it is needed. The bile is made by the liver. A gallstone is like a pebble that grows in the gallbladder. It is made from some of the chemicals in the bile. Gallstones can be painful, but they can be broken up by drugs or removed by surgery.
What is diabetes?
Diabetes is a disease that occurs when the body cannot use sugar properly. There are two types of diabetes. Type 1 diabetes happens when the pancreas cannot make enough insulin. In Type 2 diabetes there is usually enough insulin, but the body cannot use it. Diabetes causes sugar to build up in the blood, which can damage such organs as the eyes and kidneys. Some diabetes can be treated by controlling the intake of sugar or by injecting insulin.
How does chewing help digestion?
The chewing teeth, and churning stomach break food up into smaller and smaller pieces, but further action is needed to get food into a form the body can use. Breaking food up by chewing and churning allows digestive chemicals to work on finer particles of food, which makes digestion happen faster.
How much bile does the liver make?
Bile breaks up fat so that enzymes can get to the molecules of fat. The liver makes about 2 pints (1 liter) of bile every day. The bile is collected by tiny bile ducts. These join up to form the hepatic duct, which leads to the gallbladder, where the bile is stored.
Can a damaged liver be repaired?
The liver is one of the few parts of the body that can grow back if it is damaged. This is called regeneration. For example, if half of a person’s liver is removed because it is diseased, the remaining portion of the liver will grow back to its original size in a few weeks.
What does the appendix do?
The answer is probably nothing at all. The appendix is a small tube a few inches long that extends from the cecum. Scientists do not really know why it is there. Feces trapped in the appendix can cause infection and serious, painful swelling called appendicitis.
What is kidney dialysis?
Sometimes a person’s kidneys stop cleaning blood properly. This is called renal failure. It is serious because it allows poisonous waste materials to build up in the blood. One treatment for renal failure is called kidney dialysis. A dialysis machine takes blood from an artery in a patient’s arm, cleans it, and then pumps it back into a vein ’ in the patient’s arm.
What are kidney stones?
Sometimes hard lumps grow in the kidneys. These lumps are called kidney stones. They are made from chemicals in urine. Large stones can cause pain as they move along the ureters, and they can block the ureters completely, causing infection. Kidney stones can be washed out by drinking lots of water, broken up with sound waves, or removed by surgery.
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