19. Respiratory and Excretory System

The above image shows the tissue found inside of the lungs. The lungs contain alveoli. Alveoli absorb oxygen and send it to the blood vessels. They also move carbon dioxide from the blood vessels back to the lungs to be exhaled.

Alveoli look like clumps of grapes. Why do you think it is important that your body have many alveoli? What would happen if your alveoli lost their ability to function? Why do you think alveoli are shaped like spheres?

The respiratory system is important because it brings oxygen to cells in your body. But it also removes a waste, carbon dioxide. Another system in your body, the excretory system, also removes wastes. The excretory system moves waste from your digestive system and from your blood out of your body.

How do the respiratory system and excretory system work together? How does damage in one system affect the other? Consider these questions about respiration and waste removal as you read the following chapter.

The Respiratory System

Lesson Objectives

  • Identify the parts of the respiratory system.
  • Identify the main function of the respiratory system.
  • Describe how breathing works.
  • Outline how the respiratory system and the cardiovascular system work together.
  • Identify how breathing and cellular respiration are connected.

Check Your Understanding

  • What is an organ system?
  • What is the role of the circulatory system?
  • How does your blood get oxygen?


  • alveoli
  • diaphragm
  • epiglottis
  • exhalation
  • external respiration
  • gas exchange
  • inhalation
  • internal respiration
  • larynx
  • pharynx
  • respiration
  • respiratory system
  • trachea

Roles of the Respiratory System

You breathe mostly without thinking about it. Remember how uncomfortable you felt the last time you had a cold or a cough? You usually do not think about your respiratory system or how it works until there is a problem with it. Every cell in your body depends on your respiratory system.

Your respiratory system is made up of the tissues and organs that allow oxygen to enter the body and carbon dioxide to leave your body. Organs in your respiratory system include your:

  • Nose.
  • Mouth.
  • Larynx.
  • Pharynx.
  • Lungs.
  • Diaphragm.
These structures are shown in Figure below.

The respiratory system. Air moves in through the nose and mouth and down the trachea, which is a long, straight tube in the chest.

Parts of the Respiratory System

Figure above shows many of the structures of the respiratory system. Each of the parts has a specific job. The parts of the respiratory system include the following:

  • The diaphragm is a sheet of muscle that spreads across the bottom of the rib cage. When the diaphragm contracts, the chest volume gets larger and the lungs take in air. When the diaphragm relaxes, the chest volume gets smaller and air is pushed out of the lungs.
  • The nose and nasal cavity filter, warm, and moisten the air you breathe. The nose hairs and mucus produced by the cells in the nose catch particles in the air and keep them from entering the lungs. When particles in the air do reach the lungs, what do you think happens?
  • Behind the nasal cavity, air passes through the pharynx, a long tube. Both food and air pass through the pharynx.
  • The larynx, also called the "voice box," is found just below the pharynx. Your voice comes from your larynx. Air from the lungs passes across thin tissues in the larynx and produces sound.
  • The trachea, or windpipe, is a long tube that leads down to the lungs, where it divides into the right and left bronchi. The bronchi branch out into smaller bronchioles in each lung.
  • Since food goes down the pharynx, how is it stopped from entering the trachea? A flap of tissue called the epiglottis closes over the trachea when food is swallowed to prevent choking or inhaling food.
  • The bronchioles lead to the alveoli. Alveoli are the little sacs at the end of the bronchioles. They look like little bunches of grapes, as shown in Figure below. Oxygen is exchanged for carbon dioxide in the alveoli. Gas exchange is the name we give to the process that allows oxygen to enter the blood and carbon dioxide to move out of the blood - the two gases are "exchanged."

The alveoli are the tiny grape-like structures in the lungs and the sites of gas exchange.

How We Breathe

Most of the time, you breathe without thinking about it. Breathing is mostly an involuntary action that is controlled by a part of your brain that also controls your heart beat. If you swim, do yoga, or sing, you know you can also control your breathing. Taking air into the body through the nose and mouth is called inhalation. Pushing air out of the body through the nose or mouth is called exhalation. The man in Figure below is exhaling before he surfaces from the pool water.

Being able to control breathing is important for many activities, such as swimming. The man in the photograph is exhaling before he surfaces from the water.

How do lungs allow air in? As mentioned above, air moves into and out of the lungs by the movement of muscles. The diaphragm and rib muscles contract and relax to move air into and out of the lungs. During inhalation, the diaphragm contracts and moves downward. The rib muscles contract and cause the ribs to move outward. This causes the chest volume to increase. Because the chest volume is larger, the air pressure inside the lungs is lower than the air pressure outside. This difference in air pressures causes air to be sucked into the lungs. When the diaphragm and rib muscles relax, air is pushed out of the lungs. Exhalation is similar to letting the air out of a balloon.

The walls of the alveoli are very thin and allow gases to enter into them. The alveoli are lined with capillaries. These capillaries are shown in Figure below. Oxygen moves from the alveoli to the blood in the capillaries that surround the alveoli. At the same time, carbon dioxide moves in the opposite direction, from capillary blood to the alveoli.

The bronchi and alveoli. During respiration, oxygen gets pulled into the lungs and enters the blood by passing across the thin alveoli membranes and into the capillaries.

Breathing and Respiration

When you breath in, oxygen is drawn in through the mouth and down into the lungs. The oxygen then passes across the thin lining of the capillaries and into the blood. The oxygen molecules are carried to the body cells by the blood. Carbon dioxide from the body cells is carried by the blood to the lungs where it is released into the air. The process of getting oxygen into the body and releasing carbon dioxide is called respiration.

Sometimes breathing is called respiration, but there is much more to respiration than just breathing. There are actually two parts to respiration, external respiration and internal respiration. External respiration is the movement of oxygen into the body and carbon dioxide out of the body. Internal respiration is the exchange of oxygen and carbon dioxide between the blood and the cells of the body (Figure below).

The Journey of a Breath of Air

Breathing is only part of the process of bringing oxygen to where it is needed in the body. After oxygen enters the lungs, what happens?

  1. The oxygen enters the blood stream from the alveoli. Then, the oxygen-rich blood returns to the heart.
  2. Oxygen-rich blood is then pumped through the aorta.
  3. From the aorta, oxygen-rich blood travels to the smaller arteries and finally to the capillaries.
  4. The oxygen molecules move out of the capillaries and into the body cells.
  5. While oxygen moves from the capillaries and into body cells, carbon dioxide moves from the cells into the capillaries.

Gas exchange is the movement of oxygen into the blood and carbon dioxide out of the blood.

Breathing and Cellular Respiration

The oxygen that arrives at the cells from the lungs is used by the cells to help release the energy stored in molecules of sugar. Cellular respiration is the process of breaking down glucose to release energy (see the Cell Functions chapter). The waste products of cellular respiration include carbon dioxide and water. The carbon dioxide molecules move out of the cells and into the capillaries that surround the cells. As explained above, the carbon dioxide is removed from the body by the lungs.

Lesson Summary

  • Your respiratory system is made up of the tissues and organs that allow oxygen to enter and carbon dioxide to leave your body.
  • Respiratory system organs include your nose, mouth, larynx, pharynx, lungs, and diaphragm.
  • During inhalation, the diaphragm contracts and moves downward, and brings air into the lungs.
During exhalation, the diaphragm and rib muscles relax and air is pushed out of the lungs.
  • Oxygen enters the lungs, then passes through the alveoli and into the blood. The oxygen is carried around the body in blood vessels.
  • Carbon dioxide, a waste gas, moves into the blood capillaries and is brought to the lungs to be released into the air during exhalation.
  • The oxygen that arrives from the lungs is used by the cells during cellular respiration to release the energy stored in molecules of sugar.

Review Questions


1. Name the five main organs in the respiratory system.

2. What is the main function of the respiratory system?

3. In what part of the lung does gas exchange occur?

4. What is the important gas that is carried into the lungs during inhalation?

Apply Concepts

5. A classmate says that lung muscles cause the lungs to move during breathing. Do you agree with your classmate?

6. How do the respiratory system and the cardiovascular system work together?

7. Breathing is an involuntary action. Does this mean that you cannot control your breathing?

8. What is the difference between breathing and respiration?

9. What is the name of the waste gas that is released during exhalation?

Critical Thinking

10. If a disease caused the alveoli to collapse, how might this affect a person’s health?

Further Reading / Supplemental Links

Points to Consider

  • How do you think the health of your respiratory system might affect the health of other body systems?

Health of the Respiratory System

Lesson Objectives

  • Identify the organs affected by a respiratory disease.
  • Identify how a respiratory disease can affect the rest of the body.
  • Describe how asthma affects breathing.
  • Outline how smoking affects the respiratory system.
  • Identify what you can do to keep your respiratory system healthy.

Check Your Understanding

  • What is the role of the respiratory system?
  • What are some of the organs in the respiratory system?


  • allergen
  • asthma
  • bronchitis
  • chronic disease
  • chronic obstructive pulmonary disease (COPD)
  • emphysema
  • environmental tobacco smoke (ETS)
  • lifestyle disease
  • lung cancer
  • pathogen
  • pneumonia
  • respiratory disease
  • tuberculosis (TB)

Respiratory System Disease

Most of the time your respiratory system works well. But your respiratory system can sometimes be knocked out of homeostasis. Recall that homeostasis is the balancing act your body performs that keeps everything inside of your body stable. Anything that stops the respiratory system from doing its job disrupts homeostasis. When homeostasis is thrown out of balance, your respiratory system can get diseases. There are many causes of respiratory diseases, and many ways to treat such diseases.

In general, diseases that last a short time are called acute diseases. Other diseases can last for a long time, perhaps years. Diseases that last for a long time are called chronic diseases. Both acute and chronic diseases affect the respiratory system. Respiratory diseases are diseases of the lungs, bronchial tubes, trachea, nose, and throat (Figure below). These diseases can range from a mild cold to a severe case of pneumonia. Respiratory diseases are common and may cause illness or death. Some respiratory diseases are caused by bacteria or viruses, while others are caused by environmental pollutants such as tobacco smoke. Some diseases can be genetic.

This boy is suffering from whooping cough (also known as pertussis), which gets its name from the loud whooping sound that is made when the person inhales during a coughing fit.


Bronchitis is an inflammation of the bronchi, which means they become red and swollen with infection. Acute bronchitis is usually caused by viruses or bacteria, and may last several days or weeks. It is characterized by a cough that produces phlegm, or mucus. Symptoms include shortness of breath and wheezing. Acute bronchitis is usually treated with antibiotics.

Chronic bronchitis may not be caused by a bacterium or a virus. Chronic bronchitis occurs when a cough produces phlegm, for at least three months in a two-year period. Tobacco smoking is the most common cause of chronic bronchitis, but it can be caused by environmental pollution, such as smog and dust. It is generally part of a disease called chronic obstructive pulmonary disease (COPD). Treatments for bronchitis include antibiotics and steroid drugs used to reduce inflammation.


Asthma is a chronic illness in which the bronchioles are inflamed and become narrow, as shown in Figure below. The muscles around the bronchioles contract which narrows the airways. Large amounts of mucus are also made by the cells in the lungs. A person with asthma has difficulty breathing. Their chest feels tight and they wheeze. Asthma can be caused by different things, such as allergies. An allergen is any antigen that is not actually a disease, but your body responds to it as if it were a disease. Allergens can cause allergic reactions. Common allergens that cause asthma are mold, dust, or pet hair.

Asthma can also be caused by cold air, warm air, moist air, exercise, or stress. The most common asthma triggers are illnesses like the common cold. The symptoms of asthma can usually be controlled with medicine. Bronchodilators are drugs that reduce inflammation of the bronchioles and are often used to treat asthma. An inhaler is usually a bronchodilator.

Asthma is not contagious and cannot be passed on to other people. Children and adolescents who have asthma can still lead active lives if they control their asthma. Asthma can be controlled by taking medication and by avoiding contact with environmental triggers for asthma, like smoking.

Asthma occurs when the bronchioles swell and the muscles around the bronchioles contract.


Pneumonia is an illness that occurs when the alveoli become inflamed and filled with some kind of fluid. When a person has pneumonia, gas exchange cannot happen properly across the alveoli. Pneumonia can be caused by many things. Infection by bacteria, viruses, fungi, or parasites can cause pneumonia. An injury caused by chemicals or a physical injury to the lungs can also cause pneumonia. Symptoms of pneumonia include cough, chest pain, fever, and difficulty breathing. Treatment depends on the cause of pneumonia. Bacterial pneumonia is treated with antibiotics.

Pneumonia is a common illness that affects people in all age groups. It is a leading cause of death among the elderly and people who are chronically and terminally ill. Sometimes people take vaccines to prevent certain types of pneumonia.


Tuberculosis (TB) is a common and often deadly disease caused by a genus of bacterium called Mycobacterium. When a disease like TB can passed from person to person, it is called "infectious." Tuberculosis most commonly attacks the lungs, but can also affect other parts of the body. TB is a chronic disease, but most people who become infected do not develop the full disease.

The TB bacteria are spread in the air when people who have the disease cough, sneeze or spit, so it is very contagious. To help prevent the spread of the disease, public health notices, such as the one in Figure below, remind people how to stop the spread of the disease.

A public health notice from the early 20th century reminded people that TB could be spread very easily.


Lung cancer is a disease where the cells found in the lungs grow out of control. The growing mass of cells can form a tumor that pushes into nearby tissues. The tumor will affect how these tissues work. Lung cancer, which is the most common cause of cancer-related death in men and the second most common in women, is responsible for 1.3 million deaths worldwide every year (Figure below). The most common symptoms are shortness of breath, coughing (including coughing up blood), and weight loss. The most common cause of lung cancer is exposure to tobacco smoke.

The inside of lung showing cancerous tissue.


Emphysema is a chronic lung disease caused by the breakdown of the lung tissue. The surfaces of healthy alveoli are springy and flexible. They stretch out a little when full of air and relax when air leaves them. But the breakdown of the tissues that support the alveoli and the capillaries that feed the alveoli cause the alveoli to become hard and stiff. Eventually, the walls of the alveoli break down and the alveoli become larger. When alveoli become larger, oxygen cannot enter the blood as it did before. Symptoms of emphysema include shortness of breath during exercise. Damage to the alveoli, which can be seen in Figure below, is not curable. Smoking is the leading cause of emphysema.

The lung of a smoker who had emphysema (left). The black areas are enlarged alveoli, and tar, a sticky, black substance found in tobacco smoke is evident. Chronic obstructive pulmonary disease (right), is a tobacco-related disease that is characterized by emphysema.

Causes of Respiratory Diseases


Many respiratory diseases are caused by pathogens. A pathogen is an organism that causes disease in another organism. Certain bacteria, viruses, and fungi are pathogens of the respiratory system. The common cold and flu are caused by viruses. The influenza virus that causes the flu is shown in Figure below. Tuberculosis, whooping cough, and acute bronchitis are caused by bacteria. The pathogens that cause colds, flu, and TB can be passed from person to person by coughing, sneezing, and spitting.

This is the influenza virus that causes the swine flu, or H1N1. The Center for Disease Control and Prevention recommends that children between the ages of 6 months and 19 years get a flu vaccination each year.


The quality of the air can affect the health of your lungs. Asthma, heart and lung diseases, allergies, and several types of cancers are all linked to air quality. Air pollution can be either outdoor pollution or indoor pollution. Outdoor air pollution can be caused by car exhaust fumes, smoke from factories and forest fires, volcanoes, and animal feces. These pollutants contain tiny particles that can get “stuck” in the tissues of the respiratory system and irritate the lungs. Indoor air pollution can be caused by tobacco smoke, dust, mold, insects, rodents, and cleaning chemicals.

Lifestyle Choices

Smoking is the major cause of chronic respiratory disease as well as cardiovascular disease and cancer. Exposure to tobacco smoke, by smoking or by breathing air that contains tobacco smoke, is the leading cause of preventable death in the U.S. Regular smokers die about 10 years earlier than nonsmokers do. The Centers for Disease Control and Prevention (CDC) describes tobacco use as "the single most important preventable risk to human health in developed countries and an important cause of [early] death worldwide."

Dangers of Smoking

Tobacco use, particularly cigarette smoking, is a preventable cause of death in the United States. This means that people would not die if they stopped smoking. Cigarette smoking alone is directly responsible for approximately 30 percent of all yearly cancer deaths in the United States. The main health risks of using tobacco are linked to diseases of the cardiovascular system and respiratory system. Cardiovascular diseases caused by smoking include heart disease and stroke.

Diseases of the respiratory system that are caused by exposure to tobacco smoke include:

  • Emphysema.
  • Lung cancer.
  • Cancers of the larynx and mouth.
Cigarette smoking causes 87 percent of lung cancer deaths. Smoking and using tobacco is also linked to the risk of developing other types of cancer, such as pancreatic and stomach cancer.

Indoor Air Pollution

Cigarettes, like the ones shown in Figure below, are a major source of indoor air pollution. Cigarette smoke contains about 4,000 substances, including over 60 cancer-causing chemicals. Many of these substances, such as carbon monoxide, tar, arsenic, and lead, are toxic to the body. Non-smokers can also be affected by tobacco smoke. Exposure to secondhand smoke, also known as environmental tobacco smoke (ETS), greatly increases the risk of lung cancer and heart disease in nonsmokers.

Tobacco use, particularly cigarette smoking, is the single most preventable cause of death in the United States.

Chronic Obstructive Pulmonary Disease

Chronic obstructive pulmonary disease (COPD) is a disease of the lungs that occurs when the airways narrow and air cannot enter the lungs as well as it did before. This leads to shortness of breath. The lack of air entering the lungs usually gets worse over time. COPD is most commonly caused by smoking. Gases and particles in tobacco smoke trigger an inflammatory response in the lung. The inflammatory response in the larger airways is known as chronic bronchitis. In the alveoli, the inflammatory response causes the breakdown of the tissues in the lungs, leading to emphysema.

Keeping Your Respiratory System Healthy

Many of the diseases related to smoking are called lifestyle diseases, diseases that are caused by choices that people make in their daily lives. For example, the choice to smoke can lead to cancer in later life. But there are many things you can do to help keep your respiratory system healthy.

Avoid Smoking

Never smoking or quitting now are the most effective ways to reduce your risk of developing chronic respiratory diseases, such as cancer.

Eat Well, Exercise Regularly, and Get Rest

Eating a healthful diet, getting enough sleep, and being active every day can help keep your immune system strong.

Wash Your Hands

Washing your hands often, and after sneezing, coughing or blowing your nose, helps to protect you and others from diseases. Washing your hands for 20 seconds with soap and warm water can help prevent colds and flu.

Some viruses and bacteria can live from 20 minutes up to 2 hours or more on surfaces like cafeteria tables, doorknobs, and desks. A public health notice that shows people how to prevent the spread of respiratory diseases is shown in Figure below.

Avoid Contact with Others When Sick

Do not go to school or to other public places when you are sick. You risk spreading your illness to other people and getting sicker, if you catch something else.

Visit Your Doctor

Getting the recommended vaccinations can help prevent diseases such as whooping cough and flu. Seeking medical help for diseases such as asthma can help stop the disease from getting worse.

Cover your Cough - Clean your Hands is a public health campaign that reminds people of the quickest and easiest ways to avoid spreading respiratory diseases such as colds and the flu.

Lesson Summary

  • Respiratory diseases are diseases that affect the lungs, bronchial tubes, trachea, nose, and throat.
  • Respiratory diseases can reduce the amount of oxygen that gets into the blood.
  • Asthma is an illness that occurs when the bronchioles are inflamed and become narrow.
  • Difficulty breathing happens because of the inflammation, contraction of the muscles, and the production of mucus by the cells that line the bronchioles.
  • Diseases of the respiratory system that are caused by exposure to tobacco smoke include emphysema, lung cancer and cancers of the larynx and mouth.
  • Cigarette smoking causes 87 percent of lung cancer deaths.
  • Avoid smoking, get enough exercise, and wash your hands in order to protect your respiratory system from illness.

Review Questions


1. Identify two organs that can be affected by respiratory diseases.

2. What lifestyle activity has the largest health impact on the respiratory system?

3. Identify three diseases caused by smoking.

4. Identify three things besides smoking that can cause a respiratory disease.

5. What are two things you can do to keep your respiratory system healthy?

Apply Concepts

6. How might a respiratory disease affect the rest of the body?

7. How does asthma affects the bronchioles?

8. Explain how washing your hands can help prevent you from catching a cold.

Critical Thinking

9. Pneumonia is a disease that causes the alveoli to fill up with fluid. How might this affect the lungs’ ability to absorb oxygen?

10. A person who has never smoked before gets lung cancer. How might they have contracted the disease?

Further Reading / Supplemental Links

Points to Consider

Next, we move on to the excretory system.

  • The excretory system gets rids of a certain type of waste. What type of waste do you think is removed by this system?

The Excretory System

Lesson Objectives

  • Identify the functions of the excretory system.
  • List the organs of the excretory system.
  • Describe the parts of urinary system.
  • Outline how the kidneys filter blood.
  • Identify three disorders of the urinary system.

Check Your Understanding

  • What are some "wastes" that must be removed from your body?
  • Do your circulatory and respiratory systems remove "waste?"


  • excretion
  • excretory system
  • kidney
  • kidney dialysis
  • kidney failure
  • nephron
  • urinary bladder
  • urinary system
  • urinary tract infection (UTI)
  • urination
  • urine

The Excretory System

One of the most important ways your body maintains homeostasis is by keeping the right amount of water and salts inside your body. If you have too much water in your body, your cells can swell and burst. If you have too little water in your body, your cells can shrivel up like an old apple. Either extreme can cause illness and death of cells, tissues, and organs. The organs of your excretory system help to keep the correct balance of water and salts within your body.

Your body also needs to remove the wastes that build up from cell activity and from digestion. These wastes include carbon dioxide, urea, and certain plant materials. If these wastes are not removed, your cells can stop working and you can get very sick. The excretory system can also help to release wastes from the body. Excretion is the process of removing wastes from the body.

The organs of the excretory system are also parts of other organ systems. For example, your lungs are part of the respiratory system. Your lungs remove carbon dioxide from your body, so they are also part of the excretory system. More organs of the excretory system are listed in Table below.

Organs of the Excretory System
Organ(s) Function Other Organ System of which it is Part
Lungs Remove carbon dioxide Respiratory system
Skin Sweat glands remove water, salts, and other wastes Integumentary system
Large intestine Removes solid waste and some water in the form of feces Digestive system
Kidneys Remove urea, salts, and excess water from the blood Urinary system

Table 19.4: Organs of the Excretory System

Functions of the Excretory System

The excretory system controls the levels of water and salts in your body by removing wastes. This means the excretory system has an important role in maintaining homeostasis. Your body takes nutrients from food and uses them for energy, growth, and repair. After your body has taken what it needs from the food, waste products are left behind in the blood and in the large intestine. These waste products need to be removed from the body. The kidneys work with the lungs, skin, and intestines to keep the correct balance of nutrients, salts and water in your body.

The Urinary System

Sometimes, the urinary system is called the excretory system. But the urinary system is only one part of the excretory system. Recall that the excretory system is made up of the skin, lungs, and large intestine as well as the kidneys. The urinary system is the organ system that makes, stores, and gets rid of urine. It includes:

  • Two kidneys.
  • Two ureters.
  • One bladder.
  • One urethra.

The urinary system is shown in Figure below.

The kidneys filter the blood that passes through them and the urinary bladder stores the urine until it is released from the body.

Organs of the Urinary System

  1. As you can see from Figure above, the kidneys are two bean-shaped organs. Kidneys filter and clean the blood and form urine. They are about the size of your fists and are found near the middle of the back, just below your rib cage.
  2. Ureters are tube-shaped and bring urine from the kidneys to the urinary bladder.
  3. The urinary bladder is a hollow and muscular organ. It is shaped a little like a balloon. It is the organ that collects urine.
  4. Urine leaves the body through the urethra.

What is Urine?

Urine is a liquid that is formed by the kidneys when they filter wastes from the blood. Urine contains mostly water, but also contains salts and nitrogen-containing molecules. The amount of urine released from the body depends on many things. Some of these include the amounts of fluid and food a person consumes and how much fluid they have lost from sweating and breathing. Urine ranges from colorless to dark yellow, but is usually a pale yellow color. Light yellow urine contains mostly water. The darker the urine, the less water it contains.

The urinary system also removes a type of waste called urea from your blood. Urea is a nitrogen-containing molecule that is made when foods containing protein, such as meat, poultry, and certain vegetables, are broken down in the body. Urea and other wastes are carried in the bloodstream to the kidneys, where they are removed and form urine.

How the Kidneys Filter Wastes

The kidneys are important organs in maintaining homeostasis. Kidneys perform a number of homeostatic functions.

  • They maintain the volume of body fluids.
  • They maintain the balance of salt ions in body fluids.
  • They excrete harmful nitrogen-containing molecules, such as urea, ammonia, and uric acid.

There are many blood vessels in the kidneys, as you can see in Figure below. The kidneys remove urea from the blood through tiny filtering units called nephrons. Nephrons are tiny, tube-shaped structures found inside each kidney. A nephron is shown in Figure below. Each kidney has up to a million nephrons. Each nephron collects a small amount of fluid and waste from a small group of capillaries.

If the body is in need of more water, water is removed from the fluid inside the nephron and is returned to the blood. The fluid within nephrons is carried out into a larger tube in the kidney called a ureter, which you can see in Figure below. Urea, together with water and other wastes, forms the urine as it passes through the nephrons and the kidney.

Structures of the kidney; fluid leaks from the capillaries and into the nephrons where the fluid forms urine then moves to the ureter and on to the bladder.

The location of nephrons in the kidney. The fluid collects in the nephron tubules, and moves to the bladder through the ureter.

Formation of Urine

The process of urine formation is as follows:

  1. Blood flows into the kidney through the renal artery, shown in Figure below. The renal artery connects to capillaries inside the kidney. Capillaries and nephrons lie very close to each other in the kidney.
  2. The blood pressure within the capillaries causes water, salts, sugars, and urea to leave the capillaries and move into the nephron.
  3. The water and salts move along through the tube-shaped nephron to a lower part of the nephron.
  4. The fluid that remains in the nephron at this point is called urine.
  5. The blood that leaves the kidney in the renal vein has much less waste than the blood that entered the kidney.
  6. The urine is collected in the ureters and is moved to the urinary bladder, where it is stored.

Nephrons filter about ¼ cup of body fluid per minute. In a 24-hour period, nephrons filter 180 liters of fluid, and 1.5 liters of the fluid is released as urine. Urine enters the bladder through the ureters. Similar to a balloon, the walls of the bladder are stretchy. The stretchy walls allow the bladder to hold a large amount of urine. The bladder can hold about 1½ to 2½ cups of urine, but may also hold more if the urine cannot be released immediately.

How do you know when you have to urinate? Urination is the process of releasing urine from the body. Urine leaves the body through the urethra. Nerves in the bladder tell you when it is time to urinate. As the bladder first fills with urine, you may notice a feeling that you need to urinate. The urge to urinate becomes stronger as the bladder continues to fill up.

Brain Control

The kidneys never stop filtering waste products from the blood, so they are always producing urine. The amount of urine your kidneys produce is dependent on the amount of fluid in your body. Your body loses water through sweating, breathing, and urination. The water and other fluids you drink every day help to replace the lost water. This water ends up circulating in the blood because blood plasma is mostly water.

The filtering action of the kidneys is controlled by the pituitary gland. The pituitary gland is about the size of a pea and is found below the brain, as shown in Figure below. The pituitary gland is also part of the endocrine system. The pituitary gland releases hormones, which help the kidneys to filter water from the blood.

The movement of water back into blood is controlled by a hormone called antidiuretic hormone (ADH). ADH is released from the pituitary gland in the brain. One of the most important roles of ADH is to control the body’s ability to hold onto water. If a person does not drink enough water, ADH is released. It causes the blood to reabsorb water from the kidneys. If the kidneys remove less water from the blood, what will the urine look like? It will look darker, because there is less water in it.

When a person drinks a lot of water, then there will be a lot of water in the blood. The pituitary gland will then release a lower amount of ADH into the blood. This means less water will be reabsorbed by the blood. The kidneys then produce a large volume of urine. What color will this urine be?

The pituitary gland is found directly below the brain and releases hormones that control how urine is produced.

Excretory System Problems

The urinary system controls the amount of water in the body and removes wastes. Any problem with the urinary system can also affect many other body systems.

Kidney Stones

In some cases, certain mineral wastes can form kidney stones, like the one shown in Figure below. Stones form in the kidneys and may be found anywhere in the urinary system. They vary in size. Some stones cause great pain, while others cause very little pain. Some stones may need to be removed by surgery or ultrasound treatments.

A kidney stone. The stones can form anywhere in the urinary system.

Kidney failure

Kidney failure happens when the kidneys cannot remove wastes from the blood. If the kidneys are unable to filter wastes from the blood, the wastes build up in the body. Homeostasis is disrupted because the fluids in the body are out of balance. Kidney failure can be caused by an accident that injures the kidneys, the loss of a lot of blood, or by some drugs and poisons. Kidney failure may lead to permanent loss of kidney function. But if the kidneys are not seriously damaged, they may recover.

Chronic kidney disease is the slow decrease in kidney function that may lead to permanent kidney failure. A person who has lost kidney function may need to get kidney dialysis. Kidney dialysis is the process of filtering the blood of wastes using a machine. A dialysis machine (also called a hemodialyzer) filters the blood of waste by pumping it through a fake kidney. The filtered blood is then returned to the patient’s body. A dialysis machine is shown in Figure below.

During dialysis, a patient

Urinary tract infections (UTIs)

Urinary tract infections (UTIs) are bacterial infections of any part of the urinary tract. When bacteria get into the bladder or kidney and produce more bacteria in the urine, they cause a UTI. The most common type of UTI is a bladder infection. Women get UTIs more often than men. UTIs are often treated with antibiotics.

Lesson Summary

  • The excretory system controls the chemical make-up of liquids found in the body.
  • The organs of the excretory system remove wastes. They also maintain the proper levels of water, salts, and nutrients in the body.
  • The lungs, skin, kidneys, and large intestine are all organs in the excretory system.
  • The urinary system is made up of the kidneys, the ureters, the bladder, and the urethra.
  • The filtering parts of the kidneys are the nephrons.
  • Water and waste molecules move out of the blood capillaries and into the nephrons. Most of the water returns to the blood.
  • Urine collects in the nephron and moves to the urinary bladder through the ureters.
  • The filtering action of the kidneys is controlled by the pituitary gland.
  • ADH is the hormone released by the pituitary gland and controls the how water is reabsorbed by the blood from the kidneys.
  • Disorders of the urinary system include kidney stones, kidney disease, and urinary tract infections.

Review Questions


1. What is the main function of the excretory system?

2. List the organs that make up the excretory system.

3. What is urine made of?

4. What is the purpose of the urinary bladder?

5. What connects the kidneys to the urinary bladder?

Apply Concepts

6. What is the difference between the urinary system and the excretory system?

7. How do the kidneys filter the blood?

8. The walls of the urinary bladder are stretchy. What do you think is the advantage of having these stretchy walls?

9. What does antidiuretic hormone (ADH) do?

10. What is a urinary tract infection?

11. Why is kidney failure such a serious health problem?

Critical Thinking

12. If a person's urine is dark brown, what are two organs in the body that might not be functioning properly? Explain what might be wrong with the two organs.

Further Reading / Supplemental Links

Points to Consider

Next we turn our attention to the nervous system.

  • What do you think the nervous system is? What do you think it does?