Breathing and Respiration


This lesson will cover:
  1. The structure of the breathing system
  2. The mechanism of breathing
  3. Gas exchange and transport of gases
  4. Aerobic respiration
  5. The dangers of smoking


By the end of this lesson you will be able to:
  1. Identify the structure of the breathing system.
  2. Understand how air is inhaled and exhaled.
  3. Identify how carbon dioxide and oxygen are exchanged in the lungs and transported to and from the lungs.
  4. Understand respiration experiments.
  1. Structure of the Breathing System
    All organisms need energy to survive. They release energy from their food by the process of respiration. This process uses oxygen from the air and produces the waste gas, carbon dioxide.
    It is the function of the breathing system to exchange these gases between the air and the blood, taking in oxygen and excreting carbon dioxide. The lungs are in the upper part of the body called the thorax. They are protected by the ribcage.
    Given below is a brief description of the parts of the breathing system:
    • Trachea / windpipe: Keep the airways open.
    • Air sacs / alveoli: Provide a large surface area for gas exchange.
    • Diaphragm: Sheet of muscle, which is important for breathing movements.
    • Rib / intercostal muscles: Used in breathing movements.
    • Bronchi: Tubes leading into the lungs which have rings of cartilage to keep the airways open.
    • Ribs: Protect the heart and lungs and are where the intercostal or rib muscles are attached.
    The Lining of Air Passages
    The nose and air passages such as the trachea, bronchi and bronchioles are lined with mucous membrane. This is a layer of special cells which produce a sticky fluid called mucus. The mucus traps dirt and germs in the air you breathe. Tiny hairs called cilia move the mucus to the back of your throat where it is swallowed. The mucus and cilia keep the lungs clean and reduce the chance of infections.

  2. The Mechanism of Breathing
    There are two sets of muscles that help you to breathe in and out - the intercostal muscles and the diaphragm. The intercostal muscles are located between the ribs. The diaphragm separates the thorax from the abdomen.
    Inhalation and Exhalation
    Breathing in is called inhalation. When you breathe in, the intercostal muscles contract, pulling the ribcage up and outwards. At the same time, the diaphragm contracts and becomes flatter.
    These movements increase the volume of the thorax so the pressure decreases and air is pushed into the lungs through the nose and mouth. Breathing out is called exhalation.
    When you breathe out, the intercostal muscles relax so the ribcage moves down and inwards.
    The diaphragm relaxes, becoming dome-shaped. These movements decrease the volume of the thorax so the pressure increases and air is pushed out of the lungs.

  3. Gas Exchange and transport of gases
    The function of the breathing or gas exchange system is to move air containing oxygen, into the lungs. The air rich in carbon dioxide, is moved out of the lungs.
    Test For Carbon Dioxide
    Limewater can be used to test for carbon dioxide. When carbon dioxide is bubbled through limewater, it turns from clear to milky. This experiment shows that exhaled air contains more carbon dioxide than inhaled air. The carbon dioxide breathed out is a waste product of respiration. By breathing on to a mirror, you can show that water vapour is breathed out.
    If you test this vapour with a piece of cobalt chloride paper, it will turn from blue to pink.
    This shows that the liquid on the mirror is water.
    Gases Present in Inhaled and Exhaled Air
    You can compare the percentage of gases in inhaled and exhaled air. The percentage of nitrogen doesn't change because the body doesn't use nitrogen. The percentage of oxygen in exhaled air is less because it is used in respiration. The percentage of carbon dioxide increases because it is produced in respiration. The amount of water in inhaled air depends on where you are but you exhale more water vapour because this is also produced during respiration. Exhaled air is also warmer and cleaner than inhaled air. Heat energy is produced during respiration and particles of dirt are trapped by the mucus and cilia lining the air passages.
    Gas Exchange in the Lungs
    Gas exchange is the movement of oxygen from the lungs into the blood and the movement of carbon dioxide in the opposite direction, from the blood into the lungs to be excreted. Gas exchange takes place in the alveoli. The gases move by a process called diffusion. Diffusion is the movement of a substance from an area where it is in high concentration to an area of lower concentration. Oxygen and carbon dioxide are small molecules so they can diffuse across cell membranes.
    Adaptations of Alveoli for Gas Exchange
    The lungs have millions of alveoli that are situated at the ends of the bronchioles. These alveoli are specially adapted for their function of gas exchange. The large number of alveoli provide a large surface area for gas exchange. This means that a lot of gas can diffuse at the same time.
    The alveoli have a moist lining so the oxygen dissolves first, speeding up diffusion into the blood.
    The walls of the alveoli are one cell thick so the gases don't have far to travel and diffuse more quickly. The alveoli have a good blood supply to transport the gases quickly to and from the lungs.
    Transport of Gases
    Gases enter and leave the blood through capillaries, which have walls that are only one cell thick. Once the oxygen has diffused from the alveoli into the blood, it attaches to the red blood corpuscles to be transported to the respiring tissues. The red blood corpuscles are packed with a red pigment called haemoglobin, to which the oxygen attaches, forming oxyhaemoglobin.
    Blood leaving the lungs is rich in oxygen and is called oxygenated blood. The oxyhaemoglobin splits up at the tissues, releasing the oxygen from the red blood corpuscles. The oxygen then diffuses out of the blood, across the capillary walls into the cells.
    Carbon Dioxide
    Carbon dioxide is produced by respiring cells in the tissues. It must be removed quickly because it is poisonous. It diffuses out of the tissues into the blood where it dissolves in plasma and is transported to the lungs where it is breathed out. This blood, which has a high concentration of carbon dioxide and a low concentration of oxygen, is called deoxygenated blood.
    Respiration and Life Processes
    Respiration is one of the seven life processes carried out by all living organisms. It is a series of chemical reactions that occurs in all living cells to release energy from food. The energy released is used for all life processes. Energy enables muscles to contract. Energy is needed for growth. It is used to maintain a steady body temperature.

  4. Aerobic Respiration
    Respiration usually occurs in the presence of oxygen. This is called aerobic respiration.
    Fuel in the form of glucose combines with oxygen. Carbon dioxide and water are produced. Aerobic respiration releases a lot of energy.
    Comparing Respiration and Burning
    Aerobic respiration can be compared to burning or combustion. Both reactions release energy from a fuel. Both use oxygen and release carbon dioxide. The main difference between the two reactions is the rate at which they release energy. Respiration releases energy more slowly than burning and its rate can be controlled.
    To show that living organisms produce heat during respiration:
    Consider two thermos flasks, A and B. Flask A contains germinating peas. Flask B contains dead peas which had been boiled. Initially thermometers kept in the both the flasks show the same temperature. The flasks are allowed to stand for five days. It is found that there is a rise in temperature level in the thermometer in Flask A. Germinating peas respire and release heat. Dead peas do not respire and do not release heat. Therefore there is no rise in temperature in the thermometer in Flask B.
    Respiration in Plants
    In plants, some of the glucose produced during photosynthesis is used in respiration.
    Plants use energy for growth and for heat. Respiration takes place throughout both day and night. Most plants grow badly when their roots don't have enough oxygen for respiration. If fields flood after heavy rain, plants can die because there is insufficient oxygen present in the soil for the roots to respire and produce the energy required for active transport. Aquatic plants are adapted to grow in waterlogged conditions as their roots can respire without oxygen.

  5. Dangers of Smoking
    Tobacco smoke contains many harmful chemicals including nicotine which is a poisonous, addictive drug. It also damages the heart, blood vessels and nerves. Smokers become addicted to nicotine and so find it hard to give up smoking. Tar causes lung cancer and other types of cancers. This has been proved by comparing the numbers of smokers and non-smokers who develop cancer.
    Carbon monoxide is a poisonous gas. It reduces the amount of oxygen that can be carried in the blood by irreversibly combining with the haemoglobin in red blood cells. In pregnant women, this can deprive the developing foetus of oxygen resulting in a low birth mass or a premature birth.
    Tiny particles in the smoke get trapped in the lining of the trachea and bronchial tubes and extra mucus is produced.
    Chemicals in the smoke paralyse the tiny cilia which normally clear the mucus out of the air passages. The only way to clear this is by coughing. Because the lungs cannot be kept clean, smokers often develop bronchitis and chest infections. Repeated coughing causes the delicate walls of the alveoli to be damaged, which reduces the surface area for gas exchange. This is one of the reasons why smokers are often short of breath. The lungs can develop large holes which blow up like balloons. This condition is called emphysema.


This lesson has covered:
  1. In humans the organs of breathing are the lungs. Air is drawn down the windpipe to the bronchi, the bronchioles and then to tiny air sacs called the alveoli.
  2. During inhalation the intercostal muscles cause the rib cage to rise up while the diaphragm moves down.
  3. During exhalation the muscles relax; the rib cage moves down and the diaphragm moves up.
  4. Gaseous exchange occurs in the alveoli. Oxygen moves from the alveoli to the blood capillaries and carbon dioxide moves from the blood capillaries to the alveoli.
  5. Limewater turns milky when carbon dioxide gas is bubbled into it. This test is used to show that exhaled air contains more carbon dioxide than inhaled air.
  6. Water changes the colour of blue cobalt chloride paper to pink.
  7. Aerobic respiration is the process by which a fuel such as glucose is combined with oxygen to produce carbon dioxide, water and energy.
  8. Respiration is the opposite process to photosynthesis.
  9. Smoking can damage the lungs and blood vessels and tar from smoking can cause lung cancer.