Please Limited time Offer!


Gaseous Exchange

  • This is the process by which respiratory gases (oxygen and carbon IV oxide) are passed across the respiratory surface.
  • Gases are exchanged depending on their concentration gradient.
  • In simple organisms such as amoeba, diffusion is enough to bring about gaseous exchange.
  • CO2 diffuses out into the surrounding water while oxygen diffuses from the water across the plasma membrane into the amoeba.


Importance of Gaseous Exchange

  1. Promote oxygen intake for respiration.
  2. Facilitate carbon IV oxide removal from the body as a metabolic waste product.

Gaseous Exchange in Plants

  • During the day, green plants take in carbon IV for photosynthesis.
  • Oxygen is given out as a byproduct of photosynthesis and is released into the atmosphere.

Examples of respiratory Surfaces in Plants

  • Stomata in leaves
  • Roots e.g. pneumatophores
  • Lenticels in woody stems

Structure and Function of the Stomata

  • They are tiny openings on the leaf surfaces. They are made up of two guard cells.
  • Guard cells are the only epidermal cells containing chloroplasts. They regulate the opening and closing of the stomata.

Adaptations of Guard Cells

  1. They are bean shaped/sausage shaped.
  2. Contain chloroplast hence can photosynthesize.
  3. Inner walls are thicker while outer wall is thin to facilitate the opening and closing of stomata.


Mechanism of Opening and Closing of Stomata

  • There are three theories that try to explain how the stomata open and close.
  • Photosynthetic theory
  • Starch Sugar inter-conversion Theory. (effect of changes in pH of guard cells)
  • Potassium Ion Theory.

Photosynthetic theory

  • During the day, guard cells photosynthesize forming glucose.
  • This glucose increases the osmotic pressure in the guard cells.
  • Guard cells draw in water from the neighbouring epidermal cells and become turgid.
  • The stoma opens.
  • During the night, there is no photosynthesis due to absence of light.
  • Glucose is converted into starch lowering the osmotic pressure in the guard cells.
  • Guard cells lose water and become flaccid closing the stomata.
  • Starch Sugar inter-conversion Theory. (effect of changes in pH of guard cells)
  • This is under the influence of pH in the guard cells.
  • During the day CO2 is used up during photosynthesis raising the pH in the guard cells.
  • In this high pH, enzymes convert more starch into glucose.
  • Osmotic pressure of the guard cells increases and water enters into them, making them turgid hence opening the stomata.
  • During the night, there is no photosynthesis. The level of CO2 increases lowering the pH.
  • Enzymes become inactivated and starch is not converted into glucose.
  • Osmotic pressure of guard cells falls making them to lose water by osmosis.
  • Guard cells become flaccid and stoma closes.

Mechanism of Gaseous Exchange in Plants

  • Oxygen diffuses from the atmosphere where it is more concentrated into the plant.
  • CO2 diffuses out as a metabolic waste product along a concentration gradient into the atmosphere.
  • Gaseous Exchange through the Stomata
  • Stomata are modified in number of ways depending on the habitat of the plant.

Xerophytes: These are plants adapted to life in dry areas.

  • They have less number of stomata that are small in size.
  • Stomata may be sunken, hairy and in some they open during the night and close during the day.

Hydrophytes: These are the aquatic plants (water Plants)

  • They have many stomata that are large in size and mainly found on the upper leaf surface.
  • Hydrophytes have the aerenchyma tissue with large air spaces to store air for gaseous exchange.


Mesophytes: They are plants growing in areas with adequate amounts of water.

  • They have a fairly large number of stomata found on both leaf surfaces.
  • Gaseous Exchange through the Lenticels
  • They are openings found on woody stems and they are made of loosely packed cells.
  • They allow gaseous exchange between the inside of the plant and the outside by diffusion.
  • Actual gaseous exchange occurs on some moist cells under the lenticels.


  • Gaseous Exchange through the Roots
  • Plants like the mangroves growing in muddy salty waters have specialized aerial breathing roots called pneumatophores.
  • Pneumatophores rise above the salty water to facilitate gaseous exchange.

Gaseous Exchange in Animals

Types and Characteristics of Respiratory Surface

  • Different animals have different respiratory surfaces depending on the animal’s size, activity and the environment in which it operates as shown below.
Type of Respiratory SurfaceEnvironment/Medium of OperationExample of Organism
Cell Membrane.WaterAmoeba
Gill FilamentsWaterFish
Buccal CavityAirFrog
  • The respiratory surface is the basic unit of any breathing system upon which actual gaseous exchange occurs by diffusion.
  • Respiratory surfaces have the following main characteristics.
  • Must have a large surface area.
  • Must be moist to allow gases to diffuse in solution form.
  • Have a dense network of blood capillaries for efficient gaseous exchange.
  • Have a thin membrane to reduce the diffusion distance.

Gaseous Exchange in Insects

 Insects have their gaseous exchange system made of many air tubes forming the tracheal system.

  • Tracheal system is made up of spiracles and Tracheoles.
  • Spiracles are external openings found on both sides of the abdomen and thorax.
  • Spiracles have valves to control their opening and closing. They also have hairs to prevent excessive water loss from the body tissue.
  • Spiracles open into tubes called trachea. Trachea is reinforced with spiral bands of chitin to keep them open.
  • Trachea subdivides into finer air tubes called Tracheoles. Tracheoles are in direct contact with body tissues and organs and they supply individual cells with oxygen.
  • Tracheoles do not have bands of chitin and therefore they allow gaseous exchange across their thin moist walls.


 Mechanism of Gaseous Exchange in the Tracheal System of an Insect

  • Air is drawn into and out of the tracheal system by muscular movement of the abdominal wall.
  • When spiracle valves are open, air is drawn into the tracheal system. The valves close and air is forced along the system by muscle movement.
  • Oxygen diffuses into the tissue fluid and into the cells.
  • CO2 diffuses out of the cells and into the tissue fluid then into the tracheal system.

Gaseous Exchange in Fish

  • The breathing system of the fish consists of the following;
    • Mouth (buccal) cavity.
    • Gills.
    • Opercular cavity.
    • Operculum.
  • Gills are made of a long curved bone called the gill bar.
  • Gill filaments arise from one side of the gill bar. They are many and suspend freely in water providing a large surface area for gaseous exchange.
  • Gill rakers arise from the other side of the gill bar. They are teeth like and they prevent solids present in water from damaging the delicate gill filaments.
  • Blood vessels enter the gill bar and branch into the gill filaments as blood capillaries.
  • Operculum is found on either side of the body near the head and it also protects the delicate gills.


Mechanism of Gaseous Exchange in the Gills of a Bony Fish

  • Floor of the mouth cavity is lowered increasing the volume of the mouth cavity but lowering the pressure.
  • Water flows into the mouth cavity and the operculum closes.
  • Operculum on either side bulge outwards without opening. This increases volume in the gill cavity but the pressure drops.
  • Water containing dissolved oxygen flows from the mouth cavity to the gill chamber over the gills.
  • The mouth closes and the floor of the mouth cavity is raised.
  • The remaining water in the mouth is forced to flow towards the gill chamber.
  • Oxygen diffuses from the water into the blood through the thin walls of the gill filaments. It combines with haemoglobin for transportation to all body parts.
  • CO2 diffuses from the blood into the flowing water.
  • To ensure maximum gaseous exchange, the water flowing over the gills and the blood in the gills flows in opposite directions.
  • This is called counter current flow system and it ensures that at all the points, concentration of oxygen is always higher in the water than in the blood.


  • If the water and blood were flowing in the same direction, gaseous exchange will not be that effective.
  • Where the oxygen is 50% in water, there is no concentration gradient because blood also has 50% oxygen concentration.


Mechanism of Gaseous Exchange in Amphibians

  • Amphibians live on both land and water and therefore exhibit the following methods of gaseous exchange.
  • Gaseous exchange through the lining of the buccal cavity
  •  Gaseous exchange through the lungs
  • Gaseous exchange through the skin
  • Gaseous exchange through the mouth (buccal) cavity
  • Air is taken in or expelled from the mouth cavity by raising and lowering of the floor mouth.
  • Lining of the mouth cavity is moist to dissolves oxygen.
  • There is a rich supply of blood capillaries under the lining of the mouth cavity. Oxygen diffuses into the blood and is carried by haemoglobin to all parts of the body.
  • Carbon IV oxide from the tissues is brought by the blood to the mouth cavity where diffuses out.
  • Gaseous exchange through the lungs
  •  The frog has two lungs which are connected to the buccal cavity.
  • T he inner lining of the lungs is moist, thin and is richly supplied with blood capillaries.
  • During inspiration, the floor of the mouth cavity is lowered and nostrils are open. Air rushes through the open nostrils into the mouth cavity.
  • Nostrils close and the floor of the mouth cavity is raised. This reduces the volume and increase the pressure in the mouth cavity forcing air into the lungs.
  • Carbon IV oxide from the tissues diffuse into the lung while the oxygen from the lungs diffuses into the tissues.
  • Gaseous exchange through the skin
  • Frogs have a thinner and moist skin than the toads.
  • There is large network of blood capillaries below the skin to carry the respiratory gases.
  • Oxygen from the air and water diffuse through the skin into the blood stream.
  • Carbon IV oxide diffuses out of the blood capillaries through the moist skin into the surrounding water and air.

Mechanism of Gaseous Exchange in Mammals

  • The following structures are involved in gaseous exchange in mammals;
  • Nose (Nostrils)
  • Larynx
  • Trachea
  • Chest cavity (ribs and intercostals muscles)
  • Diaphragm.
  • Nose
  • It has two openings called nostrils which let in air into the air passages.
  • As air moves in the passages, it is warmed and moistened
  • The lining of the nasal cavity has also the sense organs for smell.
  • Larynx
  • It is located on top of the trachea
  • It is called the voice box. It controls the pitch of the voice.
  • Trachea
  • It is a tube made of rings of cartilage which prevents it from collapsing during breathing.
  • Inside it is lined with ciliated epithelium. Cilia beat in waves and move mucus and foreign particles away from the lungs towards the pharynx.
  • As the trachea enters the lungs, it divides into two branches called Bronchi (Bronchus).
  • Lungs
  • They are found in the chest cavity and they are enclosed by a double membrane called the pleural membrane.
  • The space between the membranes is called the pleural cavity.
  • Pleural cavity is filled with pleural fluid which reduces friction making the lungs to move freely in the chest cavity during breathing.


  • In the lungs each bronchus divides into small tubes called bronchioles.
  • Bronchioles branch further to form air sacs called alveoli (alveolus)
  • Alveolus is covered by a fine network of blood capillaries.

The mechanism of breathing

  • Breathing is achieved by changes in the volume and air pressure of the thoracic cavity.
  • Thoracic cavity is enclosed by ribs.
  • Ribs are covered by intercostals muscles.
  • The diaphragm is a muscular sheet of tissue below the chest cavity. It curves upwards in the form of a dome shape.
  •  Breathing mechanism involves two processes.
  • Inspiration (Inhalation) i.e. breathing in.
  • Expiration (Exhalation) i.e. breathing out.

Inspiration (Inhalation) i.e. breathing

  • This occurs when the volume of thoracic cavity increases and the pressure decreases.
  • External intercostals muscles contract while the internal intercostals muscles relax.
  • Ribs are pulled upwards and outwards.
  • Diaphragm flattens increasing the volume of the thoracic cavity while decreasing the pressure inside it.
  • Air rushes into the lungs through the nose and trachea inflating the lungs.

Diagrams page 62

Expiration (Exhalation) i.e. breathing out

  • Volume of thoracic cavity decreases while pressure increases. This is brought about by the following;
  • External intercostals muscles relax while internal ones contract.
  • Ribs move downwards and inwards.
  • Diaphragm relaxes and regains its original dome shape.
  • Volume of the thoracic cavity decrease and pressure increases.
  • Air is forced out of the lungs through the air passages to the atmosphere.

Gaseous exchange in the alveolus

  • Alveoli and blood capillaries are made of very thin walls.
  • The wall of the alveolus is covered b a film of moisture which dissolves oxygen in the inhaled air.
  • Oxygen diffuses through the epithelium of the alveolus, the capillary wall and through the cell membrane of the red blood cells.
  • In the red blood cells it combines with haemoglobin.
  • Carbon (iv) oxide is more concentrated in the blood capillaries than in the alveoli.
  • It therefore diffuses from the capillaries into the alveoli.
  • Water vapour also passes out of the blood by the same process.

Diagram page 64 KLB

Percentage composition of gases in inhaled and exhaled air

Gas%  in inhaled air.% in exhaled air
Carbon (iv) oxide0.034.0
Nitrogen and other gases79.9779.97

Regulation of Breathing

This is controlled by a part of the brain called Medulla oblongata.

Factors affecting the rate of breathing in humans

  1. Exercise

Breathing rate increases during vigorous activity.

  • Age

Younger people have a faster breathing because their bodies have more energy demand.

  • Emotions

Things like anxiety, fear and fright increases the breathing rate.

  • Temperature

Relatively high temperatures increase the rate of breathing.  However, very high temperatures reduce the breathing rate.

  • Health

If there is fever (high body temperature), the breathing rate increases. Some respiratory diseases however, make breathing difficult.

Lung Volumes

  1. Lung capacity

This is the total amount of air the lungs can hold when completely filled. The lungs of an adult have a capacity of about 5,500cm3

  1. Tidal volume

This is the amount of air taken in and out of the lungs during normal breathing. Tidal volume is about 500cm3

  1. Inspiratory reserve volume

This is an additional volume attained after having a forced inhalation in addition to the tidal volume. It is about 2000cm3

  1. Inspiratory capacity

This is the tidal volume +Inspiratory reserve volume.

  • Expiratory reserve volume

This is air removed after a forced exhalation. It can be up to 1,300cm3

  • Vital capacity

This is the deepest possible exhalation. This air can only be forcibly pushed out of the lungs.

  • Residual volume

This is the air that normally remains in the lungs after the deepest exhalation. It is normally about 1,500cm3


Diseases of the Respiratory System

  1. Asthma

It is caused by:

  • Allergens such as pollen grains, certain foods and drugs
  • Infections of the lungs by bacteria and viruses


  • Difficulty in breathing
  • Wheezing sound when breathing

Treatment and Control

  • Avoiding the causative agents
  •  Injection of drugs and oral application of pills
  • Spraying directly into the bronchial tubes with a muscle relaxant
  • Bronchitis

There are two types; Acute and Chronic


  • Production of thick greenish or yellowish sputum
  • Difficulty in breathing
  • Difficulty in walking and sleeping


  • Seeking early medical assistance
  • Whooping cough

It is caused by a bacterium called Bordetella pertussis.


  • Prolonged coughing and vomiting
  • Conjuctival haemorrhage (bleeding)
  • Convulsions and coma
  • Severe pneumonia in the bronchioles
  • Ulcers and heart complications
  • Emaciation due to repeated vomiting


  • Use of antibiotics
  • Use of a balanced diet on patients


  • Children immunization at early age
  • Pneumonia

It is caused by a bacterial called Streptococcus pneumoniae


  • Coughing
  • Fever
  • Chest pains
  • Deposits of fluids in the lungs


  • Use of antibiotics such as penicillin and sulphonamides


  • Avoid overcrowding.
  • Good ventilation in living houses
  • Pulmonary Tuberculosis

It is caused by a bacterium called Mycobacterium tuberculosis.


  • Weight loss
  • Coughing with blood stained sputum.
  • Fever


  • Use of antibiotics such as streptomycin


  • Pasteurization of milk
  • Immunization using BCG (Bacille Calmette Guerin)
  • Use of radiography (X-Ray)
  • Lung cancer

Cancer is uncontrolled cell growth in the body causing tumours.

Some general causes

  • Smoking
  • Inhalation of cancer causing substances such as asbestos
  •  Exposure to radiations such as X-rays, radioactive substances such as uranium and substances that alter the genetic composition of the cell such as mustard gas

Treatment and control

  • Surgery to remove the tumour
  • Radiotherapy
  • Chemotherapy
  • Use of some drugs
  • Not smoking
Scroll to Top