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.
Diagram
Importance of Gaseous Exchange
- Promote oxygen intake for respiration.
- 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
- They are bean shaped/sausage shaped.
- Contain chloroplast hence can photosynthesize.
- Inner walls are thicker while outer wall is thin to facilitate the opening and closing of stomata.
Diagram
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.
Diagrams
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.
Diagram
- 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 Surface | Environment/Medium of Operation | Example of Organism |
Cell Membrane. | Water | Amoeba |
Gill Filaments | Water | Fish |
Tracheoles | Air | Insects |
Alveoli/Lungs | Air | Mammals |
Birds | ||
Frogs | ||
Reptiles | ||
Skin | Water | Frog |
Air | Earthworm | |
Buccal Cavity | Air | Frog |
- 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.
Diagram
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.
Diagram
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.
Diagram
- 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.
Diagram
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.
Diagrams
- 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 |
Oxygen | 20 | 16.9 |
Carbon (iv) oxide | 0.03 | 4.0 |
Nitrogen and other gases | 79.97 | 79.97 |
Regulation of Breathing
This is controlled by a part of the brain called Medulla oblongata.
Factors affecting the rate of breathing in humans
- 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
- 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
- Tidal volume
This is the amount of air taken in and out of the lungs during normal breathing. Tidal volume is about 500cm3
- Inspiratory reserve volume
This is an additional volume attained after having a forced inhalation in addition to the tidal volume. It is about 2000cm3
- 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
Diagram
Diseases of the Respiratory System
- Asthma
It is caused by:
- Allergens such as pollen grains, certain foods and drugs
- Infections of the lungs by bacteria and viruses
Symptoms
- 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
Symptoms
- Production of thick greenish or yellowish sputum
- Difficulty in breathing
- Difficulty in walking and sleeping
Treatment
- Seeking early medical assistance
- Whooping cough
It is caused by a bacterium called Bordetella pertussis.
Symptoms
- Prolonged coughing and vomiting
- Conjuctival haemorrhage (bleeding)
- Convulsions and coma
- Severe pneumonia in the bronchioles
- Ulcers and heart complications
- Emaciation due to repeated vomiting
Treatment
- Use of antibiotics
- Use of a balanced diet on patients
Control
- Children immunization at early age
- Pneumonia
It is caused by a bacterial called Streptococcus pneumoniae
Symptoms
- Coughing
- Fever
- Chest pains
- Deposits of fluids in the lungs
Treatment
- Use of antibiotics such as penicillin and sulphonamides
Control
- Avoid overcrowding.
- Good ventilation in living houses
- Pulmonary Tuberculosis
It is caused by a bacterium called Mycobacterium tuberculosis.
Symptoms
- Weight loss
- Coughing with blood stained sputum.
- Fever
Treatment
- Use of antibiotics such as streptomycin
Control
- 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