RESPIRATORY SYSTEM
At the end of this topic, students should be able to:
describe the different types of respiratory systems;
list the characteristics of a respiratory surface;
draw and label some of the respiratory organs of some animals;
explain the various mechanisms of respiration in some animals e.g. mammals;
describe the mechanism of exchange of gases through the stomata of plants
LESSON PRESENTATION
RESPIRATION: This is a biochemical activity of the cell in which glucose is broken down by series reactions controlled by enzymes to release energy, in the presence or absence of oxygen.
Note: the main reason for respiration is to release energy for daily activities.
Importance of energy to living organisms
Energy is required for transportation of materials within the body;
It is needed by animals for movement;
It is required for the maintenance of body temperature with heat energy;
It is required for the synthesis of substance such as protoplasm and other body materials needed for body growth and repair of the body;
Energy is required for reproduction;
It is important in excretion in living organisms;
Energy is required in the transmission of nerve impulses and co-ordination;
It helps to produce sound e.g. speaking in human;
It helps to produce electrical shocks for offence or defence as in catfish;
Types of Respiration
Aerobic respiration: it requires oxygen to break down/oxidize glucose/simple sugar (substrates) in the cell to release carbon (IV) oxide and water.i.e.
C6H12O6 + 6O2 →6CO2 + 6H2O + 38ATP
Anaerobic respiration: it does not require oxygen to break down glucose in the cells to release alcohol, carbon dioxide and 2 ATP as the products. i.e.
C6H12O6 → C2H5OH + 2CO2 + 2ATP
NOTE:
Aerobic respiration is commonly carried out by most living things
Some micro-organisms such as yeasts, bacteria, fungi, and endoparasites such as tapeworms, and roundworms respire anaerobically.
Anaerobic respiration of yeast is referred to as alcoholic fermentation
Terrestrial trees growing in waterlogged soil respire anaerobically and are able to stay alive while the water lasts.
Conditions Necessary for Respiration
Presence of respiratory medium e.g. air and water
Presence of respiratory structure/organ e.g. body surfaces, gills, tracheae, and lungs
Presence of transport medium e.g. blood
Adequate ventilation
Presence of respiratory surface.
GENERAL PROPERTIES/CHARACTERISTICS OF RESPIRATORY SURFACES
It must have large surface area to facilitate high rate of gaseous exchange
It must be moist to enable oxygen dissolve (i.e. diffusion) before entering the blood or body fluid
It must be permeable to allow free passage of gases
It must be thin-walled to shorten diffusion distance
It must have adequate supply of transport medium i.e. blood
It must be highly vascularized i.e. richly supplied with blood vessels
Types of respiratory surfaces, examples and environment in which each functions
MECHANISM OF RESPIRATORY SYSTEM IN LOWER ANIMALS
In unicellular organisms, such as Amoeba, there are no special respiratory organs, but instead, they use plasma membrane/or body surface. Oxygen intake and expulsion of carbon dioxide is by diffusion through the entire body surface. Oxygen in water diffuses into the body while carbon dioxide diffuses out of the body. This mechanism is effective because the organisms have large surface area to volume ratio.
MECHANISM OF RESPIRATION IN EARTHWORM
The skin of an earthworm is thin and protected by a thin cuticle secreted by epidermal cells. The skin is always kept moist by a slimy mucus substance. Oxygen diffuses through the moist skin and dissolves in the blood. The dissolved oxygen combines with haemoglobin of the blood to form oxy-haemoglobin and circulated to all the cells of the body. After internal respiration, the carbon dioxide produced diffuses out of the cells into the blood. The carbon dioxide then diffuses from the blood through the skin to the environment.
Note: respiration over moist body in Annelids (e.g. earthworms) is possible because:
their cylindricalshape gives a high surface area to volume ratio so that the rate at which oxygen diffuses is sufficient to meet the worm’s requirements; and
the cells in their body surface (epidermis) have a rich supply of blood capillaries.
MECHANISM OF RESPIRATION IN INSECTS
Air enters the body through small openings called spiracles located along the sides of the body. The spiracles lead into tubes called tracheae, which branch repeatedly within the body tissues. The smaller branches are called tracheoles. The smallest tracheoles contain fluid in which oxygen dissolves before actually reaching the individual body cells.
Note:
The tracheal system includes the spiracles, tracheae, and tracheoles.
Tracheal system is used by small arthropods such as insects, because they have a small body size not more than two centimeters, hence there is large surface area to volume ratio for diffusion to take place over a very short distance.
MECHANISM OF REPIRATION IN FISH
The Gill
The organ for respiration in fish is the gill. The gills are located at both sides of the head region. Fish has three to four gills arranged in the gill chamber. Each gill has two rows of soft gill filaments. The filaments are tiny, long and numerous structures which are well supplied with blood vessels. In bony fish, such as Tilapia, the four gills of each side are covered with a bony plate called operculum. The filaments are attached to a curved bony structure called gill arch. The inner sides of the gill arch are short bony structures called gill rakers.
Functions of gill parts/or structures
Note: the nostrils of fish are not used for breathing, but rather for smelling.
How fish respires:
When the fish wants to breathe, it first closes its opercula;
and then opens its mouth and lowers the floor of the mouth;
Water with the dissolved oxygen rush into the fish’s mouth;
The fish then closes its mouth, raises the floor of the mouth and the water rush into the gill chamber; and move across the gill filaments;
Oxygen in the water then diffuses into the gill filament while carbon dioxide diffuses out of the body into the water;
After the gaseous exchange, the fish now opens its operculum and the water containing dissolved carbon dioxide then passes out of the body into the river or ocean.
Reasons why a fish will die on land, despite the fact there is plenty of oxygen (about 40 times of oxygen in water) on land
Suffocation: without the support of water, the delicate gills tissues collapsed against each other drastically reducing the gill surface area;
Drying out: the membrane of the gills must be kept moist, hence due to lack of water, it dries out i.e. desiccate.
GASEOUS EXCHANGE THROUGH EXTERNAL GILLS
Tadpoles of frogs, toads and salamanders (all belonging to the phylum Amphibia) have external gills for gaseous exchange. The external gills are well supplied with blood vessels. Oxygen dissolved in water is absorbed by diffusion through the external gills and carbon dioxide diffuses from the blood vessels of the gills into the water.
GASEOUS EXCHANGE IN TOAD
Toad respires using:
Skin i.e. cutaneous respiration
Mouth i.e. buccal respiration
Lung i.e. pulmonary respiration
Cutaneous respiration:
The toad uses its skin to respire when on land and in water.
How the skin of a toad is adapted to respiration/ or gaseous exchange
It has a large surface area and a thin covering;
It is almost moist because of continuous secretions from the mucus gland;
It is well vascularized, i.e. it is well supplied with blood capillaries, hence oxygen diffuses easily and faster and transported to cells and tissues;
Carbon dioxide from the respiratory diffuse out from the blood into the capillaries through the skin of the body.
Buccal respiration
The toad uses its mouth to respire when on land.
How the mouth of a toad is adapted to gaseous exchange
It is very large i.e. it has a large surface area
It has a thin mucus membrane for easy diffusion
It is well supplied with blood capillaries
How a toad respire using its mouth
For toad to take in air, the mouth is closed;
the nostrils are opened and the floor of the buccal cavity is lowered;
this creates a low pressure within the buccal cavity and this makes air to be drawn in from outside through the nostrils;
after this the nostrils and glottis are closed;
gaseous exchange then takes place in the buccal cavity between the blood in the capillaries and the inhaled air.
To remove the carbon dioxide from the body, the toad will raise the floor of the buccal cavity;
thereby increasing the air pressure inside the cavity;
by so doing, the nostrils are opened and air containing carbon dioxide is forced out to the environment.
Pulmonary respiration:
The toad uses its lung to respire when on land
How a toad respire using its lung
as the toad is about drawing in air, the mouth is closed while the nostrils are opened and the floor of the buccal cavity is lowered;
this action creates a low pressure within the buccal cavity;
and this makes air containing oxygen to be drawn in from outside through the nostrils.
After this, the nostrils are closed while the glottis is opened and the air is then forced into the lungs through the glottis, larynx and finally to the lungs.
Oxygen then diffuses into the blood of the capillaries of the alveoli while carbon dioxide diffuses out from the blood capillaries into the lungs where it is sent out.
The air containing carbon dioxide is drawn out of the lungs into the glottis by the lowering of the buccal cavity while the nostrils are closed.
After this, the floor of the mouth is raised several times while air is pumped out of the lungs and finally forced out through the nostrils.
RESPIRATION IN MAMMALS
Most vertebrates including birds, lizards and mammals use lungs as their only respiratory organs where gaseous exchange takes place.
The mammalian respiratory system consists of a pair of lungs enclosed in the thorax and connected to the air outside by a series of branched air tubes (trachea, bronchi, and bronchioles) and air pathways (nasal cavity, pharynx and larynx). Let’s describe the structures i.e.
Nose and mouth: they warm and moisten the in-coming air. The air entering through the mouth is not filtered. The nose also filters dust from the in-coming air through the hairs and mucus.
Pharynx: leading from the mouth and nose is the pharynx.
Note: pharynx leads to the gullet (oesophagus) and larynx (voice box).
How pharynx functions during swallowing of food and gaseous exchange
Food goes into the gullet and air into the trachea. There is a regulatory structure called epiglottis that directs food to the gullet. When food is being swallowed, the epiglottis quickly closes the glottis (upper parts of trachea) so that food does not go into the trachea. At this moment, breathing is stopped. Immediately the food passes into the gullet, the epiglottis opens the glottis for oxygen to come into the trachea.
Note: if the epiglottis fails to quickly close the glottis, some particles of food may go to the trachea causing an irritation which leads to coughing so as to throw out the food particles. Such condition include laughing, talking while swallowing food.
Larynx (voice box): this is a short and strong cartilage. Air enters the trachea through the larynx.
Trachea: it is made of C-shaped cartilage which prevents it from collapsing. It runs from the larynx and divides into right and left bronchi.
Bronchi: they are made of rings and cartilage. The right and left bronchi lead to right and left lungs respectively. The bronchi branch into bronchioles. The bronchioles in turn lead into air sacs. An air sac is made up of a cluster of alveoli
Adaptive features of alveoli for gaseous exchange
The walls are very thin and elastic to increase its surface area for diffusion of oxygen
Moist inner surface in which oxygen can easily be dissolved.
It is highly vascularized i.e. well supplied with blood
6. Lungs: it is spongy consisting mainly of bronchioles, air sacs, network of blood vessels and capillaries bound together by connective tissue. It is located in the thoracic cavity/ or chest. It is spongy, brightly red and is convoluted/ folded on the surface. Each lung is surrounded by an elastic membrane (pleural membrane) and so it can stretch.
Observable features that adapt the lung to its function
Moist/ or wet surface for dissolving gases;
Thin surface membrane for easy diffusion of gases;
Many blood vessels i.e. highly vascularized for transportation of gases.
Breathing Mechanism in Mammal
Breathing occurs due to movements of the ribs, diaphragm and intercostals muscles resulting in the increase and decrease in volume of the lungs.
Each lung is surrounded by an air tight pleural cavity. The lung membrane is elastic and so it can stretch. Two sets of muscles of the ribs (inspiratory and expiratory intercostals muscles) work in opposite directions (antagonistic).
Note: breathing is also known as external respiration
Stages in Breathing
Inspiration/inhalation:
When we breathe in, the pillar muscles of the diaphragm contract thereby flattening the diaphragm. At the same time, the inspiratory intercoastal muscles contracts while expiratory intercoastal muscles relax thus pull the ribs upward and forward. This enlarges the thoracic cavity thus creating low air pressure in the lungs. The elastic walls of the lungs are pulled out and air is drawn in through the trachea, bronchi and bronchioles. The concentration of oxygen in the lungs is now higher than in the blood. Oxygen then diffuses from the lungs into the blood. At the same time, the concentration of carbon (IV) oxide in the blood is higher than in the lungs and therefore diffuses from the blood into the lungs and expelled during expiration.
Expiration/Exhalation:
When we breathe out, the pillar muscles of the diaphragm relax thus the diaphragm returns to its curved position. At the same time, the expiratory intercostals muscles contract, the inspiratory intercostals muscles relax. This reduces the volume of the thoracic cavity and increases its air pressure. Therefore, air is forced out through the trachea.
WORKING MODELS FOR BREATHING MECHANISM IN MAMMALS
When the polythene (rubber)sheet is pulled downward, the air pressure in the bell jar decreases. Air then enters through the glass tube and the balloons are inflated. When the polythene sheet is left to return to its original domed shape, the air pressure increases and air is expelled through the glass tube and the balloons are deflated.
CLASS ACTIVITIES
Experiment to compare the amount of carbon dioxide present in inhaled and exhaled air
Set up the apparatus as shown below. Ensure that the set up is air-tight to avoid wrong reading. Place the rubber tubing from the T-tube in your mouth. Inhale and exhale repeatedly. Observe which conical flask (A or B) allows the colour of bromothymol to change rapidly (i.e. which of the exhaled air and inhaled air change the colour of bromothymol).
Which of the exhaled or inhaled air contain more carbon dioxide?
Experiment to demonstrate respiration in a small mammal usin a rat (Rattus rattus)
Method: The apparatus is arranged as shown above. It is attached to a filter pump and air is drawn in. a current of air is drawn gently through the apparatus by means of the filter pump. As the air enters ‘X’ the caustic soda (lime water) solution absorbs the carbon dioxide. The lime water remains clear in ‘Y’. This shows that there is no carbon dioxide entering where the animal is. The experiment is left for two to three hours.
A control experiment is set up in the same way but without a rat in the flask.
Result: The lime water in Z turns milky showing the liberation of carbon dioxide. The lime water in Z of the control experiment remains clear.
Conclusion: since the lime water turns milky and the lime water in Y remains clear, the air entering where the animal stays is free from carbon dioxide. The carbon dioxide that turns the lime water in Z comes from the respiration of the rat.
Note: insects or toad can also be used instead of rat.
Differences between inspired (inhaled) and expired (exhaled) air
TERMS USED IN BREATHING
Oxygen debt: This is the quick heavy breathing to take in oxygen to the muscle cells lacking oxygen (i.e. fatigued muscle cells) in other to oxidize the lactic acid built up in the muscle cells. Or the oxygen needed to oxidize lactic acid to pyruvic acid from pyruvic acid to carbon dioxide, water and chemical energy.
Causes of oxygen debt:
Vigorous exercise, leading to shortage of oxygen in muscle cells;
Lung unable to meet the demand of oxygen in the cells of the muscle;
Accumulation of lactic acid in the muscle cells;
After vigorous exercise, the lactic acid remains in the muscle cells resulting in muscle fatigue and aches.;
Sickness i.e. illness.
Respiratory quotient: This is the amount of carbon dioxide produced during aerobic respiration divided by the aount of oxygen consumed of a particular food at a particular time. i.e. RQ =
For example, respiration of glucose is:
C6H12O6 + 6O2 → 6CO2 + 6H2O +ATP(energy)
RQ = = 1.0
Table showing RQ of some foods:
Note:
The RQ in human beings ranges from 0.70 – 1.0. this indicates that fats/oils, proteins are also used for respiration;
RQ greater than 1.0 indicates that the organism is short of oxygen and is respiring both aerobically and anaerobically. For example, sprinter of 100, or 200 metres run out of oxygen in their muscles and have to complete the race respiring anaerobically;
If the organism is respiring oil or fats, RQ is less than 1 because more oxygen is needed;
Plants usually have RQ less than 1 because part of CO2 produced during respiration is used up again for photosynthesis.
Residual air: this is the volume of air left in the lung after expiration which can never be expelled naturally.
Tidal air: the volume of air inhaled and exhaled during normal breathing.
Total lung capacity: the total volume of air inspired together with the normal inspired air i.e. the additional air inspired after normal inspiration.
Class Activity
List any five diseases of the respiratory system
RESPIRATION IN PLANTS
Plants carry out gaseous exchange for:
Photosynthesis
Cellular respiration.
Photosynthesis is carried out by chloroplast-containing cells in the presence of sunlight. Cellular respiration is carried out by all plant cells all the time.
Plants do not have special gaseous exchange structures like complex animals. Instead, gases enter and leave the plant body through;
Stomata in the leaves and green part of the stem;
Lenticels in the stem; and
Root hairs in young roots.
Mechanism of gaseous exchange in plants
The shoot system of flowering plants obtains oxygen from the atmosphere and gives out carbon dioxide and water vapour to the atmosphere through the stomata of the leaves and lenticels in the stem through the process of diffusion. Due to differences in concentration gradient, oxygen is taken in through the stomata and lenticels especially during the night and carbon dioxide and water vapour are given out. But during the day, when photosynthesis is going on, oxygen and water vapour from photosynthesis diffuse out to the exterior through the stomata and lenticels.
Note:
Closing and opening of the stomata are controlled by the guard cells.
Turgidity of the guard cells thickens the opening of the stomata while flaccidity of the guard cells causes the closing of guard cells.
Gaseous exchange, especially through the stomata, is made possible when the stomata open.
DIFFERENCES BETWEEN RESPIRATION AND PHOTOSYNTHESIS
At the end of this topic, students should be able to:
describe the different types of respiratory systems;
list the characteristics of a respiratory surface;
draw and label some of the respiratory organs of some animals;
explain the various mechanisms of respiration in some animals e.g. mammals;
describe the mechanism of exchange of gases through the stomata of plants
LESSON PRESENTATION
RESPIRATION: This is a biochemical activity of the cell in which glucose is broken down by series reactions controlled by enzymes to release energy, in the presence or absence of oxygen.
Note: the main reason for respiration is to release energy for daily activities.
Importance of energy to living organisms
Energy is required for transportation of materials within the body;
It is needed by animals for movement;
It is required for the maintenance of body temperature with heat energy;
It is required for the synthesis of substance such as protoplasm and other body materials needed for body growth and repair of the body;
Energy is required for reproduction;
It is important in excretion in living organisms;
Energy is required in the transmission of nerve impulses and co-ordination;
It helps to produce sound e.g. speaking in human;
It helps to produce electrical shocks for offence or defence as in catfish;
Types of Respiration
Aerobic respiration: it requires oxygen to break down/oxidize glucose/simple sugar (substrates) in the cell to release carbon (IV) oxide and water.i.e.
C6H12O6 + 6O2 →6CO2 + 6H2O + 38ATP
Anaerobic respiration: it does not require oxygen to break down glucose in the cells to release alcohol, carbon dioxide and 2 ATP as the products. i.e.
C6H12O6 → C2H5OH + 2CO2 + 2ATP
NOTE:
Aerobic respiration is commonly carried out by most living things
Some micro-organisms such as yeasts, bacteria, fungi, and endoparasites such as tapeworms, and roundworms respire anaerobically.
Anaerobic respiration of yeast is referred to as alcoholic fermentation
Terrestrial trees growing in waterlogged soil respire anaerobically and are able to stay alive while the water lasts.
Conditions Necessary for Respiration
Presence of respiratory medium e.g. air and water
Presence of respiratory structure/organ e.g. body surfaces, gills, tracheae, and lungs
Presence of transport medium e.g. blood
Adequate ventilation
Presence of respiratory surface.
GENERAL PROPERTIES/CHARACTERISTICS OF RESPIRATORY SURFACES
It must have large surface area to facilitate high rate of gaseous exchange
It must be moist to enable oxygen dissolve (i.e. diffusion) before entering the blood or body fluid
It must be permeable to allow free passage of gases
It must be thin-walled to shorten diffusion distance
It must have adequate supply of transport medium i.e. blood
It must be highly vascularized i.e. richly supplied with blood vessels
Types of respiratory surfaces, examples and environment in which each functions
MECHANISM OF RESPIRATORY SYSTEM IN LOWER ANIMALS
In unicellular organisms, such as Amoeba, there are no special respiratory organs, but instead, they use plasma membrane/or body surface. Oxygen intake and expulsion of carbon dioxide is by diffusion through the entire body surface. Oxygen in water diffuses into the body while carbon dioxide diffuses out of the body. This mechanism is effective because the organisms have large surface area to volume ratio.
MECHANISM OF RESPIRATION IN EARTHWORM
The skin of an earthworm is thin and protected by a thin cuticle secreted by epidermal cells. The skin is always kept moist by a slimy mucus substance. Oxygen diffuses through the moist skin and dissolves in the blood. The dissolved oxygen combines with haemoglobin of the blood to form oxy-haemoglobin and circulated to all the cells of the body. After internal respiration, the carbon dioxide produced diffuses out of the cells into the blood. The carbon dioxide then diffuses from the blood through the skin to the environment.
Note: respiration over moist body in Annelids (e.g. earthworms) is possible because:
their cylindricalshape gives a high surface area to volume ratio so that the rate at which oxygen diffuses is sufficient to meet the worm’s requirements; and
the cells in their body surface (epidermis) have a rich supply of blood capillaries.
MECHANISM OF RESPIRATION IN INSECTS
Air enters the body through small openings called spiracles located along the sides of the body. The spiracles lead into tubes called tracheae, which branch repeatedly within the body tissues. The smaller branches are called tracheoles. The smallest tracheoles contain fluid in which oxygen dissolves before actually reaching the individual body cells.
Note:
The tracheal system includes the spiracles, tracheae, and tracheoles.
Tracheal system is used by small arthropods such as insects, because they have a small body size not more than two centimeters, hence there is large surface area to volume ratio for diffusion to take place over a very short distance.
MECHANISM OF REPIRATION IN FISH
The Gill
The organ for respiration in fish is the gill. The gills are located at both sides of the head region. Fish has three to four gills arranged in the gill chamber. Each gill has two rows of soft gill filaments. The filaments are tiny, long and numerous structures which are well supplied with blood vessels. In bony fish, such as Tilapia, the four gills of each side are covered with a bony plate called operculum. The filaments are attached to a curved bony structure called gill arch. The inner sides of the gill arch are short bony structures called gill rakers.
Functions of gill parts/or structures
Note: the nostrils of fish are not used for breathing, but rather for smelling.
How fish respires:
When the fish wants to breathe, it first closes its opercula;
and then opens its mouth and lowers the floor of the mouth;
Water with the dissolved oxygen rush into the fish’s mouth;
The fish then closes its mouth, raises the floor of the mouth and the water rush into the gill chamber; and move across the gill filaments;
Oxygen in the water then diffuses into the gill filament while carbon dioxide diffuses out of the body into the water;
After the gaseous exchange, the fish now opens its operculum and the water containing dissolved carbon dioxide then passes out of the body into the river or ocean.
Reasons why a fish will die on land, despite the fact there is plenty of oxygen (about 40 times of oxygen in water) on land
Suffocation: without the support of water, the delicate gills tissues collapsed against each other drastically reducing the gill surface area;
Drying out: the membrane of the gills must be kept moist, hence due to lack of water, it dries out i.e. desiccate.
GASEOUS EXCHANGE THROUGH EXTERNAL GILLS
Tadpoles of frogs, toads and salamanders (all belonging to the phylum Amphibia) have external gills for gaseous exchange. The external gills are well supplied with blood vessels. Oxygen dissolved in water is absorbed by diffusion through the external gills and carbon dioxide diffuses from the blood vessels of the gills into the water.
GASEOUS EXCHANGE IN TOAD
Toad respires using:
Skin i.e. cutaneous respiration
Mouth i.e. buccal respiration
Lung i.e. pulmonary respiration
Cutaneous respiration:
The toad uses its skin to respire when on land and in water.
How the skin of a toad is adapted to respiration/ or gaseous exchange
It has a large surface area and a thin covering;
It is almost moist because of continuous secretions from the mucus gland;
It is well vascularized, i.e. it is well supplied with blood capillaries, hence oxygen diffuses easily and faster and transported to cells and tissues;
Carbon dioxide from the respiratory diffuse out from the blood into the capillaries through the skin of the body.
Buccal respiration
The toad uses its mouth to respire when on land.
How the mouth of a toad is adapted to gaseous exchange
It is very large i.e. it has a large surface area
It has a thin mucus membrane for easy diffusion
It is well supplied with blood capillaries
How a toad respire using its mouth
For toad to take in air, the mouth is closed;
the nostrils are opened and the floor of the buccal cavity is lowered;
this creates a low pressure within the buccal cavity and this makes air to be drawn in from outside through the nostrils;
after this the nostrils and glottis are closed;
gaseous exchange then takes place in the buccal cavity between the blood in the capillaries and the inhaled air.
To remove the carbon dioxide from the body, the toad will raise the floor of the buccal cavity;
thereby increasing the air pressure inside the cavity;
by so doing, the nostrils are opened and air containing carbon dioxide is forced out to the environment.
Pulmonary respiration:
The toad uses its lung to respire when on land
How a toad respire using its lung
as the toad is about drawing in air, the mouth is closed while the nostrils are opened and the floor of the buccal cavity is lowered;
this action creates a low pressure within the buccal cavity;
and this makes air containing oxygen to be drawn in from outside through the nostrils.
After this, the nostrils are closed while the glottis is opened and the air is then forced into the lungs through the glottis, larynx and finally to the lungs.
Oxygen then diffuses into the blood of the capillaries of the alveoli while carbon dioxide diffuses out from the blood capillaries into the lungs where it is sent out.
The air containing carbon dioxide is drawn out of the lungs into the glottis by the lowering of the buccal cavity while the nostrils are closed.
After this, the floor of the mouth is raised several times while air is pumped out of the lungs and finally forced out through the nostrils.
RESPIRATION IN MAMMALS
Most vertebrates including birds, lizards and mammals use lungs as their only respiratory organs where gaseous exchange takes place.
The mammalian respiratory system consists of a pair of lungs enclosed in the thorax and connected to the air outside by a series of branched air tubes (trachea, bronchi, and bronchioles) and air pathways (nasal cavity, pharynx and larynx). Let’s describe the structures i.e.
Nose and mouth: they warm and moisten the in-coming air. The air entering through the mouth is not filtered. The nose also filters dust from the in-coming air through the hairs and mucus.
Pharynx: leading from the mouth and nose is the pharynx.
Note: pharynx leads to the gullet (oesophagus) and larynx (voice box).
How pharynx functions during swallowing of food and gaseous exchange
Food goes into the gullet and air into the trachea. There is a regulatory structure called epiglottis that directs food to the gullet. When food is being swallowed, the epiglottis quickly closes the glottis (upper parts of trachea) so that food does not go into the trachea. At this moment, breathing is stopped. Immediately the food passes into the gullet, the epiglottis opens the glottis for oxygen to come into the trachea.
Note: if the epiglottis fails to quickly close the glottis, some particles of food may go to the trachea causing an irritation which leads to coughing so as to throw out the food particles. Such condition include laughing, talking while swallowing food.
Larynx (voice box): this is a short and strong cartilage. Air enters the trachea through the larynx.
Trachea: it is made of C-shaped cartilage which prevents it from collapsing. It runs from the larynx and divides into right and left bronchi.
Bronchi: they are made of rings and cartilage. The right and left bronchi lead to right and left lungs respectively. The bronchi branch into bronchioles. The bronchioles in turn lead into air sacs. An air sac is made up of a cluster of alveoli
Adaptive features of alveoli for gaseous exchange
The walls are very thin and elastic to increase its surface area for diffusion of oxygen
Moist inner surface in which oxygen can easily be dissolved.
It is highly vascularized i.e. well supplied with blood
6. Lungs: it is spongy consisting mainly of bronchioles, air sacs, network of blood vessels and capillaries bound together by connective tissue. It is located in the thoracic cavity/ or chest. It is spongy, brightly red and is convoluted/ folded on the surface. Each lung is surrounded by an elastic membrane (pleural membrane) and so it can stretch.
Observable features that adapt the lung to its function
Moist/ or wet surface for dissolving gases;
Thin surface membrane for easy diffusion of gases;
Many blood vessels i.e. highly vascularized for transportation of gases.
Breathing Mechanism in Mammal
Breathing occurs due to movements of the ribs, diaphragm and intercostals muscles resulting in the increase and decrease in volume of the lungs.
Each lung is surrounded by an air tight pleural cavity. The lung membrane is elastic and so it can stretch. Two sets of muscles of the ribs (inspiratory and expiratory intercostals muscles) work in opposite directions (antagonistic).
Note: breathing is also known as external respiration
Stages in Breathing
Inspiration/inhalation:
When we breathe in, the pillar muscles of the diaphragm contract thereby flattening the diaphragm. At the same time, the inspiratory intercoastal muscles contracts while expiratory intercoastal muscles relax thus pull the ribs upward and forward. This enlarges the thoracic cavity thus creating low air pressure in the lungs. The elastic walls of the lungs are pulled out and air is drawn in through the trachea, bronchi and bronchioles. The concentration of oxygen in the lungs is now higher than in the blood. Oxygen then diffuses from the lungs into the blood. At the same time, the concentration of carbon (IV) oxide in the blood is higher than in the lungs and therefore diffuses from the blood into the lungs and expelled during expiration.
Expiration/Exhalation:
When we breathe out, the pillar muscles of the diaphragm relax thus the diaphragm returns to its curved position. At the same time, the expiratory intercostals muscles contract, the inspiratory intercostals muscles relax. This reduces the volume of the thoracic cavity and increases its air pressure. Therefore, air is forced out through the trachea.
WORKING MODELS FOR BREATHING MECHANISM IN MAMMALS
When the polythene (rubber)sheet is pulled downward, the air pressure in the bell jar decreases. Air then enters through the glass tube and the balloons are inflated. When the polythene sheet is left to return to its original domed shape, the air pressure increases and air is expelled through the glass tube and the balloons are deflated.
CLASS ACTIVITIES
Experiment to compare the amount of carbon dioxide present in inhaled and exhaled air
Set up the apparatus as shown below. Ensure that the set up is air-tight to avoid wrong reading. Place the rubber tubing from the T-tube in your mouth. Inhale and exhale repeatedly. Observe which conical flask (A or B) allows the colour of bromothymol to change rapidly (i.e. which of the exhaled air and inhaled air change the colour of bromothymol).
Which of the exhaled or inhaled air contain more carbon dioxide?
Experiment to demonstrate respiration in a small mammal usin a rat (Rattus rattus)
Method: The apparatus is arranged as shown above. It is attached to a filter pump and air is drawn in. a current of air is drawn gently through the apparatus by means of the filter pump. As the air enters ‘X’ the caustic soda (lime water) solution absorbs the carbon dioxide. The lime water remains clear in ‘Y’. This shows that there is no carbon dioxide entering where the animal is. The experiment is left for two to three hours.
A control experiment is set up in the same way but without a rat in the flask.
Result: The lime water in Z turns milky showing the liberation of carbon dioxide. The lime water in Z of the control experiment remains clear.
Conclusion: since the lime water turns milky and the lime water in Y remains clear, the air entering where the animal stays is free from carbon dioxide. The carbon dioxide that turns the lime water in Z comes from the respiration of the rat.
Note: insects or toad can also be used instead of rat.
Differences between inspired (inhaled) and expired (exhaled) air
TERMS USED IN BREATHING
Oxygen debt: This is the quick heavy breathing to take in oxygen to the muscle cells lacking oxygen (i.e. fatigued muscle cells) in other to oxidize the lactic acid built up in the muscle cells. Or the oxygen needed to oxidize lactic acid to pyruvic acid from pyruvic acid to carbon dioxide, water and chemical energy.
Causes of oxygen debt:
Vigorous exercise, leading to shortage of oxygen in muscle cells;
Lung unable to meet the demand of oxygen in the cells of the muscle;
Accumulation of lactic acid in the muscle cells;
After vigorous exercise, the lactic acid remains in the muscle cells resulting in muscle fatigue and aches.;
Sickness i.e. illness.
Respiratory quotient: This is the amount of carbon dioxide produced during aerobic respiration divided by the aount of oxygen consumed of a particular food at a particular time. i.e. RQ =
For example, respiration of glucose is:
C6H12O6 + 6O2 → 6CO2 + 6H2O +ATP(energy)
RQ = = 1.0
Table showing RQ of some foods:
Note:
The RQ in human beings ranges from 0.70 – 1.0. this indicates that fats/oils, proteins are also used for respiration;
RQ greater than 1.0 indicates that the organism is short of oxygen and is respiring both aerobically and anaerobically. For example, sprinter of 100, or 200 metres run out of oxygen in their muscles and have to complete the race respiring anaerobically;
If the organism is respiring oil or fats, RQ is less than 1 because more oxygen is needed;
Plants usually have RQ less than 1 because part of CO2 produced during respiration is used up again for photosynthesis.
Residual air: this is the volume of air left in the lung after expiration which can never be expelled naturally.
Tidal air: the volume of air inhaled and exhaled during normal breathing.
Total lung capacity: the total volume of air inspired together with the normal inspired air i.e. the additional air inspired after normal inspiration.
Class Activity
List any five diseases of the respiratory system
RESPIRATION IN PLANTS
Plants carry out gaseous exchange for:
Photosynthesis
Cellular respiration.
Photosynthesis is carried out by chloroplast-containing cells in the presence of sunlight. Cellular respiration is carried out by all plant cells all the time.
Plants do not have special gaseous exchange structures like complex animals. Instead, gases enter and leave the plant body through;
Stomata in the leaves and green part of the stem;
Lenticels in the stem; and
Root hairs in young roots.
Mechanism of gaseous exchange in plants
The shoot system of flowering plants obtains oxygen from the atmosphere and gives out carbon dioxide and water vapour to the atmosphere through the stomata of the leaves and lenticels in the stem through the process of diffusion. Due to differences in concentration gradient, oxygen is taken in through the stomata and lenticels especially during the night and carbon dioxide and water vapour are given out. But during the day, when photosynthesis is going on, oxygen and water vapour from photosynthesis diffuse out to the exterior through the stomata and lenticels.
Note:
Closing and opening of the stomata are controlled by the guard cells.
Turgidity of the guard cells thickens the opening of the stomata while flaccidity of the guard cells causes the closing of guard cells.
Gaseous exchange, especially through the stomata, is made possible when the stomata open.
DIFFERENCES BETWEEN RESPIRATION AND PHOTOSYNTHESIS
