GASEOUS EXCHANGE AND RESPIRATION
Gaseous exchange
Gaseous exchange is the movement of oxygen and carbon dioxide across a
respiratory surface. Unicellular organisms carry out gaseous exchange by
diffusion across the cell membrane. Large organisms cannot carry out diffusion
efficiently so they have developed specialized organs for gaseous exchange.
These are called respiratory surfaces.
Table below shows examples of
respiratory surfaces in various organisms. Respiratory surfaces in various
organisms
|
Organism |
Respiratory surface |
|
Amoeba |
Cell membrane |
|
Insects |
Tracheal system |
|
Spider |
Book lung |
|
Fish |
Gills |
|
Plants |
Leaves, stems, roots |
|
Amphibians |
Skin, gills and lungs |
|
mammals |
Lungs |
|
Birds |
Lungs |
|
Reptiles |
Lungs |
Characteristics of respiratory
surfaces
1. They are thin to reduce the
diffusion distance.
2. They are moist to dissolve gases
so that they diffuse in solution form.
3. They are highly branched, folded
or flattened in order to increase the surface area for gaseous exchange,
4. They are close to an efficient transport and exchange system so that gases
can be taken to and from the cells easily.
5. They are well ventilated so that gases can pass through them easily
GASEOUS EXCHANGE IN MAMMALS
The components of the gaseous
exchange system in mammals include the nostril, trachea, lungs, intercostals
muscles, diaphragm and ribs.
The adaptations and functions of
parts of the mammalian respiratory system
|
Part |
Adaptive features |
Functions |
|
Nose and nasal cavity |
Mucus lining and hairs (cilia) |
Trap dust and microorganisms |
|
Glottis |
Presence of epiglottis |
Closes the trachea during
swallowing to prevent food from entering the respiratory system |
|
Trachea, bronchus and bronchioles |
Blood vessels near the surface |
Warm the air |
|
Have rings of cartilage tissue
along their length |
Prevent collapse of the
respiratory tract |
|
|
Mucus lining and cilia |
Trap and filter dust and
microorganisms |
|
|
Lungs |
Spongy with air spaces (alveoli) |
Main organ of mammalian gaseous
exchange Airspaces hold inhaled air |
|
Alveoli (singular: alveolus) |
Numerous in number |
Provide large surface area for
gaseous exchange |
|
Thin membranes |
Reduce distance for diffusion of
gases |
|
|
Moist surface |
Enables gases to dissolve into
solutions before diffusing |
|
|
Has dense network of
capillaries |
Transport oxygen from the alveoli
to the tissues and carbon dioxide from the tissues to the alveoli |
|
|
Constantly contain air |
Maintain shape to avoid collapsing |
|
|
Pleural
membrane |
Contain pleural fluid |
Lubricates the membranes so that
the lungs can slide smoothly over the thoracic cavity during breathing |
|
Ribs |
Are made of hard bone tissue |
Protect the lungs from injury |
|
Intercostal muscles |
Move antagonistically: when one
muscle contracts the other relaxes and vice versa |
Allow expansion and contraction of
the thoracic cavity |
|
Diaphragm, |
Muscular sheet of tissue |
Separates the thorax from the
abdomen. Allows for gaseous exchange by becoming dome-shaped or flattens. |
The mechanism of gaseous exchange in
mammals
Gaseous exchange in mammals happens
as a result of inhalation (or inspiration) and exhalation (or expiration).
Inhalation is breathing in air into the lungs. Exhalation is breathing out
air from the lungs
During inhalation the muscles of the
diaphragm Contract, pulling the diaphragm downwards; As this happens, the
external intercostal muscles contract and pull the ribcage upwards and
outwards. The result of these movements is an increase in the volume and a
decrease in the air pressure of the thorax. This makes air rush into the lungs
through the nostrils, trachea and bronchioles.
During exhalation, the muscles of
the diaphragm relax and the diaphragm resumes its dome shape. The external
intercostal muscles relax, pulling the ribcage inwards and downwards. As a
result, the volume of the thorax decreases and the pressure inside it
increases. This forces air out through the bronchioles, trachea and nostrils
|
|
Breathing in (inhalation) |
|
Breathing out (exhalation) |
|
|
External intercostal muscles
contract |
|
The external intercostal muscles
relax |
|
|
Internal intercostal muscles relax |
|
The internal intercostal muscle
contract |
|
|
The ribcage is lifted outward and
upward |
|
The ribcage move inward and
downward |
|
|
The diaphragm contracts and
flattens |
|
The diaphragm relaxes and become
dome-shaped |
|
5 |
The volume of thoracic cavity
increase as pressure decrease This allows air to enter the
thoracic cavity |
5 |
The volume of thoracic cavity
decrease as pressure increase |
|
6 |
Air enter the alveoli through the
nostrils, pharynx, glottis, trachea, bronchioles and finally alveoli |
6 |
Air leaves the alveoli through the
bronchioles, trachea, glottis, pharynx and finally nostrils |
Gaseous exchange across the alveolus
The actual exchange of oxygen and
carbon dioxide takes place in the alveoli. One mammalian lung has millions of
alveoli. The alveoli are surrounded by network of
capillaries.
Gases exchange across alveolus
When we breathe in, air accumulates
in the alveoli. There is a higher concentration of oxygen in the air in the
alveoli than in the bloodstream.
Therefore, oxygen diffuses out the
alveoli into the blood in the capillaries. It combines with haemoglobin to form
oxyhaemoglobin
The oxygen is then transported to
the tissues. Once in the tissues, the oxyhaemoglobin breaks down to release
oxygen and haemoglobin. The tissues use released oxygen and release carbon
dioxide.
This causes the levels of carbon
dioxide to become higher in the tissues than in the blood. Carbon dioxide
therefore diffuses into the blood in the capillaries and combines with
haemoglobin to form carbaminohaemoglobin. The capillaries transport carbon dioxide
in this form to the alveoli.
The concentration of carbon dioxide
is higher in lie blood in the capillaries than in the air in the
alveoli. Carbon dioxide therefore
diffuses from the Capillaries into the alveoli. It is then transported through
the bronchioles, trachea, glottis, pharynx and finally nostrils into the
atmosphere
Composition of inspired and expired air
|
gas |
Inspired air |
Expired air |
|
Oxygen |
20.95% |
16.40% |
|
Carbon dioxide |
0.03% |
4.00% |
Factors affecting the rate of
gaseous exchange
1. Concentration of carbon dioxide
High concentration of carbon dioxide
in the blood increases the rate of gaseous exchange. This provides the tissues
with adequate amounts of oxygen and lower carbon dioxide concentration in the
blood.
2. Concentration of haemoglobin
Haemoglobin is responsible for the
transportation of gases from the lungs to the tissues and back. Efficient
transportation of gases takes place when the body has adequate amounts of
haemoglobin.
When a person is anaemic, the body
has a low concentration of haemoglobin. Only small amounts of oxygen can be
transported at a time. As a result, the rate of gaseous exchange has to
increase so that the tissues get adequate amounts of oxygen.
3. Physical activity
A more active body requires more
oxygen than a less active body. As a result, gaseous exchange takes place
faster when there is increased body activity.
4. Health status of the body
Generally, the rate of gaseous
exchange increases when somebody is sick. This is as a result of increased
metabolism by the liver in order to remove the toxins released by
disease-causing microorganisms or break down the drugs taken. Certain diseases
also make the body weak and cause slowing down of the breathing process.
5. Altitude
Altitude is the height above sea
level. At high altitudes, the concentration of oxygen is lower compared to low
altitudes. Breathing is therefore faster at high altitudes. At high altitudes,
there is also decreased atmospheric pressure. This makes breathing difficult.
Organisms therefore have to breathe in faster in order to get enough oxygen.
6. Age
Young people are generally more
active than old people. Also, a lot of growth processes take place in the
bodies of young people. This increases the demand for oxygen and therefore
increases the breathing rate.
Gaseous exchange in plants
In plants, gaseous exchange mostly
takes place through the stomata on the leaves and lenticels on the stem. Some
plants such as mangrove and ficus also carry out gaseous exchange through
breathing roots.
Gaseous exchange in the leaves
Atmospheric air moves into and out
of the leaf through the stomata. Gaseous exchange mostly takes place in the air
spaces in the spongy
mesophyll.
During the day, guard cells that
surround the stomata absorb water by osmosis.As a result, the cell sap of guard
cells becomes hypertonic and draws in water from the neighbouring cells by
osmosis.
The guard cells become turgid and
the stomata open. Air from the atmosphere enters into the air spaces in the
spongy mesophyll. The cells next to the air spaces have more oxygen (produced
by the cells during photosynthesis) but less carbon dioxide (used up during
photosynthesis).
On the other hand, carbon dioxide is
more in the air within the air spaces but oxygen is less. Carbondioxide and
oxygen diffuse in opposite directions depending on their concentration
gradients. The carbon dioxide diffuses to neighbouring cells until it reaches
the site for photosynthesis. Oxygen moves out through the open stomata into the
atmosphere.
At night, there is no light, therefore photosynthesis ceases. No glucose is
produced therefore the guard cells do not absorb water by osmosis. Hence, the
stomata remain partially closed.
However, respiration takes place in
plants at night. The partially open stomata allow in small amount of air which
accumulate in the air spaces. There is more oxygen and less carbon dioxide in
the air spaces compared to the plant cells.
Oxygen moves into the plant cells
while carbon dioxide moves into the air spaces and eventually into the
atmosphere through the partially open stomata. This explains why plants produce
carbon dioxide at night and oxygen during the day.
Gaseous
exchange through the lenticels
Lenticels made up of loosely packed
cork cells located on the bark of woody stems and roots. They are small pores
through which gaseous exchange occurs.
Gaseous
exchange in the lenticels
The loose arrangement of the cells
facilitates the movement of gases between them. The cells have a thin layer of
moisture so that gases diffuse in and out while in solution form
At night, there is a higher
concentration of oxygen in the air spaces between the cork cells than in the
ells themselves. Oxygen therefore diffuses into the cells surrounding the
lenticels. The cells use oxygen far respiration and release carbon dioxide in
the process. Thus, the concentration of carbon dioxide in the cells becomes
higher than in the air spaces. Carbon dioxide therefore diffuses out through
the cells into the air spaces and then out through the lenticel. The opposite
happens during the day.
Gaseous exchange through the roots
This occurs through breathing roots.
Plants with breathing roots have a very thin epidermal layer which enables the
root to carry out gaseous exchange.
Breathing roots
Oxygen is at a higher concentration
in the atmosphere than in the root cells. Therefore, oxygen diffuses into the
root cells through the epidermis.
During respiration, the plant uses
oxygen and releases carbon dioxide. This causes the concentration of carbon
dioxide in the root cells to be higher than in the atmosphere. Carbon dioxide
diffuses from the root cells into the atmosphere through the epidermis.
Importance of gaseous exchange in
plants
- It Enables plants to obtain carbon dioxide, which is
one of the raw materials necessary for photosynthesis.
- Plants obtain oxygen which is necessary for the
production of energy. Energy is produced during respiration.
- It enables the plant to eliminate excess carbon dioxide
at night of which if left, will harm the plant.
Respiration
Respiration
is the process by which food substances are broken down to provide energy. It
is controlled by enzymes. Enzymes are substances that affect the rate at which
a reaction occurs but are not used up in the reaction themselves. Respiration
takes place in the mitochondria of the plant cells.
There are
two types of respiration: aerobic respiration and anaerobic respiration.
Aerobic
respiration
This is a
type of respiration whereby oxygen is used to break down glucose, releasing
energy, carbon dioxide and water. The chemical reaction for aerobic respiration
is:
The energy
produced is in the form of ATP (adenosine triphosphate). Thirty-eight molecules
of ATP are produced at the end of the aerobic respiration.
Aerobic
respiration takes place in two stages: glycolysis and Kreb's cycle.
Glycolysis
takes place in the cytoplasm. It does not require oxygen so it is a phase that
is common for both aerobic and anaerobic respiration.
During
glycolysis, enzymes break down glucose into a three carbon compound called pyruvic
acid. Glycolysis produces 2 molecules of ATP per molecule of glucose.
The
pyruvic acid can further be broken down in the presence or absence of oxygen.
If there is oxygen, the pyruvic acid proceeds to the next stage of aerobic
respiration, which is Kreb's cycle. If there is no oxygen, anaerobic
respiration occurs.
Note that
pyruvic acid passes through a stage where it is decarboxylated (one
carbon dioxide molecule removed from it) before going through the Kreb's cycle.
Kreb's
cycle is also called the citric acid cycle. It involves the formation of citric
acid molecule (a six carbon) from the two carbon molecule by addition of a four
carbon molecule, i.e. oxaloacetic acid in a cyclic process.
Kreb's
cycle takes place inside the cristae of the mitochondria.
Anaerobic
respiration
Anaerobic
respiration takes place in the absence of [oxygen.
In plants,
anaerobic respiration is also called fermentation. It involves the breaking
down of glucose by bacteria or fungi to form alcohol, carbon dioxide and
energy. This is represented by the following equation:
In
animals, anaerobic respiration leads to the formation of lactic acid and
energy. This is written as
In animals anaerobic respiration
takes place during strenuous activity, for example during sports. It leads to
the accumulation of lactic acid in the muscles. Lactic acid is toxic.
Anaerobic respiration occurs when
the body's oxygen supply does not meet the body's needs. Therefore, an oxygen
debt or oxygen deficit occurs. This causes the animal to breathe fast and
deeply in order to get enough oxygen to convert the lactic acid to carbon
dioxide and water. Some of the lactic acid is converted to glucose. Breathing
goes back to normal when the acid has been broken down.
Anaerobes are organisms that respire anaerobically. They include
bacteria, yeast and fungi. There are two types of anaerobes:
Obligate anaerobes which can only live and respire in the absence of oxygen.
They die in the presence of oxygen.
Facultative anaerobes; which respire both in the presence and in the absence of
oxygen
Differences between
aerobic and anaerobic respiration
|
Aerobic respiration |
Anaerobic respiration |
|
1. Oxygen is used |
1. Oxygen is not used |
|
2. Large amounts of energy are
produced |
2. Small amount of energy are
produced |
|
3. Water molecules are produced |
3. Water is not produced |
|
4. Food substances are completely
broken down |
4.Food substances are not
completely broken down |
|
5. Takes place in the mitochondria
and cell membrane |
5.Takes place in the cytoplasm |
|
6. Carbon dioxide and water are
the end-products |
6. Lactic acid is produced in
animals and alcohol is produced in plants |
Factors affecting the rate of
respiration
The rate at which respiration takes
place varies depending on the state of an organism. Hence, respiration is
sometimes fast and at other times slow. The following factors affect the rate
of respiration:
Temperature
Respiration is controlled by
enzymes. The functioning of enzymes is affected by temperature. The rate of
respiration is slow at low temperatures and increases with increase in
temperature until the optimal temperature. Optimal temperature is the
temperature at which the enzymes function best. If the temperature is raised
above optimal temperature, the enzymes are denatured and the rate of
respiration reduces.
Activity
When an organism is involved in a
vigorous activity, it requires more energy than when it is at rest. For
example, a human being requires less energy when sitting than when taking part
in arace. Therefore, the rate of respiration changes to suit the needs of the
organism’s physical activity.
Size
Small organisms lose heat faster
than big organisms. This is because small organisms have a larger surface area
to volume ratio. Heat is a form of energy. Therefore, small organisms need to
respire faster than large organisms to replace the energy lost through heat.
Age
Generally, young organisms respire
faster than older organisms. This is because they need energy to grow. In
addition, young organisms are more active than old organisms.
Health
When we are sick, the rate of
respiration increases so as to remove the toxic materials produced by the
pathogens in our bodies.
Infections and diseases of the
respiratory system
There are several airborne
infections which affect the human respiratory system. The common ones are
influenza, pneumonia, common cold and tuberculosis.
Most of the airborne infections are
as a result of close contact with an infected person. When the sick person
breathes out, coughs or sneezes, the pathogens are released into the air.
Hence, a person who is close by may catch the infection. Sometimes, droplets
may infect bedding, clothes and surfaces used by the sick person.
Airborne infections can be
controlled by isolation of the infected patient, proper disposal of infected
secretions such as sputum, living in a well-ventilated house and avoiding
overcrowding, especially in bedrooms.
Pneumonia
Pneumonia is inflammation of the
lungs. It is caused by bacteria, viruses, fungi or by inhaling chemical toxins
or irritants. Pneumonia is normally followed by other illnesses such as cold or
flu.
Signs and symptoms of pneumonia
- Fever
- Chills
- Shortness of breath associated with pain
- Increased mucus production
- Cough
Prevention and treatment of
pneumonia
- Staying warm
- Avoiding overcrowded areas
- Avoiding cold food or drinks. Hot drinks are preferred
more as they loosen
secretions
- Get treatment as early as possible since it is curable
by antibiotics
Bronchitis
Bacteria, viruses and inhaling of
irritating substances can cause the lining of the respiratory system to become
inflamed. This causes an infection called bronchitis. Bronchitis can be acute
or chronic.
Acute bronchitis
This is caused by whooping cough or
recurrent attacks of influenza. Smoking can also cause acute bronchitis.
Signs and symptoms of acute
bronchitis
- Pain in the chest
- Rapid breathing
- Fever
- Coughing
- Headaches
Prevention and treatment of acute
bronchitis
- Staying warm. Cold temperatures make the body more
susceptible to bacterial infections
- Get treatment for all infections as fast as possible
Chronic
bronchitis
Chronic
bronchitis is caused by heavy smoking and recurrent acute bronchitis.
Signs
and symptoms of chronic bronchitis
- Coughing, with the production of thick sputum
- Breathing difficulties
Prevention
and treatment of chronic bronchitis
- Avoid smoking
- Avoid very smoky or dusty areas
- Live in a well-ventilated house
- Keep your body warm
- Seek medical help
Asthma
Asthma can
be caused by:
- Allergic reactions to dust, pollen, spores oranimal fur
- Hereditary diseases of the respiratory system
- Extremely cold weather
- Frequent viral or bacterial lung infections
Signs
and symptoms of asthma
- Narrowing of
bronchioles resulting in breathing difficulties
and a wheezing or hissing sound when breathing
- Excessive production of mucus
- Dilation of blood vessels, leading to low
bloodpressure. Low blood pressure can be fatal
Prevention
and treatment of asthma
- Avoid allergens (things that cause allergicreactions)
- Get treatment for respiratory infections asearly as
possible
- Keep the body warm
- Muscle relaxants in the
form of sprays, pills and injections are used to prevent the
narrowing of the bronchioles.
Lung
cancer
The main
cause of lung cancer is smoking. The nicotine in cigarette smoke stops the
cilia in the trachea from expelling foreign materials leading to respiratory
infection.
Signs
and symptoms of lung cancer
- Chest pain
- Breathing difficulty
- Weight loss
- Persistent cough
- Abnormal production of mucus
Prevention
and treatment of lung cancer
- Stop smoking
- There is no
cure for cancer. However, chemotherapy
and physiotherapy are used to control the disease
Emphysema
This is a
lung disease which results from destruction of the structures supporting the
alveoli leading to their collapse. This significantly reduces the surface area
available for gaseous exchange.
Causes
of emphysema
- Mainly cigarette smoke
- Air pollution
- Hereditary
- Old age
Signs
and symptoms of emphysema
- Shortness of breath
- Coughing
- Obstructive lung disease
- Difficulties when breathing, especially duringexercise
- Wheezing during breathing
Prevention
and treatment of emphysema
- Avoid cigarette smoking and exposure to smoke
- Lung surgery is usually done to relieve thesymptoms
- Use of medical drugs
- In severe cases, lung transplant is necessary
Chapter
Summary:
1.
Gaseous exchange is the exchange of
oxygen and carbon dioxide through a respiratory surface.
2.
§ thin membrane
§ large surface area
§ moist lining
§ Dense network of capillaries.Features of a gaseous exchange
surface are:
3.
The structures involved in gaseous
exchange in mammals are the nose, mouth, pharynx, glottis, trachea, lungs,
bronchioles, alveoli, ribs, pleural membranes and diaphragm.
4.
Gaseous exchange is affected by the
amount of haemoglobin in the blood and carbon dioxide concentration.
5.
In plants, gaseous exchange can take
place through the stomata in the leaves, lenticels in woody stems or in
breathing roots.
6.
Respiration is the process by which
food substances are broken down to release energy.
7.
Aerobic respiration takes
place in the mitochondria in the presence of oxygen
8.
Aerobic respiration involves
two stages: glycolysis and Kreb's cycle.
9.
Anaerobic respiration takes place in
the cytoplasm in the absence of oxygen.
10.
Diseases and infections that affect
the respiratory system include bronchitis, asthma, pneumonia, tuberculosis, and
emphysema and j influenza.
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