Chapter 6: GASEOUS EXCHANGE AND RESPIRATION

6.1 : GASEOUS EXCHANGE

Gaseous exchange is the uptake of oxygen from the environment and the release of

carbon dioxide to the environment. Or

Gaseous exchange refers to the diffusion of respiratory gases (oxygen and carbon dioxide)

between cells of living organism and the environment across the respiratory surface.

In unicellular organisms, gaseous exchange usually takes place throughout the whole

body by diffusion across the cell membrane. Large or multicellular organisms have well

developed and specialized organs for gaseous exchange called respiratory surface.

A respiratory surface is an organ or any other part of the body through which gaseous

exchange takes place.

Table 6.1: Respiratory surfaces in various organisms

Organism

Respiratory surface

Cell membrane

Amoeba

Insects

Spider

Fish

Tracheal system

Book lung

Gills

Plants

Leaves, stems, roots

Amphibians (e.g. Frog, toad)

Mammals

Skin, gills, lungs and buccal cavity

Lungs

Skin

Birds

Reptiles

Earthworms

Characteristics/Features of Respiratory Surfaces

(i) They are thin in order to reduce the diffusion distance.

(ii) They are well ventilated so that gases can pass through them easily.

(iii) They are moist in order to dissolve gases so that they diffuse in solution form.

(iv) They have large surface area to volume ratio to maximize exchange of gases.

(v) They are permeable in order to allow gases to pass through.

(vi) They have a good network of blood capillaries to facilitate easy transport of gases.

(vii) They are highly branched, folded or flattened in order to increase the surface area

for gaseous exchange.

GASEOUS EXCHANGE IN MAMMALS

Gaseous exchange in the mammals is possible through the respiratory system. The

components of the respiratory system in mammals include the nostril, nasal cavity,

pharynx, trachea, bronchi, lungs, bronchioles, alveoli, intercostal muscles, diaphragm

and ribs as shown in figure below.

Figure 6.1: Human Respiratory System

The Adaptations and Functions of Parts of the Mammalian Respiratory System

(a) Pleural membrane. The membrane secretes pleural fluid. The fluid acts as a lubricant

which reduces friction between the lungs and the thoracic muscles during breathing.

(b) Ribs. They are made of hard bone. They protect the lungs from mechanical injury.

(c) Intercostal muscles. The muscles contract and relax antagonistically to allow

expansion and contraction of the thoracic cavity.

(d) Diaphragm. It consists of a dome-shaped sheet of skeletal muscle. Diaphragm

separates the thorax from the abdomen and by becoming dome-shaped or relaxed it

allows gas exchange.

(e) Nose or nostrils and nasal cavity. The inner surface of the nostril is lined with mucus

and hairs which trap dust and microorganisms/germs inhaled with air.

(f) Pharynx. It produces mucus to trap dust and microorganisms.

(g) Glottis. This is an area which contains epiglottis which closes the trachea during

swallowing to prevent food from entering the respiratory system.

(h) Trachea, bronchus and bronchioles. The organs have the following adaptive features;

(i) They are supplied with blood vessel to warm the inhaled air

(ii) They have rings of cartilage tissue to prevent collapsing of the respiratory tract

(iii) They have mucus and cilia. Mucus traps dust and microorganisms while the cilia

sweep the trapped particles backwards into the pharynx.

(i) Lungs. They are spongy with numerous air spaces (alveoli) which hold inhaled air.

(j) Alveoli (singular: alveolus). The adaptive features of alveoli include;

(i)

They are numerous in number to provide large surface area for gaseous exchange

They have thin membranes to reduce distance for diffusion of gases.

(ii)

(iii) They have moist surface to enable gases to dissolve into solutions before diffusing

(iv) They have dense network of capillaries which transport oxygen from the alveoli to

the tissues and carbon dioxide from the tissues to the alveoli.

(v)

They constantly contain air which maintains shape to avoid collapsing.

THE MECHANISM OF GASEOUS EXCHANGE (BREATHING) IN MAMMALS

Gas exchange or breathing in mammals happens as a result of inhalation (or inspiration)

and exhalation (or expiration). Inhalation is a process of breathing in air from the

environment into the lungs while exhalation is the process of breathing out air from the

lungs into the environment.

Table 6.2: Composition of inspired and expired air

Constituent

Oxygen gas

Inhaled air

20.95%

0.03%

Exhaled air

16.40%

4.00%

Carbon dioxide gas

Nitrogen gas

78.10%

0.94%

78.10%

0.94%

Noble gases

(a) Inhalation (Breathing in or Inspiration).

During inhalation the muscles of diaphragm contract, pulling the diaphragm

downwards. The external intercostal muscles contract and the internal intercostal

muscles relax, pulling the rib cage outward and upward. These movements cause the

volume of the thorax to increase and therefore the pressure inside the lungs decreases.

This causes air to flow into the lungs through the nostrils, trachea and bronchioles

and alveoli.

Figure 6.2: Mechanism of Inhalation

(b) Exhalation (Breathing Out or Expiration).

During exhalation the muscles of diaphragm relax, pulling the diaphragm upwards

and the diaphragm resumes its dome shape. The external intercostal muscles relax

and the internal intercostal muscles contract, pulling the rib cage inward and

downward. These movements cause the volume of the thorax to decrease and therefore

the pressure inside the lungs increase. The air is then forced out of the lungs through

the bronchioles, bronchus, trachea and nostrils.

Figure 6.3: Mechanism of Exhalation

Table 6.3: Differences between inhalation and exhalation

Inhalation

Exhalation

(i) External intercostal muscles contract.

(ii) Internal intercostal muscles relax.

(iii) The ribcage is lifted outward and

upward

The external intercostal muscles relax.

The internal intercostal muscle contract.

The

ribcage

moves

inward

and

downward

(iv) The diaphragm contracts and flattens.

The diaphragm

dome-shaped.

relaxes and become

(v) The

volume

of

thoracic

cavity

The volume of thoracic cavity decreases

as pressure increases. This forces air out

of the lungs.

increases as pressure decrease. This

allows air to enter the thoracic cavity.

(vi) Air enter

nostrils,

bronchioles and finally alveoli.

the alveoli through the

Air leaves the alveoli through the

bronchioles, trachea, glottis, pharynx

and finally nostrils.

pharynx, glottis, trachea,

Table 6.4: Differences between inhaled air and exhaled air

Inhaled air

Exhaled air

(i) It contains less carbondioxide

(ii) It supplied with high percentage

volume of oxygen.

It contains more carbondioxide

by It supplied with less percentage by

volume of oxygen.

(iii) It contains little (or variable) water vapour.

(iv) It is cold

It contains more water vapour.

It is warm

GASEOUS EXCHANGE ACROSS THE ALVEOLUS

Alveoli (singular: alveolus) are small balloon-like air sacs where the actual exchange of

oxygen and carbon dioxide takes place. One mammalian lung has millions of alveoli. The

alveoli are surrounded by network of blood capillaries.

▪

When we breathe in, air accumulates in the alveoli. This brings higher concentration

of oxygen in the alveoli than in the blood capillaries. Therefore, oxygen diffuses out

the alveoli into the blood capillaries. It combines with haemoglobin to form

oxyhaemoglobin and then transported to the body tissues.

▪

Once in the body tissues, the oxyhaemoglobin breaks down to release oxygen and

haemoglobin. The body tissues use the released oxygen for respiration and release

carbon dioxide as by-product. The released carbon dioxide causes the levels of carbon

dioxide to become higher in the body tissues than in the blood.

▪

Carbon dioxide therefore diffuses from the body tissues into the blood capillaries and

combines with haemoglobin to form carbaminohaemoglobin. The capillaries

transport carbon dioxide in this form to the alveoli.

This leads to a higher concentration of carbon dioxide in the blood capillaries than in

the alveoli. Carbon dioxide therefore diffuses from the blood capillaries into the

alveoli. It is then transported through the bronchioles, trachea, glottis, pharynx and

finally nostrils into the atmosphere.

Figure 6.4: Gases Exchange Across Alveolus

Factors Affecting the Rate of Gaseous Exchange in Mammals

Concentration of Carbon dioxide. An increase in the levels of carbon dioxide in the

blood increases the breathing rate, so that more oxygen can be taken in and more

carbon dioxide is removed from the body.

(i)

(ii)

Concentration of Haemoglobin. During gas exchange, haemoglobin transports

oxygen from the lungs to the body cells and carbondioxide from body cells to the

lungs. Efficient transport of gases take place when the body has enough

concentration

of

haemoglobin.

Therefore,

the

higher

the

concentration

of

haemoglobin, the higher the rate of gas exchange.

(iii) 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 rate of gas exchange.

(iv) Altitude. At high altitude, the concentration of oxygen is lower than that in the lungs

or at sea level. Breathing rate is also higher at high altitude than at lower altitude,

this results in breathing difficult. Therefore, the rate of gas exchange has to increase

in order to obtain enough oxygen.

(v)

Exercise or physical activity of an Individual. During active exercises like running,

the body needs more of oxygen to release the energy required for the contraction of

muscles. As a result, gaseous exchange takes place faster when there is increased

body activity in order to take in oxygen for oxidation of food to release energy and

removal of carbon dioxide from the body.

(vi) Health status of the body. The rate of gaseous exchange increases when somebody

is sick. This is due to increased metabolism by the liver in order to remove the toxins

released by disease causing microorganisms or break down the drugs taken.

However, certain diseases such as asthma make the body weak and cause slowing

down of the breathing process.

GAS EXCHANGE IN PLANTS

Gas exchange in plants involves exchange of oxygen and carbondioxide. During the day,

plants take in carbondioxide and at night they take in oxygen.

Parts of the Plant Responsible for Gas Exchange

Gaseous exchange in plants takes place through the following structures;

i.

Stomata (singular: stoma). These are pores surrounded by guard cells found on

the lower side of the leaves. In plants, gas exchange mostly takes place through the

stomata.

ii.

iii.

iv.

Lenticels. These are porous tissues in the bark of the woody stem and root,

consisting of air spaces between the cells known as intercellular space.

Breathing roots (pneumatophores). These are specialized roots developed in some

plant species that grow in waterlogged or strongly compacted soils, e.g. mangroves.

Cuticles. These are waxy fluid materials secreted by plant cells and placed on the

outer parts of plant leaves and stems. They play a role of protecting the inner parts

of the plant as well as exchange of gases. The air passes in and out of the plant

body through the cuticle by diffusion.

MECHANISM OF GAS EXCHANGE IN PLANTS

(a) 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

▪

Chloroplast in the guard cells that surround the stomata undergo photosynthesis to

produce glucose. The guard cells become hypertonic, as a result, water moves by

osmosis from the neighbouring cells into the guard cells.

▪

The guard cells become turgid and the stomata open whereby the air from the

atmosphere such as carbon dioxide enters into the air spaces in the spongy

mesophyll. The carbon dioxide diffuses into neighbouring cells until it reaches the

site for photosynthesis and oxygen moves out through the open stomata to the

atmosphere.

During the night

▪

At night there is less carbon dioxide and no sunlight. Therefore, photosynthesis

ceases and there is no production of glucose.

The guard cells do not absorb water by osmosis. Therefore, the stomata remain

partially closed.

However, respiration process takes place at night in plants. The partially open

stomata allow oxygen to move into the plant cells while carbon dioxide moves into the

air spaces and eventually into the atmosphere. This explains why green plants

produce carbon dioxide at night and oxygen during the day.

Figure 6.5: Structure of a Stoma

Figure 6.6: Gaseous Exchange through the Leaf

(b) Gaseous Exchange through the Lenticels

Lenticels are small openings in the bark of the woody stems and roots. They have

loosely packed cork cell and intercellular air spaces.

▪

At night, there is a higher concentration of oxygen in the intercellular air spaces

than in the cork cells. Therefore, oxygen diffuses into the cork cells surrounding the

lenticels.

The cork cells use oxygen for respiration and release carbon dioxide. Thus, the

concentration of carbon dioxide in the cork cells becomes higher than in the

intercellular air spaces.

▪

Therefore, carbon dioxide diffuses out through the cork cells into the intercellular

air spaces and then out through the lenticel.

The opposite happens during the day.

Figure 6.6: Structure of the Lenticels