Chapter 7 Gas ExchangeOng Yee Sing

2017

Gas exchange

• Gas exchange is the biological process by which gases move by passive diffusion across a surface.

• Here, it refers to the process of diffusion of oxygen gas into the body through wet surface and the diffusion of carbon dioxide gas out of the body.

Simple diffusion

• Unicellular organisms (yeast, amoeba and paramecium) only need to carry out gaseous exchange at their body surface through a simple mechanism diffusion as they have a high total surface area to volume ratio (TSA/V ratio).

• Gas exchange by direct diffusion across surface membranes is efficient for organisms less than 1 mm in diameter.

Amoeba

Flat worm

Specialisation of organs

• Large organisms required specialized organs to undergo gas exchange.• the TSA/V ratio of large organisms

is small

• the body surface of terrestrial animals/plants is water-proof to reduce water loss

Skin Gill

Trachea Lung

Increasing complexity of respiratory system

7.1 Respiratory Surface

Respiratory surfaceRespiratory surfaces Surrounding environment Examples of organisms

1. Cell membrane Water Yeast, amoeba, paramecium

2. Gills Water Bony fish

3. Trachea Land Cockroach and grasshopper

4. Skin & lungs Water & land Frog

5. Lungs Land Reptiles, birds and human

The gills of the cartilaginous fish are exposed.

Characteristics of respiratory surfaces

• Thin surface• Facilitate the diffusion of gases

• Moist surface• Oxygen and carbon dioxide must be dissolved in water speed up rate of

diffusion.

• Large surface area• Speed up the rate of gaseous exchange.

• Network of blood capillaries which are closely packed• Facilitate transport of gases after the exchange.

Size of organisms and gaseous exchange• The increase in volume is much faster than the increase in surface area.

• This means that the surface for gaseous exchange is too small for the numerous cells found in the body.

• To overcome this problem, organisms have to change the surface area of their body or to have it increase the entry and exit of gases and the rate of transport of gases.

Lower surface-to-volume ratio = faster diffusion

Quiz

• Which of the following is not a way to increase the efficiency of a respiratory system?

A. increase the surface area available for diffusion of gases

B. decrease the distance over which the gases must diffuse

C. increase the concentration differences of gases inside and outside the system

D. dry the system out so the gases do not have to diffuse through water

E. all of the above will increase efficiency

Conclusion

• Type of respiratory surfaces includes skin, cell surface, gills, trachea and lungs.

• Characteristics of respiratory surfaces includes thin surface, moist surface, large surface area, and network of blood capillaries which are closely packed.

7.2 The mechanism of gaseous exchange

Mass flow

• Mass flow集体流动, also known as “mass transfer” and “bulk flow”, is the movement of fluids down a pressure or temperature gradient.

• Diffusion is the movement of substances down a concentration gradients.

• Examples of mass flow include blood circulation and transport of water in vascular plant tissues.

7.2.1 Gas Exchange in Insects

Tracheal system• Tracheae气管系统(air-tube) are

distribute throughout the body of the insects.

• Air enters the tracheal system through the spiracles气孔 located on both sides of the thorax胸部and abdomen腹部.

• The tracheal system contains a few main tracheal trunks气管干branch which run through the body from the head to the tail.

• The tracheal trunks branch into numerous tracheae气管(singular: trachea).

• The tracheae branches into tracheoles微气管 which directly connects to the muscles or tissue cells.

Branching of the tracheal system

Walls of tracheae and tracheoles

• The walls of the tracheae have spiral bands of chitin几丁质which thicken the walls.

• These walls can prevent the tracheae from collapsing when the pressure in the tracheae fall.

• The walls of the tracheoles are not thickened by chitin.

• Walls of tracheoles are thin and moist.

• Gas exchange occurs at the surface of tracheoles through mechanical diffusion.

Control of airflow• The opening and closure of the

spiracles of the grasshopper are controlled by muscular valves气孔瓣膜.

• Muscles contract to close the spiracle, or relax to open it.

SEM of a crickets spiracle valve. 342x

Breathing in mantis

Air sac气囊• Through the expansion and contraction of

the abdomen, the air sacs help to drive air in and out of the tracheae.

• Air sacs also provide temporary air supply so that an insect to conserve water by closing its spiracles during periods of high evaporative stress.

Unidirectional airflow during abdominal

pumping in a grasshopper. During

inspiration, air flows in through open

thoracic spiracles (sp), along the

longitudinal trachea, and into the air

sacs. At low metabolic rates, air flows

out only through the tenth abdominal

spiracles; in more active animals, air

flows out all abdominal spiracles.

Blood of insects

• Insects do not need to use blood for the transport of gases.

• The blood of insects does not contain haemoglobin.

• This colourless fluid is also known as haemolymph血淋巴.

Hemolymph of cockroach under light microscopy. It is not very cellular though it contains hemocytes (white blood cells).

Conclusion

Quiz

• Insect respiratory systems contain all of these structures EXCEPT:

A. Parabronchi

B. Spiracles

C. Tracheoles

D. Tracheae

Quiz

• When a grasshopper’s head is submerged into the water, it will

A. be drown

B. die

C. still be breathing

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7.2.2 Gas exchange in fish

Fish gills鳃

• The gills of fish are the organs for gaseous exchange.

• In bony fish, four gills are presence in each opercularcavity.• In the textbook it is refers as

“four pairs of gills” inside each opercular cavity.

• The opercular cavity鳃腔 is covered by the operculum鳃盖.

Operculum

Oncorhynchus mykiss

Pontinus nematophthalmus

Aracana aurita

Peristedion gracile

Structure of a gill

• Each gill is made up with • gill arch鳃弧

• gill filaments鳃丝

• gill rakers鳃耙

Gill arches鳃弧and gill rakers鳃耙

• Gill arches hold the gill filaments.

• Gill rakers are bony or cartilaginous processes that project from the gill arch.

• Functions of gill rakers:• To prevent the potentially damaging passage of

solid material through the gill slits and over the gill filaments.

• To divert food particles into the esophagus

Gill filaments鳃丝

• Gill filaments are fleshy processes that project from the gill arch.

• They are red in colour due to the extensive blood capillary systems in the lamella鳃板.

F: filament; L: lamella

Opercular movement鳃盖运动

• The opercular movement is responsible for fish breathing.

• Fish take in water through the mouth. The opeculum is close.

• The mouth closes, forcing the water back over the gill filaments and out through the gill slits.

Gas exchange at the gill filament• The structure of gill filaments with numerous blood capillaries in the

lamella provide a large and effective respiratory surface for gaseous exchange.

• Blood carries oxygen from the gills to other body tissues and carbon dioxide from deeply seated tissues to the gill filaments.

Counter-current exchange system逆流交换机制

• The blood flows through the blood vessels in the opposite direction to the water flowing through the lamellae.

Advantages of the counter-current exchange system • This system maximises the amount of oxygen diffused into the blood

by having the most oxygenated blood meet the most oxygenated water, and the least oxygenated blood meet the least oxygenated water to maintain the concentration gradient the whole way through.

Quiz

• A countercurrent flow system between substance A and substance B

A. maximizes the exchange by having A and B flow in the same direction

B. minimizes the exchange by having A and B flow in the same direction

C. maximizes the exchange by having A and B flow in opposite directions

D. minimizes the exchange by having A and B flow in opposite directions

Limitations of the counter-current exchange system• A limitation of this gas exchange system is that fish can only live in water.

• They need water to support the filaments and hold the lamellae apart to maintain the large surface area.

• In air, the filaments and lamellae would stick together, greatly reducing the surface area: volume ratio, and therefore decreasing the efficiency of diffusion of gases.

• This may permanently destroy the structure of the filaments as well.

• The gills would also dry out without water keeping them moist, so gases would no longer be able to dissolve in order to diffuse into the blood.

Conclusion

Quiz

• The efficiency of gills in fish is derived from

A. the countercurrent flow of water over the gills

B. the increasing temperature of blood within the gills

C. continuous diffusion of oxygen into the blood

D. a and b

E. a and c

Quiz

• A countercurrent flow system between substance A and substance B

A. maximizes the exchange by having A and B flow in the same direction

B. minimizes the exchange by having A and B flow in the same direction

C. maximizes the exchange by having A and B flow in opposite directions

D. minimizes the exchange by having A and B flow in opposite directions

Quiz

• The efficiency of gills in fish is derived from

A. the countercurrent flow of water over the gills

B. the increasing temperature of blood within the gills

C. continuous diffusion of oxygen into the blood

D. a and b

E. a and c

7.3 Gaseous exchange in mammals

Respiratory system of mammals• The respiratory system of

mammals includes lungs and other structures which help to drive gas in and out of the lung.

Nasal cavity鼻腔

• Nasal/nose hair鼻毛• Filtering foreign particles e.g. dust from entering the

nasal cavity • Collecting moisture

• Mucous membrane鼻腔黏膜• Contain microvasculatures/microvessels微血管 to

warm the absorbed air• Secrete slime to moisture the air, and trap dust and

bacteria

• Olfactory cells嗅细胞• Latin olfacere ‘to smell’ + adjective –ory• Contain nerve endings• Reception of sensory stimuli caused by odours

Quiz

• The nasal hairs and mucus

A) filter impurities from the inspired air.

B) reduce transpulmonary pressure.

C) reduce the surface tension in the alveoli.

D) keep the lungs moist so gas diffusion can occur.

Pharynx咽

• Greek phárynx ‘throat’

• connects nasal cavity, mouth cavity, middle ear中耳, larynx and oesophagus

• acts as the common passage for food and gases.

Larynx喉• from Greek larynx "the upper windpipe"

• Epiglottis会厌软骨 is a flap-like structure that covers the opening of larynx when swallowing to prevent food or liquids from entering the trachea气管.

Larynx

• Vocal cords声带 are a pair of fibrous sheets of tissue with gaps that produce sounds.

Trachea气管and bronchus支气管

• Trachea (sg.), tracheae/tracheas (pl.), from Greek τραχεῖα “windpipe”

• Bronchus (sg.), bronchi (sg.), from Latin bronchus, from Greek βρόγχος(brónkhos) "wind pipe“.

• No gas exchange occurring here.

• Part of the conducting zone.

• C-shape cartilage rings C形软骨环

• Supporting structures which open up the lumen of the trachea to conduct air

• Mucous membrane黏膜

• Secrete mucus to trap dust and germs

• Possesses cilia纤毛that lash towards the larynx, driving the mucus粘液out of the body to become phlegm痰

Lungs

• two lobes for the left lungs, three lobes for the right lungs

Respiratory bronchiolesand alveolar ducts 肺泡管

• Bronchus divides into bronchioles.

• Alveolar ducts connect bronchioles to the alveoli.

• Bronchioles and alveolar ducts have no cartilage at all.

• Gaseous exchange also takes place here.

呼吸性细支气管

Alveoli肺泡• Alveolus (sg.), alveoli (pl.)

• from Latin, ‘small cavity,’ diminutive of alveus

• Made up of a single layer of epithelial cells

• Surrounded by a network of blood capillaries and elastic fibre.

• The main site of gaseous exchange

Alveoli

SEM

TEMLight microscopy

Quiz

• Which of these is an adaptation for efficient gas exchange in the air sacs (alveoli)?

A. Thick walls

B. Few blood capillaries

C. Moist surface

Movement of air

• Movement of air into the lungs

• Nostril Nasal cavity pharynx larynx trachea bronchus lung bronchiole respiratory bronchiole alveolar duct alveolus

Lungs in motion

• An experimental device keeps a lung warm, breathing and nourished while outside the body.

• Allow the donor lung to extending the life of an organ outside the body.

Quiz

• Gas exchange in the lungs occurs in the

A. Nasal cavity

B. Larynx

C. Bronchi

D. Respiratory bronchiole

Quiz

• Alveoli are microscopic air sacs branching off the

A) tertiary bronchi.

B) bronchioles.

C) terminal bronchioles.

D) respiratory bronchioles.

Quiz

• Name the following parts.

A

B

C

DE

F

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Asthma• Asthma is a long-term inflammatory

disease of the airways.

• Symptoms include episodes of wheezing气管响声, coughing, chest tightness, and shortness of breath.

• Symptoms caused by hyperactive bronchial tube with swelling airway wall, contracting muscle (reduce diameter of airway), and increase mucosal secretion.

• Symptoms can be prevented by avoiding triggers, such as allergens敏感物质 and irritants刺激物, and by the use of inhaled corticosteroids using an inhaler吸入器.

epithelium (Ep) basement membrane (Bm) smooth muscle (Sm) blood vessel (Bv).

7.3.1 Gaseous exchange at the lung alveoli and tissues

Partial pressure气体分压

• In a mixture of gases, each gas has a partial pressure.

• Partial pressure is the hypothetical pressure of that gas if it alone occupied the entire volume of the original mixture at the same temperature.• Or the partial pressure value of a gas is the ratio of the gas occupies in a mixture of

gases and the total pressure of the mixture of gases.

• The partial pressure of oxygen or carbon dioxide varies in the blood capillaries found in alveoli, the blood in vein, the blood in artery and the body tissue.

Ptotal = Pn+Pn+1

Partial pressure of gasses in difference tissues

• The partial pressure of oxygen or carbon dioxide varies in the blood capillaries found in alveoli, the blood in vein, the blood in artery and the body tissue.

Gasses AlveoliDeoxygenated

bloodOxygenated

bloodBody tissues

Oxygen 13.3 kPa 5.3 kPa 13.3 kPa 5.3 kPa

Carbon dioxide

5.3 kPa 6.0 kPa 5.3 kPa 6.0 kPa

Partial pressure of gasses in difference tissues

Partial pressure gradient

• The driving force of gaseous exchange is the difference in partial pressure (partial pressure gradient/difference)

分压差of the gasses in the alveoli and the blood capillaries.

• Gasses will move from a region with higher partial pressure to a region of lower partial pressure.

Gas exchange in the lungs

• Inhaled air contains a high concentration of oxygen molecules.

• The oxygen partial pressure in the alveoli (13.3 kPa) is much higher than the oxygen partial pressure in the alveolar blood capillaries (5.3 kpa) (Table 7.4).

• The oxygen molecules diffuse into the blood capillaries.

Transportation of oxygen in the blood• Oxygen combine with haemoglobin血红蛋

白in the red blood cells to form oxy-haemoglobin氧合血红素.

Hb + O2 HbO2

• Oxyhaemoglobin is transport to other parts of the body.

Structure of haemoglobin血红蛋白

• Form by four polypeptides of two different type.

• Each polypeptide contain a Heme group血红素with a Fe2+.

• The Heme group can combine with oxygen.

• Each haemoglobin can carry 4 oxygen.

Quiz

• Oxygen binds to the ____ of deoxyhemoglobin.

A) alpha chains

B) beta chains

C) iron atom in the heme groups

D) organic portion of the heme group

Releasing of oxygen into the tissues• Oxyhaemoglobin is a unstable

compound.

• The partial pressure of oxygen in the tissues (5.3 kPa) is lower than the partial pressure of oxygenated blood (13.3 kPa).

• Oxyhaemoglobin releases the oxygen.

HbO2 Hb + O2

Removal of carbon dioxide

• Tissue cells produce large amount of CO2 in the metabolism processes.

• The partial pressure of CO2 in the tissues is higher (6.0 kPa) than that of the blood plasma (5.3 kPa).

• Carbon dioxide diffuses into the plasma.

• Most carbon dioxide is transported as bicarbonate / hydrogen carbonate ion (HCO3

- )碳酸氢离子 to the aveoli.

CO2 + H2O HCO3- + H+

Expel of carbon dioxide• The carbon dioxide enters the

lung with the deoxygenated pulmonary artery肺动脉.

• The carbon dioxide partial pressure in the alveoli is lower (5.3 kPa) than that of the blood capillaries (6.0 kPa).

• The HCO3- ions in the blood

capillaries are rapidly converted into carbon dioxide and water

HCO3- + H+

CO2 + H2O

• Carbon dioxide diffuses into the alveoli and is removed from the body through the respiratory tract.

Quiz

• Bicarbonate ion (HCO3-) and hydrogen (H+) ions result from a reaction

of ____ with water.

A) oxygen

B) hydrogen

C) carbon dioxide

D) carbon monoxide

Summary

• Gasses diffuse from a region of high partial pressure to a region of low partial pressure.• Aka gasses move down the partial pressure gradient.

• Oxygen is transported as oxyhaemoglobin.

• Carbon dioxide is transported as bicarbonate (HCO3-).

Gases Alveoli ⇄ deoxygenated blood Oxygenated blood ⇄ tissue

Oxygen 13.3 KPa 5.3 KPa 13.3 KPa 5.3 KPa

Carbon dioxide 5.3 KPa 6.0 KPa 5.3 KPa 6.0 KPa

7.3.2 Adaptations of the alveoli to the exchange of gases

Large surface area

• Alveoli provide an extremely large surface area for the exchange of gases.

Extensive blood capillary network

• The alveoli are surrounded by a network of blood capillary.

• Oxygen is brought away rapidly through diffusion and blood flow, thus promoting the exchange of gases.

Thin walls

• The alveolar wall and the capillary wall are only made up of a single layer of cells.

• They share a basal lamina.

• Their total thickness of about 0.0005 mm, therefore gases can diffuse through easily.

Lengthened time for exchange of gases• The diameter of the

blood capillaries covering the alveoli is slightly smaller than the diameter of the red blood cells.

• Hence the red blood cell found in the capillaries become oval in shape and they flow slowly through the capillaries• increase in the

surface area• Increase in time

When the lungs inflated, the RBC are parallel to the alveoli wall, maximizing their contact with the epithelial cells.

Moisture• A thin film of moisture is found at the inner surface of the alveolar wall.

• It can dissolve O2 and CO2 thereby facilitating the diffusion of gases

Summary

• Adaptations of the alveoli to the exchange of gases are:• Moist surface,

• Large surface area,

• Thin alveolar walls,

• Extensive narrow blood capillary network.

7.4 Gaseous exchange in plant

Gas exchange by diffusion through stomata气孔and lenticels皮孔.

Stomata• from Greek στόμα "mouth"

• A stoma is a pore, found in the epidermis of leaves, stems, and other organs, that facilitates gas exchange.

• Stomata are bordered by specialized parenchyma cells called guard cells that regulate the size of the stomatal opening.

• Most of the vascular plants, including angiosperms, gymnosperms, ferns etc. possess stomata on the epidermis of the leaves or green young stem.

Stomata of extant ferns and gymnosperms (a,

psilophyte松叶蕨类; b, fern; c, cycad; d, ginkgophyte

苏铁; e, f, conifers; all from Kew microscope slide

collection, except b and f, which are differential-

interference contrast images of cleared leaves)

Function of stomata

• The exchange of carbon dioxide and oxygen gas, and the loss of water vapour occur at stomata.

• It is estimated that about 90% of the exchange of gases between the plant body and the external environment occurs at the stomata.

Stomata on the leaves

• Most dicotyledons双子叶plants, such as sunflower, usually have more stomata on the lower epidermis than the upper epidermis.

• Most monocotyledons单子叶

plants, such as wheat, the numbers of stomata on the upper and lower epidermis are about the same.

Gas exchange in the leaf

• The stomata are connected or linked with the spaces formed between the loosely arranged mesophyll cells.

• Air moves into the leaf through the stomata, then filled the air spaces and come into contact with the surfaces of the mesophyll cells and other cells.

• The surfaces of these cells are always kept moist to facilitate the exchange of gases.

Gaseous exchange in the stem

• Stomata can also be found on young and tender stem.

• The stem of woody plants has no stoma but they have lenticels.• From Latin lens ‘lentil’ 扁豆

Corn stem.

stoma

Lenticels

• A lenticel is a porous tissue consisting of cells with large intercellular spaces in the periderm of woody stems and roots of dicotyledonous flowering plants.

Ventilation in the stem of perennial woody plants• In perennial woody plants, most of the stem are lignified dead cells,

hence oxygen is not required.

• Therefore, aeration or ventilation between the lenticels and the spaces between the cells is enough.

Drawing of a sector of a cross section through a 5-year old twig from a basswood tree (Tilia).

Gas exchange in the roots

• The root hairs of roots and the epidermis of young roots can carry out exchange of gases with the air in the soil.

• Root hair increases the surface area.

• Gas exchange in roots requires the soil to be moist.

Diffusion of air within the plant body

• The spaces between the cells in a plant are filled with gases.

• These spaces are interconnected and gases are able to diffuse to every part of the plant body through these air spaces.

Summary

• Gas exchange in plants occurs by diffusion.

• The openings on plants are stomata and lenticels.