Note0701 Breathing

8/8/2019 Note0701 Breathing

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Biology Notes Gaseous Exchange Page 1

The genera l p lan of t he human breath ing system

1. The breathing system consists of the resp i ra to ry t rac t () and the lungs. The respiratory tract servesas a passage for air while the lungs are the surface for gaseous exchange.

2. Passage of air :

Nostrils nasal cavity pharynx larynx trachea bronchi bronchiole alveoli

3. It is better to breathe through the nose than through the mouth because the nasal cavity contains

(a) ha i rs to f i l te r dust particles and germs in air ;

(b) mucus to t rap dust and germs and to moisten the air;

(c) blood capi l lar iesto w arm the a ir .

4. The inner lining of the nasal cavity, the trachea and the bronchi are covered with a layer of c i l ia ted

epi the l ium() and mucus (). The mucus traps dust particles and germs from the incoming air.The beat ing o f t he c i l ia moves the mucus with trapped particles towards the pharynx (), where it is

swa l l owed .

5. Thepharynx () is a common passage for both air and food. The l a rynx() contains voca l cords() which vibrate as air passes between them to produce sound.

6. The t rachea() is supported by C-shaped car t i la ges() which keep the trachea open and preventit from collapsing. At its lower end, the trachea divides into two bronch i () which are also supported bycartilages.

Sect ion I I I b(2) Gaseous Exchange in humans


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7. Each bronchus branches repeatedly into numerous smaller and narrower tubes called bronchio les (). Each bronchiole ends in a tiny air sac () whose surface is folded up to form many alveol i ().Each lung contains millions of alveoli which provide a very large surface area for gaseous exchange.

8. Each lung is protected by twop leura l mem branes (). In between the membranes is an air-t ight () cavity which contains thep leura l f lu id (). The fluid acts as a l ub r ican t () to reducefriction between the lungs and the ribs during breathing.

9. The lungs and the pleural membranes are protected by the r ib cage(), which is formed by(a) the ver tebra l co lumn() at the back ,(b) the ste rnum() in front , and(c) ther ibs() on the two sides.


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Adapt ive features of the a i r sac s ( a lveol i ) for gaseous exchange

Adaptation Importance

1. The alveoli are numerous and are highly folded. This provides a large surface area for diffusion of

gases.

2. The wall of the alveoli is very thin. This shortens the diffusion distance and enables

faster diffusion of gases.

3. The alveoli are surrounded by many blood

capillaries.

This allows a rapid transportation of gases to and

from the lungs. This can help to maintain a steep

concentration gradient of the respiratory gases

for rapid diffusion.

4. The inner surface is moist. This allows gases to dissolve first to facilitate

their diffusion.

Dif ferences in composi t ion betw een inhaled and exhaled a i r

Inhaled air Exhaled air

1. Oxygen 21% 16%

2. Carbon dioxide 0.03% 4%

3. Nitrogen 78% 78%

4. Other gases 1% 1%

5. Water vapour Variable Saturated ( 100% )

6. Temperature Variable Body temperature ( 37C)

1. Exhaled air contains less oxygen because some oxygen in the inhaled air has di f fused in to the b lood.

2. Exhaled air contains more carbon dioxide which is produced from resp i ra t ion by living cells.

3. The nitrogen content remains unchanged because it is neither used nor produced by body cells.

4. Exhaled air is saturated with water vapour because it is mois tened by the water f i lmon the surface of the

alveoli.

5. Exhaled air is at body temperature because it is w armed by t he b lood cap i l la r ies along the respiratory

tract.


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An experiment to compare the carbon dioxide content of inhaled and exhaled air

Results :

Note : The indicator will turn purple when the CO2concentration 0.03%

Conclusion :

Exhaled air contains more CO2 than inspired air

Tube Colour of indicator CO2 concentration

A Red 0.03%

B Yellow 0.03% ( =4% )

The mechan ism o f vent i la t ion

1. Breath ing/ vent i la t ion is the movement of air to and from the lungs. It includes inhalat ion ( inspiration / breathing in ) and exhalat ion( expiration/ breathing out ). These processes depend on thedi f ferenc es in a i r pressurebetween the lungs and the atmosphere.

2. The thorax() is an air- t ight () cavity enclosed by the rib cage at the sides and the diaphragmbelow. Movements of the ribs and diaphragm cause changes in the volume of the thorax, and this in turn leads

to corresponding changes in the volume and pressure of the lungs.

3. Inhalation occurs when the air pressure inside the lungs is lower than the atmospheric pressure. Exhalation

occurs when the air pressure inside the lungs is higher than that the atmospheric pressure.

Bicarbonate

indicatorObserve the change in colour of

bicarbonate indicator.

A B


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4. A summary of the mechanism of breathing :

Inhalation Exhalation

1. Intercostal muscles () Contract Relax2. Ribs Move upwards and outwards Move downwards and inwards

3. Diaphragm muscles () Contract Relax4. Diaphragm Flattened Returns to dome-shape5. Volume of the thorax Increases Decreases

6. Volume of the lungs Increases Decreases

7. Air pressure in the lungs Decreases and becomes lower than

atmospheric pressure

Increases and becomes higher than

atmospheric pressure

8. Movement of air Into the lungs Out of the lungs

9. State of the lungs Inflated Deflated


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Examination tips :

One breathe : P T Inspiration : R T Expiration : P Q

At H : Intercostal muscles relax, ribs move downwards and inwards . The alveoli collapse.

Diaphragm relaxes and becomes dome-shaped

At J : Interconstal muscles contract, ribs move upwards and outwards. The alveoli are fully extended.

Diaphragm contracts and becomes flattened.

J


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Using a bell jar model to show the action of the diaphragm during breathing

The bell jar model cannot truly represent the breathing movements in the human body because :

Bell jar model Actual conditions in the human body

1. The rubber sheet is moved by hand. 1. The diaphragm moves automatically.

2. Shape of rubber sheet :

Inhalation : curved downwards

Exhalation : flattened

2. Shape of the diaphragm

Inhalation : flattened

Exhalation : dome-shape

3. The wall of the bell jar is immovable. 3. The ribs can be moved by contraction of

intercostals muscles..

4. The space inside the bell jar is filled with

air.

4. The pleural cavity is filled with pleural fluid

5. No cartilage to support the glass tube. 5. The trachea is supported by cartilages.

/ Ribs / Rib cage


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Using the rib model to demonstrate the action of the intercostals muscles during breathing

Volumes of a i r exchanged (Addi t ional )

Lung volume Definition Volume ( litres )

Men Women

Tidal volume

() The volume of air breathed into and out of the lungsduring quiet breathing. 0.5 0.5Vital capacity

() The maximum volumeof air that can be expiredafter the deepest possible inhalation. 3 4 2.5 3Residual volume

() The volume of air remained in the lungs after themost forceful exhalation. 1.5 1.5Total lung capacity

() The maximum volume of air contained in the lungs 4.5 5.5 4 4.5


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Examination tips :

1. During exercise, the tidal volume increases.

2. After a long period of physical training, the vital capacity can be increased.

3. Trained athletes usually have larger vital capacities.

Ef fec t o f exerc ise on the ra te and depth o f b rea th ing

1. The breathing rate of an adult is about 12 18 minutes at rest. The depth of breathing ( = tidal volume ) is about

0.5 litre.

2. During exercise, both the ra te and dep th o f b rea th ing increase . This is important because

(a) the body can take in more oxygen for faster respirat ion in the skeletal muscles to provide more

energyto support the vigorous muscle contraction.

(b) the body can rem ove more CO2 which is produced from respiration.

3. Mechanism :

During exercise, rate of respiration in tissues increases more CO2is produced

The increase in CO2 concentration in the blood stimulates the breathing center in the brain

Which sends nerve impulses to the diaphragm and intercostal muscles

To speed up the rate and increase the force of contraction of these muscles

The rate and depth of breathing increase.


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Gaseous exchange in t he a lveol i

1. By inspiration, atmsopheric air is drawn into the lungs via the respiratory tract.

2. Since the oxygen concentration in the inhaled air is higher than that in the blood, oxygen always di f fusefrom

the alveoli to the blood.

3. The route of oxygen diffusion :

oxygen in alveoli dissolved in water f i lm lining the alveoli diffuses through a lveo la r w a l l

cap i l la ry w a l l red blood cells.

4. As the blood entering the lungs has a higher CO2 concentration than the inhaled air, CO2 always diffuse from

the blood to the alveoli.

5. As a result of gaseous exchange in the lungs, the exhaled air contains less oxygen but more CO2 than the

inhaled air.

6. Blood flow across the alveoli :

Deoxygenated blood () inpu lmonary a r t e ry () gaseous exchange in the capillaries surrounding the alveoli

oxygenated blood () carried away by thepu lmonar y ve ins ().


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Transport o f gases in blood

1. Once inside red blood cells, oxygen combines with haemoglobin () to form oxyhaemoglobin() which is bright red in colour. Oxygen is mainly transported as oxyhaemoglobin to all bodytissues.

2. Carbon dioxide produced from respiration in the tissues is transported in blood back to the lungs mainly in form

ofhydrogencarbonate ions(). The ions turn back into carbon dioxide before diffusing into thealveoli.

Note0701 Breathing

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