The Lungs

How are the lungs adapted to provide our bodies with all the

oxygen we need?

Success Criteria

• Describe the features of an efficient exchange surface with reference to diffusion of oxygen and carbon dioxide across an alveolus.

• Describe the features of the mammalian lung that adapt it to efficient gas exchange.

• Outline the mechanism of breathing (inspiration and expiration) in mammals with reference to the function of the rib cage, intercostal muscles and diaphragm.

The structure of the thorax

6.

5.

10.

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8.

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3. 7.

The structure of the thorax

Trachea

Bronchus

Diaphragm

Bronchiolus

Intercostal muscle

C-shaped cartilage

Rib

Right lung Left lung

Gaseous Exchange

• Gaseous Exchange is the movement of gases by diffusion between an organism and its environment across a barrier such as the alveolus wall

The lungs are a large pair of inflatable structures lying in the chest cavity

Air can pass into the lungs through the nose and along the trachea, bronchi and bronchioles

Finally the air reaches tiny, air-filled sacs called alveoli. The walls of the alveoli are the surface where the exchange of gases takes place

Success Criteria

• Describe the features of an efficient exchange surface with reference to diffusion of oxygen and carbon dioxide across an alveolus.

• Describe the features of the mammalian lung that adapt it to efficient gas exchange.

• Outline the mechanism of breathing (inspiration and expiration) in mammals with reference to the function of the rib cage, intercostal muscles and diaphragm.

The lungs are protected by the ribs. Movement of the ribs together with the action of the diaphragm (a layer or muscular tissue beneath the lungs) help to produce breathing movements (ventilation)

Inspiration and Expiration

The ribcage, intercostal muscles and diaphragm all work together to move air into and out of the lungs, where gas exchange occurs across the thin (single-celled) walls of the alveoli

Ventilation is a physical process, relying on the principle of Boyle’s Law – which state

“Pressure is inversely proportional to volume”

The mechanism can be illustrated using a bell jar model of the respiratory system – however, the model does not illustrate involvement of the rib cage and the intercostal muscles in ventilation

Ventilation

Breathing out(expiration / exhalation)

Breathing in (inspiration / inhalation)

Internal intecostals contract in forced

expiration

Diaphragm & external intercostals contractRib cage raised (upwards and outwards)Diaphragm lowered (becomes flatter)Volume of chest cavity increasesPressure in chest cavity drops to below

atmospheric pressure to 758 mmHg

Air moves into lungs from atmosphere

Active process

Diaphragm & external intercostals relaxRib cage lowered Diaphragm raised (dome shape) due to push

from abdominal organs Volume of chest cavity decreasesPressure in chest cavity increases to above atmospheric pressure to763

mmHg

Air forced out of lungs into atmosphere

Aided by elastic recoil and abdominal organs Passive process

INSPIRATION EXPIRATION

Atmospheric pressure = 760 mmHg

Inhaling (Inspiration) Exhaling (Expiration)

Volume of thorax

Diaphragm muscle

Diaphragm Relaxes and resumes to dome shape

External intercostal

muscles

Rib cage

Pressure in chest cavity

Decreases below atmospheric pressure

Movement of air

Inhaling (Inspiration) Exhaling (Expiration)

Volume of thorax

Increases Decreases

Diaphragm muscle

Contracts Relaxes

Diaphragm Flattens and pushes digestive organs down

Relaxes and resumes to dome shape

External intercostal

muscles

Contracts/expands Relaxes

Rib cage Upward and outward Inward and downward

Pressure in chest cavity

Decreases below atmospheric pressure

Increases below atmospheric pressure

Movement of air Into the lungs down pressure gradient

Air forced out of lungs

Exam question (5 marks)

Success Criteria

• Describe the features of an efficient exchange surface with reference to diffusion of oxygen and carbon dioxide across an alveolus.

• Describe the features of the mammalian lung that adapt it to efficient gas exchange.

• Outline the mechanism of breathing (inspiration and expiration) in mammals with reference to the function of the rib cage, intercostal muscles and diaphragm.

Gases pass both ways through the thin walls of the alveoli.

Oxygen passes from the air in the alveoli to the blood in the capillaries.

Carbon dioxide passes from the blood to theair in the alveoli.

Task

• View the selection of lung slides• Complete a lung dissection• Then answer the following questions as fully as you

can.... (you can use Text book page 46 to help you)1. State 3 ways in which the structure of the lungs

allows efficient gas exchange (4).2. Explain why the barrier to diffusion must be as thin

as possible (1).3. Describe how a steep diffusion gradient is achieved

in the lungs (4).

Answers

1. State 3 ways in which the structure of the lungs allows efficient gas exchange (4). Large surface area (1) provides more space for molecules to pass through (1), plasma membranes surrounding the cytoplasm create a permeable barrier allowing diffusion of oxygen and carbon dioxide (1), alveolus wall is one cell thick creating a thin barrier for diffusion (1)

Answers

1. Explain why the barrier to diffusion must be as thin as possible (1). It reduces the distance gases have to diffuse (1)

Answers

1. Describe how a steep diffusion gradient is achieved in the lungs (4). Blood brings carbon dioxide from the tissues to the lungs ensuring the carbon dioxide concentration in the blood is higher than that of the air in the alveoli (1), it also carries oxygen away from the lungs ensuring that the concentration of oxygen in the blood is kept lower than air inside the alveoli (1), the movement of the lungs (ventilation) ensures there is a fresh supply of oxygen entering the lungs increasing the concentration of oxygen in the alveoli (1), carbon dioxide is also removed by ventilation ensuring that the concentration in the alveoli is lower than that of the blood (1)

How do the alveoli decrease the diffusion distance?

How do the alveoli decrease the diffusion distance?

Alveolus and capillary are 1 cell thick

Alveolus and capillary are squamous epithelium.Total barrier thickness = 1 um

Capillaries in close contact.Capillaries are narrow so RBC squeeze through. Closer to alveolus and slows down movement.

Think!

Read the sentence below... Is it correct? If not, why not?

Alveoli have thin cell walls- this decreases the distance that gases have to travel and speeds up rates of diffusion

Alveoli have thin cell walls- this decreases the distance that gases have to travel and speeds up rates of diffusion

Alveoli have thin walls- their walls are one cell thick!

Success Criteria

• Describe the features of an efficient exchange surface with reference to diffusion of oxygen and carbon dioxide across an alveolus.

• Describe the features of the mammalian lung that adapt it to efficient gas exchange.

• Outline the mechanism of breathing (inspiration and expiration) in mammals with reference to the function of the rib cage, intercostal muscles and diaphragm.