Gaseous Exchange

Gaseous exchange and the respiratory system 

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Gaseous Exchange

Gaseous exchange refers to the exchange of respiratory gases between the cells of the organsim and the environment.

Animals and plants respire aerobically - oxygen is taken in and CO2 is produced as a waste product

The respiratory surface is the area where gas exchange takes place e.g. alveoli of the lungs in humans

Gaseous exchange takes place by diffusion

Ideal respiratory surface:

- permeable; so gases can pass through

- thin; diffusion is only effective over very short distances (short diffusion pathway)

- large surface area to volume ratio; so sufficient volumes of gases can be exchanged

- have an extensive capillary network; to maintain a steep diffusion gradient across the respiratory     surface (keeps removing oxygen to maintain a concentration gradient)

A given volume of air has more oxygen than an equal volume of water

There are specialised respiratory structures in larger animals as their surface area to volume ratio decreases

A respiratory pigment in the blood further increases oxygen carrying capacity e.g. haemoglobin

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Respiratory System

Make sure you can label these on a diagram

note: '*' means must learn!!

  • nasal cavity 
  • pharynx
  • epiglottis
  • larynx
  • *trachea
  • Rib
  • *intercostal muscle
  • *bronchiole
  • pleural membranes
  • *right lung
  • *diaphragm
  • *oesophagus
  • *cartilage of the trachea
  • *bronchus
  • *left lung
  • *heart
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Squamous and Ciliated Epithelium

Squamous Epithelium

- very flat, thin cells

capillaries and alveoli are lined with squamous epithelium giving a short diffusion pathway

Ciliated Epithelium

- found in the trachea

- waft mucus away from the lungs wih trapped dust and bacteria (to be coughed up or swallowed)

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Gas Exchange in Humans

1. Air passes along the following structures:

-  Nasal passages to throat (pharynx/larynx) to trachea, to bronchi, to bronchioles, to the alveoli of the    lungs

2. Trachea:

- C-shaped ring of cartilage prevents trachea collapsing during ventilation and for the oesophagus to      expand as food is swallowed

- Epiglottis (flap of cartilage) closes over the trachea when we swallow

- Lined with ciliated epithelium and goblet cells -> goblet cells produce mucus which traps bacteria and dust particles; ciliated epithelium sweeps mucus to the throat

3. Bronchi:

- Trachea branches into two bronchi which then branch into bronchioles

- Bronchi and bronchioles have cartilage, smallers ones have only muscle and elastic tissue

note: muscle tissue expands and contracts; elastic tissue stretches and recoils.

This allows the tubes to expand and contract during ventilation


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Gas Exchange in Humans Cont.

4. Alveoli:

- Smallest bronchioles end in clusters of hollow air sacs called alveoli

- Alveoli are lined with squamous epithelium and collectively form the gas exchange surface; covering a huge surface area

- Between the alveoli and capillaries is a very thin layer of tissue fluid containing elastin (elastic fibres) which allows them to stretch as they fill with air and recoil

- Also contain collagen (fibrous protein)

- The alveoli are covered in an extensive capillary network which have very thin walls (one cell thick) -> red blood cells are slowed as they pass through the capillaries allowing a greater time for uptake of oxygen (diffusion); red blood cells are squeezed through to present more of their surface area

Overview of alveoli key features:

- large surface area for increased diffusion

- thin wall of alveoli and capillaries (short diffusion pathway)

- Blood flow from extensive capillary network maintains steep diffusion gradient

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Gas Exchange in Humans Cont.

5. Mechanism of Ventilation:

- Structures involved - external intercostal muscles (slant forwards and downwards); internal intercostal muscles (slant backwards and downwards); diaphragm (circular and radial muscles)

- Inspiration (Active process)

  •    External intercostals contract (internal intercostals relax)
  • Rib cage moves forwards and outwards
  • Diaphragm muscles contract and flatten (pulls down)
  • Pressue inside decreases; volume of thorax increases
  • Air rushes in to equalise the pressure

- Expiration (Passive process)

  • Largely due to elastic recoil of lung tissue and respiratory muscles
  • External intercostals relax (internal intercostals contract)
  • Diaphragm muscles relax
  • Volume of thorax reduced; pressure inside increases
  • Air forced out of lungs

Pulmonary ventilation = tidal volume (dm^3) x ventilation rate (min^-1)

Ventilation rate = the number of breaths in one minute

Tidal volume = the volume of air breathed in and out at rest (approx 0.5 dm^3)

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