Gaseous Exchange

Gaseous exchange and the respiratory system 


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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