The respiratory system

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Gas exchange in the lungs

The alveoli, blood and the capillary network of the human lung provide an efficient gas exchange surface.

The lung system shows the following features;

  • very large surface area- in a young adult estimated to be about 70m^2 compared to total body surface area approximately 18m^2
  • barries are very thin- alveolar wall and pulmonary capillary wall are both about 0.1um thick
  • permeable to the gases involved- alveolar and capillary walls are permeable to water, oxygen and carbon dioxide
  • moist- the surface of the alveoli and the capillary walls have liquid on them that includes water so that the gases can dissolve and cross the membranes in solution
  • about 80% of the total alveolar surface is in contact with a pulmonary capillary
  • red blood cells are flexible and when forced through very narrow capillaries will change shape, curving into contact with the capillary wall- the diffusion distance is therefore very small
  • diffusion gradients exist- maintained in the correct direction
  • close to a transport system- the gases in the alveolar spaces are continously changed as a ventilation and the blood in the capillaries is continously changed as a result of blood circulation
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what features are found in what structures?

structure                feature

                              carilage         goblet cells         smooth muscle      cilia

trachea                    yes                   yes                       yes                  yes

bronchus                 yes                   yes                       yes                  yes

large bronchiole      no                     yes                       yes                  yes

alveolus                  no                      no                         no                   no

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Goblet cells

Goblet cells produce mucus. Mucus sticks dirt/microbes.


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  • Ribs and sternum- raised by contraction of external intercostal muscles
  • Diaphragm- pulled down by contraction of muscle fibres in diaphragm
  • Volume of throax- increases
  • Air pressue inside throax- decreases
  • Air movement- from the atmosphere to the lung air space
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  • Ribs and sternum- lowered by gravity and contraction of internal intercostal muscles
  • Diaphragm- moves up due to contraction of abdominal was muscle, elastic recall or resistance offered by gut
  • Volume of throax- decreases
  • Air pressure inside throax- increases
  • Air movement- from the lung air space to the atmosphere
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Peak flow meter

A peak flow meter measures how much air you can forcibly expire.

  • ensure the mouthpiece is clean/sterile
  • insert the mouthpiece into the meter
  • hold the meter horizontally so that your fingers are clear of the scale and slot
  • ensure the meter is on zero
  • take a deep breath, and blow as hard and fast as you can
  • note the number on the pointer
  • return back to zero, and repeat until you have 3 readings
  • take the highest reading and record it

A normal reading for peak respiratory flow is between 400&600 dm^3 min-1.

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  • insert a clean sterile mouthpiece
  • put nose clip on
  • put mouthpiece in
  • sit down and relax for approximately 2 minutes and get used to breathing atmospheric air (taps closed)
  • open taps so you're breathing in medical grade oxygen
  • start the recording
  • record for 1 minute to record the tital volume (should be between 0.4 - 0.5 dm^3)
  • get the patient to do a deep breath in and out to record the vital capacity (males should be 6dm^3 and females should be 4.25dm^3)
  • close the taps
  • turn the machine off and stop recording
  • remove nose and mouthpiece
  • save results and put in patients records
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Respiratory system


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Tidal Volume & Vital Capacity


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Oxygen levels

In areas of high oxygen levels (e.g. lungs); Haemoglobin + 4 Oxygen <-> oxyhaemoglobin

In areas of low oxygen levels (e.g. muscle); Oxyhaemoglobin <-> haemoglobin + 4 oxygen

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How carbon dioxide is transported

1) dissolved CO2- carbon dioxide is much more soluble in blood than oxygen. About 5% of carbon dioxide is transported unchanged, simply dissolved in the plasma.

2) Bound to haemoglobin and plasma proteins- carbon dioxide cobines reversibly with haemoglobin to form carbonmindohaemoglobin. Carbon dioxide does not bind to iron, as oxygen does, but to amino groups on the polypertide chains of plasma proteins. About 10% of carbon dioxide is transported bound to haemoglobin proteins.

3) Bicarbonate ions (HCO3) - 85% (the majority) of carbon dioxide is transported this way. Red blood cells transport carbon dioxide. Carbon dioxide reacts with water to form carbonic acid.

Carbon dioxide + water -> carbonic acid

Carbon acid -> hydrogen ion + bicarbonate ion

Hydrogen ion combines with haemoglobin to form haemoglobonic acid

Bicarbonate ion diffuses into blood (plasma) transported to the lungs

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