Exchange surfaces

Breathing rate: the number of breaths per minute.

Oxygen uptake: the volume of oxygen absorbed by the lungs in one minute.

Tidal Volume: the volume of air inhaled or exhaled in one breath; usually measured at rest.

Spirometer: a device that can meausre the movement of air into and out of the lungs.

Vital capacity: the greatest volume of air that can be expelled from the lungs after taking the deepest posisble breath.

A spirometer is a device which measures the movement of air in/out of the lungs as a person breathes. A float-chamber spirometer consists of a chamber of air or medical-grade oxygen floating on a tank of water. During inspiration, air is drawn from the chamber so the lid moves down. During expiration, the air returns to the chamber which raises the lid. These movemnets may be recordedon a data logger.

The carbon dioxide rich air exhaled, passes through a chamber of soda lime, wich absorbs the carbon dioxide. This allows the measurement of oxygen consumption.

• The subject should be healthy and free from asthma.
• The soda lime should be fresh and fuctioning.
• There should be no air leaks in the apparatus, as this would give invalid or inaccurate results.
• The mouthpiece should be sterilised.
• The water chamber must not be overfilled.

Modern spirometers may be small simple hand-held devices. They can't always measure oxygen consumption.

The total lung volume consists of the vital capacity, which can be measured and the residual volume, which cannot be meausred using the spirometer.

• Vital capacity is the maximum volume of air that can be moved by the lungs in one breath. This is measured by taking a deep breath and expiring all the air possible from the lungs. Vital capacity depends on a number of factors such as:
  • The sizeof a person (mainly height)
  • Their age and gender
  • their level of regular exercise.

Vital capacity is usually in the region of 2.5-5.0 dm^3. Trained athletes may rise above this level.

• The residual volume is the volume of air that remains in the lungs even after forced expiration. This air remains in the airways and alveoli. Approx 1.5 dm^3.

• Tidal volume is the volume of air moved in/out with each breath. It is usually measured at rest. Approx 0.5 dm^3. This is usually sufficient to supply all the oxygen required at rest.

Breathing supplies oxygen for respiration and removes carbon dioxide produced in respiration. As a person breathes from the spirometer, oxygen is absored by the blood and replaced by carbon dioxide. Thus is absorbed by the soda lime, so the volume of air in the chamber decreases.

This decrease can be observed and measured on the spirometer trace. We can assume that the volume of carbon dioxide released and absorbed by the soda lime equals the volume of oxygen absorbed in the blood. herefore, measuring the gradient of the decrease in volume enables us to calculate the rate of oxygen uptake.

• On trace (a) darw a line from point A down to the x axis and do the same for point B. Measure the length of time between these points.
• Draw a line from point A to the y axis and do the same for point B. Measure the difference in volume between these two points.
• volume/time calculation.
• The unit will be dm^3s^-1
• = rate of oxygen uptake on a trace

The breathing rate can also be measured from a spirometer trace. Simply count the number of peaks in each minute. Breathing rate at rest is usualy about 12-14 breaths per minute.

Oxygen uptake will depend on a number of factors. A higher oxygen uptake will result from an increased demand, such as during exercise when the muscles respire more.

Increased oxygen uptake will result from:

• Increased braething rate.
• Deeper breaths.