Lung Volumes



Respiratory system

Breathing volumes

The volumes of air breathed in and out in different circumstances may be measured using a spirometer. The subject breathes in and out of a sealed chamber through a mouthpiece.

As the chamber inflates and deflates, a pen recorder traces out the breathing movements on to a chart. The machine is calibrated so that breathing volumes can be calculated.

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Ventilation is said to be tidal (air moves in and out). The amount of air breathed in and then breathed out in a normal cycle is called the tidal volume, usually about 500 cm3.

We are able to breathe in and out to a greater extent than simple tidal breathing, and these extra supplies of air are called the inspiratory reserve volume, and the expiratory reserve volume, respectively.

During exercise the tidal volume increases, making use of both the inspiratory and expiratory reserve volumes. All three volumes together add up to the vital capacity - the maximum possible tidal volume - usually about 4 - 5 dm3.

When we breath out as hard as we can, some air remains in the lungs, this is called the residual volume, add this to the vital capacity and we have the total lung volume, usually between 5 – 7 dm3.

Some of the air we breathe in does not reach the alveoli, but remains in the air passages, occupying the so-called dead space.

These volumes and the effects of exercise are shown on the following spirometer trace:

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1. Use the graph to calculate the vital capacity of the subject.

2. Describe in your own words the effect of exercise on the tidal volume.

3. Describe the effect of exercise on the inspiratory and expiratory reserve volumes.

The minute ventilation is a measure of the amount of air that is breathed in in one minute.

Minute ventilation = tidal volume x breathing rate

With tidal volume being measured in litres and breathing rate in breaths per minute, minute volume must have units of litres per minute. Notice the similarity between this formula and that for cardiac output.

4. Calculate the minute ventilation for a performer at rest who is taking 15 breathes per minute and whose tidal volume is 500 mls per min.

5. During exercise the performer's breathing rate increases to 20 breaths per min and their tidal volume increases to 4.5 Litres per min. Calculate their minute ventilation.

6. What do we call the air that remains in the lungs after the deepest possible breath out (expiration)?

7. Explain why the lungs are never completely emptied.

8. Which three lung volumes make up the vital capacity ?

Control of Breathing

The muscles controlling breathing are regulated by nerve impulses from the respiratory centre in the medulla of the brain.

This respiratory centre is thought to have two sub-systems, one for causing inspiration, and the other for expiration.

This control is normally sub-consciously controlled, but can be altered through conscious control (e.g. holding your breath and during speech), or through changing circumstances (e.g. exercise).

The respiratory centre responds to a variety of different sensory signals that indicate changes in the body that demand alterations to our breathing pattern. The most important of these is a change in the blood's pH or acidity.

During exercise, both carbon dioxide (which forms carbonic acid) and lactic acid are produced as waste products, and result in the blood becoming more acidic.

In the neck, the carotid arteries contain chemoreceptors, which can detect changes in the blood's acidity, and information about these changes is sent to the respiratory centre in the medulla of the brain.

The respiratory centre responds to this by increasing the rate and depth of breathing.

When carbon dioxide levels increase in the blood because of exercise, these changes are detected by the chemoreceptors which send nerve impulses to the respiratory centre in the medulla.

The respiratory centre then sends nerve impulses along the phrenic nerve to the diaphragm and intercostal muscles which makes them contract.

This increases the volume of the chest, decreasing the pressure and sucking air into the lungs

A second signal follows from the respiratory centre to stop these muscles contracting and we breathe out.

By regulating these two series of nerve impulses, the respiratory can increase the rate and depth of breathing during exercise.

There are other receptors that also cause an increase in heart rate.

Proprioceptors, also called mechanoreceptors, are sensory receptors found in muscles, tendons and joints that provide information about movement and body position. At the start of exercise they detect an increase in muscle movement. These receptors send nerve impulses to the respiratory control centre which in turn sends impulses along the sympathetic phrenic nerves to the respiratory muscles to increase breathing rate.

Baroreceptors detect changes in blood pressure. A decrease in blood pressure occurs as exercise starts and this results in the baroreceptors sending nerve impulses to the medulla and an increase in breathing rate via sympathetic nerves. An increase in blood pressure causes a decrease in breathing rate.

The basic aim of the body during these changes to breathing rate is to make sure that sufficient oxygen is getting to cells, whilst at the same time making sure that carbon dioxide levels do not get too high.

There are several other factors that affect respiratory rate including:

• detection of changes in body temperature

• stretch receptors in the walls of the air passages are stimulated by over-stretching during inspiration, send impulses to the respiratory centre, causing expiration

9. Explain in your own words how the rate and depth of breathing are regulated.

10. The following diagram shows how breathing rate varies with the duration and intensity of exercise. Use the graph to calculate the maximum breathing rate during heavy exercise.

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11. Explain why the pulmonary ventilation (breathing rate) is different at the different levels of exercise.

12. Explain why the pulmonary ventilation at moderate and light levels of exercise level off before the end of exercise.

Gaseous Exchange

There are two locations for gaseous exchange in the body. Gas exchange occurs between the air in the alveoli of the lungs, and the blood in the capillaries that surround the alveoli.

Gas exchange also occurs at all those tissues that have a demand for oxygen and that produce carbon dioxide. In terms of this subject, that essentially means gas exchange between the blood capillaries within muscles, and the muscle tissue itself.

Diffusion of gases

Gases will move by diffusion from a region of higher concentration to a region of lower concentration, until equilibrium is reached. An increase in temperature speeds up the rate of diffusion.

With regard to gases, the correct terminology is not concentration, but partial pressure (pO2 and pCO2), which is dependent both on the concentration of a gas, and its pressure.

13. What do you understand by the term partial pressure?

Gaseous exchange in alveoli

Blood that enters the capillaries surrounding the lung’s alveoli has a lower oxygen and higher carbon dioxide content than the air in the alveoli. Therefore diffusion occurs, with oxygen moving from the alveoli into the blood, and carbon dioxide moving from the blood to the alveoli.

14. What is diffusion?

Both these processes involve the gases moving down a partial pressure or concentration gradient. During diffusion, oxygen must pass through a layer of moisture, the alveolar membrane, the thin wall of the blood capillaries, the plasma, and through the membrane of the red blood cells. Carbon dioxide diffuses in the opposite direction.

The process of diffusion is helped by having a vast network of alveoli and surrounding blood capillaries, hence producing a large surface area for diffusion.

Also the exceptionally thin alveolar and capillary membranes reduce the diffusion distance between the air in the alveoli and the blood, and the relatively slow passage of red blood cells through the blood capillaries (the transit time) allows time for diffusion to occur, even during maximum exercise.

15. Which structures within the lungs assist the process of diffusion?

The rate of diffusion is approximately 250 cm3 of oxygen per minute whilst at rest, but can increase to values of over 5 litres (dm3) of oxygen per minute during exercise.

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16. Explain why all the oxygen in the air within the alveoli could not be taken up by diffusion into the blood.

Oxygen and carbon dioxide composition of air and blood

As we breathe in and out, so the composition of the air changes as shown below:

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The air we breathe out is a mixture of air from the alveoli and air that previously occupied the dead space. Similarly, not all the air we breathe in reaches the alveoli, as it mixes with the air remaining in the lungs from the previous expiration.

17. Explain why the % oxygen in expired air decreases by about the same amount that % carbon dioxide increases.

18. Explain why the % nitrogen remains virtually unaltered in both inspired and expired air.

Effects of Lifestyle choices / smoking on the respiratory system

By smoking, people inhale substances that can damage your lungs.

Lungs lose the ability to filter harmful chemicals. No amount of coughing can clear out the toxins sufficiently, so the toxins get trapped in the lungs.

Smokers have a higher risk of respiratory infections, colds, and flu.

Smokers have an increased risk of developing chronic obstructive pulmonary disease (COPD), the most common of which is emphysema, where the alveoli damaged and destroyed by the toxins.

In another COPD, chronic bronchitis, the lining of the bronchi and bronchioles become inflamed.

Long-term smokers are also at increased risk of lung cancer.

Withdrawal from tobacco products can cause temporary congestion and respiratory pain as the lungs begin to clear out.

Children whose parents smoke are more prone to coughing, wheezing, and asthma attacks than children whose parents don’t. They also tend to have more ear infections. Children of smokers have higher rates of pneumonia and bronchitis.

19. Describe the main effects of smoking on the lungs.

Answers

1. VC = 4 L or dm3

2. TV increases;

Also more frequent/regular

3. Exercise reduces both IRV and ERV

4. MV = TV x BrR

= 500 x 15 = 7500 mls / min or 7.5 L/min

5. MV = TV x BrR

= 4.5 x 20 = 90 L/min

6. Residual volume

7. Cannot fully compress chest – bit like trying to completely flatten a drinks can!

8. Tidal volume, expiratory reserve and inspiratory reserve volumes

9. Increased levels of C02/acidity in blood; detected by chemoreceptors; movements detected by proprioceptors; changes in pressure detected by baroreceptors; impulses to medulla; phrenic nerve; diaphragm/intercostals muscles; inspiration continues until phrenic nerve stops sending impulses; passive expiration

10. 120 L/min

11. Exercise – more CO2/acidity; chemoreceptors; medulla; diaphragm/intercostals muscles – increased breathing rate

12. Reaches ‘steady state’ – where breathing rate matches need for oxygen

13. Amount of gas in a mixture of gases; equivalent to concentration

14. Movement of substances from a high concentration to a lower concentration

15. Large contact surface area; thin membranes; short distance between membranes; slow movement of red blood cells gives time for diffusion

16. Diffusion continues until equilibrium/balance between alveolar and capillary partial pressures reached – some oxygen must remain in alveoli

17. Same amount of oxygen taken in as carbon dioxide breathed out

18. Nitrogen not involved in gas exchange/inert gas

19. Causes diseases such as emphysema, bronchitis and lung cancer

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