Respiratory System

The respiratory system involves the network of airways that carry air to and from the lungs. Air moving in from the atmosphere, passes through the nose or mouth, down the trachea, into either of the two bronchioles, before reaching the alveoli in the right or left lungs. The lungs are found in the chest cavity, encased by the ribs and the diaphragm.

Mechanics of Breathing

Inspiration at Rest

To enable air to move into the lungs, the volume inside the lungs must increase. An increase in volume, decreases the pressure and as air moves from an area of high pressure, to one of a lower pressure, air rushes into the lungs. To achieve the increase the volume; the muscles in the diaphragm contract causing it to flatten. The ribs must also move upwards and outwards and this occurs because of the contraction of the external intercostal muscles.

Expiration at Rest

In order to move air out of the lungs, the volume inside the lungs must decrease to cause an increase in pressure. The volume decreases because the external intercostal muscles relax. This results in the ribs falling downwards and inwards under the effects of gravity. There is also a ‘recoil’ of the elastic connective tissue found within the lungs (much the same as when we take our fingers off the opening of a balloon after we have inflated it).The decrease in volume also occurs because the muscles in the diaphragm relax, meaning this tissue returns to a dome shape.

Inspiration during Exercise

When exercising, two changes occur to ventilation. Firstly ventilation rate (or breathing frequency) increases so we inspire (and expire) more rapidly. Secondly, the depth of each inspiration increases. The diaphragm and external intercostals again work, but during exercise they contract with more force and over a shorter period of time. Additional muscles also work to pull the ribs upwards and outwards, moving them further and at a faster rate. The additional muscles that work during inspiration whilst exercising include the pectoralis minor (found in the chest), the sternocleidomastoid and scalene muscle groups (found in the neck).

Expiration during Exercise

As muscular contractions are involved in expiration whilst exercising, it now becomes an ‘active process’. A number of muscle groups in the abdomen; internal intercostals, rectus abdominus, internal obliques, external obliques and transverse abdominus, pull the ribs downwards and inwards faster than the effects of gravity to speed up the process of exhaling air. This increases the pressure more than when resting and happens at a faster rate.

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.

During exercise ventilation rate and the depth of breathing both increase. This can be seen on a spirometer trace when exercise starts. Tidal volume reflects the depth of breathing and it can be seen that there is an increase in tidal volume. An increase in tidal volume is the result of the ribs being moved further up and out during inspiration, this leads to a reduction in IRV. During expiration the ribs are moved further downwards and inwards due to the extra muscles involved (e.g. the rectus abdominus), and this results in a reduction in the ERV. Breathing rate is reflected by the distance between the peak of each tidal volume. It can be seen on the spirometer trace that the tidal volume peaks get closer together as breathing rate increases.

Gas exchange takes place in the alveoli; oxygen diffusing into the blood and carbon dioxide passing the other way. The alveoli are structured to assist in this process as they:

• Have a thin membrane composed of epithelial cells
• Have a moist covering on the inside
• Are folded to increase the surface area
• Are surrounded by many capillaries so many red blood cells pass close to the alveoli membrane
• blood flows through the capillaries at a slow velocity; this also assists diffusion of oxygen and carbon dioxide into and out of the blood.

Gases diffuse from areas of high concentration to areas of low concentration because of the concentration gradient. To ensure that oxygen diffuses from the air in the alveoli into the blood, a high concentration gradient must be maintained between these tissues. In air in the alveoli there is a high partial pressure of oxygen, compared to the low partial pressure found in the blood. To ensure a concentration gradient is maintained, air must be constantly moved into and out of the lungs so that air which is lacking in oxygen is replaced by oxygen-rich atmospheric air.