Mechanism of breathing

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Breathing (also known as external respiration) is the first stage for supplying oxygen to the body’s cells. There are 2 phases in breathing:

(1) Inspiration = breathing in. When breathing in, the chest cavity is made larger by lifting the rib cage upwards and forwards by contracting the intercostal muscles and the diaphragm. When the diaphragm contracts, it is pushed downwards until it is flat, creating a vacuum inside the chest cavity. The lungs expand to fill the space created. Enlarging the chest cavity leads to pressure inside the lungs becoming lower than the pressure in the air outside. This results in air being sucked in, making the pressure of air inside the lungs equal to that of the air outside the body.

(2) Expiration = breathing out. The diaphragm and intercostals muscles relax. As a result, the ribs are pulled down and back, reducing the volume of the chest cavity. This squeezes the lungs to their original size resulting in a higher pressure inside the lungs than the air pressure outside. This results in forcing the air out of the body through the nose and mouth.
These 2 phases are involuntarily (it happens automatically – we do not have to think about doing it). Breathing can, however, be consciously controlled, for example, deeper and/or faster breaths can be taken or breath can be held.
inspiation and expiration

When you breathe in, the diaphragm contracts and flattens whilst the ribs lift upwards and outwards. This increases the volume of the lungs as the thoracic cavity expands and pulls them out. Because there is a bigger volume, pressure therefore decreases. As gas goes from areas of high pressure to areas of low pressure, air rushes into the lungs from the atmosphere.

When you breathe out, the diaphragm relaxes and returns to its dome-shaped position as the ribs lower back down. This decreases the volume of the lungs as the thoracic cavity reduces in size, returning to its original state. Because there is a smaller volume, pressure increases, and the air inside the lungs rushes out of the body into the atmosphere (where there is lower pressure).

An analogy that may help you picture this – imagine 10 blind-folded people walking around in random directions in a small room. These people represent air molecules. Because they are in a small room, each person will bump into others and the walls very frequently. This would represent high pressure – and if you were to open a door to the room, they would spill out. If you now transfer these same 10 people into a large sports hall and have them once again walking around at random, then they will collide with each other and the walls much less frequently – therefore low pressure. If you were to open the door, because there is so much room, other people would come in and join them.

Gaseous exchange

The lungs have an extremely large surface area. This ensures that as much as possible of the surface of the lungs is in contact with the capillaries that surround them.

This maximises delivery of oxygen from the lungs to the blood, and the delivery of carbon dioxide from the blood to the lungs. Gases exchange in the alveoli. During inspiration, air is taken into the lungs and oxygen passes from lungs into blood through the walls of the alveoli and surrounding capillaries. This allows haemoglobin in red blood cells to absorb oxygen, forming oxyhaemoglobin. At the same time, carbon dioxide passes from blood into the alveoli, through the bronchioles and out of the body through the lungs and mouth. The walls of alveoli and capillaries are only one cell thick, allowing molecules of carbon dioxide and oxygen to pass through them.

gas exchange

Vital capacity and lung volumes

A person’s vital capacity represents the maximum amount of air that can be breathed in or out in one breath, which is usually about 4½ to 5 litres for adults. The larger the person, the larger the lungs, and therefore the greater vital capacity. During exercise, vital capacity increases. A long-term adaptation to exercise is that vital capacity increases. Tidal volume represents the volume of air is that is breathed in and out per breath. Residual volume represents the volume of air that remains in the lungs after maximally breathing out. Total lung capacity represents vital capacity plus residual volume.

Oxygen debt

When anaerobic energy systems are used, an oxygen deficit is produced – muscles need more oxygen than they can get at that time. If the activity is continued, lactic acid is produced. After the activity has been finished, the performer needs to have a rest and take in the extra oxygen that is needed. This extra oxygen is known as the oxygen debt, and it helps to remove the build-up of lactic acid, replenish the stores of oxygen in the body, and build up the ATP and creatine phosphate stores in the muscles.

Exercise and the lungs

  • The lungs and heart work together to get oxygen around the body and to remove carbon dioxide. When exercising, the heart and lungs have to work harder
  • During exercise, cell respiration in the muscles increases, leading to an increase of carbon dioxide in the blood
  • The brain detects this increased carbon dioxide. It tells the lungs to breathe faster, i.e. breathing rate increases, and the depth of breathing increases up to vital capacity
  • Gas exchange in the lungs speeds up. More carbon dioxide is expelled out of the blood and more oxygen passes into it
  • The brain also tells the heart to beat faster so that more blood gets pumped into the lungs for gas exchange and more blood gets pumped to the working muscles
  • During intense anaerobic work, breathing rate does not necessarily increase (for example, during the 100 metres sprint). It only increases after the event to repay the oxygen debt. During an aerobic event, breathing rate increases to expel the excess carbon dioxide and take in the much needed oxygen
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