Respiratory system
The respiratory system is a biological system consisting of specific organs and structures used for gas exchange.
Gas exchange (in humans) is the delivery of oxygen from the lungs to the bloodstream, and the elimination of carbon dioxide from the bloodstream to the lungs.
The air enters the nasal cavity and makes it’s way down the trachea and into the lungs. The trachea branches into bronchi, which branches further into bronchioles.
The trachea is surrounded by cartilage which strengthens and supports the trachea.
The rib cage (not shown in the diagram) surrounds the lungs and its contents. It acts as a protective barrier and plays a vital role in inhalation and exhalation.
At the end of the bronchioles we find alveoli which is the gas exchange surface of the human respiratory system. The alveoli is the site at which oxygen is exchanged for carbon dioxide.
The alveoli have an extremely large surface area for gas exchange, and are surrounded by numerous capillaries to allow good ventilation with air.
Inhalation and exhalation
Inhalation allows the body to obtain oxygen from air, and exhalation allows the excretion of carbon dioxide.
To understand how inhalation and exhalation works, it is important to understand the concept of thoracic volume i.e. the volume of the thorax.
Thoracic volume and air pressure
Air will always move from higher air pressure to lower air pressure.
- When the thorax volume increases it reduces the air pressure inside the thorax (below atmospheric pressure). Air therefore travels into the body i.e. inhalation
- When the thorax volume decreases it increases the air pressure inside the thorax (above atmospheric pressure). Air therefore travels out of the body i.e. exhalation
Thorax is the part of the body of a mammal between the neck and the abdomen
The volume of the thorax is dependent on two factors:
- Position of the rib cage
- Position of the diaphragm
Inhalation
During inhalation, the thoracic volume increases to make the air pressure in the thorax to fall below atmospheric levels, and thus forcing air into the body.
This increase in thoracic volume is due to:
- Contraction of external intercostal muscles which cause the ribs to move up and out
- Contract of diaphragm which causes the dome-shape to flatten
Exhalation
During exhalation, the thoracic volume decreases to make the air pressure in the thorax above atmospheric levels, and thus forcing air out of the body.
The decrease in thoracic volume is due to:
- Relaxation of intercoastal muscles causing the rib cage to move down and in
- Relaxation of the diaphragm resulting in the original dome-shaped appearance
Air composition
The air composition of inspired and expired air is different. Inspired air has more oxygen, whilst expired air has more carbon dioxide (for obvious reasons of respiration). Nitrogen remains constant as our bodies cannot make use of nitrogen gas.
The limewater test can confirm the elevated presence of carbon dioxide in expired air. It is a clear liquid which turns milky when it reacts with carbon dioxide. Blowing into limewater with a straw will indeed cause it become milky.
Physical activity on breathing
With physical activity both the rate and depth of breathing increases.
Exercise means body cells respire more rapidly. More carbon dioxide is produced as a result and the increased carbon dioxide concentration in the blood leads to an increase in blood acidity i.e. decreased blood pH.
The brain detects the increased blood acidity and signals for the increase in breathing rate and depth to allow carbon dioxide to be excreted quicker. It also of course allows oxygen to get to respiring cells faster as well.