Structure of the respiratory system
Passage of air into the lungs
- Air enters the body and is warmed as it travels through the mouth and nose.
- It then enters the trachea.
- The trachea divides into two bronchi. One bronchus enters each lung.
- Each bronchus branches out into smaller tubes called bronchioles. Air travels through these bronchioles.
- At the end of the bronchioles, the air enters one of the many millions of alveoli where gaseous exchange takes place.
Breathing
Breathing is the term given to the process of taking air into and out of the lungs.
Two important structures for breathing are the diaphragm and intercostal muscles.
The diaphragm is a sheet of muscle that separates the chest (or thoracic) cavity from the rest of the body.
The intercostal muscles are found between the ribs and they control rib movement.
Inspiration (breathing in)
The diaphragm contracts and moves downwards. The intercostal muscles contract and move the ribs upwards and outwards. This increases the size of the chest and decreases the air pressure inside it which sucks air into the lungs.
Expiration (breathing out)
The diaphragm relaxes and moves back to its domed shape. The intercostal muscles relax so the ribs move inwards and downwards under their own weight. This decreases the size of the chest and increases the air pressure in the chest so air is forced out of the lungs.
Composition of the air we breathe
| Breathed in | Breathed out |
|---|---|
| 21% oxygen | 16% oxygen |
| 0.03% carbon dioxide | 4% carbon dioxide |
The human body is designed to take in oxygen and to remove carbon dioxide. The respiratory system, in combination with the cardiovascular system, is responsible for providing this function.
Gas exchange
Gas exchange occurs at the alveoli in the lungs and takes place by diffusion. The alveoli are surrounded by capillaries so oxygen and carbon dioxide diffuse between the air in the alveoli and the blood in the capillaries.
Diffusion is the movement of gas from an area of high concentration to an area of low concentration.
There is a high concentration of oxygen in the alveoli and a low concentration of oxygen in the blood, so oxygen diffuses from the alveoli into the blood.
There is a high concentration of carbon dioxide in the blood and a low concentration in the alveoli, so carbon dioxide diffuses from the blood into the alveoli.
Both oxygen and carbon dioxide are capable of combining with an iron-rich protein in the blood called haemoglobin. Haemoglobin carries oxygen to be exchanged at the working muscle and carbon dioxide to be exchanged at the lung.
As the blood moves through the capillaries in the alveoli, oxygen diffuses into it and carbon dioxide diffuses out of it.
Capillaries surround the alveoli in the lungs. Both the capillaries and alveoli walls are very thin – just one cell thick. They are made of semi-permeable membranes which allow oxygen and carbon dioxide to pass through them.
Lung volumes
Vital capacity is the maximum amount of air that can be breathed out after breathing in as much air as possible. Taking part in regular aerobic exercise has been shown to increase a person’s vital capacity.
Tidal volume is the amount of air breathed in with each normal breath. The average tidal volume is 0.5 litres (500 ml).
Total lung capacity is the total amount of air that the lungs can hold after the biggest possible breath in. The average total lung capacity is about 6 litres (6000 ml).
These volumes can be seen in this spirometry trace.
During exercise, tidal volume increases as the depth of breathing increases and the rate of breathing increases too. This has the effect of taking more oxygen into the body and removing more carbon dioxide.
Breathing rate (frequency, BR) is the number of breaths in a minute. The average breathing rate is 12 breaths per minute.
Minute ventilation (VE) is the total volume of air entering the lungs in a minute. The average minute ventilation is 6 litres per minute.
Minute ventilation = breathing rate × tidal volume
VE = BR × TV
6 litres per minute = 12 × 0.5
| Rest | Exercise | |
|---|---|---|
| Breathing rate | 12 breaths per minute | 30 breaths per minute |
| Tidal volume | 0.5 litres | 3 litres |
| Minute ventilation | 6 litres per minute | 90 litres per minute |
Cardio-respiratory system
The cardio-respiratory system works together to get oxygen to the working muscles and remove carbon dioxide from the body.
During exercise the muscles need more oxygen in order to contract and they produce more carbon dioxide as a waste product. To meet this increased demand by the muscles, the following happens:
Breathing depth (tidal volume) and rate increase – this gets more oxygen into the lungs and removes more carbon dioxide out of the lungs.
The graph shows that as a person goes from rest to exercise, their tidal volume increases.
Heart rate increases – this increases the rate that oxygen is transported from the blood to the working muscles and carbon dioxide is transported from the working muscles to the lungs.
This graph indicates the following:
- the person’s resting heart rate is around 60 bpm
- at 8 minutes, just before taking part in exercise their heart rate increases – this is called the anticipatory increase in heart rate which occurs when a person starts to think about taking part in exercise
- at 10 minutes the person starts to take part in exercise and there is a steep increase in heart rate which reaches 145 bpm at 13 minutes
- the heart rate remains high during exercise
- when the person stops taking part in exercise the heart rate decreases
Bibliography: BBC – Education