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In this article, we will discuss the structure of the respiratory system in detail. Moreover, we will also discuss how alveoli are adapted to allow an exchange of gases efficiently. So, let us get started.
Human Respiratory System
The human respiratory system is designed to enable the air to pass in and out of the human body so that an efficient exchange of gases can occur. Respiration refers to a process in which the energy is released from the glucose so that the life processes can continue. There are two types of respiration:
- Aerobic respiration: It needs oxygen
- Anaerobic respiration: It does not need oxygen
Our respiratory system is adapted to carry the exchange of gases efficiently.
Gaseous Exchange in the Lungs
We require oxygen from the air into our blood and need to eliminate waste carbon dioxide from our blood into the air. Moving gases in and out of the human body like this are referred to as the gas exchange.
The following structures are present in the human thorax which are explained below in detail:
- Ribs: They refer to the bone structure that safeguards internal organs, for instance, lungs.
- Trachea: It refers to the windpipe that joins the mouth and nose to the lungs.
- Larynx: It is also known as a voice box. A sound is produced when air passes across here.
- Bronchi: They are the long tubes that branch off the trachea with a single bronchus for each lung.
- Bronchioles: Bronchi divide to form smaller tubes known as bronchioles in the lungs joined to alveoli.
- Alveoli: They refer to the small air sacs where gaseous exchange occurs.
- Diaphragm: It refers to a sheet of muscle that segregates the chest or thoracic cavity from the remaining body.
- Intercostal muscles: They are present between the ribs and they regulate the movement of ribs.
Adaptations of Alveoli
The gas exchange in the lungs occurs easily and effectively due to the certain adaptations of alveoli. Some of the primary features of alveoli that enable this exchange of gases are listed below:
- Large surface area: Lungs have several alveoli. The shape of these alveoli further enhances the surface area.
- Thin walls: One cell thick alveolar walls give a short distance for the gases to diffuse through.
- Permeable walls: The permeable walls enable the gases to pass-through
- Moist walls: The dissolution of gases in the moisture helps them to pass across the gas exchange surface.
- Extensive blood supply: It ensures that the blood rich in oxygen is carried away from the lungs and the blood rich in carbon dioxide is carried to the lungs.
- Large diffusion gradient: Breathing guarantees that the concentration of oxygen in the alveoli is greater than the concentration in capillaries to ensure the movement of oxygen from the alveoli to the blood. The diffusion of carbon dioxide occurs in the reverse direction.
The movement of gases occurs through diffusion from a place of higher concentration to an area of lower concentration. The diffusion of oxygen occurs from the alveoli into the blood, whereas the diffusion of carbon dioxide occurs from the blood in the air in the alveoli. The loss of water vapours also occurs from the surface of the alveoli into the lungs. We can observe this condensing while breathing out in cold weather.
Inspiration and Expiration
The term breathing is given to describe the process of letting air into and out of the lungs. The breathing includes inspiration and expiration which are explained below in detail.
Inspiration (breathing in)
The diaphragm moves downwards as a result of contraction. The contraction of intercostal muscles results in the movement of ribs upwards and outwards. It enhances the chest size and reduces the air pressure inside it which in turn sucks air into the lungs. When exercise starts, the inspiration of air can be aided by the pectoral muscles and the sternocleidomastoid that assists in further lifting the ribs upwards and outwards.
Expiration (breathing out)
As the diaphragm relaxes, it moves back to its domed shape. The relaxation of intercostal muscles helps the ribs to move inwards and downwards under their own weight. This reduces the chest size and enhances the air pressure in the chest to force the air out of the lungs.
During the exercise, as the abdominal muscles further pull the ribs downwards and inwards, the passive procedure of relaxation becomes active.
Exhaled air
When we inhale air, the diffusion of oxygen occurs from the alveoli into the blood so that it can be used for respiration by the cells of the body. During respiration, a waste product, i.e. carbon dioxide is made by the cells of the body. Its diffusion occurs from the blood into the alveoli and is exhaled.
The expired (exhaled) air contains the following components as compared to the atmospheric air:
- More carbon dioxide
- Less oxygen
- Slightly more nitrogen
The exhaled air contains the following percentages of gases as compared to the atmospheric air:
- Nitrogen: The atmospheric air has 78% of nitrogen, whereas the expired or exhaled air contains 79% of the nitrogen in it. Hence, the change is + 1%
- Oxygen: The atmospheric air has 21% of oxygen, whereas the expired or exhaled air contains 16% of oxygen. Hence, the variation is -5%.
- Carbon dioxide: the atmospheric air has 0.04% carbon dioxide, whereas the exhaled air contains 4% of this gas. Hence, the variation is +4%/
- Other gases (mostly argon): The atmospheric air contains 1% of other gases and the exhaled air contains 1% of these gases. This shows that the variation is 0%.
Carbon dioxide gas turns lime water milky. Hence, we can use it to show the differences between inspired and expired air. When limewater comes in contact with the expired air, it immediately turns milky.
Impact of Exercise on the Rate of Breathing
Muscle cells often need more energy during exercise. Energy is released as a result of respiration. While exercising, cells require more oxygen and generate more carbon dioxide due to the increased rate of respiration.
As the blood reaches the lungs, a greater volume of air is required to replace the oxygen utilized and eliminate carbon dioxide generated by this surplus respiration. To supply greater oxygen to the exercising cells, the body enhances the rate and depth of breathing. The time consumed for the breathing rate to return to a normal rate is referred to as a recovery time. We can measure fitness from this recovery time.
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