The anatomy and function of the respiratory system
This article focuses on the structure (anatomy) and function of the respiratory system. How exactly this system responds and adapts to exercise will be the subject of the next article.
No one needs to be told how important the respiratory system (also known as the pulmonary or ventilatory system) is. All living creatures require oxygen and give off carbon dioxide. Oxygen is required for every cell in the body to function. The respiratory system serves the body much as a lifeline to an oxygen tank serves a deep-sea diver.
Think how panicked you would feel if suddenly your lifeline became blocked—if you could not breathe for a few seconds! Of all the substances that cells and therefore the body as a whole must have to survive, oxygen is by far the most crucial. A person can live a few-weeks without food, a few days without water, but only a few minutes without oxygen. Constant removal of carbon dioxide from the body is just as important for survival as a constant supply of oxygen.
The anatomy (structure) of the respiratory system
The respiratory system consists of your nose, lungs (main respiratory organs) and breathing tubes.
To get the oxygen your body needs, you inhale air through your nose (sometimes via your mouth). This air passes through a series of airways to reach the lungs.
This series of airways is referred to as the respiratory tract, and can be subdivided into two main parts.
- Upper respiratory tract. It includes the nose, nasal cavity, mouth, pharynx and larynx.
- Lower respiratory tract. It consists of the trachea, bronchi and lungs.
Sometimes we simply say that the respiratory organs consist of:
- Conducting organs or respiratory passages. These include nostrils (external nares), nasal passages, internal nares, pharynx, larynx, trachea, bronchi and bronchioles.
- Main respiratory organs or lungs.
Inhaled air passes through the chambers of this tract in the following order:
- nose
- pharynx
- larynx
- trachea
- primary bronchi
- secondary bronchi
- bronchioles
- alveolar ducts
- alveoli
I. Nasal cavity
The nasal cavity is the air passage above and behind your nose. Air enters the body through the nostrils (external nares). Small hairs within the nostrils filler out dust and foreign particles before the air passes into the two nasal passages of the nasal cavity. The mucous membranes lining the internal nose moisten and filter the air. The plentiful blood supply in these membranes also warms the air.
II. Throat or pharynx
Inhaled air passes through a pair of internal nares in the posterior portion of the internal nose. From these openings air enters the pharynx (throat). This muscular tube, about five inches in length, is lined with mucous membranes. It connects the nasal cavity and mouth to the larynx and oesophagus. It’s a passageway for food as well as air, meaning special adaptations are required to prevent choking when food or liquid is swallowed. In other words, here the food and air tubes cross each other.
III. Larynx or voice box
The larynx contains the vocal cords and connects the pharynx to the trachea. It consists of nine pieces of cartilage, two sets of muscles and two pair of vocal cords. The vibrations of vocal cords produce voice. Skeletal muscles cause them to vibrate at different frequencies, determining the pitch of voice production.
IV. Trachea or windpipe
The lower respiratory tract begins with the trachea – a tube about five inches long and one inch in diameter. The trachea or windpipe passes from the larynx to the level of the upper border of the fifth thoracic vertebra, where it divides into the two main bronchi that enter the right and left lungs. Its wall of smooth muscle is reinforced with rings of C- shaped cartilage (preventing the airway from collapsing).
V. Primary (main) bronchus
The base of the trachea subdivides into two main branches: left and right primary (main) bronchus. This arrangement resembles an inverted Y, with each bronchus entering one of the lungs. The right main bronchus is wider, shorter, and more vertical than the left main bronchus. The left main bronchus is smaller in caliber but longer than the right, being 5 cm long. Logically, the right main bronchus enters the right lung while the left main bronchus enters the root of the left lung.
Each lung is an elastic, bag-like structure that houses the remainder of the respiratory tract. The left lung is smaller than the right lung.
VI. Secondary bronchus
The left main bronchus then divides into two secondary bronchi, which supply two lobes, superior and inferior. The right main bronchus divides into three secondary bronchi, which supply three lobes of the lung, superior, inferior and middle. Therefore, there are three secondary bronchi in the right lung, one serving each lobe of the lung (three lobes). There are two secondary bronchi in the left lung, one serving each lobe of this lung (two lobes).
VII. Bronchioles
Each bronchus divides repeatedly into smaller and smaller tubes – bronchioles – numerous passageways in the lung. This pattern resembles an inverted tree. Bronchioles are very small tubes, lacking the cartilage rings found in the bronchi. The respiratory bronchioles subdivide into several alveolar ducts.The bronchioles lead ultimately to numerous alveoli.
VIII. & IX. Alveolar ducts and alveoli
Most of the interior of the lungs is filled with microscopic air sacs called alveoli (sing., alveolus). The alveoli exist as clusters. You can easily imagine them as a tiny, balloon-shaped air sacs. Each one in a group receives inhaled air from an alveolar duct. The pair of lungs have an estimated 300 million alveoli. The alveoli are close to the capillaries of the pulmonary circulation. The thin walls of the alveoli consist of one layer of cells, simple squamous epithelium.
Alveoli are an important part of the respiratory system whose function it is to exchange oxygen and carbon dioxide molecules to and from the bloodstream.
What are the functions of the respiratory system?
The primary function of the respiratory system (also known as the pulmonary system) is to bring oxygen into the lungs and remove carbon dioxide from the lungs to the outside air (external environment). Therefore, this system of organs collects oxygen from the external environment and transports it to the bloodstream to ensure proper cellular function.
Other functions of the respiratory system include:
- regulating blood pH;
- producing sounds;
- eliminating water and heat;
- housing the receptors responsible for the sense of smell;
- filtering, cleansing, warming, and humidifying air taken into the lungs;
Breathing and respiration
We need to be be very careful not to confuse respiration with breathing. Respiration is a much broader concept. The term respiration means the exchange of gasses (oxygen and carbon dioxide) between the atmosphere and the cells of the body. Therefore, respiration occurs at several different levels:
- Breathing (ventilation). During breathing we get oxygen from the environment through specialized breathing organs such as trachea, gills and lungs and release carbon dioxide. Therefore, ventilation is a term that simply means to bring in fresh air.
- External respiration is the exchange of gases between the lungs and blood. It involves diffusion of oxygen from air into blood and of carbon dioxide from blood into the air. Blood transports oxygen to the body tissues and collects carbon dioxide from them and brings it back to lungs.
- Internal respiration or tissue respiration. This is the exchange of gases, oxygen and carbon dioxide between blood and tissue cells. Oxygen from blood diffuses into tissue cells and carbon dioxide from tissue cells into the blood.
- Cellular respiration takes place inside the cells. It’s a biochemical process in which glucose is oxidised into carbon dioxide and water. Energy is released during the oxidation of glucose and transported to areas where energy is needed.
Mechanism of breathing. What happens when we breathe?
Lungs are not muscle. They cannot move of their own accord, nor are they under the control of the central nervous system. So, how do they move when we breathe in and out? The key to breathing is the diaphragm and the intercostal muscles between the ribs.
The brain sends a signal through the nerves to the breathing muscles (diaphragm and intercostals muscles), the diaphragm is pulled flat and the intercostals pull the ribcage upwards and outwards. Breath is drawn through the nose or mouth, down the throat and into the trachea. Air goes down the two bronchial tubes, down the bronchioles and into the alveoli. Oxygen passes through the paper-thin walls of the capillaries and into the bloodstream.
At the same time, waste products, e.g. carbon dioxide, pass out of the capillaries and into the alveoli. This process is known as gaseous exchange. Oxygen is picked up by the red blood cells and taken to the left side of the heart where it is pumped out through the aorta and around the body.
Interactions between the circulatory (cardiovascular) and respiratory system
The cardiovascular (cardiac) and respiratory systems work closely together and form the cardiorespiratory system.
The respiratory system works intimately with the cardiovascular system to accomplish optimal cellular function by transporting oxygen from the environment and transferring it to the bloodstream, and transferring carbon dioxide from the blood to the lungs and eventually transporting it to the environment.
This entire process is accomplished through the integrated functioning of the respiratory pump to move
air in and out of the body and of the respiratory passageways to channel the air. Therefore, the site of interaction between the two systems is the lung – this organ being responsible for the exchange of gases between them.
Easy to understand video presentation
This 10 minutes video gives a simple and easy to understand explanation of how this important process of the human body works. It also shows the complete structure (anatomy) of both upper and lower respiratory track.
Summing up: The respiratory system
The respiratory system is vital to every human being. Without it, we would cease to live outside of the womb. Like the cardiovascular system, the respiratory system undergoes specific adaptations in response to a systematic training programme, which help to maximise its efficiency.
