Main Function of the Respiratory System Lesson
Lesson Overview
The respiratory system is responsible for bringing oxygen into the body and expelling carbon dioxide. This exchange of gases is vital for cellular function, providing the oxygen needed for energy production and removing the waste gas carbon dioxide. Without a steady supply of oxygen, cells cannot make the energy they need, which is why, if breathing stops for just a few minutes, it can lead to serious harm.
The lungs and airways work together with the circulatory system to accomplish this exchange using a coordinated set of organs and processes. Key topics include the major structures of the respiratory system, the different types of respiration, how gas exchange works, the mechanics of breathing, and how breathing is regulated by the body.
Respiratory Anatomy
The main structures of the respiratory tract form a continuous path for airflow from outside the body to deep inside the lungs. Air enters through the nose, travels down the trachea, passes through branching bronchi in the lungs, and finally reaches tiny alveoli where gas exchange occurs. A large muscle called the diaphragm helps drive this airflow by changing the pressure in the chest cavity as we breathe.
- Nose: The nose is the primary entry for air into the respiratory system. It filters out dust, pollen, and other particles with tiny hairs and mucus, warms the incoming air via blood vessels in the nasal passages, and moistens the air so it does not dry out the lungs. This conditioning of the air by the nose is one reason why breathing through your nose is generally healthier than breathing through your mouth.
- Trachea: The trachea (windpipe) is a tube that connects the throat (larynx) to the bronchi of the lungs. It has sturdy rings of cartilage along its length to keep it from collapsing, and its inner lining is coated with mucus and tiny hair-like cilia that trap and sweep out particles or irritants from the air.
- Bronchi: Bronchi are the two large airways that branch off from the end of the trachea, with one bronchus entering each lung. Inside the lungs, each bronchus subdivides into smaller tubes called bronchioles, which spread throughout the lungs and direct air to all regions, ultimately leading to the alveoli.
- Alveoli: Alveoli are microscopic air sacs (shaped like clusters of tiny grapes) at the ends of the bronchioles in the lungs. They have extremely thin walls and are surrounded by capillaries, allowing oxygen to diffuse from the air into the blood (where it quickly binds to red blood cells) and carbon dioxide to diffuse from the blood into the air. The lungs contain hundreds of millions of alveoli, creating a huge combined surface area that makes gas exchange very efficient.
- Diaphragm: The diaphragm is a large dome-shaped muscle located at the bottom of the ribcage (under the lungs). It is the primary muscle used in breathing: when the diaphragm contracts and flattens downward, it expands the chest cavity and draws air into the lungs; when it relaxes and moves back up, it reduces the chest volume and helps push air out of the lungs.
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Types of Respiration
The word "respiration" can refer to several related processes that together ensure oxygen gets to our cells and carbon dioxide is removed from the body. In biology, we often break it down into four main types of respiration, each at a different level of this overall process:
- Breathing (Pulmonary Ventilation): This is the physical act of moving air into and out of the lungs. Inhalation brings fresh air (rich in oxygen) into the lungs, and exhalation expels air that is rich in carbon dioxide.
- External Respiration: The exchange of gases between the air in the lungs and the bloodstream. In the lungs' alveoli, oxygen diffuses from the inhaled air into the blood (entering the capillaries), and carbon dioxide diffuses from the blood into the alveoli to be exhaled.
- Internal Respiration: The exchange of gases between the blood and the body's tissues. Oxygen carried by the blood diffuses out of the capillaries into the cells of the tissues, and carbon dioxide produced by those cells diffuses into the blood to be carried away.
- Cellular Respiration: A chemical process inside cells that uses oxygen to convert glucose (a sugar) into ATP (the usable energy currency of the cell). This process takes place in the mitochondria of cells and produces carbon dioxide (and water) as waste products, which then diffuse into the blood to be removed by the lungs.
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Gas Exchange
Gas exchange is the fundamental purpose of breathing – it's how oxygen actually enters our bloodstream and how carbon dioxide leaves the body. This exchange of gases works by simple diffusion (gases naturally moving from higher concentration to lower concentration) and it occurs in two main places: in the lungs and in the body's tissues.
In the lungs, gas exchange takes place in the alveoli. Oxygen from the inhaled air passes through the thin walls of the alveoli into the blood in the surrounding capillaries, where it binds to hemoglobin inside red blood cells. At the same time, carbon dioxide passes from the blood into the alveoli to be exhaled. The alveolar walls are extremely thin, and the lungs contain hundreds of millions of alveoli. This gives the lungs a massive surface area, making gas exchange very efficient.
In the body's tissues, gas exchange occurs in the opposite direction. Red blood cells arriving in the tiny capillaries of the tissues release their oxygen into the surrounding cells. Meanwhile, carbon dioxide produced by the tissue cells diffuses into the blood to be carried away. The blood, now loaded with carbon dioxide waste, travels back to the lungs, where the CO2 will diffuse into the alveoli and be exhaled from the body.
Breathing Mechanics
Breathing (ventilation) is the mechanical aspect of respiration – it describes how we physically draw air into the lungs and push it out. This process works by changing the volume and pressure in the chest (thoracic) cavity, primarily through the action of the diaphragm (with help from other muscles like the intercostal muscles between the ribs). Think of your chest cavity like a bellows: expanding it draws air in, and compressing it pushes air out.
- Inhalation (Inspiration): To inhale, the diaphragm contracts and moves downward, flattening out, and at the same time the rib cage lifts up and expands. This expansion of the chest cavity increases its volume, which lowers the air pressure inside the lungs. As a result, air from outside the body is drawn into the lungs to fill the space.
- Exhalation (Expiration): To exhale, the diaphragm relaxes and moves back upward into its dome shape, and the rib cage moves down and inward. This reduces the volume of the chest cavity and raises the pressure inside the lungs, forcing air out. In normal, relaxed breathing, exhalation doesn't require muscle force – it happens mostly passively as the lungs and chest recoil to their resting size. However, during activities like vigorous exercise or blowing out candles, we actively use additional muscles (such as the abdominal muscles) to force more air out quickly.
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Control of Breathing
Even though you can change your breathing rate or hold your breath for a short time, your breathing is mostly controlled automatically. The body has a control system to make sure you get enough oxygen and get rid of carbon dioxide without you having to think about it (even when you're asleep). For example, at rest an average person breathes about 12 to 20 times per minute, all regulated unconsciously by the brain.
- Brainstem Respiratory Center: The basic rhythm of breathing is controlled by a region in the brainstem (the medulla oblongata). This respiratory center automatically sends signals to the breathing muscles (like the diaphragm and intercostal muscles) to contract and relax at a regular pace, maintaining a steady breathing rhythm.
- Chemical Regulation: The breathing control centers in the brain adjust your breathing based on the levels of certain chemicals in your blood. If carbon dioxide levels rise (which makes the blood more acidic), the brain detects this change and triggers faster, deeper breaths to expel more CO2. For instance, during exercise when your muscles produce extra CO2, you automatically start breathing faster to get rid of it. If oxygen levels drop too low, the brain will also increase the breathing rate to bring in more oxygen. These adjustments help keep the levels of oxygen and carbon dioxide in balance.
- Voluntary Control: You can temporarily control your breathing consciously using your brain's cerebral cortex. This allows actions like holding your breath, taking a deep breath on purpose, or altering your breathing pattern when speaking or singing. However, this voluntary control is limited – if you hold your breath for too long, the automatic control from the brainstem will override your effort and force you to breathe, ensuring your body gets the oxygen it needs.
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