The respiratory system is an organ system associated with respiration (breathing) or gas exchange.
In terrestrial vertebrates, the respiratory system is typically comprised of pharynx, larynx, bronchi, lungs and diaphragm. The lungs (two large organs of respiration located in the chest cavity) have about a million of alveoli (small air sacs) where gas exchange occurs.
The major function of the respiratory system is to supply the body with oxygen and dispose of carbon dioxide.
When you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. This increases the space in your chest cavity, into which your lungs expand. The intercostal muscles between your ribs also help enlarge the chest cavity. They contract to pull your rib cage both upward and outward when you inhale.
As your lungs expand, air is sucked in through your nose or mouth. The air travels down your windpipe and into your lungs. After passing through your bronchial tubes, the air finally reaches and enters the alveoli (air sacs).
Through the very thin walls of the alveoli, oxygen from the air passes to the surrounding capillaries (blood vessels). A red blood cell protein called hemoglobin helps move oxygen from the air sacs to the blood.
At the same time, carbon dioxide moves from the capillaries into the air sacs. The gas has traveled in the bloodstream from the right side of the heart through the pulmonary artery.
Oxygen-rich blood from the lungs is carried through a network of capillaries to the pulmonary vein. This vein delivers the oxygen-rich blood to the left side of the heart. The left side of the heart pumps the blood to the rest of the body. There, the oxygen in the blood moves from blood vessels into surrounding tissues.
When you breathe out, or exhale, your diaphragm relaxes and moves upward into the chest cavity. The intercostal muscles between the ribs also relax to reduce the space in the chest cavity.
As the space in the chest cavity gets smaller, air rich in carbon dioxide is forced out of your lungs and windpipe, and then out of your nose or mouth.
Breathing out requires no effort from your body unless you have a lung disease or are doing physical activity. When you’re physically active, your abdominal muscles contract and push your diaphragm against your lungs even more than usual. This rapidly pushes air out of your lungs.
The Respiratory tract is of two parts:
- Upper respiratory tract
- Lower respiratory tract
Upper respiratory tract
The upper respiratory tract is comprised of the nose and nasal cavities, paranasal sinuses, the pharynx, and the portion of the larynx above the vocal folds (cords).
The Nose
The nose is the only externally visible part of the respiratory system. It is the primary organ smell.
The external midline projection from the face. Its purpose is to warm, clean, and humidify the air that a person breathes.
The nose provides an airway for respiration, moistens and warms entering air, filters and cleans inspired air.
Nasal Cavity
It is the space inside the nose. The nasal cavity lies above the bone that forms the roof of the mouth and curves down at the back to join the throat.
It is divided into two sections called nasal passages. Air moves through these passages during breathing. The nasal passages filter and warm the air, and make it moist before it goes into the lungs. They also contain special cells that are involved in the sense of smell.
Paranasal Sinuses
The paranasal sinuses are air-filled spaces located within the bones of the skull and facial bones.
Paranasal sinuses are named after the bones that contain them: frontal (the lower forehead), maxillary (cheekbones), ethmoid (beside the upper nose), and sphenoid (behind the nose).
The paranasal sinuses open into the nasal cavity (space inside the nose) and are lined with cells that make mucus to keep the nose from drying out during breathing.
The pharynx
Pharynx is the hollow tube inside the neck that starts behind the nose and ends at the top of the trachea (windpipe) and esophagus (the tube that goes to the stomach).
It is divided into three sections: the nasopharynx, oropharynx, and laryngopharynx.
The pharynx is about 5 inches long, depending on body size and it is also called throat.
Lower Respiratory tract
The lower respiratory tract consists of larynx, trachea, bronchi, bronchioles, and alveoli.
Deep in the lungs, each bronchus divides into secondary and tertiary bronchi, which continue to branch to smaller airways called the bronchioles.
The bronchioles end in air sacs called the alveoli. Alveoli are bunched together into clusters to form alveolar sacs. Gas exchange occurs on the surface of each alveolus (plural of Alveoli) by a network of capillaries carrying blood that has come through veins from other parts of the body.
