Whether we are wide-awake while pepping for that big date or asleep during a snooze-worthy afternoon, we never think about breathing. It's so important to life that it happens automatically. Each day we breathe about 25,000 times, and by the time we are 70 years old, we would have taken at least 600 million breaths. If we don’t breathe, we wouldn’t be able to live. It's one of the most important functions our body performs. All of this breathing couldn't happen without help from the respiratory system.

The respiratory system is made up of the organs in our body that help us to breathe. Breathing is the process by which oxygen in the air is brought into the lungs and into close contact with the blood, which absorbs it and carries it to all parts of the body. At the same time the blood gives up waste matter (carbon dioxide), which is carried out of the lungs when air is breathed out. The human respiratory system consists of the nose, pharynx, larynx, trachea, smaller conducting passageways (bronchi and bronchioles), lungs and diaphragm. Now let us see the role played by each of the constituent of respiratory system, in details.

NOSE The nose or the nasal cavity is the preferred entrance for outside air into the respiratory system. Air also enter through the mouth or oral cavity, especially in people who have a mouth-breathing habit or whose nasal passages may be temporarily obstructed, as by a cold or during heavy exercise. Tiny hairs called cilia line the nasal airways. These hairs are part of the air-cleaning system. It filters out dust and other particles that enter the nose with the breathed air. As air is inhaled, the cilia move back and forth, pushing any foreign matter (like dust) toward the nostrils, from where it is blown out. Besides this, the moisture in the nose helps to heat and humidify the entering air. This increases the amount of water vapour in the air. This prevents the air entering the Nose, from drying out the lungs and other parts of our respiratory system. Hence the nose plays an important part and has three major things to do. They are:

Filters the air Warms the air Provides moisture (water vapor or humidity) to the air.

PHARYNX The Pharynx is commonly called the throat. It serves as a passage for both the respiratory and digestive systems. In respiratory system, it receives air from the nasal cavity and mouth and passes it downward to the windpipe. There are 3 regions of the pharynx: nasopharynx, oropharynx, and laryngopharynx. The nasopharynx contains the pharyngeal tonsils. The oropharynx is also a passageway for food/drink and contains the palatine tonsils. The laryngopharynx is a common pathway and extends to the larynx - where the respiratory and digestive paths diverge.

LARYNX The Larynx, commonly called the voice box or glottis, is the passageway for air between the pharynx above and the trachea below. The larynx plays an essential role in human speech. Inside the larynx, there are two highly elastic folds of tissue (Ligaments) called the vocal cords. During sound production, the vocal cords close together and vibrate as air passes between them. There is a flap of tissue that guards the entrance to the windpipe, which is called epiglottis. It prevents the food and other particles from entering the larynx. It also helps to close off the larynx when food is swallowed and has to go to oesophagus.

TRACHEA The Trachea, commonly called the windpipe, is the main airway to the Lungs. From the larynx, the warmed, filtered, and moistened air passes downward into the thoracic cavity through the Trachea. The walls of the trachea are made up of c-shaped rings of tough flexible cartilage. These rings of cartilage protect the trachea, from collapsing or over expanding. They also make the trachea flexible. The cells that line the trachea produce mucus. The internal lining of trachea also has cilia. The mucous helps to capture things that are still in the air (dust and microorganisms), and it is swept out of the air passage by cilia. Trachea divides into the right and left bronchi at the level of the fifth thoracic vertebra, channeling air to the right or left lung.

BRONCHI, BRONCHIOLES AND ALVEOLI In its bottom end, the trachea divides into left and right air tubes called bronchi, which connect to the lungs. At the level of the fifth thoracic vertebra, the trachea divides into the right and left primary bronchi, one for each lung. The bronchi are lined with cilia that have a wave-like motion. This motion carries mucous (sticky phlegm or liquid) upward and out into the throat, where it is either coughed up or swallowed. The mucous catches and holds much of the dust, germs, and other unwanted matter that has invaded the lungs. You get rid of this matter when you cough, sneeze, clear your throat or swallow. You can compare trachea with a tree trunk. It is like an upside down tree with extensive branches that become smaller and smaller. The bronchi, after entering the lungs, branches into smaller bronchi and even smaller tubes called bronchioles. Bronchioles, which are as thin as a strand of Hair, end in tiny air sacs called Alveoli. This network of Alveoli, bronchioles, and bronchi is known as the bronchial tree. The Alveoli are the very small air sacs that are the destination of air breathed in. Each lung houses about 300 to 400 million Alveoli. Group of Alveoli look like bunches of grapes. All exchange of gases in the Lungs occurs in the Alveoli. The Capillaries are imbedded in the walls of the Alveoli. Blood passes through the Capillaries, and gives off carbon dioxide through the capillary wall into the Alveoli and takes up oxygen from the air in the Alveoli.

LUNGS The Lungs are very essential organs for respiration. They are two in number, placed one on either side within the thorax. The surface area of the lungs is roughly the same size as a tennis court. Each lung is conical in shape. The substance of the lung is a light, porous, spongy texture. Only about 10% of the lung is occupied by solid tissue, whereas the remainder is filled with air and blood. On the medial surface there is hilus, where the primary bronchi enter and the Blood vessels and nerves enter/exit. The left lung is smaller than the right and has an indentation where the heart normally sits. The left lung is divided into 2 lobes whereas the right is divided into 3. The pulmonary arteries bring deoxygenated blood (for gas exchange) to the lungs while the bronchial arteries bring oxygenated blood (to supply oxygen for the structural tissue). The lungs are drained by the pulmonary Veins. At birth the lungs are pinkish white in color but in adults the colour is a dark gray. Each lung is enclosed by a double-layered serous membrane, called the pleura. The visceral pleura is firmly attached to the surface of the lung. There is also parietal pleura that lines the wall of the thorax. The small space between the visceral and parietal pleurae is the pleural cavity. It contains a thin film of serous fluid that is produced by the pleura. The fluid acts as a lubricant to reduce friction as the two layers slide against each other, and it helps to hold the two layers together as the lungs inflate and deflate. The ribs in the front of the chest and the spine in back protect the lungs. They are separated from each other by a space called the mediastinum. The Functional Unit of the lungs is the alveoli. It is here that the Circulatory and Respiratory Systems come together, for the purpose of gas exchange. Alveoli have already been explained above.

DIAPHRAGM The Diaphragm is the sheet of muscles that lies across the bottom of the chest cavity. It separates the thoracic cavity from the abdominal cavity. It is located along the bottom of the rib cage and before inhalation the diaphragm is curved upward into the chest. It controls the breathing process. During Inhalation, the Diaphragm contracts and moves down, causing the volume of the thoracic cavity to increase. The air outside is still at atmospheric pressure, to equalize the pressure inside and out, the air rushes through the trachea into the lungs. When the diaphragm relaxes, it returns to its curved position. This action causes the volume of air in the thoracic cavity to decrease. This causes the air (carbon dioxide) in the lungs to go out.

All the cells in our body require oxygen. Without it, they couldn't move, build, reproduce, and turn food into energy. In fact, without oxygen, they and we would die! But how do we get oxygen? From breathing in air which your blood circulates to all parts of the body.

So the primary function of the respiratory system is to supply the blood with oxygen in order for the blood to deliver oxygen to all parts of the body. The respiratory system does this through breathing. When we breathe, we inhale oxygen and exhale carbon dioxide. The process of taking air into the lungs is called inhalation or inspiration, and the process of breathing it out is called exhalation or expiration. Oxygen enters the respiratory system through the Mouth and the Nose. The oxygen then passes through the larynx and then to trachea. In the chest cavity, the trachea splits into two smaller tubes called the bronchi. The air passes through the tubes and finally reaches tiny sacs called alveoli. The average adult's lungs contain about 600 million of these spongy, air-filled sacs. These sacs are surrounded by capillaries. The inhaled oxygen passes into the alveoli and then diffuses through the capillaries into the arterial blood. Meanwhile, the waste-rich blood from the Veins releases its carbon dioxide into the alveoli. The carbon dioxide follows the same path out of the lungs when we exhale. The Diaphragm's job is to help pump the carbon dioxide out of the lungs and pull the oxygen into the lungs. The Diaphragm is a sheet of Muscles that lies across the bottom of the Chest cavity. As the Diaphragm contracts and relaxes, breathing takes place. When the Diaphragm contracts, oxygen is pulled into the Lungs. When the Diaphragm relaxes, carbon dioxide is pumped out of the Lungs. This exchange of gases is the respiratory system's means of getting oxygen to the Blood. Although we can't see it, the air we breathe is made up of several gases. Oxygen is the most important for keeping us alive because body cells need it for energy and growth. Without oxygen, the body's cells would die. Hence to summarize the functions of the respiratory system, they have been listed below:

Oxygen intake Expulsion of carbon dioxide Sound/voice production Regulation of plasma pH Removal/destruction of airborne pathogens and toxins.

Breathing is an involuntary action under control of the medulla oblongata in the lower part of the Brain. Although you can consciously control breathing to a limited extent-such as holding your breath but it cannot be consciously suppressed.

Asphyxia Asthma