The respiratory system is the biological system of any organism that engages in gas exchange. Even trees have respiratory systems, taking in carbon dioxide and emitting oxygen during the day, consuming carbon dioxide and producing oxygen constantly.

In humans and other mammals, the respiratory system consists of the airways, the lungs, and the respiratory muscles that mediate the movement of air into and out of the body. Within the alveolar system of the lungs, molecules of oxygen and carbon dioxide are passively exchanged between the gaseous environment and the blood. Thus, the respiratory system facilitates oxygenation of the blood with a concomitant removal of carbon dioxide and other gaseous metabolic wastes from the circulation.

Organization

The respiratory system can be conveniently subdivided into a conducting zone and a respiratory zone.

The conducting zone comprises:

¨ The nose

¨ The nasopharynx

¨ The larynx, or voicebox

¨ The trachea, an air tube that connects with the bronchi

¨ The right main bronchus and the left main bronchus tubes that carry air to and from the lungs

¨ The bronchioles, branches of the bronchi which distribute air to the alveoli

¨ The terminal bronchioles

The respiratory zone comprises:

¨ The respiratory bronchioles

¨ The alveolar ducts

¨ The alveoli, terminal sacs in which gas exchange occurs

Muscles used for inspiration include:

¨ The diaphragm, which mediates intrathoracic pressure to initiate inspiration.

¨ The external intercostal muscles, during vigorous inspiration

Although expiration is generally a passive process, muscles aiding forced expiration include:

¨ The abdominal muscles

¨ The internal intercostal muscles

Other:

The right and left bronchioles, terminal bronchioles, respiratory bronchioles, alveolar ducts, and alveoli form the right and left lungs respectively.

The pulmonary blood vessels generally accompany the airways and also undergo numerous branchings. The pulmonary circulation has a very low resistance compared to the systemic circulation, and for this reason, all the pressures within the pulmonary blood vessels are low.

Functions

The major function of the respiratory system is gas exchange. Respiration consists of a mechanical cycle of inspiration and expiration, with gaseous exchange occurring in between.

Inspiration is driven primarily by the diaphragm. When the diaphragm contracts, the ribcage expands and the contents of the abdomen are moved downward. This results in a larger thoracic volume, which in turn causes a decrease in intrathoracic pressure. As the pressure in the chest falls, air moves into the conducting zone. Here, the air is filtered, warmed and humidified as it flows to the lungs.

Expiration, on the other hand, is typically a passive process. The lungs have a natural elasticity; as they recoil from the stretch of inspiration, air flows back out until the pressures in the chest and the atmosphere reach equilibrium.

During forced inspiration, as when taking a deep breath, the external intercostal muscles and accessory muscles further expand the thoracic cavity.

During forced expiration, as when blowing out a candle, expiratory muscles including the abdominal muscles and internal intercostal muscles, generate abdominal and thoracic pressure, which forces air out of the lungs.

Upon inspiration, gas exchange occurs at the alveoli, the tiny sacs which are the basic functional component of the lungs. The alveolar walls are extremely thin (approx. 0.2 micrometers), and are permeable to gases. The alveoli are lined with pulmonary capillaries, the walls of which are also thin enough to permit gas exchange. Oxygen diffuses from the alveolar air to the blood in the pulmonary capillaries, as carbon dioxide diffuses in the opposite direction, from capillary blood to alveolar air. At this point, the pulmonary blood is oxygen-rich, and the lungs are holding carbon dioxide. Expiration follows, thereby ridding the body of the carbon dioxide and completing the cycle of respiration.

Oxygen Delivery System

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. This exchange of gases is the respiratory system's means of getting oxygen to the blood.

Respiration is achieved through the mouth, nose, trachea, lungs, and diaphragm. Oxygen enters the respiratory system through the mouth and the nose. The oxygen then passes through the larynx (where speech sounds are produced) and the trachea which is a tube that enters the chest cavity. In the chest cavity, the trachea splits into two smaller tubes called the bronchi. Each bronchus then divides again forming the bronchial tubes. The bronchial tubes lead directly into the lungs where they divide into many smaller tubes, which connect to tiny sacs, called alveoli. The average adult's lungs contain about 600 million of these spongy, air-filled sacs that 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 you 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.

Other Important facts:

In an average resting adult, the lungs take up about 250ml of oxygen every minute while excreting about 200ml of carbon dioxide.

The movement of gas through the larynx, pharynx and mouth allows us to speak, or phonate.

The respiratory tract is constantly exposed to microbes due to the extensive surface area, which is why the respiratory system includes many mechanisms to defend itself and prevent pathogens from entering the body.

Virtually all the body's blood travels through the lungs every minute. The lungs add and remove many chemical messengers from the blood as it flows through pulmonary capillary bed. The fine capillaries also trap blood clots that have formed in systemic veins.

Diseases

Diseases of the respiratory system can be classified into four general areas:

Obstructive Diseases (e.g., Emphysema, Bronchitis, Asthma)
Restrictive Diseases (e.g., Fibrosis, Sarcoidosis, Alveolar Damage, Pleural Effusion)
Vascular Diseases (e.g., Pulmonary Edema, Pulmonary Embolism, Pulmonary Hypertension)
Infectious, Environmental and Other Diseases (e.g., Pneumonia, Tuberculosis, Asbestosis, Particulate Pollutants)