Overview of the Respiratory System

The lungs are surrounded by the pleura, and then outside the pleura is the rib cage. The heart is postioned slightly to the left, causing the left lung to be slightly smaller than the right. The right lung has 3 lobes (upper, middle, lower) and the left lung has two lobes (upper, lower). The diaphragm is a muscle positioned at the bottom of the lungs which assists with breathing.

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Mechanics of Breathing
When we breathe in, the ribs move up and the diaphragm is pulled down; which expands the lungs and causes air to be pulled in. When we breathe out, the ribs and diaphragm return back to their relaxed position, decreasing the lung size, causing air to be pushed out. 

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Anatomy of the Respiratory System
The respiratory system begins at the mouth and nose. As air passes through the nasal cavity, it is warmed and humidified, and the nasal hairs filter out the solid particles. The air then travels down to the throat (pharynx), then past the epiglottis into the voice box (larynx), then into the windpipe (trachea). The trachea splits into the right and left bronchus. The bronchus then divides into smaller bronchi, and then into bronchioles, until eventually they terminate in the alveolar sacs.

The alveolar sacs are made up of a number of alveoli. The alveoli are very small in size. Each person has a few million alveoli in their lungs.

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A network of small capillaries covers the alveoli. Gas exchange takes place between the air in the alveoli and the blood in the capillaries.

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Gas Exchange at the Alveoli
During inhalation, oxygen is breathed into the mouth or nose, and travels down the respiratory track until it reaches the alveoli. Gas exchange takes place between the oxygen in the alveoli, and the blood in the capillaries surrounding it. Oxygen is passed from the alveoli, into the blood in the capillary.

Furthermore, carbon dioxide is passed from the blood in the capillary, into the alveoli. The carbon dioxide then travels up the respiratory tract and out of the mouth or nose during exhalation.

Gas Exchange inside the Organs and Cells of the Body
When discussing the respiratory system, it is also necessary to discuss the circulatory system, as both systems work closely together.

Once gas exchange has taken place at the alveoli, this oxygenated blood then leaves the lungs to return to the heart, to be distributed around the body.

The heart pumps the oxygenated blood to the organs and tissues. Once at the organs and tissues, gas exchange takes place between the blood in the capillaries, and the cells. Oxygen is transferred from the blood to the cells, and carbon dioxide is transferred from the cells to the blood. After gas exchange, the blood becomes de-oxygenated. This de-oxygenated blood returns to the heart, to then be pumped to the lungs, to begin the process again.