The respiratory system is responsible for the exchange of life-giving gases for waste products in the human body.  The human respiratory system can be divided into two major parts, the exchange of gases in the lungs and the exchange of gases in the body tissues.  The lungs serve as a gas exchange organ where blood can be supplied with oxygen and carbon dioxide can be removed.  The chest wall and diaphragm are muscles that act as a pump that expands and contracts the chest to allow the inflow and outflow of air.  At rest, the average adult inhales about half a liter of air about 15 times per minute.  This air moves into tiny air sacs in the lungs called alveoli where oxygen and carbon dioxide is exchanged with nearby blood vessels by simple diffusion.

What is the respiratory system?

The respiratory system is the major gas exchange system of the body.  The cells of the body require oxygen to operate, and produce carbon dioxide and other waste gases that are transferred through the blood.  The blood passes through tiny capillaries in the lungs where gases can be exchanged with nearby tiny air sacs called alveoli.  The major parts of the respiratory system include the airways and lungs.  Air is inhaled through the mouth and nose, travels through the trachea, into bronchi and then bronchioles.  At the farthest end of the bronchioles lie millions of tiny air sacs called alveoli.  Alveoli and capillaries lie close together where gas can be exchanged.

Air passages

Air first passes through the nose where it is moisturized and warmed before passing to the trachea.  The single trachea branches to become bronchi, bronchioles, alveolar ducts and then the terminal air sacs called alveoli.  The airway branches about 23 times before ending in the terminal alveoli.  Gas is exchanged in the respiratory bronchioles, alveolar ducts and alveoli.  The branching of the airways leads to a great increase in the surface area for gas exchange (about 70 square meters - the size of a tennis court).  The alveoli are surrounded by pulmonary capillarie.  The gas exchange occurs over a membrane two cells thick (one capillary wall cell and one alveolar wall cell).

Special lung cells

Besides the alveolar wall cells, there are other special cells in the lungs.

Granular pneumocytes secrete surfactant, a chemical that reduces the surface tension of fluid in the lungs.  

Macrophages, lymphocytes, plasma cells, and mast cells are responsible for germ-fighting and trash collection in the lungs.

Lining of the airways

The trachea and bronchi have cartilage in their walls to help hold them open.  They also have special wall cells with cilia and mucus-producing cells.  Cilia are microscopic finger-like projections that flap in rhythm to push debris and mucus out of the airways and into the back of the throat.  Here, the mucus can be swallowed (I know, gross!) or coughed up and spit out.  The bronchioles have cilia but do not have mucus glands.  The bronchioles have more smooth muscle than the upper airways and this muscle can help regulate the distribution of air in the lungs.  

Blood circulation

All of the body's blood passes through the lungs by way of the pulmonary artery.  This blood is receives oxygen and releases carbon dioxide and other waste gases.   The oxygen-rich blood returns to the heart by way of the pulmonary veins.

The lung tissue itself is nourished by the smaller bronchial arteries that come from the systemic circulation.  This blood drains into the bronchial veins.

Inspiration & expiration

Air enters the lungs due to the effects of muscle contractions.  The diaphragm is a sheet-like muscle at the bottom of the chest cavity that pulls down on the lungs causing them to expand.  The diaphragm typically accounts for about 75% of lung expansion.  The chest wall also expands as the muscles between the ribs contract.  This is due to a "bucket-handle" effect.  Imagine a bucket on its side with the handle hanging loosely.  If the handle is pulled upward, it actually has to swing outward.  Therefore the ribs swing outward when the rib muscles contract and this increases the volume of the chest cavity.

Pleura

The lungs are surrounded by membrane sacks called pleura.  These sacks have a wet, slippery surface which allow the lungs to slide within the chest easily.  Without the pleura, the lungs would rub against the inside of the chest and become irritated.

Gas exchange

Gas exchange or "respiration" occurs in two major places in the body.  First, in the lungs, oxygen is delivered to the blood while carbon dioxide is removed.  

In the body tissues, the opposite occurs... oxygen is removed from the blood by body tissue cells and carbon dioxide is released into the blood. 

 

Respiratory system -  Image used with permission from Wikimedia Commons.

Bronchial anatomy detail of alveoli and lung circulation - Patrick J. Lynch, medical illustrator, December 23, 2006.  Used with permission.