Download Gas Exchange/Alveoli














Gas Exchange/Alveoli
Gas exchange is the uptake of O2 from the environment and the discharge of CO2 to the environment. It is necessary to
support the production of ATP in cellular respiration. Alveoli are the vehicle by which some organisms perform gas
exchange, typically because they are too large or complex for direct gas exchange. Alveoli are small clusters of air sacs in the
lungs; gas diffuses across their epithelia, which in total have an area for gas exchange of about 100 m2.
O2 & CO2 Transport:
Diffusion of a gas, present in air or dissolved in water, depends on differences in a quantity called partial pressure. A gas
always diffuses from a region of higher partial pressure to a region of lower partial pressure. The partial pressure of carbon
dioxide is lower in the alveolar spaces than the partial pressure of oxygen. The higher partial pressure of carbon dioxide in
blood causes it to diffuse through the respiratory membrane into the alveolus. The higher partial pressure of oxygen in the
alveolar space causes it to diffuse through the respiratory membrane into the blood.
Vertebrate organisms commonly use respiratory pigments such as hemoglobin as the primary transport mechanism for
oxygen in the blood. It is divided into 4 protein subunits (heme) that each contain Fe 2+(iron) that binds with oxygen. Carbon
dioxide is transported as it is dissolved in the plasma, as bicarbonate ions, or attached to hemoglobin. In the process of
attaching to hemoglobin, carbon dioxide goes through several chemical changes.
Alveoli and Other Features of the Respiratory System
The trachea, or windpipe, guides air that is inhaled down to the lungs, which expand in order to allow air to pass into the
body. Networks of bronchial tubes lead air from the lungs down to the alveoli, which are sacs at the bottom of these tubes.
Gas Exchange in Organisms without Alveoli
Unicellular Organisms: Don’t have special gas exchange organs; instead, gases diffuse directly through the cell membrane.
Unicellular organisms such as a euglena (and all other Bacteria and Protists) have a large surface area to volume ratio, so
gases have a short and efficient diffusion path.
Plants: Tend to have broad leaves with large surface area and high surface area-to-volume ratios, improving light collection
and the uptake/release of gases. Stomata open and close to take in CO 2 for photosynthesis and release O2 as a by-product;
CO2 diffuses through parenchyma cells. The irregular shape and sponginess of these cells increases the internal surface area
of the leaf 10-30 times greater than the external surface area.
Animals: Multicellular organisms that are small or large but very thin, like flatworms, still use their outer surface to exchange
gases because they still have a relatively high surface area to volume ratio. Crayfish and other aquatic animals utilize gills
that allow blood vessels to directly exchange gases with water.
Insects: O2 enters spiracle, goes to the tracheal trunk, diffuses throughout a branching network of tracheal tubes reach every
body part. At the end of each tracheal branch, a special cell(the tracheole) provides a thin, moist interface for the exchange
of gasses between O2 and a living cell. O2 in the tracheal tube dissolves in the liquid of the tracheole and then diffuses into
the cytoplasm of adjacent cells. Simultaneously, carbon dioxide diffuses out of the cell and out of the body through the
tracheal system.
Other body systems the respiratory system depends on
Circulatory System: Allows inhaled oxygen to be transported through the blood to the parts of the body that need it. Also,
carbon dioxide can be transported to the lungs where it will be expelled from the body.
Nervous System: Breathing control centers in the medulla oblongata and the pons of the brain are mechanisms that
automatically regulate breathing, so organisms continue to breathe even if they are not consciously thinking about it.
Muscular System: This controls the diaphragm, which expands and contracts the lungs, allowing organisms to inhale and
exhale. Without it, your alveoli would not receive oxygen or be able to expel carbon dioxide.
Disruptions to the Respiratory System
Asthma is caused by the swelling of the airways leading to the lungs, allowing little, or no, oxygen and carbon dioxide to pass
through. Symptoms include wheezing, coughing, shortness of breath, and tightening of the chest. Asthma is chronic, so
there is no treatment, but symptoms can be managed by the use of inhalers, bronchodilators, steroids, anti-inflammatory
drugs, or a nebulizer (breathing machine).
Cystic Fibrosis is the result of a defective gene which makes the mucus and linings of the respiratory and digestive systems
thick and sticky instead of thin and slippery. The clogging up of tubes, ducts, and passageways with mucus prevents efficient
gas exchange and nutrient uptake. Symptoms: breathlessness, constant sinus and lung infections, inability to gain weight or
grow, very salty sweat, and persistent cough. Treatments include mucus-thinning drugs, bronchodilators, oral digestive
enzymes, antibiotics, and supplemental oxygen.
Diagram of the way that partial pressure moves oxygen and carbon dioxide into and out of the alveoli.
Diagram of bronchioles and alveoli in the lung.
Additional Information at:
http://faculty.stcc.edu/AandP/AP/AP2pages/Units21to23/respiration/alveolar.htm
2) http://www.cliffsnotes.com/sciences/biology/biology/gas-exchange/mechanisms-for-gas-exchange
1)