How the alveoli are adapted to quick gas exchange

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What is diffusion?

Diffusion is the passive overall movement of particles from a region of their high concentration to a region of their lower concentration across a partially permeable membrane (e.g. along the concentration gradient). 

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Diffusion in the alveoli

Oxygen is inspired into the alveoli. Oxygen diffuses from the alveoli to the capillaries. Carbon dioxide diffuses from the capillaries to the alveoli.Carbon dioxide is expired from the alveoli. The walls of the alveoli are only one cell thick. Alveoli are covered with a network of capillaries. The walls of the capillaries are also only one cell thick. 

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Large surface area

Within the lungs, alveoli provide a large surface area for exchange of gases between the air and the blood. The rate of diffusion is directly proportional to the surface area. As the surface area increases the rate of diffusion increases.

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Diffusion and calculating S.A

Living organisms have to exchange surfaces with their surroundings. For example, they take in oxygen and nutrients and get rid of waste materials such as carbon dioxide. In unicellular organisms the whole cell surface membrane is the exchange surface. Substances that diffuse into or out of a cell move down a concentration gradient (from a high to a low concentration). The gradients are maintained by the cell continuously using the substances absorbed and producing waste. For example, oxygen diffusing into a cell is used for respiration which produces carbon dioxide.

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Diffusion and calculating S.A continued

         The larger an organism, the more exchange has to take place to meet the organism’s needs. Larger multicellular organisms have more problems absorbing substances because of the size of the organism’s surface area compared with its volume. This is shown as the surface compared with its volume. This is known as the surface area to volume ratio, calculated by dividing an organism’s total surface area by its volume. 

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Thickness of surface

The walls of the alveoli and the capillaries are only one cell thick. This increases the rate at which diffusion happens by decreasing the distance over which it happens. The rate of diffusion is inversely proportional to the thickness of the gas exchange surface. The thicker the surface the slower the rate of diffusion

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The alveoli are connected to a network of capillaries

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Concentration gradient

There is a high concentration gradient between the alveoli and the capillaries maintained by ventilation of the alveoli. The rate of diffusion is directly proportional to the difference in concentration across the gas exchange surface. The greater the concentration gradient the faster the diffusion.


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Role of Surfactant

Surfactants are compounds that lower the surface tension (or interfacial tension) between two liquids or between a liquid and a solid. Surfactant reduces surface tension, so that the alveoli in the lungs are able to expand. It is basically a biological detergent. Surfactant reduces surface tension. Without surfactant, the wet surfaces of the alveoli in your lungs would stick together and your lungs would not be able to expand- so you would not be able to breathe.

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Flick's law

Rate of diffusion((surface area×difference in concentration)/ (thickness of gas exchange surface))
This is known as Fick’s law. The surface area of the alveoli, the steep concentration gradient between the alveolar air and the blood (maintained by ventilation of the alveoli) and the thin walls of the alveoli and capillaries to ensure rapid diffusion across the gas exchange surface. 

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