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What is the process involved in the gaseous exchange between the alveoli and the body cells?

Respiration occurs when there is an exchange of gases between a living thing and its environment. In the case of humans, the structure for gas exchange is called an alveolus. The oxygen gas which is inhaled is exchanged for carbon dioxide gas-- a waste product that will be excreted when it is exhaled.


The respiratory system is composed of the nose, pharynx, larynx, trachea, bronchi, and lungs. It is within the lungs that the millions of alveoli take part in gas exchange.


Air moves from the nose to the pharynx, which is the tube in the throat. From there, it enters the trachea which is held open by cartilaginous rings. This airway divides into two tubes called bronchi. Each bronchus leads to each lung. Bronchi subdivide into bronchioles which get progressively smaller. Eventually, at the end of the bronchioles are the millions of alveoli which are surrounded by tiny capillaries-- blood vessels from the circulatory system. Because capillaries and alveoli are thin-walled, gas exchange can occur between them by the process of diffusion.


Oxygen can diffuse from high concentration in the alveoli to lower concentration in the blood stream by passing through the thin capillary walls. As long as a gradient is present, passive transport will occur. With carbon dioxide in higher concentration in the bloodstream relative to the amount in the alveoli, it will pass from the bloodstream to the alveoli also by diffusion. 


When the muscle called the diaphragm located between the chest and abdominal cavity contracts, the chest cavity expands. This creates a difference in air pressure--it is greater outside the body relative to the inside. This causes the lungs to fill with air and inhalation occurs. When the diaphragm muscle relaxes, the size of the chest cavity decreases. There will be greater air pressure inside the body relative to the outside. This allows air to be pushed out of the lungs and exhalation occurs. 


Breathing is regulated by the medulla oblongata in the brain. As carbon dioxide levels rise, nerve signals are sent to the diaphragm to increase its contractions which regulates breathing rate.


I have included a link of a cross section of an alveolus that shows its relation to the blood capillary and gas exchange.

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