Why is alveoli surrounded by capillaries

If the oxygen level in the blood is mm Hg and the oxygen level in the alveoli is mm Hg, What is the role of surfactant in respiration? What allows air to flow into or out of the lungs? How do the cells lining the Trachea and Bronchus protect the alveoli from damage? See all questions in Alveoli in the Lungs.

Impact of this question views around the world. Why are oxygen and carbon dioxide such important gasses? All cells of the body need energy to do their work. They get energy by combining sugars or other food materials with oxygen. This chemical reaction is something like burning. Inside the body cells the chemical reaction gives off heat and other forms of energy.

This energy provides the power we need to talk and move and think. When a fire burns, carbon dioxide is formed. When a body cell combines sugar with oxygen to get energy, carbon dioxide is formed there, too. But too much carbon dioxide could poison a cell. They need some way to get rid of carbon dioxide. The blood brings oxygen to the body cells and takes away their carbon dioxide.

The blood that travels back to the heart and lungs is dark red. It has picked up carbon dioxide from the body cells, and it has left most of its oxygen with the cells. This is the blood that the heart pumps into the lungs. The carbon dioxide in the blood is exchanged for oxygen in the alveoli. These tiny air sacs in the lungs are only one cell thick and they are surrounded by capillaries that are also only one cell thick.

At the very end of the air passages are clusters of small air filled sacs called alveoli singular: alveolus. These air sacs are surrounded by tiny blood vessels called capillaries Image 1. When fresh air containing oxygen enters the alveoli, the oxygen is able to move into the capillaries to be carried back to the left side of the heart. Normally, the alveoli remain open all the time. Problems can occur that can make the alveoli close. Fluid edema around alveoli, secretions in the airway or fluid or air in the sac around the lung can put pressure on the alveoli and make them collapse.

Because oxygen cannot enter alveoli that are collapsed, alveolar collapse causes the concentration of oxygen in the blood to fall.