Red blood cells are filled with oxygen at the level of the pulmonary capillaries. They are the junction between the small arteries and veins of the pulmonary circulation. It is at that microscopic site that ‘’respiration” really occurs.

Junction between the pulmonary arteries and veins

The pulmonary capillaries are the junction points between the pulmonary arteries and the pulmonary veins. They are microscopic vessels in which red blood cells pass in single file.

The plumbing system of the human body

One can compare the blood circulation of the human body to the plumbing network of a house. It brings water to all the required sites. Once used, the waste water returns to the sewers by the drains.

The human body has a plumbing system made up of arteries, veins and capillaries. Blood is brought to the organs by the arteries, and the use of this blood by the cells occurs at the level of the smallest vessels:  the capillaries.

A multitude of small air sacs

The lungs are composed of more than 300 million of small air sacs called alveoli. They have a very thin wall covered with pulmonary capillaries.

Alveoli have a diameter of 0.2 mm.

The capillaries are tiny blood vessels that act as perforated drains which prevent blood cells from leaving the circulation, but which allow fluids from the blood to escape and to return to the circulation. Under normal conditions, the movement of fluids is balanced and there is no accumulation of water in the lungs.

The thin layer of cells lining the interior of the alveoli is covered by this fluid, which creates the contact between the capillary and alveoli cells. This is what makes the exchange of oxygen and carbon dioxide (CO2) possible.

The effect of breathing

The oxygen that gets into the lungs during inspiration moves from the alveoli to the capillaries. In the opposite direction, carbon dioxide (CO2) moves from the capillaries to the alveoli and it is eliminated during expiration.