The systemic capillaries are the junction points between the arteries and the veins of the great circulation. It is at that this microscopic level that breathing and feeding of the cells of the human body occur.

Cell needs

The heart is a pump that makes the blood circulate throughout the human body. Organs and muscles need nutrients and a constant oxygen supply to function properly. In the same way, they need a system to get rid of their waste. That’s the role of blood circulation.                    

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.

Systemic capillaries

The systemic capillaries are the junction points between the arteries and the veins.

They are microscopic vessels in which red blood cells pass in single file. Capillaries allow the transfer of nutrients and oxygen to organ and muscle cells, and the removal of waste and  carbon dioxide (CO2). If all the capillaries were put end to end, they would cover  a distance of nearly 81.000 km.

The capillaries 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 body.

All the cells are surrounded by this fluid. This is where oxygen and nutrients are distributed (diffusion) and waste is removed.

The cells take oxygen and produce CO2

In summary, the cells need fuel to function properly. Oxygen is absorbed by the cells. Cell activity generates a waste called carbon dioxide (CO2) which must be evacuated out of the body.

The toxic products and CO2 must be eliminated

Toxic wastes and CO2 are recovered by the red blood cells and the plasma in the capillaries. The capillaries bring them to the veins and then to the heart.  Waste is eliminated by the liver and the kidneys, whereas CO2 is eliminated through the lung by breathing.

High concentration to low concentration

All the exchanges, nutrients and waste, are done using a process called diffusion. Diffusion means that substances move from high to low concentration zones.

Exchanges are made at the level of the capillaries

It is through the capillaries that all exchanges are made. That also includes  exchange of fluids. The capillaries collect fluids in excess by ensuring a balance between what comes in and what leaves. Under normal conditions, there is no accumulation of fluids (swelling) in the body.