The substance of the liver was tender, and full of bile and venous blood.
The liver and the lungs, therefore, are the great purifiers of the venous blood.
There is thus no mixing of arterial and venous blood in the heart.
And the first of these is the proportion of arterial to venous blood in the capillaries.
The object being to aid the return of venous blood to the heart.
In the course of this process it acquires the dingy tint of venous blood.
The impeded return circulation of venous blood now comes into play.
It is arterial blood when it flows in, and venous blood when it comes back.
For, as you know, venous blood is still impure and dirty blood.
Venous blood contains more carbonic acid, and less oxygen and nitrogen than arterial blood.