The blood is the vehicle for the cardiovascular system, it is used to transport substances around the body.
Transport of Gases - General
One very important transport function of the blood is to carry gases back and forth between the lungs and the individual cells of the body.
The alveoli and the individual body cells are the sites of exchange of gases to and from the blood.
At these sites, the gases move according to the directions of pressure of concentration gradients.
That is, each gas moves from an area where it is in higher concentration to an area of lesser concentration.
Oxygen is in the air filling the alveolus of the lung.
The oxygen passes through the walls of the alveolus and capillary to become dissolved in the
plasma of the blood.
Most of the dissolved oxygen is rapidly picked up by the hemoglobin of the RBCs.
Thus, the RBC is the main transporting element for oxygen in the blood.
Carbon dioxide is produced during metabolic oxidation within the individual cell.
It passes through the cell membrane and the wall of the capillary to become dissolved in the plasma of the blood.
Through action of an enzyme in the RBCs, most of the carbon dioxide (CO2) is transformed into bicarbonate ions (HCO3).
Transport of Nutrients
Other substances, such as the end products of digestion, are also carried by the blood.
They are either dissolved or suspended in the plasma.