circulation
Meaning of circulation: A closed loop that delivers oxygen and fuel to every cell and carries away what those cells no longer need.
Definition of circulation
Circulation is the continuous movement of blood around the body, driven by the pumping of the heart through a closed network of vessels. It delivers oxygen, nutrients, hormones and immune cells to tissues, and carries carbon dioxide and waste away for disposal. The word also covers the slower one-way flow of lymph, the fluid that drains from tissue back into the bloodstream.
What are the two main circuits?
The heart is really two pumps side by side, each serving a separate loop, and blood passes through both on every trip around.
| Circuit | Route | Purpose |
|---|---|---|
| Pulmonary | Right heart to lungs and back | Pick up oxygen, release carbon dioxide |
| Systemic | Left heart to the body and back | Deliver oxygen and fuel to tissue |
| Coronary | Aorta to the heart muscle itself | Feed the pump |
| Portal | Gut to the liver | Process absorbed food before it spreads |
Where does the exchange actually happen?
Not in the large vessels. Arteries carry blood away from the heart under pressure and branch into progressively smaller ones, ending in capillaries whose walls are a single cell thick. Oxygen and nutrients cross that thin wall into the surrounding tissue, and waste crosses back. Capillaries then join into veins, which return blood at low pressure and rely on valves and the squeeze of surrounding muscle to keep it moving upward against gravity. A resting blood pressure reading captures the force inside the arteries during this cycle, not inside the capillaries where the work is done.
What is the lymph side of it?
Not all fluid pushed out of the capillaries is reabsorbed. The remainder collects between cells and drains into a second network of thin vessels. This lymph passes through lymph nodes, where immune cells inspect it, and empties back into a large vein near the collarbone. Unlike blood, lymph has no pump. It moves only when muscles contract and vessels are compressed, which is why a limb that stays still swells and why damage to a blood vessel network is not the only cause of persistent swelling.
Used in a sentence
Examination found reduced circulation in the lower limb with absent pedal pulses.
Good to know
Blood circulates in a closed loop under pressure from the heart, while lymph moves one way and has no pump, relying on muscle movement to drain tissue fluid.
Word origin
Where “circulation” comes from
From the Latin circulatio, from circulus, a small ring or circle.