The circulatory system
The circulatory system
- Mammals have a closed double circulation.
- Closed: blood stays inside vessels the whole time.
- Double: blood passes through the heart twice per full trip.
Practice
A "closed double circulation" means that:
Closed = blood always in vessels; double = it passes through the heart twice (pulmonary then systemic).
Two loops
- The pulmonary circulation carries blood from the heart to the lungs and back.
- The systemic circulation carries blood from the heart to the rest of the body and back.
- Blood flows: arteries → arterioles → capillaries → venules → veins.
Practice
The pulmonary circulation carries blood:
Pulmonary = to the lungs; systemic = to the rest of the body.
The main vessels
- pulmonary artery — heart → lungs (low oxygen).
- pulmonary vein — lungs → heart (oxygen-rich).
- aorta — heart → body (oxygen-rich).
- vena cava — body → heart (oxygen-poor).
Vessels suited to their jobs
| Vessel | Structure | Function |
|---|---|---|
| artery | thick muscle + elastic wall, narrow lumen | high-pressure flow away from heart; walls recoil to smooth flow |
| capillary | wall just one cell thick | short distance for exchange with cells |
| vein | thin wall, wide lumen, valves | low-pressure return to heart; valves stop backflow |
Practice
Arteries have thick, elastic, muscular walls because they:
High-pressure blood leaving the heart needs strong walls that stretch and recoil to smooth the flow.
Practice
Veins contain valves in order to:
Blood in veins is at low pressure, so valves prevent backflow as it returns to the heart.
Practice
A capillary wall is just one cell thick because this:
A one-cell-thick wall means a short diffusion distance, so substances exchange quickly.
You've got it
Key idea
- closed double circulation: blood in vessels, through the heart twice
- pulmonary loop = lungs; systemic loop = body
- artery: thick wall, high pressure, away from heart; vein: valves, low pressure, back to heart
- capillary: one cell thick → fast exchange