In short: Animals move materials around the body in a circulatory system. Mammals have a closed, double circulation: the heart pumps blood to the lungs and then to the body, arteries carry blood away from the heart, veins bring it back, and capillaries exchange materials with cells.
Starting point: what you already know
- Large animals have a small surface area to volume ratio, so they need a transport system (see Levels of Organisation).
- Oxygen enters the blood in the lungs. Nutrients enter it in the small intestine (see Nutrient and Gas Exchange).
- Blood is pumped by the heart through blood vessels.
Open and closed systems
| Open system | Closed system | |
|---|---|---|
| Examples | Insects, crabs, snails | Humans, fish, earthworms, octopus |
| Blood | Pumped into a body cavity and bathes the organs directly | Stays inside blood vessels |
| Pressure | Low | High, so it moves fast |
| Suits | Small animals, or ones like insects that get oxygen by tracheae | Larger, active animals |
Single and double circulation
A fish has single circulation: heart, then gills, then body, then back to the heart. Blood passes through the heart once per lap.
Mammals and birds have double circulation: blood passes through the heart twice per lap.
- Pulmonary circuit: right side of the heart pumps blood to the lungs to pick up oxygen.
- Systemic circuit: left side of the heart pumps oxygen-rich blood to the rest of the body.
Returning to the heart after the lungs lets blood be pumped out again at high pressure, which an active, warm-blooded animal needs.
The human heart
The heart has four chambers. The atria (top) receive blood. The ventricles (bottom) pump it out. Valves stop blood flowing backwards; their closing makes the "lub-dub" sound.
The path of blood: body → vena cava → right atrium → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → left ventricle → aorta → body.
The left ventricle has the thickest wall because it pumps blood all around the body.
Blood vessels
| Arteries | Capillaries | Veins | |
|---|---|---|---|
| Carry blood | Away from the heart | Between arteries and veins, past every cell | Back to the heart |
| Wall | Thick, muscular, elastic | One cell thick | Thin |
| Pressure | High | Low | Lowest |
| Valves | No | No | Yes, stop backflow |
Exchange of materials happens only in capillaries, because their walls are so thin.
What blood carries
| Part | Job |
|---|---|
| Plasma (55%) | Yellow liquid carrying glucose, amino acids, , urea, hormones, salts and heat |
| Red blood cells | Carry oxygen joined to haemoglobin |
| White blood cells | Fight infection |
| Platelets | Help blood clot |
- Open: insects
- Closed: vertebrates
- Single: fish
- Double: mammals and birds
- Pulmonary and systemic
- Atria receive
- Ventricles pump
- Valves stop backflow
- Left ventricle thickest
- Arteries: away, high pressure
- Capillaries: exchange
- Veins: valves, low pressure
- Plasma, red and white cells, platelets
Quick summary
- Open systems pump blood into a body cavity. Closed systems keep it in vessels.
- Fish have single circulation. Mammals have double circulation: pulmonary and systemic.
- Atria receive blood, ventricles pump it, valves stop backflow. The left ventricle is thickest.
- Arteries carry blood away from the heart at high pressure. Veins return it and have valves. Capillaries exchange materials.
- Plasma carries dissolved substances. Red blood cells carry oxygen.
Quiz
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Try 3 questions now
Which animal has an open circulatory system?
- A Human
- B Fish
- C Grasshopper
- D Dog
Show answer
C: Grasshopper
Insects have open systems.
Which vessels carry blood away from the heart?
- A Arteries
- B Veins
- C Capillaries
- D Lymph vessels
Show answer
A: Arteries
Arteries carry blood Away.
Where are materials exchanged between blood and body cells?
- A Arteries
- B Veins
- C Capillaries
- D Aorta
Show answer
C: Capillaries
Capillary walls are one cell thick.