Circulatory Systems Notes

Cambridge IGCSE Biology: Revision notes

Key facts

  • The circulatory system is blood vessels, a heart (pump) and valves, giving one-way flow.
  • Single circulation (fish): blood passes through the heart once per circuit: heart, gills, body, heart.
  • Double circulation (mammals): blood passes through the heart twice, through the pulmonary and systemic circuits.
  • Double circulation keeps blood pressure high and oxygenated and deoxygenated blood separate.
  • Valves stop backflow; arteries have none because their pressure is high.

Pump, vessels and valves

Blood vessels, a pump and valves work together to keep blood flowing one way.

The circulatory system is a closed network of blood vessels, with the heart as a muscular pump and valves as one-way gates. The pressure gradient made by the heart keeps blood flowing forward.

One-way flow keeps transport of oxygen, nutrients and waste efficient and ensures every cell gets blood at the right pressure.

Labelled diagram of the human heart
The heart is the pump; notice the valves that keep the blood flowing one way and the vessels that carry it in and out
  • Vesselscarry the blood
  • Heartpumps it
  • Valveskeep it one-way

What is the job of valves in the circulatory system?

Single circulation

In fish, blood passes through the heart once per circuit, going to the gills before the body.

Single circulation is found in fish. The heart pumps blood to the gills for gas exchange, and it then flows on to the body tissues and back to the heart. Blood loses pressure after the gills, so it moves slowly through the body.

This suits fish, which are generally less active and have lower metabolic demands than mammals.

  1. 1

    Heart

    blood leaves via the ventricle

  2. 2

    Gills

    gas exchange occurs

  3. 3

    Body tissues

    oxygenated blood flows directly to them

  4. 4

    Heart

    deoxygenated blood returns via the atrium

Then back to step 1

Single circulation in a fish

In a fish, which comes first after the heart?

Double circulation

In mammals, blood passes through the heart twice per circuit: once to the lungs and once to the body.

Double circulation is found in mammals.

  • Pulmonary circulation: the right side pumps deoxygenated blood to the lungs; oxygenated blood returns to the left side.
  • Systemic circulation: the left side pumps oxygenated blood to the body; deoxygenated blood returns to the right side.

A septum keeps the two sides apart.

  1. 1

    Right side of heart

    pumps deoxygenated blood

  2. 2

    Lungs

    gas exchange

  3. 3

    Left side of heart

    receives oxygenated blood and pumps it out

  4. 4

    Body tissues

    oxygen delivered

Then back to step 1

Double circulation in a mammal

Pulmonary

  • Heart to lungs only
  • Leaves heart deoxygenated
  • Lower pressure

Systemic

  • Heart to all body tissues
  • Leaves heart oxygenated
  • Higher pressure

How many times does blood pass through a mammal heart in one complete circuit?

Why double circulation

The left ventricle re-pressurises blood after the lungs, so fully oxygenated blood reaches active tissues quickly.

Blood returning from the lungs has lost pressure. The left ventricle pumps it out again at high pressure, so oxygen is delivered faster and more forcefully.

The septum keeps oxygenated and deoxygenated blood apart, and each side can be regulated independently. This supports the high metabolic rate and activity of mammals.

  1. 1

    Right side of the heart

    pumps blood to the lungs

  2. 2

    Lungs

    blood picks up oxygen, and its pressure drops

  3. 3

    Left side of the heart

    pumps the blood again at higher pressure

  4. 4

    Body

    oxygenated blood reaches active tissues quickly

Then back to step 1

Double circulation in a mammal

Single (fish)

Blood pressure to body:
Lower; drops through the gills
Oxygen delivery:
Slower delivery
Metabolic support:
Lower metabolic rate

Double (mammal)

Blood pressure to body:
Higher; re-pressurised after the lungs
Oxygen delivery:
Faster delivery
Metabolic support:
Supports high activity

Why does double circulation allow higher pressure to the body?

Valves

Valves close when blood tries to flow backwards, so blood moves in one direction only.

Valves lie between the chambers of the heart and between the heart and the arteries. Veins also have valves, especially in the limbs; the squeezing of skeletal muscles pushes blood through them. Arteries have no valves because their high pressure keeps blood moving forward. Backflow is stopped by the flaps closing.

  1. 1

    Forward flow

    blood pressure pushes the flaps open

  2. 2

    Attempted backflow

    blood pushes the flaps shut

  3. 3

    Result

    blood moves in one direction only

How a valve works

Why are valves not needed in arteries?

Try an exam question

Explain why mammals have a double circulatory system but fish have a single circulatory system.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.