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Course: SQA National 5 Biology
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SQA National 5 Biology

1. Unit 1 — Cell Biology

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2. Unit 2 — Multicellular Organisms

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3. Unit 3 — Life on Earth

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Text lesson

2.6.4 Circulatory System

2.6.4 Circulatory System: Describe the circulatory system as a closed network of vessels and trace the movement of blood between the heart, lungs and body tissues.

2.6.4 Circulatory System
A Closed Network
Organ system
  • The circulatory system is an organ system whose role is to transport blood around the body.
Closed vessels
  • Blood stays inside a closed network of vessels that are connected to the heart.
Arteries
  • Arteries carry blood away from the heart towards the organs and tissues.
Veins
  • Veins carry blood back towards the heart from the organs.
1

Arteries carry blood away from the heart and become narrower as they enter an organ, branching into arterioles.
2

Arterioles lead into capillaries, where respiring cells take oxygen from the blood and give carbon dioxide to it.
3

The capillaries join to form venules, which then widen into veins as they leave the organ.
4

Veins return the deoxygenated blood to the heart.
To the lungs
  • Deoxygenated blood is pumped from the heart to the lungs, where it picks up oxygen and loses carbon dioxide.
Back to the heart
  • Oxygenated blood returns from the lungs to the heart, which pumps it out again to the body.
To the body
  • Blood leaving the heart for the body travels in arteries to all the respiring tissues.
Two circuits
  • Blood passes through the heart twice on each complete trip around the body, so it is a double circulation.
What You Need to Know
Vessels at the heart
  • You need to know the names of the blood vessels that enter and leave the heart.
Others
  • The names of other vessels in the circulatory system are not required at this level.
Direction rule
  • Arteries carry blood away from the heart and veins carry blood towards it, whatever the oxygen content.
Gas exchange
  • In the lungs oxygen diffuses into the blood and carbon dioxide diffuses out.
Delivery
  • Oxygen and nutrients must reach all of the respiring cells, and the circulation delivers them continuously.
Waste removal
  • Carbon dioxide and other wastes are carried away from tissues so that they do not build up.
Pump
  • The heart acts as the pump that keeps blood moving round the whole system.
Gas exchange link
  • The circuit through the lungs is where blood picks up fresh oxygen and gets rid of carbon dioxide.

2.6.4 – Key Points for the Exam

  • Circulatory system: The organ system that transports blood around the body.
  • Artery: A vessel that carries blood away from the heart.
  • Vein: A vessel that carries blood towards the heart.
  • Capillary: A tiny vessel that links arteries to veins and allows exchange with cells.
  • Closed system: A system in which blood stays inside vessels.

Common Mistakes

  • Defining arteries as vessels carrying oxygenated blood; the correct rule is blood away from the heart.
  • Thinking that blood goes through the heart only once on a complete circuit of the body.
  • Saying that veins always carry deoxygenated blood, forgetting the pulmonary vein.
  • Leaving out the capillaries when describing the path between arteries and veins.
YOUR STUDY PATH

2.6.4 Circulatory System

SQA National 5 Biology / 2. Unit 2 — Multicellular Organisms

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