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2.6.1Unit 2 — Multicellular Organisms

The Blood — SQA National 5 Biology Revision Notes & Topical Questions

5 min read 5 parts Lesson 27 of 39

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Part 1 of 5Components of Blood

2.6.1 The Blood

Blood is a transport medium that carries essential substances around the bodies of mammals.

Plasma
  • The pale liquid that makes up just over half the volume of blood and carries many dissolved substances.
Red blood cells
  • These make up most of the remaining volume and are specialised for carrying oxygen to respiring cells.
White blood cells
  • These form less than one per cent of the volume and are involved in defence against pathogens.
Other components
  • Small fragments such as platelets are also present, but only the three main components need detailed study here.
ComponentApproximate share of blood volume
PlasmaJust over half
Red blood cellsMost of the rest
White blood cellsLess than 1%

Part 2 of 5Substances Transported

Substances Transported
Oxygen
  • Oxygen is carried from the lungs to every respiring cell, where it is used to release energy from food.
Nutrients
  • Digested nutrients such as glucose are carried in the plasma from the small intestine to the cells that need them.
Carbon dioxide
  • Carbon dioxide made by respiring cells is carried to the lungs so that it can be removed from the body.
Other wastes
  • Other waste substances, including urea, are also carried in the blood to the organs that remove them.

Part 3 of 5Role of Each Component

1
Plasma carries dissolved substances, including glucose and carbon dioxide, around the body in the circulatory system.
2
Red blood cells pick up oxygen where its concentration is high, in the lungs, and release it in the tissues.
3
White blood cells patrol the blood and body tissues, ready to attack and destroy pathogens that have entered the body.
4
All three components are pumped around the body by the heart through a closed network of blood vessels.

Part 4 of 5Why Blood Is Needed

Why Blood Is Needed
Large organism
  • In a large animal most cells are far from the surface, so diffusion alone could not supply them with oxygen quickly enough.
Respiration
  • Cells need a constant supply of oxygen and glucose for respiration, which releases the energy they use.
Removing waste
  • Wastes such as carbon dioxide would harm cells if they built up, so the blood removes them continuously.
Pumped flow
  • The heart keeps the blood moving, which speeds delivery far beyond what diffusion could achieve.

Part 5 of 5Exam Checklist & Tips

2.6.1 – Key Points for the Exam

  • Plasma The liquid part of blood that transports dissolved substances.
  • Red blood cell A cell that carries oxygen around the body.
  • White blood cell A cell of the immune system that defends against pathogens.
  • Transport medium A substance that carries materials from one place to another.

Common Mistakes

  • Saying that red blood cells make up the largest part of blood; plasma is just over half the volume.
  • Forgetting that blood carries carbon dioxide away from cells as well as oxygen to them.
  • Stating that nutrients are carried by red blood cells; they travel in the plasma.
  • Thinking that white blood cells carry oxygen, which is the role of red blood cells.

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