8.1 The Circulatory System

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. State that the mammalian circulatory system is a closed double circulation consisting of a heart, blood, and blood vessels (arteries, arterioles, capillaries, venules, and veins).
  2. Describe the functions of the pulmonary artery, pulmonary vein, aorta, and vena cava.
  3. Recognise arteries, veins, and capillaries from microscope slides and micrographs, and draw plan diagrams of arteries and veins in transverse section (TS) and longitudinal section (LS).
  4. Explain how the structures of muscular arteries, elastic arteries, veins, and capillaries relate to their functions.
  5. Recognise and draw red blood cells, monocytes, neutrophils, and lymphocytes from micrographs.
  6. State that water is the main component of blood and tissue fluid, and relate its properties (solvent action and high specific heat capacity) to transport in mammals.
  7. State the functions of tissue fluid and describe how it is formed in a capillary network.

1. Why Do Mammals Need a Circulatory System?

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Every cell in your body needs a constant supply of substances like oxygen and glucose for metabolism (all the chemical reactions keeping you alive). It also needs to get rid of waste products like carbon dioxide.

  • Single-celled organisms are tiny, so substances can simply diffuse in and out across their surface. Diffusion is fast over very short distances.
  • Mammals are large, made of billions of cells. If you relied on diffusion alone, it would take far too long for oxygen to reach cells deep inside the body. The diffusion distance is simply too great.
  • The solution is a circulatory system — a network of tubes (blood vessels) that rapidly carries substances to every cell. This is called mass transport: the bulk movement of fluids in one direction through a system of vessels.

2. The Closed Double Circulatory System

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What is "closed"?

In a closed circulatory system, blood is always contained inside blood vessels. It never floods freely into body cavities. All mammals have a closed system.

What is "double"?

In a double circulatory system, blood passes through the heart twice in every single complete circuit around the body. There are two separate loops:

Loop 1 — Pulmonary Circulation

  • The right side of the heart pumps deoxygenated blood (low in oxygen, high in carbon dioxide) to the lungs.
  • In the lungs, the blood picks up oxygen and releases carbon dioxide (gas exchange).
  • The now oxygenated blood returns to the left side of the heart.

Loop 2 — Systemic Circulation

  • The left side of the heart pumps oxygenated blood to all other body tissues (muscles, organs, brain, etc.).
  • The blood delivers oxygen and collects carbon dioxide from the tissues.
  • The now deoxygenated blood returns to the right side of the heart.

Why is double better than single? Because the blood returns to the heart between the two loops, the heart can re-pressurise it. This means oxygenated blood is delivered to the body at high pressure, making transport much more efficient.

The Components of the Mammalian Circulatory System

ComponentRole
HeartA muscular pump that pushes blood around both circuits
ArteriesCarry blood away from the heart
ArteriolesSmaller branches of arteries; connect to capillaries
CapillariesTiny vessels where exchange of substances with cells occurs
VenulesSmall vessels that collect blood from capillaries
VeinsCarry blood back to the heart

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