4BI1

Transport in Humans

Structures and Functions in Living Organisms · 6 question types

Practise

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 15% of your exam marks.

stable
Very High
Stable15%

The heart, blood vessels, and blood components are regularly tested, particularly structure-function links.

When a blood vessel is cut, the body has to act fast to stop blood escaping and to seal off the wound before bacteria can enter. The system that does this involves and a chain of plasma proteins.

The steps of blood clotting:

  1. Platelets at the site of the wound release chemicals that trigger the clotting cascade.
  2. A soluble plasma protein called fibrinogen is converted into long insoluble fibres of fibrin.
  3. The fibrin fibres form a tangled mesh across the wound.
  4. get trapped in the mesh, forming a blood clot.
  5. The clot dries out at the surface to form a scab, which stays in place while new skin grows underneath.

The clot does two things: it stops further blood loss, and it provides a physical barrier that keeps bacteria out of the wound while it heals.

Blood clotting shown in three stages at a broken blood vessel wall: red blood cells and soluble fibrinogen proteins at the wound, then platelets releasing chemicals that convert fibrinogen into insoluble fibrin, then a mesh of fibrin trapping red blood cells and activated platelets to form a blood clot
Source: Platelets & Blood Clotting by Save My Exams

A condition called haemophilia is caused by inheriting a defective version of one of the clotting proteins. People with haemophilia bleed for much longer than normal after even minor cuts, because their clots form very slowly.

Exam tip

Describing the role of platelets

What comes up: "describe the role of platelets in the body" (3 marks).

Write (three marks): (1) Platelets help blood to clot at a wound (for example by triggering the conversion of fibrinogen to fibrin) (1). (2) Clotting prevents further blood loss / stops bleeding (1). (3) The clot acts as a physical barrier that prevents pathogens / bacteria from entering the wound (1).

Watch out: a vague answer such as "platelets seal wounds" scores at most one mark. You need to name at least one consequence of clotting (stops blood loss) and the infection-prevention role to reach full marks.