Ch 4 · Blood · O Level Biology 5090
Topic 4 of 4 in Transport in Animals — O Level Biology 5090 — 48 lessons.
Introduction
Blood is a tissue: living cells suspended in a liquid. Roughly 55 per cent is plasma and 45 per cent is cells, almost all of them red blood cells. Each component has a distinct job, from carrying oxygen to sealing wounds and destroying invading microorganisms. You must recognise the components in a photomicrograph, state their functions in syllabus wording and distinguish the two main types of white blood cell and describe how a clot forms.
Key ideas — The four components and how to recognise them
🧑🏫 Teacher's explanation
Think of blood as a busy highway system. The plasma is the liquid road surface that everything floats on. The red blood cells are the delivery trucks, specifically designed to carry oxygen cargo. The white blood cells are the police force, patrolling for intruders and fighting them off. The platelets are the emergency repair crews, rushing to any 'potholes' or leaks in the highway to patch them up immediately.
📖 Definitions
🔍 Worked example — Easy
A student observes a blood smear under a light microscope. They identify a cell that is significantly larger than a red blood cell and contains a large, dark-staining nucleus. Identify the component and state its general function.
🔍 Worked example — Standard
A patient has a low count of platelets. Explain the physiological consequence of this condition in terms of blood function.
🔍 Worked example — Exam-style
Calculate the percentage of white blood cells in a sample if there are 5 white blood cells for every 700 red blood cells (assume platelets and plasma volume are negligible for this calculation).
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Describe two structural features that allow a student to distinguish a white blood cell from a red blood cell. [2] (b) State the function of platelets in the blood. [1] [3 marks]
Key ideas — Functions of the components
🧑🏫 Teacher's explanation
Think of your circulatory system as a massive city delivery network. The plasma is the river flowing through the streets, carrying small packages like nutrients and hormones. The red blood cells are specialized delivery trucks specifically designed to carry oxygen. White blood cells are the police force, patrolling for intruders, and platelets are the emergency repair crews that patch up leaks in the pipes.
📖 Definitions
🔍 Worked example — Easy
A student observes a blood smear under a microscope. They identify a cell that has a lobed nucleus and is much larger than a red blood cell. Identify the type of white blood cell and its function.
🔍 Worked example — Standard
Compare the structure of a red blood cell to its function in oxygen transport.
🔍 Worked example — Exam-style
Explain the process of blood clotting and why it is essential for survival.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Name the component of blood that is responsible for the production of antibodies. [1] (b) Describe how the structure of a red blood cell is adapted to its function of transporting oxygen. [3] [4 marks]
Key ideas — Two kinds of white blood cell
Phagocytes have a lobed nucleus and granular cytoplasm. They change shape, squeeze out of capillaries into tissues and carry out phagocytosis: the cell flows around a pathogen, engulfs it into a vacuole and digests it with enzymes. They arrive first at an infection and are not specific to any pathogen.
🧑🏫 Teacher's explanation
Think of your blood as a high-security highway system. While red blood cells are the delivery trucks carrying oxygen, white blood cells are the specialized security forces patrolling the lanes.
📖 Definitions
🔍 Worked example — Easy
A student observes a blood smear under a light microscope. They identify a cell with a large, spherical nucleus that occupies most of the cell volume. Identify this cell type and state its primary function.
🔍 Worked example — Standard
Compare the response of phagocytes and lymphocytes to a new pathogen introduced into the blood.
🔍 Worked example — Exam-style
A patient has a high concentration of a specific antibody in their plasma. Explain how this relates to the presence of a pathogen.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Describe two structural differences between a phagocyte and a lymphocyte. [2] (b) Explain how lymphocytes help to defend the body against pathogens. [2] [4 marks]
Key ideas — Blood clotting
how a clot forms. When a vessel is damaged, platelets stick to the exposed surface and release chemicals. These trigger reactions in which the soluble plasma protein fibrinogen is converted into insoluble fibrin. Fibrin threads build a mesh across the wound, trapping red blood cells and platelets to form a clot, which dries to a scab.
🧑🏫 Teacher's explanation
Imagine you have a leaking water pipe in your garden. If the water is flowing freely, it's like blood inside your veins—it needs to stay liquid to move around. But if there is a hole, you need something to plug it immediately. Blood clotting is like a rapid-response repair kit that turns a liquid into a solid mesh.
📖 Definitions
🔍 Worked example — Easy
Identify the two main biological purposes of blood clotting in humans.
🔍 Worked example — Standard
Describe the change in the state of the protein involved in clotting during the formation of a clot.
🔍 Worked example — Exam-style
Explain why blood must remain liquid while circulating in the blood vessels, but must solidify at the site of a wound.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Name the component of the blood that is involved in the clotting process. [1] (b) Describe how the structure of the proteins involved changes to form a clot. [2] (c) State two reasons why the formation of a clot is essential for survival. [2] [5 marks]
Worked example
Question. A stained blood smear is examined. (a) Identify cell P, which has no nucleus and a pale centre, and state its function. [2] (b) Name three substances, other than cells, transported by plasma. [3] (c) Cell Q has a large round nucleus filling most of the cell. Identify Q and state its function. [2] (d) Describe how a blood clot forms at a cut and explain why clotting is important. [4]
🔍 Worked example — Easy
A student observes a blood smear under a light microscope. They identify a cell that lacks a nucleus and has a biconcave shape. Identify this cell and state its primary function.
🔍 Worked example — Standard
Calculate the percentage increase in the concentration of a soluble nutrient in the plasma if the initial concentration is 0.5 g/dm^3 and the final concentration is 0.7 g/dm^3.
🔍 Worked example — Exam-style
Compare the roles of phagocytes and lymphocytes in the body's immune response.
⚠️ Common errors
Common mistakes
Practice
Answers