Biology (0610) | Blood

🩸 The Incredible Journey: Study Notes for Blood (IGCSE 0610)

Welcome to the chapter on Blood! This is one of the most vital topics in the "Transport in Animals" section. Why is blood so important? Think of your blood as the body’s highly efficient delivery and waste disposal system. Without it, none of your cells would receive the oxygen and nutrients they need, and toxins would quickly build up.

In these notes, we will break down the four main components of blood, explore their jobs, and see how they keep you alive, healthy, and patched up when you get a cut!

1. Components of Blood (The Big Four)

Blood is actually a type of tissue, specifically a transport tissue. It's made up of a liquid component and three main types of cell/cell fragments floating within it.

Key Component 1: Plasma (The Liquid Highway)

The plasma is the pale yellow liquid component of blood, making up about 55% of the total volume.

• What is it? Primarily water (about 90%), making it an excellent solvent.
• Function (Transport!): Plasma is the carrier fluid for almost everything.

Plasma Transports:

1. Blood Cells: Red blood cells, white blood cells, and platelets.
2. Nutrients: Such as glucose, amino acids, and fatty acids (absorbed from the small intestine).
3. Ions: Mineral ions (like sodium and chloride).
4. Hormones: Chemical messengers (e.g., insulin).
5. Waste Products: Especially urea (a waste product of metabolism, transported to the kidneys for excretion).
6. Carbon Dioxide ($\text{CO}_2$): Transported from the body cells to the lungs (mostly as hydrogencarbonate ions, but dissolved in plasma is also acceptable).

2. Red Blood Cells (RBCs): The Oxygen Delivery Service

Red blood cells (also called erythrocytes) are the most numerous cells in the blood. Their sole purpose is to move oxygen around the body.

Structure and Function (9.4 Core 3a)

• Transport: They transport oxygen from the lungs to all respiring tissues.
• Key Molecule: The transport is carried out by the red pigment called haemoglobin.
• Haemoglobin + Oxygen: In the lungs, oxygen binds loosely with haemoglobin to form oxyhaemoglobin.
• Adaptations for Maximum Efficiency:
  • No Nucleus: This means they have more space inside for haemoglobin molecules. (Did you know? This is why they only live for about 120 days!)
  • Biconcave Shape: This dimpled disc shape gives them a large surface area to volume ratio, which increases the rate of oxygen diffusion in and out.

Analogy: Haemoglobin is like the magnet inside the taxi (RBC) that picks up the oxygen passenger.

3. White Blood Cells (WBCs): The Immune Army

White blood cells (also called leukocytes) are the body's defence system. Unlike RBCs, they have a nucleus and are generally larger. We focus on two main types:

Type 1: Phagocytes (The 'Engulfers') (9.4 Core 3b, Supplement 6b)

Phagocytes protect the body by literally eating invaders.

• Mechanism: They perform phagocytosis, which means they engulf (swallow) foreign material and pathogens (like bacteria) and destroy them using digestive enzymes.
• Shape: They have an irregular, lobed nucleus and can change shape to squeeze through capillary walls to reach sites of infection.

Analogy: Phagocytes are the "Pac-Man" cells of your blood, continuously sweeping up and consuming anything harmful.

Type 2: Lymphocytes (The 'Antibody Producers') (9.4 Core 3b, Supplement 6a)

Lymphocytes are responsible for specific immunity—a highly targeted response.

• The Threat: Pathogens have unique marker molecules on their surface called antigens. Each pathogen has a specific antigen shape.
• The Response: Lymphocytes produce specific proteins called antibodies.
• Function of Antibodies: Antibodies have a complementary shape to a specific antigen. They bind to the antigens on the pathogen, which can lead to:
  1. Direct destruction of the pathogen.
  2. Marking the pathogen so that phagocytes can easily find and destroy it.

4. Platelets and Blood Clotting (The Repair Crew)

When you cut yourself, it is crucial that the body stops blood loss quickly and seals the entry point against pathogens. That is the job of platelets and the clotting process.

Core Function of Clotting (9.4 Core 4)

The two main roles of blood clotting are:

1. Preventing Blood Loss: Essential for maintaining blood pressure and survival.
2. Preventing Entry of Pathogens: Sealing the wound stops bacteria and viruses from entering the body.

The Clotting Process (Extended/Supplement Only - 9.4 Supplement 7)

Platelets are small fragments of cells (they are not full cells). When they encounter a damaged blood vessel wall, they initiate a rapid cascade of chemical reactions.

Step-by-Step Clot Formation:

1. Damage to the blood vessel stimulates platelets to become active.
2. The platelets release chemicals that start a reaction cascade (you don't need to recall these specific enzymes/chemicals).
3. This cascade leads to the conversion of a soluble plasma protein called fibrinogen into an insoluble protein called fibrin.
4. The insoluble fibrin forms a sticky, strong network of threads (a mesh).
5. This fibrin mesh traps red blood cells and more platelets, creating a solid clot or scab over the wound.

Common Mistake Alert: Fibrinogen is soluble (a protein floating in the plasma). Fibrin is insoluble (the mesh that forms the clot).

Quick Review Summary Table (9.4 Blood)

Remember, understanding the function of each component is key to nailing those exam questions. Good luck with your revision!