Blood
Components of Blood
Blood is a specialized connective tissue that transports substances around the body. It consists of four main components: plasma, red blood cells, white blood cells, and platelets. Understanding the structure and function of each component is essential for explaining how the circulatory system maintains homeostasis.
| Component | Description / Structure |
|---|---|
| Red Blood Cells (Erythrocytes) | Biconcave disc shape; no nucleus; contains haemoglobin. |
| White Blood Cells (Leucocytes) | Have a nucleus; larger than red blood cells; irregular shape. Includes phagocytes and lymphocytes. |
| Platelets | Small, irregular cell fragments (not complete cells); no nucleus. |
| Plasma | Yellowish liquid component; mostly water; contains dissolved substances. |
Haemoglobin
Haemoglobin is an iron-containing protein found inside red blood cells. It has a specific shape that allows it to bind reversibly with oxygen molecules. This binding is what gives blood its red colour.
Phagocytosis
Phagocytosis is the process by which certain white blood cells (phagocytes) engulf and digest pathogens (such as bacteria). The cell membrane extends around the pathogen to form a vacuole, which then fuses with lysosomes containing digestive enzymes.
Antibodies
Antibodies are Y-shaped proteins produced by lymphocytes. Each antibody has a specific shape that is complementary to a specific antigen (a marker on the surface of a pathogen). This allows antibodies to bind to and neutralize or mark pathogens for destruction.
Fibrinogen and Fibrin
Fibrinogen is a soluble protein dissolved in plasma. During clotting, it is converted into fibrin, which is an insoluble protein that forms long strands.
Functions of Blood Components
Red Blood Cells: Transport oxygen from the lungs to tissues. Haemoglobin binds to oxygen in the high-concentration environment of the lungs and releases it in the low-concentration environment of the tissues.
White Blood Cells: Defend against infection. Phagocytes engulf pathogens via phagocytosis. Lymphocytes produce antibodies that target specific antigens on pathogens.
Platelets: Initiate blood clotting to prevent blood loss and pathogen entry.
Plasma: Transports blood cells (suspension), ions, nutrients (e.g., glucose, amino acids), urea, hormones (e.g., adrenaline), and carbon dioxide. Note: Plasma is the medium that carries the cellular components.
⚠︎ Confusing Fibrinogen and Fibrin
Error: Students often state that fibrinogen forms the mesh or that fibrin is soluble.
Correct Understanding: Fibrinogen is the soluble protein in plasma. It is converted into insoluble fibrin strands which form the mesh. The key distinction is solubility: fibrinogen = soluble; fibrin = insoluble.
Correct Understanding: Fibrinogen is the soluble protein in plasma. It is converted into insoluble fibrin strands which form the mesh. The key distinction is solubility: fibrinogen = soluble; fibrin = insoluble.
⚠︎ Misidentifying White Blood Cell Types
Error: In diagrams, students often label any white blood cell as a 'lymphocyte' or fail to distinguish between phagocytes and lymphocytes.
Correct Understanding: Phagocytes typically have a multi-lobed nucleus (irregular shape). Lymphocytes typically have a large, round nucleus that takes up most of the cell, leaving very little cytoplasm. Both are white blood cells, but their structures differ.
Correct Understanding: Phagocytes typically have a multi-lobed nucleus (irregular shape). Lymphocytes typically have a large, round nucleus that takes up most of the cell, leaving very little cytoplasm. Both are white blood cells, but their structures differ.
Describing Blood Clotting
When asked to describe the process of clotting, you must mention the conversion of fibrinogen to fibrin.
Why this is accepted: Examiners look for the specific chemical change. Stating 'platelets form a plug' is incomplete because it misses the protein mechanism. The correct phrase is: 'Platelets trigger the conversion of soluble fibrinogen into insoluble fibrin, which forms a mesh to trap blood cells and seal the wound.' This directly addresses the syllabus requirement for the role of plasma proteins in clotting.
Why this is accepted: Examiners look for the specific chemical change. Stating 'platelets form a plug' is incomplete because it misses the protein mechanism. The correct phrase is: 'Platelets trigger the conversion of soluble fibrinogen into insoluble fibrin, which forms a mesh to trap blood cells and seal the wound.' This directly addresses the syllabus requirement for the role of plasma proteins in clotting.
Identifying Plasma Functions
When asked for the function of plasma, always include 'transport of blood cells' alongside dissolved substances.
Why this is accepted: Plasma is not just a solvent; it is the suspension medium. If you only list dissolved ions or hormones, you miss the fact that red and white blood cells are suspended in it. A complete answer is: 'Plasma transports blood cells, ions, nutrients, urea, hormones, and carbon dioxide.'
Why this is accepted: Plasma is not just a solvent; it is the suspension medium. If you only list dissolved ions or hormones, you miss the fact that red and white blood cells are suspended in it. A complete answer is: 'Plasma transports blood cells, ions, nutrients, urea, hormones, and carbon dioxide.'
Past Paper Style Questions
Q:
State the function of haemoglobin in red blood cells. [1]
A:
Transports / carries oxygen.
Q:
Identify the component of blood responsible for producing antibodies. [1]
A:
Lymphocytes (or White blood cells).
Q:
Describe how platelets help to prevent disease. [2]
A:
- They trigger clotting / convert fibrinogen to fibrin.
2. This prevents entry of pathogens / seals the wound.
Q:
Name two substances transported by plasma. [1]
A:
Any two from: blood cells, ions, nutrients, urea, hormones, carbon dioxide.
Q:
Explain why a low platelet count might be dangerous. [1]
A:
Blood would not clot properly / risk of excessive bleeding.