The four components of blood
Blood is a liquid with three solid components in it. Plasma is the pale yellow liquid carrying the blood cells plus dissolved nutrients, hormones, waste and carbon dioxide. Red blood cells carry oxygen. White blood cells are fewer but larger and defend against disease (fighting pathogens). Platelets are cell fragments that clot the blood at a wound. State each component AND its function, not just the names.
Haemoglobin: binding and releasing oxygen
Haemoglobin is the red pigment/protein inside every red blood cell, and it -- not the cell itself -- carries the oxygen. In the lungs, where oxygen concentration is high, haemoglobin binds with oxygen to form oxyhaemoglobin. At a working muscle, where oxygen is low, oxyhaemoglobin releases (dissociates) its oxygen. Examiners want this bind-and-release mechanism named precisely, not a vague "carries oxygen".
Red blood cell structure and carbon dioxide
A red blood cell's shape suits its job. It is a biconcave disc, giving a larger surface area for faster gas exchange, and has no nucleus, leaving more room for haemoglobin so it carries more oxygen. Carbon dioxide from respiring muscles returns to the lungs two ways: most dissolved in plasma, and some carried by haemoglobin. Link structure to function rather than just describing the shape, and remember haemoglobin's role in BOTH gases.
Drawn from real examiner reports.
Haemoglobin "carries" is too vague
Examiners want the mechanism, not a paraphrase. "Haemoglobin carries oxygen" or "holds oxygen" is not enough. The credited version names the chemistry: haemoglobin binds (combines) with oxygen in the lungs to form oxyhaemoglobin. The marks are in bind/combine (not "hold" or "carry") and oxyhaemoglobin -- so use them.
w22 P12 Q2a(ii)
Plasma does not carry the oxygen
A recurring slip is crediting plasma with carrying oxygen. Plasma transports dissolved substances -- nutrients, hormones, waste and carbon dioxide -- and the blood cells. It is the red blood cells, via haemoglobin, that carry oxygen. Keep the roles separate: plasma carries dissolved substances and cells; oxygen belongs to red blood cells and their haemoglobin.
Vague blood-cell functions
Blood-cell functions are often given vaguely -- "white blood cells fight things" -- not the precise wording. White blood cells defend against disease (fighting pathogens); platelets clot the blood at a wound to stop blood loss and pathogens entering. A generic answer for any component loses the mark even if the idea is right.
Haemoglobin is not a blood component
Asked to name the components of blood, some candidates list haemoglobin as a fifth. Haemoglobin is not a component -- it is the pigment/protein carried inside the red blood cells. The four components are plasma, red blood cells, white blood cells and platelets. Name haemoglobin when the question is about oxygen transport, not when it asks for blood's components.
RBC shape given without its effect
Describing a red blood cell as "a biconcave disc with no nucleus" earns fewer marks than linking each feature to its effect. The biconcave shape gives a larger surface area for gas exchange; no nucleus leaves more room for haemoglobin, so more oxygen is carried. On a "structure suits function" question, pair each feature with what it achieves.
Only plasma credited for carbon dioxide
On blood gas transport, candidates often credit only plasma with carrying carbon dioxide and forget haemoglobin. The syllabus gives haemoglobin a role in both gases: it carries most of the oxygen, and also some carbon dioxide back to the lungs (the larger share is dissolved in plasma). A full answer on carbon dioxide transport should not leave haemoglobin out.
Oxygen is not carried dissolved in plasma
Students often picture oxygen "dissolving" in the plasma. For oxygen this is wrong: it dissolves poorly in plasma -- nowhere near enough to meet demand. Almost all oxygen is instead chemically bound to haemoglobin inside red blood cells, as oxyhaemoglobin. That is why red blood cells exist as an oxygen-transport system that plasma alone could not provide.
Use the exact credited wording
For state/describe parts, use the exact credited wording: the component, then its precise function -- "haemoglobin binds with oxygen to form oxyhaemoglobin", not "blood carries oxygen". A vague synonym is not rewarded; use the precise terms from the first line.
Build the haemoglobin explain chain
For an "explain" question, build a chain: haemoglobin binds oxygen in the lungs to form oxyhaemoglobin; it is carried to the working muscles; there it releases the oxygen for aerobic respiration. Each linked step earns a mark a one-liner misses.
Apply to a named endurance activity
When the question names a performer, apply the mechanism. For an endurance event, the muscles need a steady oxygen supply throughout, so efficient transport delays fatigue. A general answer with no named activity is capped lower.
Link structure to function
When red blood cell structure is asked, pair each feature with its effect: biconcave disc = larger surface area for gas exchange; no nucleus = more room for haemoglobin, so more oxygen carried. Naming the feature alone scores less.
Blood is a liquid (plasma) with three types of solid component suspended in it.
