Blood

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 2.59 — The composition of blood: red cells, white cells, platelets and plasma
• 2.60 — The role of plasma in transporting carbon dioxide, digested food, urea, hormones and heat
• 2.61 — Adaptations of red blood cells for oxygen transport

Specification section 2(h) Transport — also taught in Why Transport Systems Exist, and the Two Plant Vessels, Water Uptake and Transpiration, Immunity and Clotting, The Heart, and What Goes Wrong With It.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

Blood looks like a red liquid and is nothing of the sort. Spin a sample in a centrifuge and it separates: rather more than half of it is a pale straw-coloured fluid, and the rest is a packed red sediment of cells. The fluid is plasma and it carries almost everything dissolved; the cells do the jobs a dissolved substance cannot. Four components, four quite different tasks.

What each component is for

1. PLASMA — the straw-coloured liquid, mostly water. Everything that can dissolve travels in it: carbon dioxide, digested food such as glucose and amino acids, urea, hormones, and the plasma proteins. It also carries HEAT, distributing warmth from the liver and muscles around the body.
2. RED BLOOD CELLS — carry oxygen, and nothing else. They are by far the most numerous.
3. WHITE BLOOD CELLS — defend against pathogens, some by engulfing them and some by producing antibodies. Far fewer in number, and much larger.
4. PLATELETS — cell fragments rather than whole cells, involved in clotting blood at a wound.

Now the interesting one. A red blood cell is a specialist so extreme that it has thrown away parts most cells cannot manage without, and every one of those sacrifices buys something specific for carrying oxygen.

Three adaptations, and what each one buys

1. IT CONTAINS HAEMOGLOBIN, an iron-containing protein that binds oxygen where the concentration is high, in the lungs, and releases it where the concentration is low, in the respiring tissues. Without it, blood could carry only the small amount of oxygen that dissolves in plasma — nowhere near enough.
2. IT HAS NO NUCLEUS. Removing the nucleus frees up space, so the cell can be packed with more haemoglobin and carry more oxygen. The cost is severe: with no nucleus it cannot make new proteins or divide, so it cannot repair itself and lives only about four months before being destroyed and replaced.
3. IT IS A BICONCAVE DISC — round and dimpled on both sides rather than spherical. This gives it a larger surface area for its volume than a sphere would have, so oxygen diffuses in and out across more surface and over a shorter distance to the centre. The flexible shape also lets it squeeze single-file through capillaries narrower than the cell itself.

It is worth noticing what the plasma is doing while all this happens, because it is easy to think of blood as being 'for' oxygen. Oxygen is the one thing plasma cannot handle, which is exactly why a specialised cell exists to carry it. Everything else — the glucose from your last meal, the urea your liver made from surplus amino acids, the hormones controlling your growth, the carbon dioxide from every respiring cell, and the heat itself — is simply dissolved and carried along in the fluid.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Plasma

The pale yellow liquid part of blood, mostly water, in which the blood cells are suspended and in which dissolved substances and heat are transported.

Learn this definition · Haemoglobin

The red, iron-containing protein inside red blood cells that binds oxygen where its concentration is high and releases it where its concentration is low.

The three cell components — do not confuse them

What plasma transports — learn all five

• Carbon dioxide, from respiring tissues to the lungs
• Digested food, such as glucose and amino acids, from the small intestine to the tissues
• Urea, from the liver to the kidneys
• Hormones, from glands to their target organs
• Heat, distributed from the liver and active muscles around the body

The red blood cell's three adaptations, each with what it buys

Contains haemoglobin

binds oxygen in the lungs and releases it in respiring tissues, allowing far more oxygen to be carried than could dissolve in plasma

No nucleus

leaves more room for haemoglobin, so each cell carries more oxygen; the cost is a lifespan of about four months and no ability to divide

Biconcave disc shape

a larger surface area to volume ratio than a sphere, so oxygen diffuses in and out faster; also flexible enough to pass through narrow capillaries

Model answer [4 marks]

Explain how a red blood cell is adapted to transport oxygen efficiently. [4]

It contains haemoglobin, which binds oxygen where the concentration is high in the lungs and releases it where the concentration is low in the tissues. It has no nucleus, which leaves more space for haemoglobin so each cell carries more oxygen. It is a biconcave disc, which gives a large surface area to volume ratio so oxygen diffuses in and out quickly. That shape is also flexible enough to squeeze through narrow capillaries.

Model answer [3 marks]

State three substances transported by plasma and where each is travelling from and to. [3]

Carbon dioxide is carried from the respiring tissues to the lungs. Digested food such as glucose is carried from the small intestine to the tissues. Urea is carried from the liver to the kidneys.

Mark-losing trap. Red blood cells have no nucleus. They are still cells, and they are not 'dead' — but they cannot divide or repair themselves.

Mark-losing trap. Plasma carries carbon dioxide; haemoglobin carries oxygen. Swapping the two gases between them is the classic error here.

Mark-losing trap. Biconcave means a larger surface area to VOLUME ratio, not simply a larger surface. Say the ratio and say why it helps.

Mark-losing trap. Platelets are fragments of cells, not whole cells, and they clot blood — they do not fight infection.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · Identify [1 mark] — Which component of the blood transports oxygen?
2. Grade 7 · Explain [2 marks] — Red blood cells have no nucleus. Explain the advantage this gives.
3. Grade 8 · Explain [4 marks] — Explain how a red blood cell is adapted to transport oxygen efficiently. Select every statement that belongs in a full-mark explanation.
4. Grade 9 · Calculate [4 marks] — A person has 5.2 dm³ of blood. 45% of that volume is blood cells, and 99% of those cells are red blood cells. Calculate the volume of red blood cells, in cm³, to three significant figures.
5. 9+ · Analyse [5 marks] — At high altitude the air contains less oxygen. Someone who moves there feels breathless and exhausted for the first week, but after a month feels almost normal, and a blood test then shows a much higher number of red blood cells per mm³ than before. Select every statement that belongs in a full-mark explanation.

The people behind this science

Three ways into the same idea — the one who first saw a red blood cell, and the one who identified the gas the blood exists to carry, and the one who proved the blood comes back. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Antonie van Leeuwenhoek — the one who first saw a red blood cell

In 1674 Leeuwenhoek looked at blood through lenses he had ground himself and found that it is not a red liquid at all but a clear fluid crowded with countless small red bodies. He estimated their size with remarkable accuracy and watched them pass single file through vessels finer than a hair, bending to get through — the flexibility this lesson explains. What he could never say was what any of them was for; he supposed the red bodies were simply a colouring matter.

• “What did you actually see the first time you examined blood?”
• “How did you work out how big the little red bodies were?”
• “What happened when you watched them enter the very finest vessels?”
• “What did you think the red bodies were doing?”
• “Why did the clear liquid interest you so much less than the bodies in it?”

Antoine Lavoisier — the one who identified the gas the blood exists to carry

Lavoisier established that only one component of air is consumed by a living body, and that blood changes colour as it takes it up — dark blood becoming bright as it passes through the lungs. He could not know about haemoglobin, but he identified what the red cells of this lesson are built entirely around, and he did it by chemistry rather than by dissection.

• “Why does blood change colour as it passes through the lungs?”
• “How did you establish which part of the air the body actually consumes?”
• “What did you think was carrying that substance around inside the body?”
• “Could you tell how much of it a body used in a day?”

William Harvey — the one who proved the blood comes back

Before Harvey, blood was thought to be manufactured in the liver and used up by the body. If that were true, blood could not be a transport medium at all — it would be a supply. He proved by measuring the heart's output that the same blood must return and be reused, which is precisely what allows plasma to collect carbon dioxide here and deliver it there, and to carry heat from a warm organ to a cold one.

• “How did counting heartbeats prove that the blood must return?”
• “What did people believe happened to blood before you, and why was it wrong?”
• “What did you think joined the arteries to the veins, since you could not see it?”
• “Why does it matter so much whether blood is used up or reused?”

Carry on through the course

• Previous lesson: Water Uptake and Transpiration
• Next lesson: Immunity and Clotting
• Levels of Organisation — From organelle to organism — the five-step ladder that every later topic in biology is built on.
• Comparing Cells, and Stem Cells — What plant and animal cells share and where they differ — then why a cell specialising is one of the most powerful ideas in biology.
• Carbohydrates, Proteins and Lipids — The three big food molecules — what they are made of, what they are built from, and how to test for them.
• Enzymes: Catalysts With a Shape — Why a protein with a hole in it speeds up a reaction, and why heat destroys that ability permanently.
• Enzymes and pH — Why acid ruins one enzyme and is the only condition another will work in — and how to find an enzyme's best pH by experiment.
• All of Biology · Biology key terms · required practicals · Incandio Science · The 4BI1 specification