CDS Biology · Human Physiology
Blood and Breathing
What blood is made of, how haemoglobin carries the gases of breathing, why high altitude makes you breathless, what lymph is, the ABO blood groups, and how a vaccine prepares the body.
Why this matters
Seven CDS questions from 2016 (II) to 2026 (II). Five are about the blood itself: haemoglobin, red cells, platelets, lymph and altitude sickness. One asks the four ABO groups and one is about vaccination.
Concept 1 of 2: Blood, haemoglobin and lymph
Blood is a liquid, the plasma, carrying three kinds of cell. Red cells are bags of haemoglobin that pick up oxygen in the lungs and drop it in the tissues, and haemoglobin brings some carbon dioxide back. White cells fight germs, and platelets start a clot at a cut. Some plasma leaks out of the thin capillaries into the spaces between cells; this fluid is collected again as lymph.
Definition
The parts of blood:
- Plasma: the liquid part (mostly water) carrying digested food, hormones, salts and much of the carbon dioxide.
- Red blood cells (RBCs): full of haemoglobin, an iron-containing protein. Haemoglobin carries oxygen and also some carbon dioxide. A mature mammalian RBC has no nucleus and no ribosomes, so it cannot make protein or divide, but it can squeeze through narrow capillaries.
- White blood cells (WBCs): fight infection.
- Platelets: release the clotting factors that start a blood clot.
- Lymph: tissue fluid that has leaked out of the capillaries, collected in lymph vessels. It has white cells but no red cells, carries absorbed fat from the intestine, and is moved by body muscles and valves, not by the heart.
Breathing:
- In the lungs, oxygen passes from tiny air sacs, the alveoli, into the blood, and carbon dioxide passes out.
- At high altitude the air is thinner. It still has about 21 per cent oxygen, but the partial pressure of oxygen is low, so haemoglobin picks up less of it: this causes altitude sickness.
| Part or situation | What to remember |
|---|---|
| Haemoglobin | Carries both oxygen and carbon dioxideQ CDS 2019 (I): haemoglobin carries both gases, not oxygen alone. |
| Mature red blood cell | No nucleus: cannot make protein or divideQ CDS 2026 (II): protein synthesis and cell division are what an erythrocyte cannot do. |
| Platelets | Release clotting factorsQ CDS 2024 (I): clotting factors come from platelets, not RBCs, eosinophils or monocytes. |
| Lymph | Fluid leaked from capillaries; no red cellsQ CDS 2018 (I): lymph forms from leakage of blood through capillaries. |
| High altitude | Low partial pressure of oxygenQ CDS 2016 (II): altitude sickness is caused by low partial pressure of oxygen. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2019 · CDS (I) 2019 — General Knowledge · Q87Easy
Example 1 · Human Physiology · Blood and Breathing
Which one of the following statements regarding haemoglobin is correct?
The percentage of oxygen does not fall
High up, air still has about 21 per cent oxygen. What falls is the pressure, and so the partial pressure of oxygen. 'High partial pressure' or 'low haemoglobin' are the wrong causes.
Lymph is not pumped by the heart
Lymph has no red cells, is moved by muscle squeezing and valves, and carries fat and white cells, not hormones only.
Concept 2 of 2: ABO blood groups and vaccination
Red cells carry markers called antigens on their surface. A person may have the A marker, the B marker, both, or neither, and that gives the four blood groups. The plasma holds antibodies against whichever marker the person lacks, which is why the wrong blood in a transfusion makes the cells clump. A vaccine uses the same defence system: it shows the body a harmless form of a germ so the body is ready for the real one.
Definition
ABO blood groups (Karl Landsteiner):
- Four groups: A, B, AB and O, named by the antigen on the red cells.
- O has no A or B antigen: the universal donor. AB has no anti-A or anti-B antibody: the universal recipient.
- The Rh factor is a separate antigen: Rh positive or Rh negative.
Vaccination:
- A vaccine contains killed or weakened germs, or parts of them.
- The immune system responds by making antibodies and memory cells.
- When the real germ enters later, the body recognises it and responds much faster and more strongly. Edward Jenner made the first vaccine, against smallpox.
| Blood group | Antigen on red cells | Antibody in plasma | Can receive from |
|---|---|---|---|
| A | A | Anti-B | A and OQ CDS 2025 (II): the four ABO types are A, B, AB and O. |
| B | B | Anti-A | B and O |
| AB | A and B | Neither | All groups (universal recipient) |
| O | Neither | Anti-A and anti-B | O only (universal donor) |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2025 · CDS (II) 2025 — General Knowledge · Q43Easy
Example 2 · Human Physiology · Blood and Breathing
What are the four human blood types in ABO blood group typing ?
AO and BO are not blood groups
AO and BO describe the genes a person carries. The four blood groups are A, B, AB and O.
Donor O, recipient AB
Group O cells have no A or B antigen, so they can be given to anyone. Group AB plasma has no anti-A or anti-B, so it can receive any group. Swapping the two is the usual mistake.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (2)
Blood, haemoglobin and lymph5 rows
| Part or situation | What to remember |
|---|---|
| Haemoglobin | Carries both oxygen and carbon dioxideQ CDS 2019 (I): haemoglobin carries both gases, not oxygen alone. |
| Mature red blood cell | No nucleus: cannot make protein or divideQ CDS 2026 (II): protein synthesis and cell division are what an erythrocyte cannot do. |
| Platelets | Release clotting factorsQ CDS 2024 (I): clotting factors come from platelets, not RBCs, eosinophils or monocytes. |
| Lymph | Fluid leaked from capillaries; no red cellsQ CDS 2018 (I): lymph forms from leakage of blood through capillaries. |
| High altitude | Low partial pressure of oxygenQ CDS 2016 (II): altitude sickness is caused by low partial pressure of oxygen. |
ABO blood groups and vaccination4 rows
| Blood group | Antigen on red cells | Antibody in plasma | Can receive from |
|---|---|---|---|
| A | A | Anti-B | A and OQ CDS 2025 (II): the four ABO types are A, B, AB and O. |
| B | B | Anti-A | B and O |
| AB | A and B | Neither | All groups (universal recipient) |
| O | Neither | Anti-A and anti-B | O only (universal donor) |
Watch out for (4)
- The percentage of oxygen does not fall→ Blood, haemoglobin and lymph
- Lymph is not pumped by the heart→ Blood, haemoglobin and lymph
- AO and BO are not blood groups→ ABO blood groups and vaccination
- Donor O, recipient AB→ ABO blood groups and vaccination
Test yourself on Human Physiology
15 past CDS questions from this chapter, timed at 15 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.