1. Composition of blood
Blood is a fluid connective tissue: an adult has about 5–6 litres.
Plasma (about 55%) is a pale yellow liquid: about 90% water, with plasma proteins (albumin, globulins including antibodies, fibrinogen and prothrombin for clotting), digested food (glucose, amino acids), salts, urea, hormones and dissolved carbon dioxide. It transports these substances and distributes heat.
Formed elements (about 45%):
| Red blood cells (erythrocytes) | White blood cells (leucocytes) | Platelets (thrombocytes) | |
|---|---|---|---|
| Shape | Biconcave disc | Irregular, amoeboid | Small fragments |
| Nucleus | Absent in mammals | Present | Absent |
| Number per cubic mm | About 5 million | 5000–10 000 | 2.5–4 lakh |
| Formed in | Red bone marrow | Bone marrow; lymph nodes | Bone marrow |
| Life | About 120 days | A few days | About a week |
| Function | Carry oxygen (as oxyhaemoglobin) and some CO₂ | Defence | Start clotting |
Kinds of white cells
- Granulocytes (lobed nucleus, granular cytoplasm): neutrophils (3–5 lobes; most numerous; engulf bacteria by phagocytosis), eosinophils (2 lobes; rise in allergies and worm infections), basophils (S-shaped nucleus; release histamine and heparin).
- Agranulocytes: lymphocytes (large round nucleus; make antibodies) and monocytes (largest; kidney-shaped nucleus; become scavenger cells).
- White cells squeeze through capillary walls by diapedesis to reach infected tissue.
Key terms
- Plasma
The liquid part of blood, about 55% of its volume.
- Phagocytosis
Engulfing and digesting of germs by white cells.
- Diapedesis
Squeezing of white cells through capillary walls.
- Antibodies
Proteins made by lymphocytes that destroy germs or their toxins.
2. Why mammalian red blood cells are efficient
| Feature | Advantage |
|---|---|
| No nucleus | More space for haemoglobin, so more oxygen per cell |
| No mitochondria | Respires anaerobically, so it does not use up the oxygen it carries |
| Biconcave disc | Large surface area for gas exchange; thin centre gives a short diffusion distance |
| Small and flexible | Squeezes through narrow capillaries |
Because they have no nucleus, red cells cannot divide or repair themselves; they live about 120 days and are broken down in the spleen and liver. More red cells are made when the body needs more oxygen, for example at high altitude.
Key terms
- Haemoglobin
Iron-containing red pigment that combines with oxygen.
- Oxyhaemoglobin
Haemoglobin combined with oxygen.
3. Blood coagulation
When a vessel is cut, blood clots and seals the wound.
- Damaged tissue and platelets release thromboplastin (thrombokinase).
- Thromboplastin, with calcium ions, converts prothrombin into thrombin.
- Thrombin converts soluble fibrinogen into insoluble fibrin threads.
- The fibrin mesh traps blood cells, forming a clot. As the clot shrinks, serum (plasma without fibrinogen) oozes out.
- Vitamin K is needed by the liver to make prothrombin.
- Blood does not clot inside healthy vessels: the smooth lining does not release thromboplastin, and heparin prevents clotting.
- Blood banks add sodium citrate, which removes calcium ions, to stop stored blood clotting.
Key terms
- Thromboplastin
Substance from platelets and damaged tissue that starts clotting.
- Fibrin
Insoluble protein threads that form the clot.
- Serum
Plasma without fibrinogen and other clotting factors.
- Heparin
A natural substance that prevents clotting inside vessels.
4. Tissue fluid, lymph, spleen and tonsils
Tissue fluid. At the arterial end of a capillary, pressure forces plasma (without red cells and most proteins) out into the spaces between cells. This tissue fluid supplies food and oxygen and collects wastes. Most of it re-enters the capillary at the venous end.
Lymph. The extra tissue fluid drains into lymph capillaries (closed at one end) and is now called lymph: colourless, with few proteins, many lymphocytes and no red cells.
Lymphatic system: lymph capillaries → lymph vessels (with valves, so flow is one way, towards the heart) → lymph nodes → main lymph ducts (the thoracic duct) → large veins near the heart.
Functions: returns tissue fluid to the blood (blockage causes swelling); lacteals absorb digested fats; lymph nodes filter germs and make lymphocytes.
- Spleen — the largest lymphoid organ: destroys worn-out red cells (the graveyard of red cells), makes lymphocytes, filters blood and stores blood.
- Tonsils — lymphoid tissue at the back of the throat: trap and destroy germs entering through the mouth and nose.
Key terms
- Tissue fluid
Fluid filtered from plasma that bathes the body cells.
- Lymph
Tissue fluid that has entered lymph vessels.
- Lymph node
Swelling on a lymph vessel that filters germs and makes lymphocytes.
- Lacteal
Lymph vessel in a villus that absorbs fats.
5. ABO blood groups and the Rh factor
Red cells may carry antigens A and B; plasma carries antibodies against the antigens a person lacks (discovered by Karl Landsteiner).
| Group | Antigen on red cells | Antibodies in plasma | Can receive from |
|---|---|---|---|
| A | A | anti-B | A, O |
| B | B | anti-A | B, O |
| AB | A and B | none | A, B, AB, O |
| O | none | anti-A and anti-B | O |
- Agglutination (clumping) happens when donor antigens meet the matching antibody in the recipient's plasma.
- O is the universal donor (no antigens); AB is the universal recipient (no antibodies).
Rh factor. Another antigen on red cells: Rh-positive (most people) or Rh-negative. An Rh-negative person forms anti-Rh antibodies after receiving Rh-positive blood.
Rh incompatibility in pregnancy. An Rh-negative mother carrying an Rh-positive baby may form anti-Rh antibodies when the baby's blood mixes with hers at birth. In a later Rh-positive pregnancy these antibodies cross the placenta and destroy the baby's red cells (anaemia, jaundice, possible death). An anti-Rh injection given to the mother soon after the first delivery prevents this.
Key terms
- Antigen
A substance on red cells that can trigger antibody formation.
- Agglutination
Clumping of red cells by matching antibodies.
- Universal donor
Group O: red cells with no A or B antigen.
- Rh factor
An antigen on the red cells of Rh-positive people.
Exam tips
Common mistakes
- Saying white cells have no nucleus: it is mature red cells that lack one.
- Writing the clotting steps out of order, or forgetting calcium and vitamin K.
- Putting antibodies on the red cells: antigens are on the cells, antibodies in the plasma.
- Calling AB the universal donor: O is the donor, AB the recipient.
- Saying the first Rh-positive baby is at risk: it is usually a later one.
How marks are usually awarded
- 1 mark: a term (agglutination, diapedesis, serum), an odd one out with its category, a blood group.
- 2 marks: a feature with its advantage, two differences, a transfusion decision with a reason.
- 3 marks: the steps of clotting, the Rh problem and its prevention, or a labelled drawing of blood cells.