One Blood Group Gives to Everyone and Takes From Almost Nobody
Learn what blood is made of and what each component does, follow the chain of reactions that seals a cut, understand the ABO groups and the Rh factor, and compare arteries, veins and capillaries.
Why can blood group O be given to anyone?
Because its red cells carry no markers for another person's blood to object to.
Red blood cells carry chemical markers on their surface, and a person's plasma carries substances that attack any marker it does not recognise. Group O red cells have neither the A nor the B marker, so no recipient's blood finds anything to attack — which makes group O the universal donor.
The same logic run backwards gives the opposite case. Group AB plasma has no attacking substances at all, so it accepts every kind of red cell — making AB the universal recipient. This page covers the first part of the ICSE Class 8 Biology chapter on the circulatory system: what blood is made of, how it clots, how it is grouped, and the vessels that carry it.
Red blood cells carry chemical markers on their surface, and a person's plasma carries substances that attack any marker it does not recognise. Group O red cells have neither the A nor the B marker, so no recipient's blood finds anything to attack — which makes group O the universal donor.
The same logic run backwards gives the opposite case. Group AB plasma has no attacking substances at all, so it accepts every kind of red cell — making AB the universal recipient. This page covers the first part of the ICSE Class 8 Biology chapter on the circulatory system: what blood is made of, how it clots, how it is grouped, and the vessels that carry it.
What is blood made of, and what does each part do?
Blood is a fluid connective tissue with two portions: a liquid part, plasma, forming about 55 per cent of it, and the blood cells, forming about 45 per cent.
Plasma is a pale straw-coloured liquid, about 90 per cent water, with a great deal dissolved in it — proteins, glucose, amino acids, minerals, hormones, vitamins, and the wastes urea and carbon dioxide.
Its function is transport. Plasma is the medium in which digested food, hormones, wastes and dissolved gases travel, and by carrying warmth from active organs to cooler parts it also helps regulate body temperature.
Red blood cells (erythrocytes) are small biconcave discs — thinner in the middle than at the edges. A mature mammalian red cell has no nucleus, which leaves more room for its contents. It is packed with haemoglobin, a red pigment containing iron.
Its function is to carry oxygen. Haemoglobin combines with oxygen in the lungs to form oxyhaemoglobin and releases it in the tissues, and it carries some carbon dioxide back. Red cells are the most numerous blood cells, are made in the bone marrow, live about 120 days, and are destroyed in the liver and spleen.
The biconcave shape is not a detail: it gives a larger surface area for the same volume, so oxygen can be loaded and unloaded faster.
White blood cells (leucocytes) are larger than red cells and far fewer. Each has a nucleus, and many can change shape and squeeze out through capillary walls into the tissues.
Their function is defence. Some engulf and digest germs — a process called phagocytosis — and others produce antibodies against them. They are sometimes called the soldiers of the body, and their number rises during an infection, which is why a blood test can reveal one.
Platelets (thrombocytes) are the smallest, with no nucleus — they are cell fragments rather than whole cells.
Their function is blood clotting, which is the next section.
The three cells compared in one line. Red cells carry, white cells defend, platelets seal. Everything else in this chapter follows from those three jobs.
A boundary case that explains a common illness. Too little iron in the diet means too little haemoglobin, so the blood carries less oxygen and the person tires easily. That is anaemia, and it is a shortage of a dietary mineral rather than a disease of the blood itself — which is exactly why iron-rich food was stressed in the chapter on adolescence.
Plasma is a pale straw-coloured liquid, about 90 per cent water, with a great deal dissolved in it — proteins, glucose, amino acids, minerals, hormones, vitamins, and the wastes urea and carbon dioxide.
Its function is transport. Plasma is the medium in which digested food, hormones, wastes and dissolved gases travel, and by carrying warmth from active organs to cooler parts it also helps regulate body temperature.
Red blood cells (erythrocytes) are small biconcave discs — thinner in the middle than at the edges. A mature mammalian red cell has no nucleus, which leaves more room for its contents. It is packed with haemoglobin, a red pigment containing iron.
Its function is to carry oxygen. Haemoglobin combines with oxygen in the lungs to form oxyhaemoglobin and releases it in the tissues, and it carries some carbon dioxide back. Red cells are the most numerous blood cells, are made in the bone marrow, live about 120 days, and are destroyed in the liver and spleen.
The biconcave shape is not a detail: it gives a larger surface area for the same volume, so oxygen can be loaded and unloaded faster.
White blood cells (leucocytes) are larger than red cells and far fewer. Each has a nucleus, and many can change shape and squeeze out through capillary walls into the tissues.
Their function is defence. Some engulf and digest germs — a process called phagocytosis — and others produce antibodies against them. They are sometimes called the soldiers of the body, and their number rises during an infection, which is why a blood test can reveal one.
Platelets (thrombocytes) are the smallest, with no nucleus — they are cell fragments rather than whole cells.
Their function is blood clotting, which is the next section.
The three cells compared in one line. Red cells carry, white cells defend, platelets seal. Everything else in this chapter follows from those three jobs.
A boundary case that explains a common illness. Too little iron in the diet means too little haemoglobin, so the blood carries less oxygen and the person tires easily. That is anaemia, and it is a shortage of a dietary mineral rather than a disease of the blood itself — which is exactly why iron-rich food was stressed in the chapter on adolescence.
How does blood clot, and why does it matter?
By converting a soluble protein in the plasma into an insoluble mesh that traps blood cells and plugs the wound.
The sequence at a cut:
- Platelets collect at the injured site and disintegrate, releasing an enzyme called thromboplastin.
- Thromboplastin, helped by calcium ions and vitamin K, converts the plasma protein prothrombin into thrombin.
- Thrombin converts the soluble plasma protein fibrinogen into insoluble fibrin.
- Fibrin forms a network of fine threads across the wound, trapping blood cells in its mesh.
- The mass hardens into a clot, which dries and becomes a scab.
The key change is soluble fibrinogen becoming insoluble fibrin. Nothing is added from outside — the clot is built from a protein that was already dissolved in the plasma, waiting.
Why clotting matters:
- It prevents excessive loss of blood from a wound, which could otherwise be fatal.
- It keeps germs out of the wound, acting as a temporary seal.
- It lets the tissue heal underneath while protected, and the scab falls off once new skin has formed.
Two substances the process cannot do without. Calcium and vitamin K both appear in the chain, which is why a severe deficiency of either slows clotting. Vitamin K is made by bacteria in the intestine as well as being present in green leafy vegetables.
A disorder that proves the mechanism. In haemophilia one of the substances in this chain is missing or faulty, so the blood clots very slowly or not properly. A minor injury that would seal itself in minutes in another person can bleed for a long time. The disorder is inherited, and it shows that clotting depends on a specific chain of reactions rather than on the blood simply drying.
And why blood does not clot inside a healthy vessel. The chain is set off by injury, and an intact vessel lining does not trigger it. A clot forming inside an undamaged blood vessel is dangerous — it can block the vessel and cut off the supply to whatever lies beyond, which is what happens in a heart attack or a stroke.
The sequence at a cut:
- Platelets collect at the injured site and disintegrate, releasing an enzyme called thromboplastin.
- Thromboplastin, helped by calcium ions and vitamin K, converts the plasma protein prothrombin into thrombin.
- Thrombin converts the soluble plasma protein fibrinogen into insoluble fibrin.
- Fibrin forms a network of fine threads across the wound, trapping blood cells in its mesh.
- The mass hardens into a clot, which dries and becomes a scab.
The key change is soluble fibrinogen becoming insoluble fibrin. Nothing is added from outside — the clot is built from a protein that was already dissolved in the plasma, waiting.
Why clotting matters:
- It prevents excessive loss of blood from a wound, which could otherwise be fatal.
- It keeps germs out of the wound, acting as a temporary seal.
- It lets the tissue heal underneath while protected, and the scab falls off once new skin has formed.
Two substances the process cannot do without. Calcium and vitamin K both appear in the chain, which is why a severe deficiency of either slows clotting. Vitamin K is made by bacteria in the intestine as well as being present in green leafy vegetables.
A disorder that proves the mechanism. In haemophilia one of the substances in this chain is missing or faulty, so the blood clots very slowly or not properly. A minor injury that would seal itself in minutes in another person can bleed for a long time. The disorder is inherited, and it shows that clotting depends on a specific chain of reactions rather than on the blood simply drying.
And why blood does not clot inside a healthy vessel. The chain is set off by injury, and an intact vessel lining does not trigger it. A clot forming inside an undamaged blood vessel is dangerous — it can block the vessel and cut off the supply to whatever lies beyond, which is what happens in a heart attack or a stroke.
What are the ABO blood groups and the Rh factor?
Blood is grouped by the markers present on the red blood cells, called antigens, and by the antibodies present in the plasma.
The four ABO groups:
- Group A — antigen A on the red cells
- Group B — antigen B on the red cells
- Group AB — both antigens A and B
- Group O — neither antigen
Each person's plasma carries antibodies against the antigens they lack. So group A plasma attacks B cells, group B plasma attacks A cells, group O plasma attacks both, and group AB plasma attacks neither.
The two special cases:
- Group O is the universal donor. Its red cells carry no antigens, so no recipient's antibodies find a target. But a group O person can receive only group O blood, because their plasma attacks both A and B cells.
- Group AB is the universal recipient. Its plasma carries no antibodies, so it accepts red cells of any group. But AB blood can be given only to another AB person, since its cells carry both antigens.
Why matching matters so much. Given the wrong group, the recipient's antibodies make the donated red cells clump together — a reaction called agglutination. The clumps block small blood vessels and the cells are destroyed, which can be fatal. This is why blood is always tested and cross-matched before a transfusion, and why a donor's group is recorded.
The Rh factor is a separate antigen, present on the red cells of most people.
- A person with it is Rh positive
- A person without it is Rh negative
An Rh-negative person given Rh-positive blood begins producing antibodies against the Rh antigen. The first transfusion may pass without trouble, but a later one causes a serious reaction — so an Rh-negative person must receive Rh-negative blood.
Where it matters in pregnancy. If an Rh-negative mother carries an Rh-positive foetus, she may develop Rh antibodies, which can affect a later Rh-positive pregnancy. It is a known and manageable situation, which is why a mother's Rh status is checked routinely.
So a complete blood group has two parts — the ABO group and the Rh status — which is why it is written as a letter and a sign together, such as A positive or O negative. Giving only the letter is an incomplete answer.
The four ABO groups:
- Group A — antigen A on the red cells
- Group B — antigen B on the red cells
- Group AB — both antigens A and B
- Group O — neither antigen
Each person's plasma carries antibodies against the antigens they lack. So group A plasma attacks B cells, group B plasma attacks A cells, group O plasma attacks both, and group AB plasma attacks neither.
The two special cases:
- Group O is the universal donor. Its red cells carry no antigens, so no recipient's antibodies find a target. But a group O person can receive only group O blood, because their plasma attacks both A and B cells.
- Group AB is the universal recipient. Its plasma carries no antibodies, so it accepts red cells of any group. But AB blood can be given only to another AB person, since its cells carry both antigens.
Why matching matters so much. Given the wrong group, the recipient's antibodies make the donated red cells clump together — a reaction called agglutination. The clumps block small blood vessels and the cells are destroyed, which can be fatal. This is why blood is always tested and cross-matched before a transfusion, and why a donor's group is recorded.
The Rh factor is a separate antigen, present on the red cells of most people.
- A person with it is Rh positive
- A person without it is Rh negative
An Rh-negative person given Rh-positive blood begins producing antibodies against the Rh antigen. The first transfusion may pass without trouble, but a later one causes a serious reaction — so an Rh-negative person must receive Rh-negative blood.
Where it matters in pregnancy. If an Rh-negative mother carries an Rh-positive foetus, she may develop Rh antibodies, which can affect a later Rh-positive pregnancy. It is a known and manageable situation, which is why a mother's Rh status is checked routinely.
So a complete blood group has two parts — the ABO group and the Rh status — which is why it is written as a letter and a sign together, such as A positive or O negative. Giving only the letter is an incomplete answer.
How do arteries, veins and capillaries differ?
By the direction they carry blood, the thickness of their walls, the pressure inside them, and whether they have valves.
Arteries carry blood away from the heart.
- Walls: thick, muscular and elastic
- Lumen (the internal space): narrow
- Pressure: high, and the blood flows in spurts with each heartbeat
- Valves: absent, except at the base of the aorta and pulmonary artery
- Position: deep-seated, lying well below the skin
- Blood carried: oxygenated — with one exception
The thick elastic wall is needed because the blood arrives under high pressure; elasticity lets the artery stretch with each surge and recoil, smoothing the flow.
Veins carry blood towards the heart.
- Walls: thin, with much less muscle
- Lumen: wide
- Pressure: low, with a steady, slow flow
- Valves: present throughout, to prevent backflow
- Position: superficial, often visible under the skin
- Blood carried: deoxygenated — with one exception
The valves matter because the pressure is too low to push blood upwards from the legs on its own. Movement of the surrounding muscles squeezes the veins, and the valves ensure the blood squeezed along can only go one way — towards the heart.
Capillaries connect the smallest arteries to the smallest veins.
- Walls: just one cell thick
- Lumen: extremely narrow, barely wider than a red blood cell
- Pressure: lowest, with a very slow flow
- Function: this is where the exchange of materials happens — oxygen and nutrients pass out to the tissues, and carbon dioxide and wastes pass in
Every feature serves that exchange. A one-cell-thick wall is short enough for substances to diffuse across, and a slow flow gives them time to do it. The arteries and veins are only plumbing; the capillaries are where the blood actually does its job.
The two exceptions that are always asked. The pulmonary artery carries deoxygenated blood from the heart to the lungs, and the pulmonary vein carries oxygenated blood from the lungs back to the heart.
This shows the definitions rest on direction, not on the kind of blood. An artery is an artery because it leads away from the heart, whatever it happens to be carrying — and answering arteries carry oxygenated blood without the exception is the commonest error in this chapter.
Arteries carry blood away from the heart.
- Walls: thick, muscular and elastic
- Lumen (the internal space): narrow
- Pressure: high, and the blood flows in spurts with each heartbeat
- Valves: absent, except at the base of the aorta and pulmonary artery
- Position: deep-seated, lying well below the skin
- Blood carried: oxygenated — with one exception
The thick elastic wall is needed because the blood arrives under high pressure; elasticity lets the artery stretch with each surge and recoil, smoothing the flow.
Veins carry blood towards the heart.
- Walls: thin, with much less muscle
- Lumen: wide
- Pressure: low, with a steady, slow flow
- Valves: present throughout, to prevent backflow
- Position: superficial, often visible under the skin
- Blood carried: deoxygenated — with one exception
The valves matter because the pressure is too low to push blood upwards from the legs on its own. Movement of the surrounding muscles squeezes the veins, and the valves ensure the blood squeezed along can only go one way — towards the heart.
Capillaries connect the smallest arteries to the smallest veins.
- Walls: just one cell thick
- Lumen: extremely narrow, barely wider than a red blood cell
- Pressure: lowest, with a very slow flow
- Function: this is where the exchange of materials happens — oxygen and nutrients pass out to the tissues, and carbon dioxide and wastes pass in
Every feature serves that exchange. A one-cell-thick wall is short enough for substances to diffuse across, and a slow flow gives them time to do it. The arteries and veins are only plumbing; the capillaries are where the blood actually does its job.
The two exceptions that are always asked. The pulmonary artery carries deoxygenated blood from the heart to the lungs, and the pulmonary vein carries oxygenated blood from the lungs back to the heart.
This shows the definitions rest on direction, not on the kind of blood. An artery is an artery because it leads away from the heart, whatever it happens to be carrying — and answering arteries carry oxygenated blood without the exception is the commonest error in this chapter.
Exam tip
Exam tip: define arteries by direction, not by oxygen
Define an artery as a vessel carrying blood away from the heart and a vein as one carrying blood towards it. Then add the exceptions: the pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood.
Give the function with each blood component: plasma transports, red cells carry oxygen, white cells defend, platelets clot.
Name haemoglobin as the pigment, say it contains iron, and call the oxygen-carrying form oxyhaemoglobin.
For clotting, give the chain in order with the substances named: platelets release thromboplastin, which with calcium and vitamin K turns prothrombin into thrombin, which turns soluble fibrinogen into insoluble fibrin. The soluble-to-insoluble step is the heart of the answer.
Say O is the universal donor because its cells have no antigens, and AB is the universal recipient because its plasma has no antibodies — the reason matters as much as the name.
Use agglutination for the clumping caused by a wrong transfusion.
State that a complete blood group needs both the ABO letter and the Rh sign.
And when comparing vessels, answer in pairs across all four points — direction, wall, pressure, valves — rather than describing each vessel separately.
Give the function with each blood component: plasma transports, red cells carry oxygen, white cells defend, platelets clot.
Name haemoglobin as the pigment, say it contains iron, and call the oxygen-carrying form oxyhaemoglobin.
For clotting, give the chain in order with the substances named: platelets release thromboplastin, which with calcium and vitamin K turns prothrombin into thrombin, which turns soluble fibrinogen into insoluble fibrin. The soluble-to-insoluble step is the heart of the answer.
Say O is the universal donor because its cells have no antigens, and AB is the universal recipient because its plasma has no antibodies — the reason matters as much as the name.
Use agglutination for the clumping caused by a wrong transfusion.
State that a complete blood group needs both the ABO letter and the Rh sign.
And when comparing vessels, answer in pairs across all four points — direction, wall, pressure, valves — rather than describing each vessel separately.
Did you know
Why can you see veins on the back of your hand but never arteries?
Look at the back of your hand and the veins stand out as faint bluish lines. No artery anywhere on the body is visible in the same way.
The difference is depth, and it is not accidental. Arteries carry blood at high pressure, so a cut artery loses blood fast and dangerously. They are therefore deep-seated, protected under muscle and often alongside bone. Veins carry blood at low pressure, so a cut vein bleeds far more slowly, and they can afford to run near the surface.
That placement also helps the veins do their work. Running close to the skin puts them among the muscles whose movement squeezes them, and the valves inside convert that random squeezing into steady one-way flow towards the heart.
It is also why blood is taken from a vein and why a pulse is felt at only a few places — the wrist and the neck — where an artery happens to run close enough to the surface to be pressed against a bone.
The difference is depth, and it is not accidental. Arteries carry blood at high pressure, so a cut artery loses blood fast and dangerously. They are therefore deep-seated, protected under muscle and often alongside bone. Veins carry blood at low pressure, so a cut vein bleeds far more slowly, and they can afford to run near the surface.
That placement also helps the veins do their work. Running close to the skin puts them among the muscles whose movement squeezes them, and the valves inside convert that random squeezing into steady one-way flow towards the heart.
It is also why blood is taken from a vein and why a pulse is felt at only a few places — the wrist and the neck — where an artery happens to run close enough to the surface to be pressed against a bone.
Key takeaways
Blood, clotting and blood vessels: quick revision
- Blood is about 55 per cent plasma and 45 per cent cells.
- Plasma — about 90 per cent water, with proteins, glucose, minerals, hormones and wastes. It transports and helps regulate temperature.
- Red blood cells — biconcave, no nucleus, containing haemoglobin with iron; they carry oxygen as oxyhaemoglobin. Made in bone marrow, they live about 120 days and are destroyed in the liver and spleen. Too little iron gives anaemia.
- White blood cells — larger, fewer, with a nucleus; they defend by phagocytosis and by making antibodies, and their number rises during infection.
- Platelets — smallest, no nucleus; they cause clotting.
- Clotting: platelets release thromboplastin, which with calcium and vitamin K turns prothrombin into thrombin, which turns soluble fibrinogen into insoluble fibrin, forming a mesh that traps cells and becomes a scab.
- It prevents blood loss, keeps germs out, and lets tissue heal underneath. In haemophilia the chain is faulty and blood clots poorly.
- ABO groups by the antigens on red cells: A, B, AB (both), O (neither). Plasma carries antibodies against the antigens it lacks.
- O is the universal donor (no antigens) but receives only O. AB is the universal recipient (no antibodies) but donates only to AB.
- A wrong transfusion causes agglutination — clumping that blocks vessels and can be fatal.
- Rh factor is a separate antigen: Rh positive if present, Rh negative if absent. An Rh-negative person must receive Rh-negative blood, and an Rh-negative mother carrying an Rh-positive foetus needs monitoring. A full group needs both the letter and the sign.
- Arteries — away from the heart, thick elastic walls, narrow lumen, high pressure in spurts, no valves, deep-seated.
- Veins — towards the heart, thin walls, wide lumen, low pressure, valves to prevent backflow, superficial.
- Capillaries — one cell thick, extremely narrow, slowest flow, and the site of exchange of materials.
- The exceptions: the pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood — because the definitions rest on direction, not on oxygen.
Try writing the clotting chain in order and the three-vessel comparison in pairs — those two answers between them carry most of the marks in this chapter.
- Plasma — about 90 per cent water, with proteins, glucose, minerals, hormones and wastes. It transports and helps regulate temperature.
- Red blood cells — biconcave, no nucleus, containing haemoglobin with iron; they carry oxygen as oxyhaemoglobin. Made in bone marrow, they live about 120 days and are destroyed in the liver and spleen. Too little iron gives anaemia.
- White blood cells — larger, fewer, with a nucleus; they defend by phagocytosis and by making antibodies, and their number rises during infection.
- Platelets — smallest, no nucleus; they cause clotting.
- Clotting: platelets release thromboplastin, which with calcium and vitamin K turns prothrombin into thrombin, which turns soluble fibrinogen into insoluble fibrin, forming a mesh that traps cells and becomes a scab.
- It prevents blood loss, keeps germs out, and lets tissue heal underneath. In haemophilia the chain is faulty and blood clots poorly.
- ABO groups by the antigens on red cells: A, B, AB (both), O (neither). Plasma carries antibodies against the antigens it lacks.
- O is the universal donor (no antigens) but receives only O. AB is the universal recipient (no antibodies) but donates only to AB.
- A wrong transfusion causes agglutination — clumping that blocks vessels and can be fatal.
- Rh factor is a separate antigen: Rh positive if present, Rh negative if absent. An Rh-negative person must receive Rh-negative blood, and an Rh-negative mother carrying an Rh-positive foetus needs monitoring. A full group needs both the letter and the sign.
- Arteries — away from the heart, thick elastic walls, narrow lumen, high pressure in spurts, no valves, deep-seated.
- Veins — towards the heart, thin walls, wide lumen, low pressure, valves to prevent backflow, superficial.
- Capillaries — one cell thick, extremely narrow, slowest flow, and the site of exchange of materials.
- The exceptions: the pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood — because the definitions rest on direction, not on oxygen.
Try writing the clotting chain in order and the three-vessel comparison in pairs — those two answers between them carry most of the marks in this chapter.