Blood and the internal environment
Every cell in your body sits in a watery fluid. Blood, this fluid between cells (tissue fluid) and lymph together are the internal environment of the body. It must stay steady: the right temperature, the right amount of water, salts and sugar. Keeping it steady is called homeostasis. Blood helps because it moves everywhere and carries things to cells and away from cells.
What blood does
- Carries oxygen, food, hormones and waste.
- Protects by fighting germs and by clotting.
- Balances heat and water in the body.
What blood is made of
Blood is a liquid tissue. A grown-up has about 5 litres.
Plasma (about 55%)
A yellow liquid, about 90% water. It carries digested food, salts, hormones, carbon dioxide and urea (a waste), and also proteins.
Red blood cells (RBC)
Round, flat, with a dip in the middle. They have no nucleus, so there is more room for haemoglobin, a red protein with iron that grabs oxygen. They live about 120 days. Low haemoglobin is called anaemia.
White blood cells (WBC)
Bigger than red cells and fewer in number. They have a nucleus. They destroy germs and make antibodies.
Platelets
Tiny cell pieces that help blood clot.
Blood groups: ABO and Rh
Red cells can carry marker molecules called antigens. Plasma may hold antibodies that attack a marker the person does not have.
| Group | Markers on red cells | Antibodies in plasma |
|---|---|---|
| A | A | anti-B |
| B | B | anti-A |
| AB | A and B | none |
| O | none | anti-A and anti-B |
Rh factor: another marker (D). People with it are Rh positive (+). People without it are Rh negative (โ).
Who can give to whom?
Blood is safe only if the donor's red cells have no marker that the receiver's plasma can attack. If not, red cells clump and block small vessels. Group Oโ is called the universal donor and AB+ the universal receiver. Hospitals always test before giving blood.
Blood clotting
When a vessel is cut, platelets stick to the wound and form a soft plug. Chemicals then change a plasma protein into long sticky threads called fibrin. The threads make a net that traps red cells. This is a clot. It dries into a scab and protects the wound while new skin grows. Vitamin K helps the body make clotting proteins. In haemophilia a clotting protein is missing, so bleeding does not stop easily. Clotting inside a vessel without a cut is dangerous.
Where blood cells are made
Blood cells are made in red bone marrow, the soft red stuff inside flat bones (ribs, breastbone, skull, hip) and the ends of long bones. A baby before birth also makes blood in the liver and spleen. The spleen breaks down old red cells. Adults make millions of new cells every second. Eating iron-rich food (spinach, jaggery, lentils) helps make haemoglobin.
Try it
Predict first: if you press your finger on a small cut, why does the bleeding stop sooner? Then use the slider in the 3D at step 5 to check.
Key formulas and definitions
- Blood = plasma (about 55%) + cells (about 45%)
- Plasma is about 90% water
- RBC: carry oxygen (haemoglobin). WBC: fight germs. Platelets: clotting
- Universal donor: Oโ. Universal receiver: AB+
- Adult blood: about 5 litres. RBC life: about 120 days
Worked examples
1. A person has 5 litres of blood. About how much of it is plasma?
Plasma is about 55%. 55/100 ร 5 = 2.75 litres.
2. Can a person with group A blood receive blood from group B?
No. Group A plasma has anti-B antibodies. Group B red cells carry marker B, so they would clump.
3. Why can group Oโ be given to almost anyone in an emergency?
O red cells carry no A or B marker, and Rh negative cells carry no D marker. So the receiver's antibodies have nothing to attack.
4. A blood sample is spun. The bottom layer is 2.4 cm tall in a 6 cm column. What percent is red cells?
2.4/6 ร 100 = 40%.
Common mistakes
- Thinking blood is only red cells. More than half is yellow plasma.
- Saying red cells have a nucleus. Human red cells have none.
- Mixing up the markers and antibodies: group A has marker A and anti-B antibodies.
- Thinking platelets and white cells do the same job. Platelets stop bleeding; white cells fight germs.