Blood
Blood carries oxygen, defends against germs and clots wounds.
About this topic
Blood is a tissue made of plasma (about 55%), red blood cells, white blood cells and platelets. Plasma carries nutrients, hormones, waste and heat. Red blood cells, biconcave and without nuclei, contain haemoglobin to carry oxygen. White blood cells fight infection by phagocytosis or antibody production, and platelets help clot blood to seal wounds.
An adult has about 5 litres. Anaemia is low red cell or haemoglobin levels. Blood is made in bone marrow.
A drop of blood holds millions of red cells, which carry oxygen as oxyhaemoglobin, a bright red form. White cells include phagocytes and lymphocytes that fight infection, and platelets are cell fragments that help clots form, sealing wounds with a fibrin mesh. Plasma, which is mostly water, also carries carbon dioxide mainly as dissolved bicarbonate.
Anaemia, a lack of red cells or haemoglobin, causes tiredness, often from low iron. Blood donation helps patients after surgery or accidents, and the body replaces donated blood within weeks. Athletes sometimes train at altitude to make more red cells. Students often think blood is blue inside veins; it is always red, darker when low in oxygen, and it only looks blue through the skin.
Key ideas
- Red cells carry oxygen
- White cells fight infection
- Platelets clot
Quick check
Use cases: where Blood helps
- Homework: review the key idea behind Blood before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Blood without notes.
- Visual learning: play with the interactive lab to see how Blood changes when you adjust its inputs.
- Teaching a friend: explain Blood in your own words, then compare your version with the key ideas.
Biology underpins medicine, farming, conservation and biotechnology.
Frequently asked questions
What is Blood?
Blood carries oxygen, defends against germs and clots wounds.
What will I learn in Blood?
Red cells carry oxygen. White cells fight infection. Platelets clot.
Is Blood part of Biology?
Yes. It is one of 150 Biology topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Blood lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Blood?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.