Blood is a living organ that flows. It carries oxygen, fights infection, seals wounds, and moves heat and nutrients everywhere in your body, about 5 litres of it in an average adult, roughly 7–8% of body weight. Understanding what it is made of explains everything else on this site: why donations are separated into components, why intervals exist, and why certain patients need certain donors.
The four parts
Plasma, about 55%. A pale yellow liquid that is 90% water. The rest: proteins (clotting factors, antibodies, albumin), salts, hormones and nutrients. Plasma is the delivery system, and on its own, a lifesaver for burn victims, patients with liver disease and clotting disorders.
Red cells, about 45%. The workhorses. Each red cell is packed with haemoglobin, an iron-containing protein that grabs oxygen in your lungs and releases it everywhere else. Red cells live about 120 days, then your spleen recycles them, which is why your body needs a steady supply of iron, and why donated red cells are useful for about 35–42 days in storage.
White cells, under 1%. Your immune defence: they hunt bacteria, viruses and abnormal cells. Donated blood is filtered of most white cells during processing, because donor white cells can cause reactions in patients.
Platelets, a tiny fraction. Cell fragments that plug leaks. When you bleed, platelets clump and trigger clotting. They live only 8–10 days in your body, and about 5 days in storage, which is why platelet shortages appear suddenly (dengue season is the classic Assam example) and why platelet donors are asked for again and again.
Where blood comes from
Your bone marrow is the factory, spongy tissue inside bones (breastbone, hips, ribs, spine) producing billions of new cells daily. It needs raw materials: iron, protein, folate, vitamin B12. Eat well and the factory keeps up; donate and the factory simply ramps up, red cells are fully replaced within weeks, which is exactly why the donation interval (3 months for men, 4 for women) guarantees complete recovery before the next donation.
What blood does, in four lines
- Delivers oxygen from lungs to every cell (haemoglobin).
- Delivers nutrients and hormones; carries wastes to kidneys and liver.
- Clots when you bleed (platelets + plasma proteins).
- Defends against infection (white cells + antibodies).
Where groups come from
Your red cells carry inherited markers (antigens) on their surface. The ABO system (A, B, AB, O) and the Rh factor (+/−) are the two that matter for transfusion; together they make the eight common groups. Compatibility is your immune system reacting to markers it doesn’t recognise, which is why matching matters and why the centre runs a crossmatch before every transfusion. How blood groups are inherited →
Why this biology matters for donors
- You regenerate. Donation is not depletion, the interval rules are designed around your biology.
- Your single unit reaches up to three patients once separated into red cells, platelets and plasma.
- Your group decides who you can help, and negative-group donors (O− above all) are needed most.
- Haemoglobin is the gate: 12.5 g/dL minimum at the health check, because low-Hb blood helps nobody and hurts you. How to keep haemoglobin up →
The takeaway
Blood is not mysterious, it is a factory product with a supply chain. Assam’s hospitals run short not because bodies fail, but because the supply chain has always depended on emergency appeals. A register of healthy, informed, regular donors fixes that. Join it →