Blood Components

Why Blood Components Matter in CDS

Blood is classified as a fluid (liquid) connective tissue because, like bone and cartilage, its cells are separated by a non-living matrix — here the matrix is the liquid plasma. An average adult carries about 5 to 6 litres of blood, roughly 7–8% of body weight. This blood circulates continuously, completing one full round of the body in under a minute when you are at rest.

The CDS & OTA paper rarely asks deep physiology. Instead it tests crisp factual recall: which cell carries oxygen, which one fights infection, what gives blood its red colour, and which group is the universal donor. The questions are usually single statements you must mark true or false, so a clear mental map of the four components and their jobs is exactly what you need. Lock these facts in and this becomes a guaranteed-marks topic that costs very little study time.

Plasma: The Liquid Highway

Plasma is the pale-yellow liquid that makes up about 55% of blood by volume. It is roughly 90–92% water, with the remaining 8–10% being proteins, mineral salts, glucose, hormones, urea, dissolved gases and antibodies. Because plasma is mostly water, it is the medium in which everything else floats and travels around the body.

Think of plasma as a busy highway. Substances absorbed from the gut, hormones squeezed out by glands, and wastes picked up from tissues all dissolve in this fluid and are carried to wherever they are needed or removed. Without plasma, the cells of the blood would have no way to move from one organ to another.

Key plasma proteins

• Albumin — the most abundant plasma protein; maintains osmotic pressure and keeps water from leaking out of blood vessels into tissues.
• Globulins — include the antibodies (immunoglobulins) that defend the body against infection.
• Fibrinogen and prothrombin — clotting proteins made in the liver that are vital for sealing wounds.

Plasma also transports carbon dioxide (mostly as bicarbonate ions) back to the lungs and distributes heat, helping spread warmth evenly so that organs deep inside the body stay at the same temperature as the skin.

Red Blood Cells (Erythrocytes)

RBCs are the most numerous cells in the blood and give it the characteristic red colour. They are biconcave, disc-shaped — that is, pushed in on both sides like a tiny doughnut without a hole. In mammals they lack a nucleus when mature, and this extra internal space lets them pack in more oxygen-carrying pigment. Their flexible shape also lets them squeeze through the narrowest capillaries.

• They contain the red pigment haemoglobin, an iron-rich protein that binds oxygen in the lungs to form oxyhaemoglobin and releases it in the tissues.
• They are manufactured in the red bone marrow of flat bones such as the sternum and ribs.
• Average lifespan is about 120 days; worn-out cells are broken down in the spleen, which is therefore nicknamed the “graveyard of RBCs”.
• The iron freed from old haemoglobin is recycled to make fresh cells, while the rest forms the bile pigment bilirubin.

White Blood Cells (Leucocytes)

WBCs are the body’s soldiers — they fight infection and provide immunity. They are larger than RBCs, nucleated and colourless (no haemoglobin), and far fewer in number.

Two main families

• Granulocytes — neutrophils (most numerous, eat bacteria), eosinophils (allergy/parasites), basophils (release histamine).
• Agranulocytes — lymphocytes (make antibodies) and monocytes (largest WBC, become macrophages).

WBCs are special in that many of them can squeeze out through the walls of capillaries and move into the tissues to reach the site of an infection — a process called diapedesis. This is why pus, which is mostly dead neutrophils and bacteria, collects at a wound. Unlike RBCs, which stay inside vessels, WBCs patrol the whole body, making them the front line of the immune system.

Platelets and Blood Clotting

Platelets (thrombocytes) are the smallest formed elements — tiny cell fragments without a nucleus, also made in the bone marrow. Their job is to trigger clotting (coagulation) and stop bleeding.

How a clot forms (simplified)

1. Injured platelets release thromboplastin.
2. With calcium ions (Ca²⁺) and vitamin K, prothrombin → thrombin.
3. Thrombin converts soluble fibrinogen → insoluble fibrin, forming a mesh that traps cells — the clot.

It also helps to know what stops blood clotting inside healthy vessels. The natural anticoagulant heparin, produced by the body, prevents unwanted clots, while in the laboratory sodium citrate is added to stored blood to keep it liquid. A clot that forms inside an unbroken blood vessel is called a thrombus and can block circulation, which is why both clotting and anti-clotting balance is so important.

ABO and Rh Blood Groups

Blood groups were discovered by Karl Landsteiner. They are based on antigens on the RBC surface and antibodies in the plasma.

The ABO system

• Group A — antigen A, antibody b.
• Group B — antigen B, antibody a.
• Group AB — both antigens, no antibodies → universal recipient.
• Group O — no antigens, both antibodies → universal donor.

If the wrong blood group is transfused, the recipient’s antibodies attack the donor RBCs and make them clump together — a dangerous reaction called agglutination. This is why every transfusion is preceded by careful cross-matching of both the ABO group and the Rh factor. Knowing the names of the antigens and antibodies for each group lets you predict instantly whether two samples are compatible.

Main Functions of Blood

Group the functions under three simple headings so they are easy to recall in the exam.

1. Transport

• Oxygen (by haemoglobin) and carbon dioxide.
• Digested food, hormones and waste like urea (to the kidneys).

2. Protection

• WBCs and antibodies fight germs.
• Platelets clot blood to prevent loss after injury.

3. Regulation

• Distributes body heat to keep temperature uniform across organs.
• Buffers maintain blood pH around 7.4 (slightly alkaline).
• Helps balance water and salt levels by carrying excess to the kidneys.

If you can state at least one example under each of these three headings, you can answer almost any “function of blood” question. Examiners often disguise the question by asking which function a particular component performs, so always link the function back to the cell or fluid responsible for it.

Worked Example: Reading Blood Numbers

Numerical-style reasoning sometimes appears. Practise interpreting standard values.

This matches the textbook figure of roughly 600 : 1, confirming RBCs vastly outnumber WBCs — a fact examiners love to test.

Common Mistakes to Avoid

• Confusing serum and plasma — serum has no fibrinogen.
• Thinking WBCs carry oxygen — only RBCs (haemoglobin) do.
• Mixing up universal donor and recipient — O donates to all, AB receives from all.
• Forgetting that mature mammalian RBCs have no nucleus.
• Believing platelets are full cells — they are cell fragments.

Previous-Year Style Question

Blood Disorders Worth Knowing

A few disorders recur in general-science questions. Memorise the one-line cause.

• Anaemia — low haemoglobin or RBCs, often from iron deficiency.
• Leukaemia — cancer of WBCs (uncontrolled multiplication).
• Haemophilia — inability to clot; genetic, sex-linked.
• Thrombocytopenia — low platelet count (notable in dengue).
• Jaundice — excess bilirubin from RBC breakdown, yellowing skin and eyes.
• Sickle-cell anaemia — an inherited disorder in which RBCs become sickle-shaped and carry less oxygen.

For the exam, you only need the single most striking fact about each disorder rather than full clinical detail. Pair the disorder with the component it affects: anaemia and sickle-cell with RBCs, leukaemia with WBCs, and haemophilia and thrombocytopenia with the clotting mechanism. This pairing trick turns a long list into something you can recall in seconds.

Quick Revision

Revise these six lines and you can confidently attempt almost any Blood Components question in CDS & OTA.