🩸 Blood
Study blood composition — RBCs, WBCs, platelets and plasma for CUET Agriculture. ABO blood groups, Rh factor and clotting cascade covered.
Blood — Overview
Blood is a specialized fluid connective tissue that circulates throughout the body, performing vital transport, defense, and regulatory functions. Unlike other connective tissues that are stationary, blood is always in motion.
- Total volume: 5–7 litres (average ~5.5 L in adults; slightly more in males than females due to larger body size)
- pH: 7.35–7.45 (slightly alkaline). Even a small deviation outside this range can be life-threatening — the body uses buffer systems to maintain this narrow range.
- Temperature: ~38°C (slightly above core body temperature of 37°C, because blood absorbs metabolic heat)
- Specific gravity: 1.052–1.060 (denser than water due to dissolved proteins and cells)
Composition
Blood is composed of two main fractions — a liquid portion and a cellular portion:
| Component | Percentage | Details |
|---|---|---|
| Plasma | ~55% | The liquid portion — a straw-colored fluid that carries dissolved substances and suspends the formed elements |
| Formed elements | ~45% | The cellular components: RBCs, WBCs, and Platelets. The percentage of blood volume occupied by RBCs is called the hematocrit (normally 40–45% in males, 36–44% in females). |
Blood Plasma
Plasma is the liquid matrix of blood — it makes up about 55% of total blood volume and is essential for transporting nutrients, hormones, waste products, and heat throughout the body.
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Blood — Overview
Blood is a specialized fluid connective tissue that circulates throughout the body, performing vital transport, defense, and regulatory functions. Unlike other connective tissues that are stationary, blood is always in motion.
- Total volume: 5–7 litres (average ~5.5 L in adults; slightly more in males than females due to larger body size)
- pH: 7.35–7.45 (slightly alkaline). Even a small deviation outside this range can be life-threatening — the body uses buffer systems to maintain this narrow range.
- Temperature: ~38°C (slightly above core body temperature of 37°C, because blood absorbs metabolic heat)
- Specific gravity: 1.052–1.060 (denser than water due to dissolved proteins and cells)
Composition
Blood is composed of two main fractions — a liquid portion and a cellular portion:
| Component | Percentage | Details |
|---|---|---|
| Plasma | ~55% | The liquid portion — a straw-colored fluid that carries dissolved substances and suspends the formed elements |
| Formed elements | ~45% | The cellular components: RBCs, WBCs, and Platelets. The percentage of blood volume occupied by RBCs is called the hematocrit (normally 40–45% in males, 36–44% in females). |
Blood Plasma
Plasma is the liquid matrix of blood — it makes up about 55% of total blood volume and is essential for transporting nutrients, hormones, waste products, and heat throughout the body.
- 95% water + 5% solutes (proteins, electrolytes, nutrients, gases, waste products)
- Plasma proteins constitute approximately 7–9 g/100 ml and are primarily synthesized by the liver:
| Protein | Function |
|---|---|
| Albumin (~60% of plasma proteins) | Maintains colloid osmotic pressure (keeps fluid inside blood vessels, preventing oedema); also acts as a transport carrier for fatty acids, bilirubin, and drugs |
| Globulins (~36%) | Three subclasses: α and β globulins transport lipids, hormones, metal ions, and fat-soluble vitamins; γ-globulins are immunoglobulins (antibodies) produced by B-lymphocytes for immune defense |
| Fibrinogen (~4%) | Essential for blood clotting — converted to insoluble fibrin threads during coagulation, forming the structural framework of a blood clot |
| Prothrombin | A clotting factor synthesized in the liver (requires vitamin K); converted to thrombin during the clotting cascade |
- Other solutes: glucose, amino acids, urea, creatinine, electrolytes (Na⁺, K⁺, Ca²⁺, Cl⁻, HCO₃⁻), hormones, dissolved gases (O₂, CO₂)
- Serum = Plasma minus clotting factors (fibrinogen removed). When blood clots in a test tube and the clot is removed, the remaining liquid is serum.
TIP
Memory aid: Serum = Plasma − Fibrinogen. Serum is what remains after the clot has formed and been removed. It contains all plasma components except clotting factors.
Red Blood Cells (RBCs / Erythrocytes)
RBCs are the most abundant cells in blood and are specialized for one primary purpose: oxygen transport. Their unique structure — no nucleus, no organelles, biconcave shape — maximizes their ability to carry hemoglobin and exchange gases.
| Feature | Details |
|---|---|
| Origin | Red bone marrow (process called erythropoiesis) — in adults, primarily in flat bones (sternum, ribs, pelvis, vertebrae) |
| Destruction | Spleen — often called the "graveyard of RBCs". Old or damaged RBCs are phagocytosed by splenic macrophages. The liver (Kupffer cells) also destroys some. |
| Shape | Biconcave disc — this unique shape increases the surface area-to-volume ratio by ~20–30%, enhancing gas exchange efficiency |
| Size | Diameter: 7–8 μm |
| Count | Males: 4.7–6.1 million/μL; Females: 4.2–5.4 million/μL (lower in females partly due to menstrual blood loss and lower androgen levels) |
| Lifespan | ~120 days — about 2.4 million RBCs are produced and destroyed every second |
| Nucleus | Absent in mature mammalian RBCs (unique among vertebrates — birds, reptiles, amphibians, and fish all have nucleated RBCs) |
| Organelles | No mitochondria, no endoplasmic reticulum, no nucleus — this maximizes internal space for hemoglobin molecules (~280 million Hb molecules per RBC) |
Hemoglobin (Hb)
Hemoglobin is the oxygen-carrying conjugated protein inside RBCs. It is what gives blood its red color and is one of the most studied proteins in biology.
- Structure: 4 polypeptide chains — 2α + 2β chains in adult HbA (hemoglobin A). Each chain is associated with a heme group, an iron-containing porphyrin ring.
- Each heme group contains 1 Fe²⁺ atom (ferrous iron) → each hemoglobin molecule can bind 4 O₂ molecules simultaneously
- Hüfner's constant: 1 g Hb carries 1.34 ml O₂ — this is the theoretical oxygen-carrying capacity per gram of hemoglobin
- Normal concentration: 13–17 g/100 ml (males); 12–16 g/100 ml (females)
Forms of hemoglobin — hemoglobin exists in several forms depending on what is bound to it:
- Oxyhemoglobin (HbO₂) — bright cherry-red; O₂ reversibly bound to Fe²⁺. This is the form in arterial blood.
- Deoxyhemoglobin (reduced Hb) — dark red/purplish; O₂ has been released. This is the form in venous blood.
- Carbaminohemoglobin (HbCO₂) — CO₂ bound to the amino groups of the globin chains (NOT to the heme iron). About 20–25% of CO₂ is transported this way.
- Carboxyhemoglobin (HbCO) — carbon monoxide bound to Fe²⁺. This is toxic because CO has ~200× greater affinity for Hb than O₂, effectively blocking oxygen transport. This is why CO poisoning is so dangerous.
- Methemoglobin (MetHb) — iron is oxidized to Fe³⁺ (ferric) state; cannot carry O₂. Normally kept below 1% by the enzyme methemoglobin reductase.
Erythropoiesis
The production of RBCs is a tightly regulated process driven by oxygen demand:
- Stimulus: low O₂ in blood (hypoxia) → detected by kidney cells → kidneys release erythropoietin (EPO) → EPO travels to red bone marrow → stimulates increased RBC production
- Sequence of development: Stem cell → Proerythroblast → Basophilic erythroblast → Polychromatic erythroblast → Orthochromatic erythroblast → Reticulocyte (still retains some RNA; makes up 1–2% of circulating RBCs — a useful clinical indicator of marrow activity) → Mature RBC
- Essential nutrients: Iron (for heme synthesis), Vitamin B₁₂ and Folic acid (for DNA synthesis during rapid cell division), Vitamin B₆ (for globin synthesis)
- Fetal erythropoiesis: In the fetus, the liver is the primary site of RBC production (not bone marrow). The spleen and yolk sac also contribute in early fetal life.
IMPORTANT
EPO (Erythropoietin) is sometimes misused as a performance-enhancing drug in sports because it boosts RBC production, increasing oxygen-carrying capacity and endurance. This practice is banned in competitive athletics.
White Blood Cells (WBCs / Leukocytes)
WBCs are the soldiers of the immune system. Unlike RBCs, they are nucleated, can move independently (amoeboid movement), and can leave blood vessels to enter tissues (diapedesis) to fight infections at the site of invasion.
| Feature | Details |
|---|---|
| Origin | Bone marrow (all WBCs) and thymus (T-lymphocytes mature here) |
| Shape | Irregular / amoeboid — they can change shape to squeeze through capillary walls |
| Size | 8–15 μm (larger than RBCs) |
| Count | 6,000–8,000 per mm³ (an elevated count is called leukocytosis, often indicating infection; a decreased count is leukopenia) |
| Lifespan | Variable: 4–7 days for most types (but memory lymphocytes can survive for years or decades) |
| Nucleus | Present (nucleated) — essential for protein synthesis and immune functions |
| Function | Defense against infection — through phagocytosis, antibody production, and cell-mediated killing |
Classification
WBCs are classified into two major groups based on whether they contain visible cytoplasmic granules when stained:
Granulocytes (have cytoplasmic granules)
| Type | % of WBC | Size | Nucleus | Lifespan | Function |
|---|---|---|---|---|---|
| Neutrophils | 60–65% | 12–15 μm | 2–5 lobes (hence called PMNL — polymorphonuclear leukocyte) | 2–4 days | Phagocytosis — the first responders to bacterial infection; they engulf and destroy bacteria using enzymes and reactive oxygen species. In female neutrophils, an inactive X chromosome appears as a small "drumstick" appendage on the nucleus — this is the Barr body. |
| Eosinophils | 2–3% | 10–15 μm | Bilobed (2 lobes connected by a thin strand) | 8–12 days | Defense against parasitic worms (helminths); modulate allergic and inflammatory responses. Their granules stain red-orange with eosin (an acidic dye). Elevated eosinophil counts (eosinophilia) suggest parasitic infection or allergy. |
| Basophils | 0.5–1% (rarest WBC) | 10–15 μm | 2–3 lobes, often S-shaped | 12–15 days | Release histamine (causes vasodilation and increased capillary permeability), serotonin (contributes to inflammation), and heparin (natural anticoagulant). Play a key role in allergic reactions and inflammation. Functionally similar to tissue mast cells. |
Agranulocytes (lack visible granules)
| Type | % of WBC | Size | Nucleus | Function |
|---|---|---|---|---|
| Lymphocytes | 20–25% | 8–16 μm | Large, round, occupying most of the cell | Two main types: B-lymphocytes (mature in bone marrow) produce antibodies → humoral immunity. T-lymphocytes (mature in thymus) → cell-mediated immunity (T-helper cells coordinate immune responses; T-killer/cytotoxic cells destroy virus-infected cells and cancer cells). |
| Monocytes | 4–10% | 15–20 μm (largest WBC) | Kidney-shaped or horseshoe-shaped | The most powerful phagocytes. Monocytes circulate in blood for 1–3 days, then migrate into tissues and differentiate into macrophages — long-lived tissue-resident phagocytes. Tissue-specific names: Kupffer cells (liver), alveolar macrophages (lungs), microglia (brain), osteoclasts (bone). |
What is the difference between humoral and cell-mediated immunity?
**Humoral immunity** (antibody-mediated): B-lymphocytes encounter an antigen, become activated (often with T-helper cell assistance), and differentiate into **plasma cells** that secrete antibodies (immunoglobulins) into the blood and body fluids. These antibodies neutralize toxins, opsonize bacteria for phagocytosis, and activate complement. Effective against extracellular pathogens.Cell-mediated immunity: T-lymphocytes directly attack infected cells. Cytotoxic T-cells (T-killer cells) recognize and destroy cells displaying foreign antigens on their surface (virus-infected cells, transplanted cells, cancer cells). This type does not involve antibody secretion. Effective against intracellular pathogens and abnormal cells.
Platelets (Thrombocytes)
Platelets are not true cells but rather cell fragments — tiny pieces that break off from giant precursor cells. Despite their small size, they play a critical role in stopping bleeding.
| Feature | Details |
|---|---|
| Origin | Fragments of megakaryocytes (very large cells) in red bone marrow. Each megakaryocyte can produce 2,000–3,000 platelets. |
| Size | 2–4 μm (smallest formed element in blood) |
| Shape | Disc-shaped / biconvex when inactive; become irregular with pseudopods when activated |
| Count | 1.5–3.5 lakh (150,000–350,000) per mm³. Low count (thrombocytopenia) → bleeding tendency; high count (thrombocytosis) → clotting risk. |
| Lifespan | 5–7 days |
| Nucleus | Absent (since they are cell fragments, not complete cells) |
| Function | Blood clotting (hemostasis): form a temporary platelet plug at injury sites; release clotting factors, serotonin (vasoconstriction), and ADP (attracts more platelets). |
Functions of Blood (10 Key Functions)
Blood is truly a multifunctional tissue. Its functions can be grouped into transport, regulation, and protection:
- Transport of O₂ — from lungs to tissues (bound to hemoglobin as oxyhemoglobin)
- Transport of CO₂ — from tissues to lungs (mainly as bicarbonate ions in plasma, also as carbaminohemoglobin)
- Transport of nutrients — glucose, amino acids, lipids absorbed from the gut are carried to all body tissues
- Transport of waste — metabolic wastes (urea, creatinine, uric acid) are carried from tissues to kidneys for excretion
- Transport of hormones — hormones from endocrine glands are carried through blood to their distant target organs
- Defense — WBCs destroy pathogens through phagocytosis and immune responses; antibodies neutralize antigens
- Blood clotting — prevents blood loss (hemostasis) through the coordinated action of platelets and clotting factors
- Temperature regulation — blood distributes metabolic heat throughout the body; vasodilation in skin releases heat, vasoconstriction conserves it
- pH regulation — buffer systems in blood (bicarbonate buffer, hemoglobin buffer, phosphate buffer) maintain the critical pH range of 7.35–7.45
- Osmotic regulation — plasma proteins (especially albumin) maintain colloid osmotic pressure, preventing excessive fluid loss from capillaries into tissues
Blood Groups (ABO System)
The ABO blood group system was discovered by Karl Landsteiner in 1900, for which he received the Nobel Prize in Physiology or Medicine in 1930. This discovery made safe blood transfusions possible and remains one of the most important advances in medicine.
Blood groups are determined by the presence or absence of specific antigens (glycoproteins) on the surface of RBCs and corresponding antibodies (agglutinins) in the plasma:
| Blood Group | Antigens on RBC | Antibodies in Plasma | Can Donate To | Can Receive From |
|---|---|---|---|---|
| A | A antigen | Anti-B | A, AB | A, O |
| B | B antigen | Anti-A | B, AB | B, O |
| AB | A and B antigens | None | AB only | A, B, AB, O — Universal Recipient |
| O | No antigens (H antigen only) | Anti-A and Anti-B | A, B, AB, O — Universal Donor | O only |
WARNING
The "universal donor" and "universal recipient" concepts are simplified. In clinical practice, cross-matching is always performed before transfusion to check for additional antibody incompatibilities beyond ABO and Rh.
Rh Factor
- Discovered by Landsteiner and Wiener in 1940 in the Rhesus monkey (hence the name "Rh")
- Rh+ (Rh positive): approximately ~80% of the population possess the Rh antigen (D antigen) on their RBCs
- Rh− (Rh negative): ~20% of the population lack the Rh antigen. Rh-negative individuals do not naturally have anti-Rh antibodies but will develop them if exposed to Rh+ blood.
Erythroblastosis fetalis (Hemolytic Disease of the Newborn — HDN):
This is a serious condition that occurs when an Rh− mother carries an Rh+ fetus (the father being Rh+). During the first pregnancy, small amounts of fetal Rh+ blood may enter the mother's circulation during delivery. The mother's immune system recognizes the Rh antigen as foreign and produces anti-Rh antibodies (IgG). During a second pregnancy with another Rh+ fetus, these maternal IgG antibodies cross the placenta and attack fetal RBCs, causing their destruction (hemolysis). This leads to severe fetal anemia, jaundice, and potentially death.
Prevention: An injection of Anti-D immunoglobulin (RhoGAM) is given to the Rh− mother within 72 hours after the first delivery (or miscarriage/abortion). This injection destroys any fetal Rh+ RBCs in the mother's blood before her immune system can mount a response, preventing sensitization.
Blood Clotting (Hemostasis)
Blood clotting is a complex cascade mechanism that prevents excessive blood loss when a blood vessel is damaged. It involves a series of enzyme activations, often described as a "cascade" because each step amplifies the next.
Clotting Factors
- There are 13 clotting factors (numbered I to XIII in order of discovery, not in order of action)
- Important factors to remember: Factor I (Fibrinogen), Factor II (Prothrombin), Factor III (Thromboplastin/Tissue factor), Factor IV (Ca²⁺ — calcium ions, required at multiple steps), Factor VIII (Anti-hemophilic factor — deficient in Hemophilia A)
Three-Stage Cascade
| Stage | Events |
|---|---|
| Stage 1: Formation of Thromboplastin | Injured tissue releases tissue factor (Factor III) → combines with platelet factors → forms prothrombinase complex (thromboplastin). This can occur via the intrinsic pathway (contact with exposed collagen) or extrinsic pathway (tissue damage). |
| Stage 2: Prothrombin → Thrombin | Prothrombinase + Factor VIII + Ca²⁺ (Factor IV) + Factor X → converts prothrombin (Factor II) → thrombin (active enzyme) |
| Stage 3: Fibrinogen → Fibrin | Thrombin + Ca²⁺ → converts soluble fibrinogen (Factor I) → fibrin (insoluble protein threads) → fibrin threads form a meshwork that traps RBCs, WBCs, and platelets → blood clot (thrombus) |
Key clinical facts about clotting:
- Normal clotting time: 3–6 minutes
- Vitamin K is essential for the liver to synthesize prothrombin and factors VII, IX, and X. Newborns often receive a vitamin K injection at birth because they lack gut bacteria to produce it.
- Heparin — a natural anticoagulant released by mast cells and basophils; prevents clotting by inhibiting thrombin. Used clinically as a blood thinner.
- Hemophilia A — an X-linked recessive disorder caused by deficiency of Factor VIII. Almost exclusively affects males. Patients bleed excessively from even minor injuries.
- Hemophilia B (Christmas disease) — caused by deficiency of Factor IX; also X-linked recessive
Why is hemophilia called the 'Royal Disease'?
Hemophilia B was prevalent in European royal families, traced back to **Queen Victoria of England**, who was a carrier. She passed the gene to several of her children, who married into royal families across Europe — including those of Spain, Germany, and Russia. The most famous case was **Tsarevich Alexei of Russia**, whose hemophilia indirectly contributed to the influence of Rasputin and the Russian Revolution.Lymph
Lymph is a colorless fluid that forms when blood plasma filters out of capillaries into the interstitial spaces (the spaces between cells). This fluid is then collected by lymphatic vessels and eventually returned to the bloodstream, maintaining fluid balance.
| Feature | Details |
|---|---|
| Colour | Colourless to pale yellow (no RBCs, no hemoglobin — hence no red colour) |
| RBCs | Absent (lymphatic capillaries are too selective to allow RBCs in) |
| Lymphocytes | More than in blood — lymph passes through lymph nodes where lymphocytes are concentrated |
| CO₂ | More than in blood (reflects tissue metabolic activity) |
| Nutrients | Less than in blood (nutrients have already been delivered to tissues) |
| Proteins | Less than in blood plasma (large proteins do not easily filter out of capillaries) |
Functions of Lymph (3 Key)
- Returns interstitial fluid to blood — about 3 litres of fluid leaks out of capillaries into tissues daily. The lymphatic system collects and returns this fluid, maintaining blood volume and preventing tissue swelling (oedema).
- Transports absorbed fats from the intestine — dietary fats are absorbed as chylomicrons into lacteals in intestinal villi. These fat-laden lymph vessels (called lacteals because the milky appearance of fat makes the lymph look like milk) drain into larger lymphatic vessels and eventually into the bloodstream.
- Immune defense — lymph carries lymphocytes and is filtered through lymph nodes, where pathogens and foreign particles are trapped and destroyed by immune cells. Swollen lymph nodes during an infection indicate active immune responses.
NOTE
Unlike the blood circulatory system, the lymphatic system has no pump (no heart equivalent). Lymph flow depends on skeletal muscle contractions, respiratory movements, smooth muscle in lymphatic vessel walls, and one-way valves that prevent backflow.
Summary Cheat Sheet
| Concept / Topic | Key Details / Explanation |
|---|---|
| Blood Composition | Plasma ( Formed elements ( |
| Plasma Proteins | Albumin (~60%): maintains osmotic pressure, transport carrier Globulins (~35%): alpha, beta (transport), gamma (immunoglobulins/antibodies) Fibrinogen (~4%): blood clotting (converted to fibrin) Prothrombin: clotting factor (requires vitamin K) |
| RBCs (Erythrocytes) | Origin: red bone marrow (adults); liver and spleen in foetus Destruction: spleen ("graveyard of RBCs") Shape: biconcave disc (increases surface area for gas exchange) Count: Males 5–5.5 million/mm³, Females 4.5–5 million/mm³ Lifespan: ~120 days No nucleus in mature mammalian RBCs (exception: camels, llamas have nucleated RBCs) Rouleaux formation: RBCs stack like coins in slow-moving blood |
| Haemoglobin | 4 subunits: 2 alpha + 2 beta chains, each with a haem group containing Fe²⁺ Normal level: Males 14–18 g/dL, Females 12–16 g/dL Each Hb binds 4 O₂ molecules → oxyhaemoglobin (bright red) Deoxyhaemoglobin (dark red/bluish) |
| WBCs (Leucocytes) | Total count: 4000–11,000/mm³ Granulocytes: Neutrophils (60–70%, phagocytosis, first responders), Eosinophils (2–4%, allergy and parasites), Basophils (0.5–1%, release histamine and heparin) Agranulocytes: Lymphocytes (20–30%, T and B cells, immunity), Monocytes (3–8%, largest WBC, become macrophages) |
| Platelets (Thrombocytes) | Count: 1.5–3.5 lakh/mm³ Lifespan: 8–10 days Origin: megakaryocytes in bone marrow Function: blood clotting and wound sealing No nucleus (cell fragments) |
| ABO Blood Groups | A: antigen A on RBC, anti-B antibodies in plasma B: antigen B on RBC, anti-A antibodies AB: both antigens, no antibodies → universal recipient O: no antigens, both antibodies → universal donor Discovered by Karl Landsteiner (1900, Nobel 1930) |
| Rh Factor | Rh+ (antigen D present): ~85% of population Rh− (antigen D absent): ~15% Discovered in Rhesus monkey (1940, Landsteiner & Wiener) |
| Erythroblastosis Fetalis | Occurs when Rh− mother carries Rh+ foetus (2nd pregnancy) Mother's anti-Rh antibodies (IgG) cross placenta → destroy foetal RBCs Prevention: Anti-D immunoglobulin (RhoGAM) injection to mother within 72 hours of first delivery |
| Blood Clotting Cascade | Step 1: Injury → platelets + damaged tissue release thromboplastin Step 2: Thromboplastin + Ca²⁺ + clotting factors convert prothrombin → thrombin Step 3: Thrombin converts fibrinogen → fibrin (insoluble threads) → fibrin mesh traps RBCs → clot Requires: Vitamin K (for prothrombin synthesis), Ca²⁺ ions |
| Haemophilia | Haemophilia A: deficiency of Factor VIII (most common, X-linked recessive) Haemophilia B (Christmas disease): deficiency of Factor IX Affects males predominantly; females are carriers |
| Lymph | Colourless fluid, no RBCs, more lymphocytes than blood, more CO₂ than blood Functions: returns interstitial fluid to blood (~3 L/day), transports absorbed fats via lacteals, immune defense via lymph nodes No pump — flow depends on muscle contractions and valves |
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