15.1 Blood — A Special Connective Tissue
Blood is a fluid connective tissue with a liquid matrix called plasma in which formed elements are suspended.
| Component | Approx. share | Role |
|---|
| Plasma | ~55% | Liquid matrix, transport |
| Formed elements | ~45% (PCV) | RBC, WBC, platelets |
Plasma (90–92% water; 6–8% proteins)
- Fibrinogen — needed for blood clotting.
- Globulins — defence (antibodies).
- Albumins — maintain osmotic balance.
- Ions (Na⁺, Ca²⁺, Mg²⁺, HCO₃⁻, Cl⁻), glucose, amino acids, lipids and hormones are also transported.
Serum = plasma minus the clotting factors (fibrinogen).
15.2 Formed Elements — RBC, WBC & Platelets
Formed elements constitute about 45% of blood.
| Element | Count | Key facts |
|---|
| RBC (erythrocyte) | 5–5.5 million/mm³ | Biconcave, no nucleus (mammals), haemoglobin, life 120 days |
| WBC (leucocyte) | 6000–8000/mm³ | Nucleated, colourless, immunity, life 1–2 weeks |
| Platelet (thrombocyte) | 1.5–3.5 lakh/mm³ | Cell fragments, blood clotting |
RBC (Erythrocytes)
- Formed in the red bone marrow; destroyed in the spleen ("graveyard of RBCs").
- Carry haemoglobin (12–16 g per 100 mL blood) which transports O₂ and CO₂.
WBC (Leucocytes)
| Type | Group | % | Function |
|---|
| Neutrophil | Granulocyte | 60–65% | Phagocytosis (most abundant) |
| Eosinophil | Granulocyte | 2–3% | Allergy, resist infection |
| Basophil | Granulocyte | 0.5–1% | Release histamine, heparin (rarest) |
| Lymphocyte | Agranulocyte | 20–25% | Immunity (B & T cells) |
| Monocyte | Agranulocyte | 6–8% | Phagocytosis (largest WBC) |
🧠 Memory Hook
"Never Let Monkeys Eat Bananas"
WBC abundance, high → low: Neutrophil > Lymphocyte > Monocyte > Eosinophil > Basophil.
15.3 Blood Groups — ABO & Rh
The ABO system depends on surface antigens (A, B) on RBCs and natural antibodies in plasma.
| Group | Antigen (RBC) | Antibody (plasma) | Can donate to |
|---|
| A | A | anti-B | A, AB |
| B | B | anti-A | B, AB |
| AB | A and B | none | AB only |
| O | none | anti-A & anti-B | All (universal donor) |
O = universal donor; AB = universal recipient.
Rh Factor
- Rh antigen (like the rhesus monkey): Rh⁺ have it, Rh⁻ lack it.
- Erythroblastosis fetalis: an Rh⁻ mother carrying an Rh⁺ foetus may make anti-Rh antibodies that attack the foetal RBCs in a later Rh⁺ pregnancy.
🧠 Memory Hook
"O gives to all, AB takes from all"
O = no antigens → universal donor. AB = no antibodies → universal recipient.
15.4 Blood Clotting (Coagulation)
A clot or coagulum is a network of fibrin threads trapping blood cells. It seals injured vessels.
Injury → platelets release factors + thromboplastin→Prothrombin → Thrombin (needs Ca²⁺)→Fibrinogen → Fibrin→Fibrin mesh traps cells = clot
- Requires Ca²⁺ ions and clotting factors (numbered I–XIII).
- Heparin is a natural anticoagulant; vitamin K is needed for prothrombin synthesis.
Prothrombin --(thrombokinase + Ca²⁺)--> Thrombin
Fibrinogen --(thrombin)--> Fibrin (insoluble threads)
15.5 Lymph (Tissue Fluid)
Lymph is a colourless fluid formed when plasma (without large proteins and RBCs) filters out of capillaries into tissue spaces.
- Contains WBCs — mainly lymphocytes; no RBCs.
- Returns tissue fluid, proteins and absorbed fats (via lacteals) back to the blood.
- Part of the immune system; nodes filter pathogens.
| Feature | Blood | Lymph |
|---|
| RBC | Present | Absent |
| Proteins | High | Low |
| Direction | Circulates in vessels/heart | Tissue → back to blood |
15.6 Circulatory Patterns
Animals show open or closed circulation.
| Open | Closed |
|---|
| Blood flows through open spaces (haemocoel) | Blood confined to vessels |
| Low pressure, less efficient | High pressure, precise control |
| Arthropods, molluscs | Annelids, vertebrates |
Heart-based circulation
- Single circulation — fishes; blood passes through the heart once per cycle (2-chambered heart).
- Incomplete double — amphibians & most reptiles (3-chambered; mixing).
- Complete double — birds & mammals (4-chambered; no mixing).
15.7 The Human Heart — Structure
The heart is mesodermal, lies in the thoracic cavity between the lungs, protected by a double-walled pericardium with pericardial fluid.
- Four chambers: two thin-walled atria (upper) and two thick-walled ventricles (lower); the left ventricle is thickest (pumps to the whole body).
- Septa: inter-atrial, inter-ventricular and atrioventricular.
Valves (prevent backflow)
| Valve | Location | Cusps |
|---|
| Tricuspid | Right atrium → right ventricle | 3 |
| Bicuspid (mitral) | Left atrium → left ventricle | 2 |
| Pulmonary semilunar | Right ventricle → pulmonary artery | 3 |
| Aortic semilunar | Left ventricle → aorta | 3 |
🧠 Memory Hook
"LAB RAT"
Left Atrium → Bicuspid; Right Atrium → Tricuspid.
15.8 Conducting System & Cardiac Cycle
The heart is myogenic — it generates its own beat through specialised tissue.
SA node (pacemaker)→AV node→Bundle of His→Bundle branches→Purkinje fibres
- The SA node in the right atrium wall is the pacemaker, setting the rhythm (~70–75/min).
Cardiac cycle (~0.8 s at 72 bpm)
| Phase | Duration |
|---|
| Atrial systole | 0.1 s |
| Ventricular systole | 0.3 s |
| Joint diastole | 0.4 s |
- Heart sounds: "lub" (S1 — AV valves close) and "dub" (S2 — semilunar valves close).
Stroke Volume ≈ 70 mL
Cardiac Output = Stroke Volume × Heart Rate ≈ 70 × 72 ≈ 5000 mL/min (5 L)
15.9 ECG & Double Circulation
An electrocardiogram (ECG) records the heart’s electrical activity as standard waves.
| Wave | Represents |
|---|
| P wave | Atrial depolarisation (atria contract) |
| QRS complex | Ventricular depolarisation (ventricles contract) |
| T wave | Ventricular repolarisation (relaxation) |
Counting QRS complexes over a known time gives the heart rate.
Double circulation
Pulmonary: RA → RV → lungs → LA||Systemic: LA → LV → body → RA
- Pulmonary circuit carries deoxygenated blood to the lungs and oxygenated blood back.
- Systemic circuit delivers oxygenated blood to body tissues and returns deoxygenated blood.
15.10 Blood Vessels, Regulation & Disorders
Three vessel types form the closed circuit.
| Vessel | Carries | Wall | Valves |
|---|
| Artery | Blood away from heart | Thick, muscular, elastic | Absent |
| Vein | Blood toward heart | Thinner | Present |
| Capillary | Exchange site | One cell thick | — |
The pulmonary artery is the only artery carrying deoxygenated blood; the pulmonary vein is the only vein carrying oxygenated blood.
Regulation
- The cardiac centre in the medulla oblongata regulates heart activity via the autonomic nervous system.
- Sympathetic nerves & adrenaline increase the rate/force; parasympathetic (vagus) decreases it.
Disorders
- Hypertension — persistent high blood pressure (above ~140/90 mmHg).
- Coronary Artery Disease (atherosclerosis) — narrowing of coronary arteries by plaque.
- Angina pectoris — chest pain from insufficient O₂ to the heart muscle.
- Heart failure — the heart cannot pump enough blood for body needs.