Learning Objectives
- Understand the composition and functions of blood and lymph
- Learn about blood groups (ABO and Rh) and their significance
- Study the structure and functioning of the human heart
- Understand the cardiac cycle, cardiac output, and ECG
- Learn about circulatory pathways and disorders
Key Concepts
Blood Composition
Blood is a fluid connective tissue (~5-5.5 litres in adults). Components: Plasma (~55%) and Formed elements (~45%). Plasma: 90-92% water, 6-8% proteins (albumin — osmotic balance, globulin — immunity, fibrinogen — clotting), minerals, glucose, amino acids, hormones, gases. Serum = Plasma - Clotting factors.
Formed elements: (1) RBCs (Erythrocytes): Biconcave, enucleated (in mammals), contain haemoglobin. Count: 5-5.5 million/mm3 (males). Lifespan: 120 days. Formed in red bone marrow, destroyed in spleen (graveyard of RBCs). (2) WBCs (Leucocytes): 6000-8000/mm3. Types: Granulocytes (Neutrophils — most abundant 60-65%, phagocytic; Eosinophils — allergic reactions, anti-parasitic; Basophils — release histamine, heparin) and Agranulocytes (Lymphocytes — B cells produce antibodies, T cells — cell-mediated immunity; Monocytes — largest WBCs, become macrophages). (3) Platelets (Thrombocytes): 1.5-3.5 lakhs/mm3. Cell fragments, involved in blood clotting.
Blood Groups
ABO system: Discovered by Karl Landsteiner. Based on antigens on RBC surface and antibodies in plasma. Group A (antigen A, anti-B antibody), Group B (antigen B, anti-A antibody), Group AB (both antigens, no antibodies — universal recipient), Group O (no antigens, both antibodies — universal donor). Controlled by gene I with three alleles: IA, IB, i.
Rh factor: Rh+ (antigen present), Rh- (antigen absent). Erythroblastosis foetalis: Rh- mother carrying Rh+ foetus; mother develops anti-Rh antibodies that can destroy RBCs of subsequent Rh+ foetuses.
Blood Coagulation
Cascade of events: Injured tissue releases thromboplastin → Prothrombin → Thrombin (requires Ca2+) → Fibrinogen → Fibrin (insoluble threads forming clot). Platelets release platelet factors. Vitamin K required for synthesis of prothrombin.
Human Heart
Four-chambered: 2 atria (upper) + 2 ventricles (lower). Myogenic heart (contraction initiated by heart muscle itself). SAN (Sinoatrial Node) in right atrium — pacemaker, initiates heartbeat. Impulse pathway: SAN → AVN (Atrioventricular Node) → Bundle of His → Purkinje fibres. Heart rate: ~72 beats/min. Tricuspid valve (right side), Bicuspid/Mitral valve (left side), Semilunar valves (at base of aorta and pulmonary artery).
Cardiac Cycle
One complete heartbeat (~0.8 seconds). Phases: Atrial systole (0.1s — atria contract, blood pushed to ventricles), Ventricular systole (0.3s — ventricles contract, blood pushed to arteries), Joint diastole (0.4s — all chambers relax, blood fills atria). Cardiac output: Stroke volume (70 mL) x Heart rate (72/min) = ~5 L/min. Lub sound: AV valves close (start of ventricular systole). Dub sound: semilunar valves close (start of ventricular diastole).
ECG (Electrocardiogram)
Graphical representation of electrical activity of heart. P wave: Atrial depolarization (atrial systole). QRS complex: Ventricular depolarization (ventricular systole). T wave: Ventricular repolarization (ventricular relaxation).
Double Circulation
Pulmonary circulation: Right ventricle → Pulmonary artery → Lungs (oxygenation) → Pulmonary veins → Left atrium. Systemic circulation: Left ventricle → Aorta → Body tissues → Vena cava → Right atrium. Hepatic portal system: Blood from gut passes through liver before returning to heart.
Disorders
Hypertension: Blood pressure higher than 120/80 mmHg. Atherosclerosis: Deposition of cholesterol plaques in arteries. Coronary Artery Disease (CAD): Narrowing of coronary arteries. Heart failure: Heart cannot pump blood effectively.
Summary
Blood consists of plasma and formed elements (RBCs, WBCs, platelets). ABO and Rh blood grouping systems are critical for transfusion. The human heart is myogenic with the SAN as pacemaker. The cardiac cycle involves atrial and ventricular systole and diastole. Double circulation involves pulmonary and systemic circuits. ECG records electrical activity of the heart.
Important Terms
- Pacemaker (SAN): Sinoatrial node that initiates heartbeat impulse
- Cardiac output: Volume of blood pumped per minute (~5 L/min)
- Erythroblastosis foetalis: Haemolytic disease of Rh+ newborn from Rh- mother
- Double circulation: Two separate circuits — pulmonary and systemic
- ECG: Electrocardiogram showing electrical activity of heart
- Thrombocytes: Platelets involved in blood clotting
- Serum: Plasma without clotting factors
- Atherosclerosis: Fatty deposit build-up in artery walls
Quick Revision
- Blood: Plasma (55%) + Formed elements (45%)
- RBCs: 5 million/mm3, 120 days lifespan, made in bone marrow, destroyed in spleen
- Universal donor: O; Universal recipient: AB
- SAN = pacemaker → AVN → Bundle of His → Purkinje fibres
- Cardiac cycle: 0.8s (atrial systole 0.1s + ventricular systole 0.3s + diastole 0.4s)
- Cardiac output = Stroke volume x Heart rate = ~5000 mL/min
- ECG: P wave (atrial), QRS complex (ventricular), T wave (repolarization)
- Lub = AV valves close; Dub = semilunar valves close