Blood is a fluid connective tissue constituting 6-8% of body weight, comprising plasma (55%) and formed elements (45%), with plasma containing albumin (osmotic pressure), globulins (immunity), and fibrinogen (clotting). Red blood cells are biconcave, anucleate discs with a 120-day lifespan, formed in red bone marrow and destroyed in the spleen — the "graveyard of RBCs." White blood cells are classified as granulocytes (neutrophils at 60-65%, eosinophils at 2-3%, basophils at 0.5-1%) and agranulocytes (lymphocytes at 20-25%, monocytes at 6-8%), with basophils uniquely releasing histamine and heparin. In the ABO system, Group O (universal donor, no antigens) and Group AB (universal recipient, no antibodies) are the most clinically important; the true universal donor is O-negative and the true universal recipient is AB-positive. Erythroblastosis fetalis occurs in the second or later pregnancies of an Rh-negative mother carrying an Rh-positive foetus, preventable by RhoGAM administration within 72 hours after first delivery. The sinoatrial node (SAN), located in the right atrium, is the heart's primary pacemaker at 70-75 bpm, and the impulse travels to the AVN (0.1-second delay), Bundle of His, and Purkinje fibres for coordinated ventricular contraction from apex upward. One cardiac cycle takes 0.8 seconds: atrial systole (0.1 s), ventricular systole (0.3 s), and joint diastole (0.4 s — the longest phase), with cardiac output equalling stroke volume × heart rate ≈ 5 L/min at rest. ECG records P (atrial depolarization), QRS (ventricular depolarization — not contraction), and T (ventricular repolarization) waves, with atrial repolarization masked within the QRS complex. The most critical exception in this chapter: pulmonary arteries carry deoxygenated blood and pulmonary veins carry oxygenated blood — the reverse of all other vessels. Major circulatory disorders include hypertension (>140/90 mmHg, the silent killer), CAD (atherosclerosis), angina pectoris (reversible ischemia), and heart failure (inadequate pump function causing breathlessness and edema).