Polycythaemia (Erythrocytosis) is a condition where there is an abnormal increase in RBC count, making the blood thicker and more viscous. This increases the risk of blood clots, stroke, and heart attack. Types: Relative Polycythaemia (due to dehydration - plasma volume decreases), Absolute Polycythaemia (true increase in RBC mass), Secondary Polycythaemia (due to chronic low oxygen - high altitude, lung disease), and Polycythaemia Vera (a bone marrow cancer).

Cardiac Tamponade is a life-threatening emergency where excess fluid accumulates in the pericardial sac (space between the heart and pericardium), compressing the heart and preventing it from filling properly. This severely reduces cardiac output. Classic signs: Beck's Triad - low blood pressure, distended neck veins, and muffled heart sounds. Causes include trauma, cancer, and infections. Treatment: emergency drainage (pericardiocentesis).

The Thoracic Duct is the largest lymphatic vessel in the body. It collects lymph from the left side of the head, neck, and chest, both legs and the abdomen, and the left arm. It empties lymph into the left subclavian vein (returning it to bloodstream). The right lymphatic duct drains the right side of the upper body into the right subclavian vein. Together they complete the return of lymph to circulation.

The Spleen is a multifunctional organ: it filters blood (removes old, damaged RBCs and foreign particles), destroys old RBCs (graveyard of RBCs - after 120 days), stores blood (acts as a reservoir, releases blood during emergencies), performs immune function (produces lymphocytes and antibodies), and performs fetal haematopoiesis (produces RBCs in the fetus). The spleen is located in the upper left abdomen and is the largest lymphoid organ in the body.

The Bundle of His (Atrioventricular Bundle) is a collection of specialized cardiac muscle fibres that conducts the electrical impulse from the AV node to the ventricles. The cardiac conduction pathway: SA Node > AV Node > Bundle of His > Right and Left Bundle Branches > Purkinje Fibres > Ventricular muscle contraction. This system ensures that the ventricles contract in a coordinated, efficient manner from the apex upward to eject blood into the arteries.

Haematocrit (HCT / Packed Cell Volume / PCV) is the percentage of total blood volume occupied by Red Blood Cells. Normal values: Males: 40-52%, Females: 36-48%, Average: ~45%. Low haematocrit indicates Anaemia. High haematocrit indicates Polycythaemia (increased risk of clotting). Haematocrit is measured by centrifuging a blood sample - RBCs settle at the bottom, plasma floats on top. It is a key indicator of oxygen-carrying capacity of blood.

Normal platelet count is 1,50,000-4,00,000 per mm3 of blood. Platelet abnormalities: Thrombocytopenia = count below 1,50,000/mm3 (risk of bleeding - common in dengue fever). Thrombocytosis = count above 4,00,000/mm3 (risk of clotting). Critical low = below 20,000/mm3 (risk of spontaneous bleeding). Platelets are produced from large cells called Megakaryocytes in the bone marrow and have a lifespan of 7-10 days.

Capillaries have walls that are only one endothelial cell thick - making them the site of exchange between blood and tissues: Oxygen and nutrients diffuse FROM blood INTO tissues, CO2 and waste products diffuse FROM tissues INTO blood, and water and small molecules move by osmosis and diffusion. Their extremely thin walls and vast surface area (total surface area approximately 6,000 m2 in the human body) make them perfectly designed for efficient exchange. Blood flows slowest through capillaries - allowing maximum time for exchange.

The Coronary Arteries arise from the ascending aorta just above the aortic valve (in the coronary sinuses). They supply oxygenated blood to the myocardium: Left Main Coronary Artery divides into Left Anterior Descending (LAD) and Left Circumflex arteries. Right Coronary Artery (RCA) supplies the right ventricle and inferior left ventricle. The LAD is called the Widow Maker because its blockage causes a massive, often fatal heart attack.

Veins have several unique features: they contain valves (semi-lunar valves) that prevent backflow of blood - especially important in leg veins where blood must travel upward against gravity. They have thinner walls than arteries (less muscle and elastic tissue), carry blood towards the heart (mostly deoxygenated, except pulmonary veins), have larger lumens than arteries of the same size, and blood flows at lower pressure in veins than arteries.