The heart is the central organ of the circulatory system. It acts as a muscular pump that continuously pumps blood throughout the body. The human heart beats approximately 72 times per minute (at rest) and pumps about 5 litres of blood per minute. It is located in the thoracic cavity, slightly to the left of the centre of the chest.

The human heart has 4 chambers: Right Atrium (receives deoxygenated blood from the body), Right Ventricle (pumps deoxygenated blood to the lungs), Left Atrium (receives oxygenated blood from the lungs), and Left Ventricle (pumps oxygenated blood to the entire body). The left ventricle has the thickest wall as it pumps blood against the highest pressure.

The normal resting heart rate for a healthy adult is 60-100 beats per minute (bpm). The average is approximately 72 bpm. Athletes may have lower resting heart rates (40-60 bpm) due to stronger, more efficient hearts. A heart rate above 100 bpm is called Tachycardia and below 60 bpm is called Bradycardia.

Arteries carry blood away from the heart to the body tissues (mnemonic: Arteries = Away). Most arteries carry oxygenated blood EXCEPT the pulmonary artery, which carries deoxygenated blood from the right ventricle to the lungs. Arteries have thick, muscular, elastic walls to withstand high blood pressure. The largest artery is the Aorta.

Veins carry blood towards the heart. Most veins carry deoxygenated blood EXCEPT the pulmonary veins, which carry oxygenated blood from the lungs to the left atrium. Veins have thinner walls than arteries and contain valves to prevent backflow of blood. The largest veins are the Superior and Inferior Vena Cava.

Plasma is the liquid component of blood, making up approximately 55% of total blood volume. It is a straw-yellow coloured fluid composed of 90% water and contains proteins (albumin, globulin, fibrinogen), hormones, nutrients, waste products, gases, and electrolytes. Serum is plasma without clotting factors (fibrinogen removed).

The Aorta is the largest and main artery in the human body. It arises directly from the left ventricle and carries oxygenated blood to the entire body (except lungs). The aorta has a diameter of approximately 2.5-3.5 cm. It arches over the heart and descends through the chest and abdomen, branching into smaller arteries that supply all organs and tissues.

The Spleen is called the Graveyard of RBCs because it is the primary site where old, damaged, and worn-out RBCs (after their 120-day lifespan) are destroyed and removed from circulation. The spleen also stores blood, filters blood, and produces lymphocytes. The liver also destroys some old RBCs. The iron from destroyed haemoglobin is recycled for new RBC production.

The AV (Atrioventricular) Node is located at the junction of the atria and ventricles in the right atrium. After receiving the electrical impulse from the SA node, the AV node delays the impulse by ~0.1 second before passing it to the ventricles via the Bundle of His. This delay allows the atria to fully contract and empty before the ventricles contract - ensuring efficient, coordinated pumping.

The Inferior Vena Cava (IVC) is the largest vein in the human body. It carries deoxygenated blood from the lower body (below the diaphragm - legs, abdomen, pelvis) back to the right atrium of the heart. The Superior Vena Cava (SVC) carries deoxygenated blood from the upper body (head, neck, arms) to the right atrium. Together they are the main venous drainage of the body.