The Circulatory System

The human heart is a muscular, four-chambered organ located in the thoracic cavity, slightly to the left of the sternum. It acts as a double pump, ensuring efficient circulation of blood throughout the body. The four chambers are the right atrium, right ventricle, left atrium, and left ventricle. Valves (tricuspid, bicuspid/mitral, pulmonary semilunar, aortic semilunar) prevent the backflow of blood. The cardiac cycle is the sequence of events in one complete heartbeat, involving a period of contraction (systole) and relaxation (diastole). The sinoatrial (SA) node, located in the right atrium, acts as the natural pacemaker, initiating the electrical impulses that cause the heart to contract. These impulses spread through the atria, causing them to contract (atrial systole), then pass to the atrioventricular (AV) node, and down the Bundle of His and Purkinje fibres to the ventricles, causing ventricular systole. Diastole is the relaxation phase when the heart chambers fill with blood.

Blood vessels form a closed network that transports blood. There are three main types: 1. Arteries: Carry blood away from the heart. They have thick, muscular, elastic walls to withstand high pressure. The lumen (internal space) is relatively narrow. The aorta is the largest artery. 2. Veins: Carry blood towards the heart. They have thinner, less muscular walls and a wider lumen compared to arteries, as blood pressure is lower. Veins contain valves to prevent the backflow of blood, especially in the limbs. The vena cavae are the largest veins. 3. Capillaries: Microscopic vessels that connect arterioles (small arteries) and venules (small veins). Their walls are only one cell thick, facilitating the efficient exchange of gases, nutrients, and waste products between blood and tissue cells.

Blood is a specialised connective tissue composed of a liquid matrix called plasma and various cellular components: 1. Plasma: The yellowish liquid component (about 55% of blood volume) consisting mainly of water (90%), proteins (e.g., albumin, globulins, fibrinogen), salts, hormones, nutrients, and waste products. It transports all blood cells and other substances. 2. Red Blood Cells (Erythrocytes): Biconcave discs, lacking a nucleus in mature form, containing haemoglobin. Their primary function is oxygen transport from the lungs to the tissues and carbon dioxide transport from tissues to the lungs. 3. White Blood Cells (Leucocytes): Larger than red blood cells, with a nucleus. They are part of the immune system, defending the body against infection and disease. Types include phagocytes (e.g., neutrophils, macrophages) and lymphocytes (B and T cells). 4. Platelets (Thrombocytes): Small, irregular cell fragments involved in blood clotting (haemostasis). When a blood vessel is damaged, platelets aggregate at the site, forming a plug. They also release clotting factors that initiate a cascade of reactions, converting prothrombin to thrombin, which then converts soluble fibrinogen into insoluble fibrin threads. These threads form a mesh that traps blood cells, forming a clot to prevent blood loss.

The lymphatic system is a network of vessels, nodes, and organs that works alongside the circulatory system. It has three main functions: 1. Drainage: It collects excess tissue fluid (interstitial fluid) that leaks from capillaries and returns it to the bloodstream. Once inside lymphatic vessels, this fluid is called lymph. 2. Immunity: Lymph nodes, located along lymphatic vessels, filter lymph and contain lymphocytes and phagocytes that destroy pathogens and foreign particles. Organs like the spleen, thymus, and tonsils are also part of the lymphatic system and play crucial roles in immune responses. 3. Fat Absorption: Specialised lymphatic capillaries called lacteals in the villi of the small intestine absorb digested fats and fat-soluble vitamins, transporting them to the bloodstream.