Deep Vein Thrombosis (DVT) is the formation of a blood clot (thrombus) in a deep vein, most commonly in the legs. Risk factors include prolonged immobility (long flights, bed rest), surgery, and obesity. The main danger is Pulmonary Embolism - when the clot breaks off and travels to the lungs. Symptoms include leg pain, swelling, and redness. Treatment: anticoagulants (blood thinners).

Angina Pectoris is chest pain or discomfort caused by reduced blood flow (ischaemia) to the heart muscle - usually due to partial narrowing of coronary arteries by atherosclerotic plaques. Unlike a heart attack, the heart muscle is not permanently damaged. Pain is typically triggered by physical exertion or stress and relieved by rest or nitroglycerin. It is a warning sign of significant coronary artery disease.

The Cardiac Conduction System pathway: SA Node (right atrium) generates impulse and atria contract. AV Node (junction of atria and ventricles) delays impulse. Bundle of His conducts impulse to interventricular septum. Right and Left Bundle Branches carry impulse down septum. Purkinje Fibres spread impulse throughout ventricular walls causing ventricles to contract. This ensures coordinated, sequential contraction of atria then ventricles.

Erythrocyte is another name for Red Blood Cell (RBC) - NOT a White Blood Cell. Types of WBCs (Leucocytes): Granulocytes include Neutrophils, Eosinophils, and Basophils. Agranulocytes include Lymphocytes and Monocytes. WBCs differ from RBCs in that they are larger, fewer in number, nucleated (have a nucleus), and colourless (lack haemoglobin). Each type of WBC has a specific role in immunity.

Blood pressure terminology: Systolic pressure = pressure during heart contraction (systole) - the higher value (120 mmHg normal). Diastolic pressure = pressure during heart relaxation (diastole) - the lower value (80 mmHg normal). Written as Systolic/Diastolic = 120/80 mmHg. Pulse Pressure = Systolic - Diastolic = 120 - 80 = 40 mmHg. Mean Arterial Pressure (MAP) = Diastolic + 1/3 Pulse Pressure = approximately 93 mmHg.

Thalassaemia is an inherited blood disorder caused by mutations in globin genes, resulting in reduced or absent haemoglobin production. Types: Alpha Thalassaemia (mutations in alpha-globin genes) and Beta Thalassaemia (mutations in beta-globin genes, more common). Thalassaemia Major (severe) requires regular blood transfusions. Thalassaemia Minor (trait/carrier) usually has mild or no symptoms. Common in Mediterranean, Middle East, South Asian, and African populations. Bone marrow transplant can be curative.

Lymphocytes (20-30% of WBCs) are the key cells of the adaptive immune system: B Lymphocytes (B cells) produce antibodies (immunoglobulins) against specific antigens. T Lymphocytes (T cells) perform cell-mediated immunity - kill infected cells directly (Cytotoxic T cells) or regulate immune response (Helper T cells). Natural Killer (NK) cells destroy cancer cells and virus-infected cells. Lymphocytes are produced in bone marrow and mature in lymph nodes, spleen, and thymus.

The Rh blood group system: Rh positive (Rh+) means the Rh antigen (D antigen) IS present on RBCs (~85% of population). Rh negative (Rh-) means the Rh antigen is ABSENT on RBCs (~15% of population). Named after the Rhesus monkey in which this antigen was first discovered. Clinical importance: blood transfusion compatibility and Erythroblastosis Fetalis (Rh- mother + Rh+ baby). Rh- individuals must receive Rh- blood during transfusion.

The Lymphatic System works closely with the circulatory system by: returning excess interstitial fluid (fluid that leaks from capillaries into tissues) back to the bloodstream via lymphatic vessels and the thoracic duct, absorbing dietary fats from the small intestine via lacteals into the lymphatic system, providing immune defense (lymph nodes filter lymph and trap pathogens), and transporting lymphocytes throughout the body. Without the lymphatic system, fluid would accumulate in tissues causing oedema.

Thrombocytopenia is a condition where the platelet count falls below normal (less than 1,50,000 per mm3). It leads to prolonged bleeding, easy bruising, and petechiae (small red spots on skin). Severe thrombocytopenia (below 20,000/mm3) can cause spontaneous internal bleeding. Causes include dengue fever (very common cause in India), bone marrow disorders, autoimmune conditions, and certain medications.