Circulation: The Heart, Blood, and Blood Types

The heart is a muscular organ, located in the centre of the chest, that acts as a pump to circulate blood throughout the body. It is divided into four chambers: two upper chambers called atria (singular: atrium) and two lower chambers called ventricles. The right side of the heart pumps deoxygenated blood to the lungs, while the left side pumps oxygenated blood to the rest of the body. Valves ensure blood flows in one direction, preventing backflow. Major blood vessels connected to the heart include the aorta, vena cavae (superior and inferior), pulmonary artery, and pulmonary veins.

Blood is a vital fluid composed of several key components, each with specific functions: 1. Plasma: The liquid matrix of blood, mostly water (about 90%), which carries dissolved nutrients, hormones, waste products, and proteins. 2. Red Blood Cells (Erythrocytes): Biconcave disc-shaped cells, lacking a nucleus, that contain haemoglobin. Their primary function is to transport oxygen from the lungs to the body tissues and carry some carbon dioxide back to the lungs. 3. White Blood Cells (Leucocytes): Larger cells with a nucleus, forming part of the immune system. They defend the body against infection by engulfing pathogens (phagocytes) or producing antibodies (lymphocytes). 4. Platelets (Thrombocytes): Small, irregular-shaped cell fragments that are crucial for blood clotting (haemostasis) to prevent excessive blood loss from injuries.

The ABO blood grouping system classifies human blood based on the presence or absence of specific antigens (A and B) on the surface of red blood cells and corresponding antibodies (anti-A and anti-B) in the blood plasma. There are four main blood types: 1. Type A: Has A antigens on red blood cells and anti-B antibodies in plasma. 2. Type B: Has B antigens on red blood cells and anti-A antibodies in plasma. 3. Type AB: Has both A and B antigens on red blood cells and no anti-A or anti-B antibodies in plasma. 4. Type O: Has no A or B antigens on red blood cells but has both anti-A and anti-B antibodies in plasma. Blood type compatibility is critical for safe blood transfusions, as transfusing incompatible blood can lead to a severe immune reaction (agglutination or clumping of red blood cells).

Blood circulates through two main pathways: 1. Pulmonary Circulation: Deoxygenated blood from the body enters the right atrium, then the right ventricle, and is pumped to the lungs via the pulmonary artery. In the lungs, carbon dioxide is released, and oxygen is picked up. Oxygenated blood then returns to the left atrium via the pulmonary veins. 2. Systemic Circulation: Oxygenated blood from the left atrium enters the left ventricle, which then pumps it into the aorta. The aorta branches into smaller arteries that distribute the oxygenated blood to all body tissues. After delivering oxygen and nutrients and collecting waste, deoxygenated blood returns to the right atrium via the vena cavae.