Heart and circulation
Mammals have a double circulation: blood passes through the heart twice per circuit, once to the lungs (pulmonary) and once to the body (systemic), keeping body pressure high. Deoxygenated blood enters the right atrium from the vena cava, passes to the right ventricle, and is pumped through the pulmonary artery to the lungs. Oxygenated blood returns in the pulmonary vein to the left atrium, then the left ventricle, which pumps it out through the aorta. Valves stop backflow. The left ventricle wall is thickest. Coronary heart disease happens when fatty deposits block the coronary arteries.
≈ 45 minBlood vessels and blood
Arteries carry blood at high pressure away from the heart: thick muscular, elastic walls, narrow lumen. Veins carry blood at low pressure back to the heart: thin walls, wide lumen, valves. Capillaries are one cell thick, allowing exchange with tissues. Blood contains red blood cells (haemoglobin carries oxygen), white blood cells (phagocytes engulf pathogens; lymphocytes make antibodies), platelets (clotting) and plasma (carries CO₂, glucose, urea, hormones, heat).
≈ 40 min