Respiratory and circulatory systems - gas exchange, blood and lymph
Gas exchange
Air passes the nose (warming, filtering), pharynx, larynx (voice), trachea, bronchi, bronchioles and alveoli - some 300 million, giving 70–80 m² of surface. Exchange is by diffusion across a wall one cell thick: alveolar oxygen pressure is high and blood oxygen pressure low, so oxygen enters the blood while carbon dioxide leaves it. Tidal volume is about 500 mL, vital capacity 3.5–4.5 L, adult rate 12–20 breaths per minute. Breathing is controlled by the medulla oblongata.
Blood
Plasma forms about 55%. Red cells carry haemoglobin, which transports roughly 97% of oxygen as oxyhaemoglobin; white cells defend; platelets clot. Carbon dioxide travels mostly as bicarbonate in plasma. Clotting: platelet factors convert prothrombin to thrombin (needing calcium and vitamin K), which turns fibrinogen into a fibrin mesh. ABO and Rh groups matter; O negative is the universal donor, AB positive the universal recipient.
Heart and lymph
Four chambers with double circulation; the left ventricle wall is thickest; valves stop backflow; the SA node is the pacemaker; normal blood pressure is about 120/80 mmHg and pulse about 72 per minute. Lymph is tissue fluid collected in lymph vessels, filtered in lymph nodes and returned to the blood, carrying lymphocytes.
Worked example
A road-crash victim is bleeding heavily and needs blood before grouping is complete. O negative red cells carry no A, B or D antigen, so the recipient's antibodies cannot attack them - the safest emergency choice.
Marks-losing traps
Anaemia lowers oxygen content although pulse oximetry may look normal; arteries carry blood away from the heart, so the pulmonary artery carries deoxygenated blood; the left side of the heart handles oxygenated blood.