Transport in mammals
Blood vessels and haemoglobin
Arteries: thick muscle and elastic tissue, narrow lumen, high pressure. Veins: wide lumen, thin walls, valves, low pressure. Capillaries: one-cell-thick endothelium, short diffusion distance. Tissue fluid forms at the arterial end of capillaries where hydrostatic pressure exceeds the water potential effect of plasma proteins; it returns at the venous end and via lymph. Haemoglobin binds O₂ cooperatively, giving an S-shaped (sigmoid) dissociation curve: high saturation in lungs, unloading in respiring tissues. Bohr shift: more CO₂ lowers affinity (curve shifts right), so more O₂ is released where it's needed. Fetal haemoglobin has higher affinity (curve left). CO₂ is carried mostly as HCO₃⁻ (carbonic anhydrase in red cells), with the chloride shift balancing charge.
≈ 50 minThe heart and cardiac cycle
Mammals have a double circulation: pulmonary (right ventricle → lungs) and systemic (left ventricle → body). The left ventricle wall is thickest to pump blood at high pressure. Cardiac cycle: atrial systole (atria contract, AV valves open), ventricular systole (ventricles contract; pressure rises; AV valves close when ventricular pressure exceeds atrial; semilunar valves open when it exceeds arterial), diastole (relaxation; semilunar valves close; atria fill). Valves open and close only due to pressure differences. Heartbeat is myogenic: the SAN (pacemaker) sends a wave of excitation over the atria; the AVN delays it, then passes it down the bundle of His and Purkyne fibres so ventricles contract from the apex up. Cardiac output = stroke volume × heart rate.
≈ 45 min