Evolution of the circulatory organs of animals
The circulatory system carries substances and gases, spreads heat and gives defence. It is absent in sponges, cnidarians and flatworms, where substances pass by diffusion. Annelids evolved a closed system; molluscs and arthropods have a heart but blood leaves the vessels into spaces among tissues (open system), and in insects haemolymph mainly carries food while tracheae deliver oxygen. In chordates the system is closed: in fish the heart has 2 chambers (one atrium, one ventricle) and blood flows in one circuit; in amphibians 3 chambers (2 atria, 1 ventricle), 2 circuits, but blood partly mixes; in reptiles the ventricle is partly divided (fully in crocodiles, though blood still mixes partly in the body); in birds and mammals the heart has 4 chambers, the systemic and pulmonary circuits are completely separate and arterial and venous blood do not mix. As a result tissues get more oxygen and a constant body temperature is possible (bird body temperature about 40–42 °C). Three steps of evolution: the origin of the heart and growth in the number of chambers, two circuits, and division of the heart into left (arterial) and right (venous) parts.
Count your pulse: count beats on the wrist artery for 30 seconds and multiply by 2, at rest and after 20 squats. Record results in a table. Safety: a child with a heart or breathing condition should not do the exercise.