Mammals have a closed double circulation system.

Mark scheme

Marking points from the official mark scheme (May/June 2017). The mark or mark type is shown in brackets.

1. [2] blood contained in (blood) vessels AW or blood contained in any three of heart, arteries, veins, capillaries ; systemic and pulmonary, systems / circulation ; A described if circulations not named e.g. for each complete circuit (round the body) blood passes through heart twice blood transported from heart to lungs and back, then to (rest of) body and back
2. [2] 1st and 5th boxes (2 and 5) correct; 2nd and 4th boxes (4 and 3) correct ;
3. [max 2] assume answer refers to arteries unless stated otherwise withstand / AW, higher pressure (of blood) ; prevent rupturing / bursting (from high blood pressure); I collapsing one from thicker/AW, tunica media ; more elastic, tissue / fibres, and (smooth) muscle tissue ; more / AW, elastic, tissue / fibres, to maintain, blood pressure / blood flow ; more (smooth) muscle to maintain, blood pressure / blood flow;
4. [max 4] carbon monoxide max 3 forms carboxyhaemoglobin ; A binds to haemoglobin less haemoglobin available to bind oxgen / haemoglobin has greater affinity for carbon monoxide / AW ; reduces, percentage saturation of haemoglobin (with oxygen) / AW A less oxygen binds to haemoglobin I prevents oxygen binding or reduces oxygen carrying capacity of, haemoglobin / blood ; nicotine max 3 increases heart rate; increases blood pressure; constricts, arteries / arterioles ; A vasoconstriction makes platelets sticky / promotes clotting / promotes thrombosis; for either damages, endothelium / tunica intima / lining of blood vessels ;