Everything from the 4 lessons of this topic on a few pages: the key idea of each lesson, the diagram to remember, the points to learn and the traps examiners set. Read it, cover it, test yourself.
The circulatory system is a system of blood vessels with a pump (the heart) and valves that ensure a one-way flow of blood. Extended: a fish has a single circulation, in which blood passes through the heart once in each complete circuit of the body. A mammal has a double circulation: blood passes through the heart twice in each circuit, once on the way to the lungs and once on the way to the rest of the body.
A circulatory system is not just "the heart and blood". The syllabus description has three parts: blood vessels, a pump, and valves to ensure one-way flow. Name all three.
"Twice through the heart" is a description, not an advantage. For an explain the advantages question, say what the second pumping achieves: higher pressure, faster delivery of oxygen and glucose to respiring cells.
The heart is a muscular pump with a muscular wall, a septum, left and right atria, left and right ventricles, one-way valves and coronary arteries that supply its own muscle. Blood is pumped away from the heart in arteries and returns to the heart in veins. Heart activity can be monitored by an ECG, the pulse rate and listening to the sounds of the valves closing. Blockage of the coronary arteries causes coronary heart disease.
Diagrams show the heart as if you are facing the person, so the left ventricle is on the right of the page. Find the thick-walled ventricle or the aorta first, then label left and right.
Do not write that the heart beats faster "to give the muscles more energy" or "to make more oxygen". Blood delivers oxygen and glucose faster so the muscle cells can release more energy by respiration.
Coronary arteries carry blood to the heart muscle, not blood through the chambers. The heart cannot use the blood inside its chambers for its own oxygen supply, which is why a blocked coronary artery is so serious.
Arteries have thick walls and a narrow lumen; veins have thin walls, a wide lumen and valves; capillaries have walls only one cell thick and a very narrow lumen. Capillaries exchange substances between the blood and the tissues. The main vessels are the vena cava, aorta, pulmonary artery and pulmonary vein (heart and lungs) and the renal artery and renal vein (kidneys).
Do not say arteries carry oxygenated blood and veins carry deoxygenated blood. The definition is direction: arteries carry blood away from the heart and veins carry it back. The pulmonary artery carries deoxygenated blood.
"Capillaries are thin" is too vague. Say the wall is one cell thick, so the diffusion distance is short. The lumen is narrow, which is a different point.
The hepatic portal vein runs from the gut to the liver, not from the liver to the heart. Blood leaves the liver in the hepatic vein.
Blood is made of red blood cells, white blood cells, platelets and plasma. Red blood cells transport oxygen using haemoglobin; white blood cells carry out phagocytosis and antibody production; platelets are involved in clotting; plasma transports blood cells, ions, nutrients, urea, hormones and carbon dioxide. Clotting prevents blood loss and the entry of pathogens.
Plasma, not red blood cells, carries most carbon dioxide, along with glucose, urea and hormones. Red blood cells carry oxygen. Do not write that plasma carries oxygen.
Lymphocytes make antibodies; they do not engulf pathogens. Phagocytes engulf pathogens; they do not make antibodies. Match the cell to its job.
Fibrinogen is soluble and always present in plasma; fibrin is the insoluble form that makes the mesh. Write "fibrinogen is converted to fibrin", not the other way round.