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 mammalian circulatory system is a closed double circulation made of a heart, blood and blood vessels: arteries, arterioles, capillaries, venules and veins. Closed means the blood stays inside vessels. Double means it passes through the heart twice per complete circuit: the pulmonary circulation (right ventricle, pulmonary artery, lungs, pulmonary vein, left atrium) and the systemic circulation (left ventricle, aorta, body, vena cava, right atrium). Elastic arteries near the heart stretch and recoil to smooth the flow; muscular arteries have thick smooth muscle to control flow to organs; veins carry low-pressure blood in a wide lumen with thin walls and valves; capillaries have walls one endothelial cell thick for rapid exchange.
Arteries carry blood away from the heart and veins carry blood towards it. Do not define them by oxygen: the pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood.
An artery has a small lumen relative to its thick wall; a vein has a wide lumen and often looks squashed or irregular on a slide because its thin wall does not hold its shape.
Blood contains red blood cells (biconcave, no nucleus, full of haemoglobin) and white cells: neutrophils (lobed nucleus, granular cytoplasm, phagocytes), monocytes (the largest, with a kidney-shaped nucleus, becoming macrophages) and lymphocytes (small, with a large round nucleus filling most of the cell). Water is the main component of blood plasma and of tissue fluid. Its solvent action lets it carry glucose, ions, amino acids, urea and hormones in solution, and its high specific heat capacity lets blood carry heat without large temperature changes. Tissue fluid bathes the cells, supplying them with oxygen, glucose and ions and removing wastes such as carbon dioxide. It forms at the arterial end of a capillary, where hydrostatic pressure forces fluid and small solutes out; plasma proteins stay in, so at the venous end, where hydrostatic pressure has fallen, water returns by osmosis. Excess fluid drains into lymph vessels.
A lymphocyte's nucleus nearly fills the cell, leaving a thin rim of cytoplasm; a monocyte is much bigger with a bean-shaped nucleus. Do not call any cell with a large nucleus a monocyte.
Tissue fluid is not the same as plasma. It has almost no plasma proteins and no red blood cells, because these are too large to leave the capillary.
Each haemoglobin molecule has four polypeptide chains, each with a haem group that binds one O2, so it carries up to four oxygen molecules. Binding is cooperative: once one O2 binds, the molecule changes shape and the next bind more easily, giving the S-shaped (sigmoid) oxygen dissociation curve. In the lungs (high partial pressure of oxygen) haemoglobin is almost fully saturated; in respiring tissues (low pO2), on the steep part of the curve, a small fall in pO2 releases a lot of oxygen. Carbon dioxide diffuses into red blood cells, where carbonic anhydrase catalyses CO2 + H2O → H2CO3, which dissociates to H+ and HCO3-. H+ combines with haemoglobin to form haemoglobinic acid, releasing oxygen and buffering the blood. HCO3- diffuses into the plasma, balanced by Cl- moving in: the chloride shift. Some CO2 binds haemoglobin as carbaminohaemoglobin and a little dissolves in plasma. High CO2 lowers haemoglobin's affinity for oxygen and moves the curve to the right: the Bohr shift.
A curve shifted to the right means lower affinity: haemoglobin is less saturated at any given pO2 and gives up oxygen more readily. Do not say the Bohr shift helps haemoglobin pick up oxygen.
The chloride shift moves chloride ions into the red blood cell as hydrogencarbonate ions move out. It is about balancing charge, not about carrying carbon dioxide.
The heart has four chambers: thin-walled atria receive blood from the vena cava (right) and pulmonary vein (left) and pump it a short distance into the ventricles. The left ventricle has a thicker muscular wall than the right because it pumps blood at high pressure all round the body, while the right ventricle pumps only to the nearby lungs at lower pressure. The septum keeps oxygenated and deoxygenated blood apart. Atrioventricular valves stop backflow into the atria; semilunar valves stop backflow from the aorta and pulmonary artery. Valves open and close only because of pressure differences. In the cardiac cycle, atrial systole tops up the ventricles, ventricular systole closes the AV valves and opens the semilunar valves, and in diastole the semilunar valves close and the AV valves open as the heart refills. The sinoatrial node starts each wave of excitation, the atrioventricular node delays it, and the Purkyne tissue carries it to the apex so the ventricles contract from the bottom up.
The ventricles have thicker walls than the atria because of the distance and resistance they pump against, not because they hold more blood. Both ventricles pump the same volume per beat.
The AVN does not start the heartbeat; the SAN does. The AVN's job is the delay.