in our last video we introduced the circulatory system and saw how the heart acts as the system's pump in this video though we'll take a closer look at the blood vessels specifically the arteries capillaries and veins to quickly summarize the arteries carry blood away from the heart the capillaries exchange nutrients and oxygen with the tissues and veins carry the blood back to the heart as you take a closer look at each the key thing to notice is how the structure of each type of vessel is related to its function as arteries carry blood directly from the heart the blood inside them is going to be at really high pressures because it's just been pumped out to the ventricles in order to bear these high pressures the arteries need to be both strong and elastic and to achieve this they have a thick layer of muscle tissue to keep them strong and a thick layer of elastic tissue so they can stretch and recoil these two layers though mean that the wall of the artery is quite thick when compared to lumen which is what we call the hole or space in the middle of the vessel once the arteries have transported the blood to the different parts of the body it enters the capillaries these vessels are really small and come into close contact with all of the cells in our body their role is to exchange substances with our cells giving them useful nutrients and oxygen and taking away waste products like carbon dioxide if we take a closer look at a single capillary we can see how they're adapted to this function for example their walls are only a single cell thick and they're also permeable which means that substances can easily diffuse through them because they're so tiny the lumen of an individual capillary is also tiny but there are so many of them that a total cross-sectional area which is the area of all the capillaries added together it's actually huge far higher than for arteries this means that the pressure of the blood is much lower than in the arteries and so it flows more slowly which gives the blood time to exchange things with the tissues finally these tiny capillaries start to join up into larger vessels called veins which transport the blood back to the heart veins are relatively large with by far the biggest lupines of any blood vessels they also have relatively thin walls with only small layers of elastic fibers and smooth muscle this works fine though because the blood that they carry is at very low pressures so the walls don't need to be very strong the important thing to remember about veins though is they have valves which prevent blood from flowing backwards ensuring that it always flows towards the heart before we finish let's quickly cover how to calculate the rate of blood flow a typical question could look something like this 2500 ml of blood passes through the aorta which is the big artery that leaves the heart in eight minutes calculate the rate of blood flow through the aorta in milliliters per minute so in a question like this we would just divide how much blood has flowed so 2560 by the time it took so eight minutes which gives us 320 milliliters per minute and that's all for the blood vessels if you haven't seen our other videos on the heart and the blood yet we put the links for those in the description below otherwise cheers for watching and i'll see you next time you