 We've learnt about how various animal functions rely on the transport of molecules at the cell level by active, passive or bulk transport across the cell membrane. This style of transport is very important, but it's not efficient over long distances to keep a whole animal functioning. Nidarians, animals including coral, hydra and jellyfish, have such thin bodies that they only need a gastrovascular cavity, a space inside them that's open to the ocean that allows for the flow of fluids including digestion and excretion. They have shapes that allow all their cells to be quite close to the cavity in order for diffusion to work. For long distance bulk transport, most animals need a circulatory system. This includes a special fluid like blood, plumbing like arteries and a pump system like a heart. Vertebrates including humans, earthworms, squids and octopuses have blood which travels around the body and cells that sit in interstitial fluid. The interstitial fluid acts as a go-between both for molecules coming or going between blood and other cells. This is called a closed circulatory system or cardiovascular system. Insects and mollusks have open circulatory systems. This means they have just one fluid, hemolymph, that does the job of our blood and interstitial fluid. All vertebrates have a heart with two or four chambers or atria and ventricles. They also have blood vessels of three types, arteries, veins and capillaries. Arteries carry blood away from the heart and towards capillaries. Veins carry blood from capillaries and towards the heart. The capillaries are where the diffusion of molecules into the interstitial fluid around cells can happen. The circulatory system is interlinked with the respiratory system. Blood cells pick up oxygen in the lungs and then travel to the heart to be pumped around the body. Arteries and veins are made of the same tissue types but are put together in different ways that suit their function. They have three layers. The outermost layer is made of stretchy, connective tissue that allows the vessel to be flexible. The middle layer is made of smooth muscle which gives the vessel strength and shape. The inner layer is a one cell thick lining called endothelium. While their tissue types are the same, the tissues of arteries are thicker to cope with the uneven speed and pressure of the blood rushing within them. Veins on the other hand are thinner and also have one-way valves that stop blood flowing back towards capillaries. Capillaries only have the endothelium layer. It's smooth and thin and allows for diffusion and active transport in and out. While capillaries are very tiny, they more than make up for their carrying capacity in number. There are so many of them arranged in intricate networks that blood flow actually slows down in them. The combined surface area of capillaries in any area compared with the arteries leading to the area is very large. This slow down of movement enables more transport and exchange across the membranes into the interstitial fluid in both directions. Some molecules move out of the blood down a concentration gradient. Some molecules including water, sugar and salts are pushed out with the blood pressure. And other molecules are carried by endocytosis by the endothelial cells. As the blood with its new solute concentrations all recombines into the veins, it speeds up again because of the decreased surface area or pipe diameters.