Transport in animals

• The purpose of a transport system is to transport nutrients and oxygen to the body cells and waste products from the cells to the lungs. It also helps to stabilise pH and temperature, fight disease, provide nourishments and maintain homeostasis.
• In single-celled organisms, diffusion, osmosis, active transport, endocytosis and exocytosis can meet the organism needs due to it's large SA:V ratio, smaller transport distances and lower levels of activity meaning a lower metabolic demand.
• In multi-celled organisms, their metabolic demand is higher, their SA:V ratio is higher and their transport distance is larger therefore diffusion and other passive transports are not enough to suffice, so a transport system is required.
• The features of a circulatory system include a liquid transport medium which circulates the system, a vessel that transports the medium and a pumping mechanism.
• An open circulatory system has very few vessels, it's main being the dorsal vessel. The haemolymph is pumped straight from the tubular heart into the dorsal vessel which delivers it to the main body cavity, haemcoel. Their is no particular direction of flow of the medium, though this inefficiency is counter-acted by the direct contact with the tissues + cells, so oxygen is delivered directly to them.
• Haemolymph is made up of little O2 and CO2, mostly of nutrients and nitrogenous waste.
• Closed circulatory system have their blood enclosed in vessels, therefore doesn't have direct contact with cells and tissues. The blood is pumped under high-pressure and returns directly to the heart. Substances enter and leave the body by difusion through the walls of the blood vessels.
• Blood vessels can vasoconstrict and vasodilate to adjust blood flow to particular tissues.
• A single closed circulatory system occurs when blood is pumped through the heart only once, such as in a fish. Deoxygenated blood is pumped to the gills capillaries where gases are exchanged. The oxygenated blood then travels through the rest of the body, delivering oxygen before it reaches the heart again.
• In an incomplete closed circuit, the blood passes through the heart twice, blood gets pumped to the lungs and to the body, but there is only one ventricle so oxygenated and deoxygenated blood are mixed.