B5.1

• cell→tissue→organ→organ system→organism
• the digestive system is made up of the mouth, oesophagus, stomach, pancreas, intestines and rectum
• the digestive system is responsible for breaking down food into soluble particles and removing undigested food from the body
• the excretory system is made up of the kidneys and bladder
• the excretory system is responsible for removing waste products that have been made by the body
• the circulatory system is made up of the heart, blood and blood vessels
• the circulatory system transports useful and waste products around the body between other systems
• the gas exchange system is made up of the nose, mouth,trachea and lungs
• the gas exchange system is responsible for oxygenating the blood, and removing carbon dioxide from it
• substances cells need: water and food molecules, oxygen
• waste products: excess water and salts, urea and  undigested food, carbon dioxide
• exchange surfaces are surfaces that are adapted to maximise the efficiency of gas and solute exchange across them
• multicellular organisms cannot rely on diffusion to reach all their cells, so they have specialised surfaces
• where substances are exchanged through blood in animals, exchange surfaces are densely packed with blood vessels
• blood vessels replenish the blood supply to maintain a high concentration gradient by bringing in new blood as diffusion starts to even out the concentrations
• exchange surfaces have a large surface area that allows more of a substance to diffuse at the same time
• in animals, if a gas is exchanged, the surface is ventilated (through breathing) to maintain a high concentration gradient and increase the rate of exchange
• a thin membrane on an exchange surface reduces the diffusion distance
• white blood cells, platelets and red blood cells are suspended in blood plasma
• red blood cells transport oxygen from the lungs to all body cells
• haemoglobin is found in red blood cells, it binds with oxygen in the lungs
• haemoglobin carries oxygen which will then be released to the body cells to be used for respiration
• red blood cells have a biconcave shape which creates a large surface area and allows for the rapid diffusion of oxygen
• red blood cells do not have a nucleus, this frees up more room for haemoglobin and maximises the amount of oxygen that they can carry
• red blood cells are small so that they can pass through tiny capillaries
• white blood cells defend against infections
• unlike red blood cells, white blood cells have a nucleus
• white blood cells can change shape, this allows them to squeeze through the walls of blood vessels into body tissues and to engulf harmful microorganisms
• platelets are small cell fragments that do not have a nucleus
• platelets are responsible for triggering blood clotting at the sites of wounds
• blood moves around the body in arteries, capillaries and veins
• arteries transport blood from the heart to the organs
• arteries all carry oxygenated blood apart from the pulmonary artery
• artery walls have thick layers of muscle, this makes them strong and able to cope with the high pressure at which blood is pumped out by the heart
• artery walls have elastic fibres, allowing them to stretch and spring back (recoil)
• arteries branch into much smaller vessels, called capillaries
• capillaries have thin walls and pass very close to the body cells
• in capillaries, waste products, such as carbon dioxide, move out of the cells and into the blood
• in capillaries, food and oxygen moves out of the blood and into the cells
• veins form when capillaries join up after passing through the body