Transportation

Blood carries food, oxygen, carbon dioxide and wastes around the body. It has a fluid part, plasma, with red blood cells (which carry oxygen) and platelets floating in it. The heart pumps it through tubes: arteries, capillaries and veins.

The heart has four chambers. One rule makes it easy, LO–RD: the Left side carries Oxygenated blood, the Right side Deoxygenated blood.

• Oxygen-rich blood comes from the lungs into the left atrium, then the left ventricle, which pumps it to the whole body.
• Used blood comes back from the body into the right atrium, then the right ventricle, which pumps it to the lungs.

Valves stop blood flowing backwards.

So blood passes through the heart twice in each round of the body: once on the way to the lungs, once on the way to the body. This is double circulation. It keeps oxygen-rich and oxygen-poor blood apart, which gives a steady supply of oxygen. Birds and mammals need this because they use a lot of energy to keep their bodies warm. Amphibians and many reptiles have three chambers; fish have two.

The three kinds of tube:

Blood pressure is the force of blood on the walls of the vessels, measured with a sphygmomanometer. Normal is about 120/80 mm of mercury. High blood pressure (hypertension) happens when small arteries narrow; it can burst an artery and cause internal bleeding.

Fall off a bicycle and the cut soon stops bleeding: platelets help the blood clot. Lymph is a clear fluid that drains from the spaces between cells back into the blood; it also carries the fat absorbed from the intestine.

Plants have two transport tissues:

• xylem carries water and minerals up from the roots. At night the roots push it up (root pressure); by day, water evaporating from the leaves (transpiration) pulls it up;
• phloem carries food (sugars) from the leaves to wherever it's needed, up or down. This is translocation, and it uses energy from ATP.