Blood and its functions
The body’s internal environment is made of three fluids – blood, tissue fluid and lymph – which circulate into one another: part of the plasma seeps out of capillaries between the cells, the extra fluid becomes lymph and flows back into the bloodstream along lymph vessels. Blood has three main functions. Transport: carries oxygen and carbon dioxide, nutrients, hormones and waste products. Regulation: delivers hormones and helps keep body temperature and water–salt balance steady. Protection: leukocytes swallow and neutralise microbes, and antibodies in plasma bind them. Blood is about 5 times more viscous than water, due to red cells and proteins. Osmotic pressure depends on the salts in plasma; 0.9% salt solution (isotonic solution) does not harm blood cells, while pure water can make cells swell and burst. An adult has about 5 litres of blood; losing a lot of blood is dangerous.
In your notebook draw the scheme plasma → tissue fluid → lymph → blood. On each arrow write how the passage occurs (capillary wall, lymph vessel).