Every cell in your body needs a supply of oxygen and glucose for respiration and small amounts of minerals and other substances, they also need water and to get rid of waste products that build up in the like carbon dioxide. All these things are delivered, or carried away, through the blood.
Transfers of substances like water and oxygen happen by diffusion or osmosis but things such as sodium ions may need to be pumped into the celll by active transport.
Firstly I’m going to discuss how oxygen moves around the circulatory system. Haemoglobin molecules contains four iron atoms that can bind with oxygen to transport it through the blood. One of the properties of Haemoglobin that makes it such a suitable Oxygen carrier is how it’s saturation depends on the concentration of oxygen. That is in high concentrations of oxygen (such as in the lungs) it is easy for it to bind with Oxygen it generally leaves the lungs about 98.99% saturated. In low concentrations of Oxygen but high concentrations of carbon dioxide it releases the oxygen. The Haemoglobin binds with some carbon dioxide molecules others dissolve in the blood plasma to be transported back to the lungs. Oxyhaemoglobin is bright red bacause it is bound with oxyge deoxyhaemoglobin is darker.
Water travels through the blood in the blood plasma, as do most substances the body needs. The make up of blood plasma is something like 92% water, 7% protein, 0.8% salts, 0.6% fats and 0.1% glucose. Most of the substances in the blood are being trasported from their source, like the liver or digestive tract, to a sink that can be every cell or something specific such as the kidney depending on the substance. When blood is forced into tiny blood cells called capillaries it seeps through gaps in the capillary wall to bathe the tissues in fluid. Then the cells can take up the products they need directly from this ‘lymph’, most of this eventually find it’s way back into the blood at the other end of the capillaries once the arterial pressure squeezing blood through the capillary vessels has decreased. The part that does not eventually finds it’s way into your lymphatics system. The lymphatics system is a network of blunt ended tubes throgh your tissues that collect and carry away lymph. The system does not have a pump like the heart and so moves very slowly. The contractions of your muscles squeeze fluid along the tubes and valves prevent it moving backwards. Eventually it drains into your sub-clavian veins, that is the veins coming from your arms.