Blood is a tissue in the body that plays a variety of roles in transport and in defending the body against disease.  It is an unusual tissue since it is a liquid, with many different kinds of cells suspended in a watery solution called plasma.

Plasma makes up 55% of the volume of blood and is a solution of many different chemicals in water.  For example, the plasma contains dissolved glucose, amino acids and other products of digestion from the intestines.  It also transports the waste molecule urea from the liver where it is made to the kidney where it is excreted.  Blood plasma contains dissolved carbon dioxide, mostly in the form of hydrogencarbonate ions.  Many hormones (for example testosterone, ADH, adrenalin) are transported in the blood plasma and because the plasma is mostly water, it provides a good way of moving heat around the body from respiring muscles to the skin where it can be lost.

The most common cell in blood are the red blood cells (or erythrocytes).  These tiny cells are adapted for the transport of oxygen.  Each red blood cell contains around 270 million molecules of a transport protein, haemoglobin.  Each molecule of haemoglobin can bind up to four molecules of oxygen in the lungs and then unload the oxygen when the red blood cell passes through a capillary in an actively respiring tissue.

(Don’t worry too much about the structure of the protein – this is A level stuff really…. Just remember haemoglobin is a transport protein for oxygen found in red blood cells)

As well as being packed full of haemoglobin molecules, red blood cells have other adaptations for transporting oxygen.  Red blood cells lose their nucleus during their development as this allows more haemoglobin to be packed into each cell.  Having no nucleus means the red blood cell cannot divide nor repair damage to its structure.  This is why each red blood cell only lives for 100-120 days in the body.

Red blood cells have a characteristic shape.  It is called a biconcave disc and they have an especially flexible shape.  Remember that a capillary is actually smaller in diameter than a red blood cell, so the cells have to squeeze through capillaries in single file…..