The blood. Fair enough it can gross quite a few people out, but it’s super important to humans and many other animals because it delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells, so even if you’re squeamish – you still need to know all about it.
In humans the blood is composed of red blood cells which are biconcave shaped discs, and these discs are suspended in blood plasma.
The biconcave shape of blood cells is crucial as it gives them a larger surface area over which diffusion of oxygen can take place.
The plasma mainly consists of water but it also contains both red and white blood cells with platelets suspended in it.
The plasma also contains dissolved substances including glucose, amino acids, vitamins and minerals and hydrogen carbonate ions (the form which carbon dioxide takes in the blood). Heat energy is also carried by the plasma, so areas of the body which have extensive blood supplies (like the core) will be warmer than those with reduced blood supply (the extremities for example).
The platelets within the plasma help with the clotting of blood by producing a network of protein threads that eventually lead to the formation of a scab at the site of the wound. It’s probably worth mentioning at this point that platelets are not cells themselves but are cell fragments.
The red blood cells in the plasma contain haemoglobin and carry oxygen. They do not have a nucleus which allows for more space within the cell for haemoglobin molecules.
The blood is transported through the body by veins. Veins carry blood back towards the heart which means it has very little oxygen within it. This means that they are purply-red in colour as opposed to the bright red colour of oxygenated blood.
Hopefully that didn’t make anyone too squeamish!