Blood

Now we’re going to talk about blood. Blood can be defined as the liquid that fills the vascular compartment. And it’s where we transport the blood cells and the substances dissolved within it. The average human contains about 5 litres of blood. And if we allow this blood to settle out, the heaviest cells will fall to the bottom. The red blood cells form about 45% of the volume, and the remaining 55% is a liquid that’s known as plasma. Plasma is a straw-coloured liquid that contains substances dissolved within it such as salts, sugars, and transports proteins around the body. Floating within the plasma are the three main types of blood cells, the red blood cells, the white blood cells, and the platelets.

Let’s start with the red blood cells. The red blood cells are the most common blood cell. In one drop of blood, there’s 500 million red blood cells. And in the average adult human, there’s 25 trillion of them. Red blood cells are anucleate and biconcave. That means that they don’t have a nucleus, and they’re shaped a bit like a doughnut. The function of red blood cells is to transport oxygen around the body. And they do this using a protein called haemoglobin. And red blood cells are packed full of haemoglobin. There’s 250 million molecules of haemoglobin in each red blood cell. And each haemoglobin can carry up to four molecules of oxygen.

As well as transporting oxygen, red blood cells also transport carbon dioxide back to the lungs. Red blood cells are made in the bone marrow. They circulate in the blood for about 120 days. Every second, 2.5 million red blood cells are being made and destroyed in your body. The white blood cells are involved in the immune system, patrolling the blood, looking for things to fight if they see them as being foreign. There are different types of white blood cell, and each is specialised to attack different things such as viruses, bacteria, and to clear up dead or dying tissue. White blood cells are bigger than red blood cells, and they’re round. And this time they have a nucleus.

There’s far fewer white blood cells in the blood than red blood cells, and in one drop there’s only about 750,000. If we spin the blood and allow the heavy cells to fall to the bottom, the white blood cells form a tiny layer that sits above the red cells. White blood cells are also made in the bone marrow, and they circulate in the blood for either days, weeks, months, years, or even decades. And this provides the immune system with some memory. This is why the second time you get exposed to an infection, you can often fight it off faster. The last type of blood cell we’ll talk about is the platelet.

Platelets are the tiniest blood cells, and they circulate in the blood and are involved in clotting. Platelets are anucleate. They don’t have a nucleus, and they’re able to change their shape. At rest, platelets are a disc shape. But when they become activated, they send out these spiny protrusions, which enable them to stick together at sites of injury and help stop bleeding. Despite their tiny size, platelets have an important role in the cardiovascular system. When they go wrong, this can result in either excessive bleeding or perhaps inappropriate clot formation. And this can happen in conditions such as heart attacks and strokes. We’ll find out more about these later in the course.