General Facts – Blood Formation

00:00 Hello! Welcome to this lecture, which will serve as an introduction to blood.

00:06 Blood is a complex mixture of cells which circulate in plasma.

00:12 We'll learn in this lecture that plasma contains a range of proteins that help the blood to clot, and serum is the fluid that remains after blood has clotted. Blood cells are made from haemopoietic stem cells and the three main cell types are red cells, white cells and platelets.

00:37 Let's start with some general facts about blood volume and blood composition.

00:44 First of all, the amount of blood within your body. The blood volume is around 70 ml/kg.

00:54 A typical person might be 70 kg meaning 4900 or 5 litres of blood in the average person.

01:05 Now blood is composed of a range of cells which circulate in fluid called plasma.

01:13 If you take blood from somebody's vein, put it in a tube and then in a centrifuge, You'll see the picture on the right. At the bottom, the red blood cells, by far the most common cell in the blood, representing around 45% of the blood volume.

01:33 Above them, white blood cells and platelets, a small fraction. They're very important as we shall see.

01:42 And above them, the straw colored fluid - the plasma, which makes up the great volume, 55% of the blood.

01:53 Here's another representation of the blood flowing down a vessel and you'll see the three main cell types: red cells, white cells and platelets within the plasma. And this is an example of the closed circulation of blood from arteries and capillaries and back into veins.

02:15 As we'll see later, there is one organ - the spleen which has a very unusual open blood circulation which is of great interest.

02:26 Now I do want to just emphasize the difference between these terms serum and plasma.

02:33 Let me show you the difference. On the left diagram that we've already seen, plasma at the top, making up 55% of blood volume. But that's what we see if we centrifuge blood which is being anticoagulated, but cannot clot. We take blood into a tube, it starts to clot very rapidly as you can see on the right - clotted blood. All the cells have been drawn down into that clot and coagulation factors, the range of proteins involved in blood coagulation have been taken out of plasma and incorporated into that blood clot. and what is left is the serum above there.

03:19 So serum is deficient in blood coagulation factors. So if we need to find a liquid to give to patients who have blood coagulation factors, you'll see immediately that we need to give plasma, fresh frozen plasma Let's spend some time discussing haemopoiesis - the formation of blood.

03:46 This is a classic chart of haemopoiesis that you would probably find in every haematology laboratory around the world and it starts at the top as always with a haemopoietic stem cell.

04:00 It's termed PLURIPOTENT - that means it has the ability to develop into many different types of cells and of course it has to self-replicate. Everytime it divides, it has to create a daughter cell to maintain its numbers. Now a key point for us to remember at this stage is these two major lineages during haemopoiesis. You have the lymphoid lineage on the left leading to T-cells, B-cells and NK or natural killer cells and on the right the great mass of myeloid cells that are derived from that stem cell. Working from the left, you see monocytes, neutrophils, basophils, eosinophils, megakaryocytes leading to platelets and of course, the red cells - the erythrocytes.

04:53 Let's just spend a minute talking about the haemopoietic stem cell in a little more detail because this is such a crucial cell. It's a rare subtype of cell within the bone marrow despite the fact that all the blood cells are derived from it. It expresses a range of proteins on its surface, but the key one that we use in science and also clinically is this molecule CD34.

05:23 Now that's interesting because we can do stem cell transplants where we take blood from one person and put it into another person and often we will select the stem cells on the basis of CD34 selection and that can lead to reconstitution of haemopoiesis after transfer.

05:44 Now as we will see later on, haemopoietic stem cells exist within a specialized microenvironment.

05:52 It is called a stem cell niche, and look at the range of cells, which are involved in supporting the survival and division of the haemopoietic stem cell.

06:04 We will also see that stem cells get disturbed in a range of conditions and that can lead to dysfunction of haemopoiesis and a reduction in the blood count in the blood.