Erythrocytes – Blood Cells

00:01 Hello.

00:02 In the second lecture in our series on hematology, we're going to look at the physiology and function of some of the major blood cells.

00:11 And the learning outcomes that we'll be getting from this lecture are as follows.

00:16 We'll look at red cell production, and how that's regulated by erythropoietin.

00:22 We will explore how red cells contain hemoglobin, and how that helps them to carry oxygen to tissue.

00:30 We will look at the subsets of white cells which play specific roles in protection against infection.

00:36 And we'll explore how lymphocytes mediate the function of the immune system.

00:42 And find out how neutrophils are critical in the first line of defense against bacteria and fungi.

00:49 In this lecture, we're not going to focus on platelets, because they're the subject of a later lecture on blood clotting.

00:56 Let's look at red cells in a little bit more detail.

00:59 By far, the most common cell within the blood.

01:05 Red blood cells derive of course from a nucleated cell within the bone marrow.

01:10 And the first cell that's committed to forming red cells is the erythroblast.

01:15 And there are rare forms of leukemia, are in fact derived from these cells.

01:21 Now, red cells are packed with hemoglobin.

01:24 And they carry oxygen to tissue.

01:27 The reason that we've evolved red cells is it allows hemoglobin to be packaged effectively within these blood cells, rather than being free within the blood.

01:40 Now the major production of red cells is through this hormone called erythropoietin.

01:45 I will explore the regulation of erythropoietin in some detail.

01:50 On the right, you'll see a lovely scanning electron micrograph of red cells.

01:54 And you'll see that classic biconcave discs structure of the red cell, which allows it to be flexible and flow through the capillaries as well as diffusing oxygen into tissues.

02:10 Now hematologist can't stop making blood films.

02:12 And there you'll see a blood film on the right, and you'll see how red cells look down the microscope.

02:19 And you'll see the characteristic pale center because of their shape.

02:25 They're around seven microns in diameter, and a highly flexible structures with the biconcave shape.

02:33 Now, some red cells in the blood are very young because they've just come out of the bone marrow.

02:40 And we call those reticulocytes.

02:43 Now, they carry a lot of RNA.

02:45 And that RNA gradually gets degraded over two or three days.

02:50 And the final red cell then becomes apparent.

02:55 So you'll see on the left that reticulocytes can live for two or three days normally within the blood around 2% of the red cells are reticulocytes.

03:05 And we can detect them by staining, what we call a supravital staining with this wonderful dye called brilliant crystal blue.

03:14 And you'll see on the right, those cells with the intense blue stain, those are reticulocytes.

03:21 There's an awful lot in that picture and many more than we'd find in your own blood, I'm sure.

03:27 Now, as well as being physiologically important reticulocytes are important clinically, because they can give us a guide as to the type of anemia that we may be dealing with in a patient.

03:39 Because if the reticulocyte count is increased, it shows that the patient's bone marrow is very active and pouring out a lot of red cells into the blood.

03:50 Whereas, if we can't find many reticulocytes, it means that there's a problem with the production of red cells from the bone marrow.

03:58 So when we discuss anemia in later lectures, you can see why the reticulocyte count is a very useful test for trying to understand the etiology of the anemia.

04:10 Now, the production of red cells is described by the erythropoiesis.

04:17 And again, it's represented by those blue cells.

04:22 Normoblasts have a nucleus.

04:24 But then as they themselves differentiate, the nucleus is objective.

04:30 And that stage that reticulocytes can be released into the blood.

04:35 Now, we don't know why the nucleus is ejected in red cell production, but it may be that that allows more room for hemoglobin production.

04:45 And that has been selected during evolution as a very effective way to increase oxygen transport within the blood.

04:54 Now the regulation of this process of hemopoiesis is really control through this protein called erythropoietin or EPO.

05:07 This hormone EPO is produced largely from the kidney.

05:12 Again, very surprising finding of the kidney should be regulating our blood production.

05:18 But if cells in the kidney detect that the blood is hypoxic in any way, they will then release erythropoietin into the blood.

05:29 Erythropoietin circulate to the bone marrow, and it stimulates the production of red cells.

05:36 As you can probably imagine, EPO, erythropoietin levels are increased.

05:41 And people who are short of oxygen for instance, those who live at high altitude or patients who have lung disease, or perhaps smoke.

05:51 And you'll see on the right there representation of the stimuli to hypoxia factors such as anemia or low atmospheric oxygen tension.

06:04 That hypoxia is detected by the kidney and erythropoietin is produced.

06:12 Erythropoietin stimulates hemopoiesis through a number of mechanisms.

06:17 It increases the differentiation pathway through erythropoiesis to make more erythropoietic cells, to accelerate cell division and it accelerates the release of cells into the blood.

06:33 That can usually address the hypoxia, of course, the negative feedback loop is established.

06:41 Erythropoietin, it's actually very useful molecule for clinical therapy and we can use it to stimulate hemopoiesis in patients with some forms of anemia.