0610 · 9.4Blood

Blood components and red cells

0:00Blood looks like a single red liquid. But if you put a sample of blood in a tube and spin it very fast, it separates. At the bottom is a thick dark red layer. At the top is a clear, pale yellow liquid. And between them is a very thin pale layer. So blood is not one thing at all. It is a mixture, and each part of it does a completely different job.

0:20By the end of this video you will be able to list the components of blood, recognise red and white blood cells under a microscope, tell apart the two main types of white blood cell, state what each component does, and describe how blood clots. There are four components of blood you need to know: red blood cells, white blood cells, platelets and plasma. The dark red layer in the tube is the red blood cells.

0:42The thin pale layer is the white blood cells and the platelets. And the yellow liquid on top is the plasma. More than half of blood is plasma. Let's start with red blood cells. They are by far the most common cell in the blood. For every white blood cell, there are several hundred red blood cells. A red blood cell is a small, round disc that is pinched in on both sides, like a doughnut where the hole never quite goes through.

1:05This shape is called a biconcave disc. And unusually for a cell, a mature red blood cell has no nucleus. The job of red blood cells is to transport oxygen, and the key to that is a red protein called haemoglobin. Each red blood cell is packed with haemoglobin, and haemoglobin contains iron. That is where the red colour of blood comes from. Here is how it works. In the lungs, where there is a lot of oxygen, haemoglobin combines with oxygen to form oxyhaemoglobin.

1:32The red blood cells carry it round the body. Then, in the tissues, where cells are using up oxygen for respiration and there is very little of it, the oxyhaemoglobin releases its oxygen, which diffuses into the cells. The structure of the red blood cell helps it do this. Having no nucleus leaves more room for haemoglobin, so each cell can carry more oxygen. The biconcave shape gives a large surface area, so oxygen can diffuse in and out quickly.

1:56And the cells are small and flexible, so they can squeeze through the narrowest capillaries.

White blood cells and immunity

2:01Now let's look at blood under a microscope, which is what you will see in an exam photograph, called a photomicrograph. The blood is usually stained so the parts show up. You will see huge numbers of small pink discs, often with a paler centre where they are thinner. Those are the red blood cells. Here and there you will see a larger cell with a dark purple stained nucleus.

2:20That is a white blood cell. The quickest way to tell them apart is the nucleus. Red blood cells have no nucleus. White blood cells always have a nucleus. White blood cells are also bigger, and there are far fewer of them. White blood cells defend the body against pathogens. A pathogen is a microorganism that causes disease, such as a harmful bacterium or virus. The syllabus says white blood cells work in two ways: phagocytosis and antibody production.

2:47And each of those is done by a different type of white blood cell. The first type is the phagocyte. Under the microscope, a phagocyte has a nucleus with several lobes, so it looks like a few blobs joined together, rather than one round shape. Its cytoplasm often looks slightly grainy. Phagocytes engulf pathogens by phagocytosis. A phagocyte moves towards a pathogen, and its cytoplasm flows out around it until the pathogen is completely surrounded and taken inside the cell.

3:15Then the phagocyte releases enzymes that digest the pathogen and destroy it. Phagocyte literally means eating cell, and that is exactly what it does. The second type is the lymphocyte. It looks quite different. A lymphocyte has one large, round nucleus that fills almost the whole cell, with only a thin rim of cytoplasm around the edge. Lymphocytes produce antibodies. Antibodies are proteins, not cells. They are released into the blood, and they attach to a specific pathogen.

3:42That helps to destroy the pathogen, or it marks it so that phagocytes can find it and engulf it more easily. You will learn more about exactly how antibodies work in the topic on diseases and immunity. So to identify them in a photo-micrograph, look at the nucleus again. A lobed nucleus means a phagocyte. One large round nucleus that fills most of the cell means a lymphocyte. And to state their functions, lymphocytes produce antibodies, and phagocytes engulf pathogens by phagocytosis.

Platelets and plasma transport

4:10Next, platelets. Platelets are not whole cells. They are tiny fragments of cells, much smaller than red blood cells, and they have no nucleus. Their job is to help the blood to clot. For the core syllabus, that is all you need to state: platelets are involved in clotting. The fourth component is plasma. Plasma is mostly water, with a lot of substances dissolved in it. You can think of it as the river that carries everything else.

4:34The syllabus lists six things that plasma transports, so let's go through them. Plasma transports the blood cells themselves, the red cells, the white cells and the platelets, which are carried along in the liquid. It transports ions, such as sodium and chloride ions. It transports nutrients, like glucose and amino acids, from the gut to the cells that need them. It transports urea, a waste product made in the liver, to the kidneys, where it is removed in urine.

4:58It transports hormones, such as insulin and adrenaline, from the glands that make them to the organs they act on. And it transports carbon dioxide from the respiring cells to the lungs, where it is breathed out. So red blood cells carry the oxygen, but most of the carbon dioxide is carried dissolved in the plasma. That is a detail that exam questions like to test.

How blood clotting works

5:17Now the last part: blood clotting. If you cut your finger, it bleeds for a short while, and then the bleeding stops. A clot forms, which dries to a scab. Why does that matter so much? The syllabus gives two roles of blood clotting. The first is to prevent blood loss. Without clotting, even a small cut could keep bleeding. The second is to prevent the entry of pathogens. A cut is an opening in the skin, and the skin is normally a barrier that keeps microorganisms out.

5:44The clot seals the wound, so pathogens cannot get in through it. So how does a clot actually form? Plasma contains a protein called fibrinogen. Fibrinogen is soluble, so normally it just stays dissolved in the plasma, and it does not get in the way. When a blood vessel is damaged, the platelets and the damaged tissue set off a series of reactions. The result is that the soluble fibrinogen is converted into fibrin.

6:07Fibrin is insoluble. It forms long, thin threads. The threads of fibrin tangle together into a mesh over the wound, like a net. Red blood cells and platelets get caught in the mesh, and that forms the clot. The clot plugs the wound, stopping blood from leaking out and pathogens from getting in. Later it dries and hardens into a scab, which protects the skin while it heals underneath. So the key phrase to remember is this: clotting is the conversion of soluble fibrinogen into insoluble fibrin, which forms a mesh.

6:36Why is it so important that fibrinogen is soluble to begin with? Because if it were already insoluble fibrin, it would form clots all the time, inside healthy blood vessels, and block them. Keeping it soluble until it is needed means clots only form where there is damage.

Exam questions and recap

6:51Let's see how this can be tested. A typical question shows a photo-micrograph of blood and points to a cell. Suppose the cell is large, with a nucleus in several lobes. First, it has a nucleus, so it is not a red blood cell. It is a white blood cell. The nucleus is lobed, not one large round shape, so it is a phagocyte. Its function is to engulf pathogens by phagocytosis and digest them.

7:13Another question might ask you to explain how a red blood cell is adapted to its function. A strong answer links each feature to the job. It contains haemoglobin, which combines with oxygen. It has no nucleus, which leaves more space for haemoglobin, so it can carry more oxygen. Its biconcave shape gives a large surface area for diffusion of oxygen. And its small, flexible shape lets it pass through narrow capillaries.

7:36Now some common mistakes. First, calling platelets cells. They are fragments of cells, with no nucleus. Second, saying that antibodies are white blood cells. Antibodies are proteins, and they are made by lymphocytes. The lymphocyte is the cell. The antibody is what it releases. Third, mixing up which white blood cell does what. Phagocytes engulf. Lymphocytes make antibodies. A lymphocyte does not eat pathogens. Fourth, saying that platelets form the mesh in a clot.

8:06The mesh is made of fibrin. Platelets help set off the process, but the net itself is fibrin threads. And fifth, saying plasma is just water. It is mostly water, but its whole job is to carry the substances dissolved and suspended in it. Let's recap. Blood has four components: red blood cells, white blood cells, platelets and plasma. Red blood cells have no nucleus and are full of haemoglobin, which carries oxygen from the lungs to the tissues.

8:32White blood cells have a nucleus and defend the body. Phagocytes, with their lobed nucleus, engulf pathogens by phagocytosis. Lymphocytes, with their large round nucleus, produce antibodies. Platelets are cell fragments involved in clotting. Plasma transports blood cells, ions, nutrients, urea, hormones and carbon dioxide. And clotting prevents blood loss and the entry of pathogens. It happens when soluble fibrinogen is converted into insoluble fibrin, which forms a mesh that traps blood cells and seals the wound.

9:03So blood is not just a red liquid. It is a delivery service, a defence force and a repair kit, all flowing through the same tubes.