Blood: Introduction

00:01 In this lecture, I am going to talk about blood. Blood is a very specialized connective tissue.

00:07 It is a fluid and it flows within blood vessels throughout the entire body.

00:16 At the end of this lecture, I want you to be able to understand that blood consists of plasma, a fluid which either has lots and lots of proteins and other substances that have very important roles throughout the body. Blood also has cells, red blood cells and the white blood cells. And I want you also to understand their functions and how to identify them.

00:43 Blood contains platelets and also it would be a good idea for you to be able to understand the relative numbers of each of these blood cells in blood and also some idea of their relative sizes. Well, blood has many different functions. As I mentioned earlier, it is a fluid flowing within blood vessels right throughout the body and therefore it can deliver certain substances to all the tissues of the body. It can take away waste products. Importantly it carries cells with it that helps us fight, a defence against invading pathogens, the immune cells. It also carries around lots of chemical messengers like hormones, that are secreted from endocrine glands and then deliver to their target organs some distance away from those glands. We will learn that it is very important in maintaining various roles in hemostasis in the body. For instance, maintaining a relatively even tissue volume of fluid proportions. Also it is involved with thermoregulation like sweat glands in skin. And we will also appreciate that the erythrocyte or the red blood cell is the important cell that transport oxygen and carbon dioxide to the tissues and from the tissues.

02:21 Well let us first of all look at the general characteristics of blood. If we take a blood sample, if we take some blood out of our vein for instance and we put it in a test tube and then centrifuge that blood, it separates blood into three major components. The greatest volume of blood is the plasma. It occupies somewhere around 55 percent of the total blood. The other major proportion of blood are the erythrocytes, those cells that transport oxygen and also carbon dioxide and they occupy roughly about 45 percent of total blood. And then there is this very very tiny layer called the buffy coat. That has less than 1 percent of total blood volume. That little buffy coat contains the white blood cells and platelets, very small proportion of blood. But those white cells as we will see later on are very very important because they are involved with protecting our body against invading pathogens and they also have other roles that I will talk about in this lecture and also when we talk about the immune system. Sometimes hematologists will measure the percentage of total blood volume that is occupied by erythrocytes. Here it is about 45 percent, but it varies.

03:50 It varies depending on whether you are a male or a female. Also, it varies depending on whether or not you may have some diseases of blood such as anemia where the proportion of red blood cells is decreased, it gets very low and that is called the hematocrit.

04:11 The hematocrit is the percent of total blood that is occupied by erythrocytes. And that hematocrit if it is found to be low, can be an indication of blood diseases such as anemia. Well let us have a look at plasma. As I said earlier in the lecture, it is the property of blood, the fluid property that allows blood to circulate through blood vessel to all parts of the body.

04:43 It is basically water, 95 percent water, but it contains very very important components.

04:51 One of the most important components are the proteins in blood. Blood proteins, by far the most abundant protein in the blood is albumin. Albumin is the smallest protein in blood, but it has a very important role. It is involved in what we call the colloid osmotic pressure gradient in the body. Albumin is responsible for making sure that there is an even balance between fluid in the blood, the plasma, water in the plasma and water in the interstitium, that is the interstitial compartment that surrounds cells. It is important that the amount of water in that interstitial component is controlled and regulated very very finely.

05:43 If albumin leaks out of blood or is lost, then fluids starts to accumulate within the tissues and it can give rise to edema. Sometimes that colloid osmotic pressure can actually be inhibited or at least overridden for instance when we stand for a long time without moving.

06:06 We tend to get fluid then accumulating in our ankles, we get swollen ankles and it is not if we moved around. That fluid can then be returned into the vascular system.

06:18 By the way, I will talk about when we look at the immune system. Now albumin also has another role. It can transport substances through the blood to the different parts of the body.

06:32 For instance, they can transport a hormone called thyroxine. They can transport metabolites such as bilirubin and they can also transport drugs such as the barbiturates.

06:47 The globulins, those plasma proteins termed globulins are antibodies and we will talk about those again when we look at the immune system. Fibrinogen is the largest blood protein. It is a very important blood protein. It is soluble, but when a blood vessel becomes damaged, that fibrinogen can then be transformed into fibrin and that has a role in the clotting mechanism and I will mention that later on. Well listed are there other substances that blood contains, the plasma contains and I just want to focus on the bottom one, chemical messengers because that is really an important component of blood that I will refer to when we talk about the endocrine glands and the secretion of these products from the endocrine glands called hormones that as I mentioned earlier, have their effect on target tissue elsewhere in the body.