Blood

Introduction Blood is an essential component of the body. The average human has approximately 5 litres of blood which contributes to 7% of body weight. The blood performs many vital functions but the most well-known of its functions is the transportation of oxygen from the lungs to cells in the body. However, blood is not just the red liquid perceived by humans it has a much more complex composition and carries out several roles to keep the body in a good state of health. In this book, you will gain a deeper insight into the composition of blood and the functions it carries out.

What is blood? Blood is a type of connective tissue which is composed of living cells or formed elements (red blood cells, white blood cells etc.) suspended in a plasma matrix. To understand this, let us first look at the different components of blood:

Erythrocytes (red blood cells) Erythrocytes are more commonly referred to as red blood cells and they make up 45% of the total blood. This is the densest component in the blood. The main function of erythrocytes is to transport the gases involved in respiration (carbon dioxide and oxygen). They transport oxygen from the lungs to all the cells in the body (which they use in respiration to release energy) and they transport carbon dioxide (a waste product of respiration) from cells to the lungs in order to be removed from the body. The erythrocytes are specialised to transport respiratory gases through the body. Firstly, they have a bi-concave disc shape which increases the surface area of the cell. Erythrocytes are also enucleate (they don’t have a

nucleus). Both of these adaptions means that more oxygen can be transported by the red blood cells. This is because it allows erythrocytes to have space for a high concentration of haemoglobin. This is a protein that allows oxygen to be transported around the body. Understanding the structure of haemoglobin will allow us to comprehend how it helps in the transport of oxygen. Haemoglobin is made up of four globin proteins: two alpha chains and two beta chains. A haem group is

bound to each of these chains. A haem group is a ring- like compound called porphyrin to which an iron atom is attached. The oxygen atoms bind to this iron atom. The oxygen is also able to detach from iron. Four haem groups on each haemoglobin means that each haemoglobin can carry four oxygen molecules. At the lungs, oxygen binds to the iron in the haem group to form oxyhaemoglobin. At the cells, the oxygen detaches from the iron and can then be used by cells in respiration. Buffy Coat The buffy coat makes up less than 1% of the blood but contains Leukocytes (White Blood Cells) and platelets. Leukocytes (White Blood Cells) Leukocytes are cells that are part of your immune system and help to protect your body against harmful foreign pathogens and prevent infections from occurring. There are different types of white blood cells that defend the body in different ways but they can be classified into 5 major types.

Neutrophils Neutrophils make up 55-73% of the white blood cell count in an average human body. These kill bacteria by using the toxic granules (they are granulocytes) in their cytoplasm which fuse with the bacteria; the bacteria are killed by the toxic granulocytes which are then phagocytosed (engulfing a foreign pathogen). Lymphocytes There are three types of lymphocytes: Natural Killers These are a type of white blood cell that are involved in containing viral infections before antigen-specific cells kill the virus (or cancer cells) in a specific immune response. NK cells have a protein called perforin in their cytoplasm. These perforin proteins bind to the target cell’s plasma membranes and start to form pores in the target cell. This allows granules from the NK cells to enter the target cell which induces a process known as apoptosis. Apoptosis is a type of programmed cell death which occurs due to a variety of biochemical changes

within the cell. During apoptosis, the cell shrinks and form blebs (outgrowths on the surface of the cell). Activated proteins then break down the cellular components and enzymes break down the nucleus. Macrophages (white blood cells that engulf cellular debris) receive signals from these dying cells. When the macrophages reach the dying cell, the cell would have been broken down into small pieces with the cell fragmented components. The macrophages recognise the components and engulf them, removing them from the body. This is how natural killers initiate the death of target cells. T cells (T-Lymphocytes) T-cells are produced in the bone marrow and mature in the thymus gland. They are involved in the specific immune response (response to a specific foreign pathogen). There are two ways that T cells help to kill foreign pathogens.  T cells secrete cytokines (small proteins involved in signalling), which cause B- Lymphocytes to turn into plasma cells (see B cells).  T cell receptors bind to the antigen of a specific virus and produce cytotoxins to kill the cell. Apoptosis may be induced to kill the virus or cancer cells.

B cells (B- Lymphocytes) B cells are part of the humoral immune response. This means that they are involved in the production of antibodies against specific antigenic pathogens. B- Lymphocytes detect soluble antigens on a pathogen. They then differentiate into plasma cells that secrete antibodies specific to the pathogen or cancer cell. The plasma cell may also differentiate into memory lymphocytes that remember the antigen on the pathogen so that if the pathogen enters the bod again, the pathogen can be killed in a quick secondary immune response. Cytokines released by T- cells also stimulate the differentiation of B-Lymphocytes into plasma cells. Monocytes Monocytes are the largest white blood cells in the body. They differentiate into macrophages and myeloid lineage dendritic cells. The macrophages help to remove pathogens that have been killed by lymphocytes, neutrophils and other white blood cells. Myeloid dendritic cells capture antigens (special molecules on antigens) and transport them to the lymphoid organs where immunity is initiated (by producing lymphocytes).

Eosinophil Eosinophils are leukocytes which are responsible for helping defend the body against multicellular parasites. They are granulocytes. They are also responsible for the inflammatory response during bronchospasms (asthma attacks). For example, during an asthma attack, Interleukin-5 proteins (produced by T- helper cells) help increase the production of Eosinophils. Basophils Basophils are leukocytes which are involved in inflammatory reactions, especially ones which cause allergic reactions. They are granulocytes; the granules contain histamine and heparin. Histamine is an organic nitrogenous compound causes vasodilation – the blood vessels near the infection to dilate. This allows leukocytes to have greater access to the pathogens. Heparin prevents blood clots near the infection by thinning the blood.

Plasma Making about 55% of the total blood volume, plasma is the liquid part of the blood (95% of which is composed of water) in which blood cells and proteins are suspended and carried around in the body. Plasma also contains salts, enzymes and antibodies. Below are the main comments of plasma. Albumins Albumins are important proteins in plasma that help regulate oncotic pressure. Proteins in the plasma displace water from the blood, creating a concentration gradient between the blood and the surrounding fluid. This results in water flowing into the blood; the force causing water to enter the blood from the surrounding fluid is known as the oncotic pressure – it helps regulate the water content in the blood. Coagulation Factors When a blood vessel wall, skin or other tissue is damaged, platelets form an initial seal of the cut followed by the activation of coagulation factors (like factor 10). Fibrinogen is a soluble clotting factor which forms fibrin strands when it comes into contact with the wound. The fibrin traps platelet fragments resulting in the formation of a clot. This prevents pathogens from

entering the body and repairs essential blood vessels (e.g. ones that are involved in gas exchange). Immunoglobulins Immunoglobulins are protein antibodies which have a distinct Y shape. They are produced by plasma cells which have differentiated from B-lymphocytes. Antibodies bind to the antigens located on pathogens; this helps other white blood cells to kill the pathogen.

As you can see above, it is due to the antigen-binding site on each arm of the Y shaped antibody that allows the antibody to bind to the antigen. Globulins Alpha globulins partially consist of HDL which transport fats to cells in the body. HDL prevents the build-up of plaque which could cause the arteries to be blocked, causing cardiovascular diseases. Beta globulins are known as LDL (Low-Density Lipoproteins). They transport fats which are used in cell membrane synthesis. Gamma globulins are known as immunoglobulins. Platelets Thrombocytes (also known as platelets) are fragments which help in the process of clotting. This prevents bleeding and hence excess blood loss. When the skin is damaged due to injury, the blood and platelets form a clot to prevent pathogens from entering the body. This is an initial seal before inflammation.

Where are these different cells produced? Now we know the composition of the blood, we need to understand where its components are produced. Red blood cells, white blood cells and platelets are produced in the bone marrow (which is a type of connective spongy tissue in some bones) through the process of haematopoiesis. Haematopoiesis Haemopoietic stem cells differentiate into two types of cell groups: lymphoid group cells and myeloid group cells. The myeloid group includes erythrocytes, neutrophils, eosinophil, monocytes, basophils and megakaryocytes (which eject fragments of their cytoplasm to produce platelets). Lymphoid group of cells includes T-Lymphocytes and B-Lymphocytes. These cells can be produced because stem cells have the ability to differentiate into particular types of specialised cells.

Functions of the blood Through learning about the various components of the blood, you have also had an insight into the functions of the blood. As mentioned in the beginning, the blood transports oxygen and carbon dioxide in gas exchange. However, the substances in plasma (salts, proteins, antibodies, enzymes etc.) are also transported by the blood. The blood also carries hormones and nutrients. It also plays an important role in regulating body conditions such as pH and body temperature. Conclusion Now you have a detailed understanding of the components of the blood and some of its important functions in the body. Hopefully, this gives you a sense of the importance of blood for humans.

References The Composition and Function of Blood – https://www.youtube.com/watch?v=yj7bfZKlIp8 Blood, Part 1 - True Blood: Crash Course A&P #29 - https://www.youtube.com/watch?v=HQWlcSp9Sls Blood Clot Formation - Coagulation Factors & Platelets - https://www.youtube.com/watch?v=_yQD0U3ZtCs How a wound heals itself - Sarthak Sinha https://www.youtube.com/watch?v=TLVwELDMDWs Hematopoiesis | Hematologic System Diseases | NCLEX- RN | Khan Academy https://www.youtube.com/watch?v=XVWOlKdpF_I

https://www.healthline.com/health/neutrophils Oncotic Pressure - https://www.youtube.com/watch?v=wh-RQOOSLus Lymphocytes | Your Specialized Immunity | White Blood Cells – https://www.youtube.com/watch?v=07S5VSeciOk https://en.wikipedia.org/wiki/Basophil https://www.news-medical.net/health/Blood-Plasma- Components-and-Function.aspx

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