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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Blood Composition
Blood is often called the 'river of life' because it flows through our bodies carrying essential substances to our cells and removing waste products. An average adult has about 5 litres of blood, making up around 7-8% of body weight. Understanding blood composition helps us see how this amazing tissue supports nearly every function in our body.
Key Definitions:
Blood is classified as a connective tissue and consists of cells and cell fragments suspended in plasma. If we spin blood in a centrifuge, it separates into layers: plasma (55%), red blood cells (45%) and a thin buffy coat of white blood cells and platelets (less than 1%).
Blood performs several vital functions including: transporting oxygen from lungs to tissues, carrying carbon dioxide from tissues to lungs, delivering nutrients from the digestive system, removing waste products to excretory organs, distributing hormones, regulating body temperature and defending against disease.
Red blood cells are the most abundant cells in blood, with about 5 million per cubic millimetre. These specialised cells have a unique biconcave disc shape (like a doughnut with a thin centre instead of a hole) that gives them a large surface area for gas exchange.
The primary function of red blood cells is to transport oxygen from the lungs to body tissues and to help remove carbon dioxide. Haemoglobin, the iron-containing protein that gives blood its red colour, binds to oxygen in the lungs where oxygen concentration is high. When the red blood cells reach tissues where oxygen concentration is low, the haemoglobin releases the oxygen.
Red blood cells have a lifespan of about 120 days. Your body produces about 2 million new red blood cells every second to replace those that die. Old red blood cells are broken down in the liver and spleen and the iron from their haemoglobin is recycled for making new red blood cells.
White blood cells are the body's defence force against infection and disease. Unlike red blood cells, they have nuclei and can move independently, squeezing through blood vessel walls to reach infected tissues.
The most common type (60-70% of all white blood cells). They are the first responders to infection, engulfing bacteria through phagocytosis.
Make up 20-30% of white blood cells. They include B cells (which produce antibodies) and T cells (which coordinate immune responses and kill infected cells).
Monocytes (become macrophages), eosinophils (fight parasites) and basophils (release histamine during allergic reactions) make up the remaining white blood cells.
White blood cells are produced in bone marrow and lymphatic tissue. Their numbers increase during infection, which is why doctors often check your white blood cell count when you're ill. A normal count is about 4,000-11,000 white blood cells per cubic millimetre of blood.
Platelets are not complete cells but rather small fragments of larger cells called megakaryocytes. These tiny disc-shaped structures play a crucial role in blood clotting (coagulation), which prevents excessive bleeding when blood vessels are damaged.
Plasma is the often-overlooked hero of blood, making up about 55% of total blood volume. This straw-coloured liquid is mostly water (90%) but contains hundreds of different proteins and other substances that are essential for life.
Plasma serves as the transport medium for blood cells and carries nutrients, waste products, antibodies, clotting factors, hormones and proteins. It helps maintain blood pressure and body temperature and keeps the pH of blood balanced. Without plasma, the cellular components of blood would be unable to move through the circulatory system.
Blood groups are determined by the presence or absence of certain antigens on the surface of red blood cells. The most important blood group system is the ABO system, which has four main types: A, B, AB and O. Another important factor is the Rhesus (Rh) factor, which can be positive or negative.
Blood transfusions must be carefully matched because the wrong type can cause a severe immune reaction. Type O negative blood is considered the universal donor because it lacks A, B and Rh antigens, while type AB positive is the universal recipient because it has all these antigens and won't react against any blood type.
This understanding of blood groups has revolutionised medicine, making safe blood transfusions possible and saving countless lives during surgeries and emergencies.
The components of blood don't function in isolation but work together as part of the circulatory system. Red blood cells carry oxygen to tissues, where white blood cells patrol for infections. Platelets stand ready to seal any leaks, while plasma transports everything from nutrients to waste products. This coordinated effort ensures that every cell in your body receives what it needs and gets rid of what it doesn't need.
Understanding blood composition is essential for diagnosing many diseases. Blood tests can reveal anaemia (low red blood cell count), infections (elevated white blood cell count), clotting disorders (abnormal platelet numbers) and many other conditions. Modern medicine relies heavily on our knowledge of blood to treat everything from cancer to heart disease.
A complete blood count (CBC) measures levels of all blood components and can help diagnose many conditions. Other tests look at blood chemistry, clotting ability, or the presence of specific substances. Blood tests are one of the most common and valuable diagnostic tools in medicine.
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