Blood Revision Notes

Blood is a specialised connective tissue that circulates throughout the body, delivering substances and removing wastes. It consists of cells suspended in a liquid called plasma. The four main components are red blood cells, white blood cells, platelets, and plasma. Each plays a unique role in transport, defence, and repair mechanisms.

• Blood is a mixture of cells and plasma.
• Cells include red blood cells, white blood cells, and platelets.
• Plasma is the fluid portion carrying dissolved substances.

Red blood cells are the most abundant cells in blood. Their primary function is to transport oxygen from the lungs to body tissues and carry some carbon dioxide back to the lungs for exhalation. They have a distinctive biconcave disc shape, which increases surface area for gas exchange and allows flexibility to pass through narrow capillaries.

These cells contain haemoglobin, a protein that binds oxygen molecules. Haemoglobin's iron atoms temporarily hold oxygen, enabling efficient transport. Red blood cells lack a nucleus, maximising space for haemoglobin.

An example: When you breathe in, oxygen enters the lungs and binds to haemoglobin in red blood cells. These cells then travel through arteries to deliver oxygen to muscles during exercise.

• Carry oxygen using haemoglobin.
• Biconcave shape increases surface area.
• No nucleus to maximise haemoglobin content.
• Transport carbon dioxide back to lungs.

White blood cells are part of the immune system and defend the body against infection. They are fewer in number compared to red blood cells but are crucial for identifying and destroying pathogens such as bacteria and viruses.

There are several types of white blood cells, including phagocytes that engulf harmful microbes and lymphocytes that produce antibodies. Their ability to move out of blood vessels into tissues allows them to reach sites of infection.

For example, if you get a cut, white blood cells migrate to the wound to fight invading bacteria and prevent infection.

• Defend against infection and disease.
• Include phagocytes and lymphocytes.
• Can move out of blood vessels to infection sites.
• Produce antibodies to target pathogens.

Platelets are small cell fragments that play a key role in blood clotting. When a blood vessel is damaged, platelets gather at the site and stick together to form a temporary plug. They also release chemicals that trigger a series of reactions leading to the formation of a stable clot, preventing excessive blood loss.

Though much smaller than red or white blood cells, platelets are essential for wound healing. For instance, if you scrape your knee, platelets help stop bleeding quickly by forming clots.

• Small cell fragments involved in clotting.
• Form plugs at injury sites to stop bleeding.
• Release chemicals to strengthen clots.
• Essential for wound repair.

Plasma is the pale yellow liquid that makes up about 55% of blood volume. It is mostly water but contains dissolved substances such as nutrients (glucose, amino acids), hormones, carbon dioxide, urea, ions, and blood cells.

Plasma transports these substances to and from cells. For example, it carries glucose absorbed from the digestive system to cells for energy and transports carbon dioxide from cells to the lungs for removal.

Plasma also helps maintain blood pressure and volume and plays a role in temperature regulation.

• Liquid portion of blood, mostly water.
• Transports nutrients, hormones, and waste products.
• Carries blood cells throughout the body.
• Helps maintain blood pressure and temperature.

Blood is a vital transport medium in animals, composed of several cell types suspended in plasma. Among these, red blood cells (erythrocytes) and white blood cells (leukocytes) are the primary cellular components with distinct roles. Recognising these cells in images and diagrams is essential for understanding their functions and for practical biology assessments.

• Blood contains red blood cells, white blood cells, platelets, and plasma.
• Red and white blood cells differ in structure and function.
• Identification relies on recognising key morphological features.

Red blood cells are specialised for oxygen transport. They have a unique biconcave disc shape, which increases their surface area for gas exchange. Unlike most cells, mature red blood cells lack a nucleus, allowing more room for haemoglobin, the oxygen-carrying protein. In images, red blood cells appear as small, round, pale-centred discs without a visible nucleus.

The absence of a nucleus and the biconcave shape are critical identification markers. Their uniform size and smooth edges help distinguish them from other blood cells.

• Biconcave disc shape increases surface area.
• No nucleus present in mature cells.
• Contain haemoglobin for oxygen transport.
• Appear as pale-centred discs in stained images.

White blood cells are part of the immune system and vary in size and appearance. Unlike red blood cells, they have a nucleus, which is often irregularly shaped and visible under a microscope. White blood cells are generally larger than red blood cells and may have granules in their cytoplasm depending on the type (e.g., granulocytes).

In diagrams and photomicrographs, white blood cells can be identified by their larger size, presence of a nucleus, and sometimes granular cytoplasm. Different types include lymphocytes with a large round nucleus and phagocytes with lobed nuclei.

• Larger than red blood cells.
• Contain a visible nucleus, often irregularly shaped.
• May have granules in cytoplasm depending on type.
• Involved in immune responses such as phagocytosis and antibody production.

When identifying blood cells in images, first compare size: white blood cells are noticeably larger. Next, look for the presence or absence of a nucleus: red blood cells lack one, white blood cells have one. The shape is also a clue; red blood cells are smooth and biconcave, while white blood cells have irregular shapes due to their nucleus and cytoplasmic features.

Colour staining in photomicrographs can help: nuclei often stain darker, highlighting white blood cells. Red blood cells usually appear lighter and more uniform. Label diagrams carefully by marking these features to avoid confusion.

• Compare cell size: white blood cells are larger.
• Check for nucleus presence: only in white blood cells.
• Observe cell shape and staining patterns.
• Use these features to label cells accurately in diagrams.

The structural differences between red and white blood cells relate directly to their functions. Red blood cells' lack of nucleus and biconcave shape maximise oxygen transport efficiency. White blood cells' nucleus and variable shape enable them to perform complex immune functions, such as engulfing pathogens or producing antibodies.

Understanding these links helps reinforce why these features are important and aids memory when identifying cells in exams.

• Red blood cells' shape and no nucleus optimise oxygen transport.
• White blood cells' nucleus supports immune functions.
• Structural features reflect specialised roles in the body.

Red blood cells (RBCs) are specialised cells responsible for carrying oxygen from the lungs to tissues throughout the body. Their biconcave shape increases surface area for gas exchange and allows flexibility to pass through narrow capillaries.

Each red blood cell contains haemoglobin, a protein that binds oxygen molecules reversibly. Haemoglobin contains iron atoms that form bonds with oxygen, enabling efficient oxygen uptake in the lungs and release in tissues where oxygen concentration is lower.

For example, when blood passes through lung capillaries, oxygen binds to haemoglobin forming oxyhaemoglobin. In body tissues, where oxygen is used up, haemoglobin releases oxygen to cells for respiration. This process is vital for producing energy in cells.

• Red blood cells have no nucleus to maximise space for haemoglobin.
• Haemoglobin binds oxygen in the lungs and releases it in tissues.
• The biconcave shape aids oxygen diffusion and capillary flow.

White blood cells (WBCs) are key players in the immune system, defending the body against pathogens such as bacteria and viruses.

One type of white blood cell performs phagocytosis, engulfing and digesting harmful microorganisms. This process helps remove infections before they spread.

Another type produces antibodies—specialised proteins that recognise and bind to specific antigens on pathogens. Antibodies neutralise pathogens or mark them for destruction by other immune cells, providing targeted defence and immunological memory.

• Phagocytes engulf and digest pathogens.
• Lymphocytes produce antibodies specific to antigens.
• White blood cells can move out of blood vessels to reach infection sites.

Platelets are small cell fragments that play a crucial role in blood clotting, which prevents excessive blood loss after injury.

When a blood vessel is damaged, platelets gather at the site and stick to the vessel wall, forming a temporary plug.

They also release chemicals that trigger a cascade of reactions, leading to the formation of fibrin threads that strengthen the clot and seal the wound. This process stops bleeding and blocks entry points for pathogens, reducing infection risk.

• Platelets initiate clot formation at injury sites.
• They release substances that activate clotting factors.
• Clotting prevents blood loss and infection.

Plasma is the pale yellow liquid component of blood, making up about 55% of its volume. It serves as the transport medium for various substances.

Plasma carries blood cells suspended within it, allowing them to circulate throughout the body.

It transports dissolved ions (such as sodium and potassium), nutrients (like glucose and amino acids), waste products (including urea and carbon dioxide), and hormones to target organs or excretion sites.

• Plasma is mostly water, facilitating transport.
• It carries nutrients from digestion to cells.
• Waste products are transported to excretory organs.
• Hormones travel in plasma to regulate body functions.

When a blood vessel is damaged, blood escapes from the circulatory system, which can lead to dangerous blood loss. Blood clotting is the body's natural response to seal the wound quickly and prevent excessive bleeding. Without clotting, even minor injuries could cause severe blood loss, threatening survival.

Moreover, the open wound created by injury is a potential entry point for harmful microorganisms such as bacteria and viruses. Blood clotting forms a physical barrier that blocks these pathogens from entering the bloodstream, reducing the risk of infection and illness. Thus, clotting serves both to preserve blood volume and protect against disease.

• Prevents excessive blood loss after injury
• Forms a barrier to block pathogens
• Maintains internal environment stability

The clotting process involves several components working together. Platelets, small cell fragments circulating in the blood, are the first responders to vessel injury. They stick to the damaged site and release chemicals that attract more platelets and activate clotting proteins.

Clotting factors are proteins in plasma that trigger a cascade of reactions, ultimately converting fibrinogen, a soluble protein, into fibrin. Fibrin forms a mesh of fibres that traps blood cells and platelets, creating a stable clot that seals the wound.

This coordinated action ensures rapid and effective clot formation to stop bleeding and protect the body.

• Platelets initiate clotting by adhering to injury
• Clotting factors activate a cascade of reactions
• Fibrin forms a mesh to stabilise the clot

1. Injury causes blood vessel damage, exposing collagen fibres beneath the vessel lining.

2. Platelets adhere to the exposed collagen and become activated, releasing chemicals like ADP and thromboxane.

3. These chemicals attract more platelets, forming a temporary platelet plug at the injury site (platelet aggregation). 4. Clotting factors in plasma are activated in a cascade, converting prothrombin into thrombin (an enzyme). 5. Thrombin converts fibrinogen into insoluble fibrin strands. 6. Fibrin strands weave through the platelet plug, trapping red blood cells and strengthening the clot. 7. The clot contracts to reduce its size and firmly seal the wound until tissue repair occurs.

• Platelet adhesion and activation
• Platelet aggregation forming a plug
• Clotting factor cascade producing thrombin
• Fibrin mesh formation stabilising the clot

The clot acts as a physical barrier that seals the wound, preventing microorganisms from entering the bloodstream. This is crucial because the blood circulates throughout the body, and pathogens inside it can spread rapidly, causing systemic infections.

By quickly forming a clot, the body limits the time the wound is open and exposed to the external environment. Additionally, some clotting factors and platelets release substances that can help inhibit microbial growth locally, providing an extra layer of defence.

Therefore, clotting not only stops bleeding but also plays a vital role in the immune protection of the body.

• Clot seals wound to block pathogen entry
• Reduces risk of systemic infection
• Platelets may release antimicrobial substances

In conditions where clotting is impaired, such as haemophilia, even small injuries can cause prolonged bleeding, demonstrating the importance of clotting in survival.

Conversely, excessive clotting inside blood vessels can cause blockages leading to heart attacks or strokes, showing that clotting must be carefully regulated.

For example, when you accidentally cut your finger, platelets quickly gather at the site and form a clot within minutes, stopping the bleeding and preventing germs from entering your body.

• Haemophilia illustrates dangers of poor clotting
• Excessive clotting can cause vessel blockage
• Normal clotting is rapid and localised to injury