Chapter: Blood, Tissue Fluid and Blood Groups
Welcome to this chapter on your body's amazing internal transport liquid! Think of your bloodstream as a super-fast motorway network delivering essential supplies to trillions of cells and carrying away waste products. In this chapter, we will break down what blood is made of, how it delivers oxygen, how it protects us from blood loss and pathogens, how tissue fluid forms to bathe our cells, and how the ABO blood grouping system works for safe blood transfusions.
Note: Details of the heart and blood vessels are covered in the separate chapter "Heart, blood vessels and circulation".
---1. Composition of the Blood
If you take a sample of blood and spin it in a centrifuge, it separates into liquid and cellular layers. Blood is made up of four main components:
• Plasma: The pale yellow liquid portion that makes up just over half of the blood volume.
• Red blood cells (erythrocytes): The most numerous cells, responsible for oxygen transport.
• White blood cells (phagocytes and lymphocytes): Immune cells that defend against disease.
• Platelets: Tiny cell fragments involved in blood clotting.
Plasma and What It Transports
Plasma is mostly water with many dissolved substances. Don't worry if this seems like a lot to remember; just think of plasma as the body's delivery van. It carries five essential things specified in your syllabus:
1. Carbon dioxide: A waste product of aerobic respiration, carried away from respiring cells to the lungs for excretion.
2. Digested food: Soluble nutrients such as glucose and amino acids carried from the small intestine to cells throughout the body.
3. Urea: A toxic waste product produced by the liver from excess amino acids, transported to the kidneys for excretion in urine.
4. Hormones: Chemical messengers (such as insulin or adrenaline) released by endocrine glands and carried to target organs.
5. Heat energy: Distributed from active organs (like the liver and contracting muscles) around the body to maintain an even body temperature.
Memory Trick: Remember the five things plasma transports with C-D-U-H-H: Carbon dioxide, Digested food, Urea, Hormones, and Heat energy!
Key Takeaway: Blood consists of plasma, red blood cells (erythrocytes), white blood cells (phagocytes and lymphocytes), and platelets. Plasma transports carbon dioxide, digested food, urea, hormones, and heat.
---2. Red Blood Cells (Erythrocytes)
Red blood cells (erythrocytes) have one primary job: transporting oxygen from the lungs to every respiring cell in the body. They are exquisitely adapted to do this efficiently.
Key Adaptations of Red Blood Cells
• Contain haemoglobin: This iron-rich protein binds reversibly with oxygen in the lungs to form oxyhaemoglobin, releasing the oxygen when it reaches respiring tissues where oxygen levels are low.
• Biconcave disc shape: The cell is dented inward on both sides. This increases the surface area to volume ratio, allowing oxygen to diffuse into and out of the cell much more quickly.
• No nucleus: By not having a nucleus, there is more space inside the cell to pack in haemoglobin, maximising the amount of oxygen carried.
• Flexible and small: Their flexibility allows them to squeeze through tiny capillary walls in single file without bursting, bringing them right next to body cells.
Common Mistake to Avoid: Never say red blood cells "have no organelles at all because they are dead." They are living, specialised cells! State clearly that they lack a nucleus to create more room for haemoglobin.
Key Takeaway: Erythrocytes are adapted for oxygen transport by having a biconcave shape (high surface area to volume ratio), no nucleus (more room for haemoglobin), and containing haemoglobin to bind oxygen reversibly.
---3. White Blood Cells
White blood cells protect the body against pathogens (disease-causing microorganisms). For your exam, you need to know the roles of two specific types:
1. Phagocytes (Phagocytosis)
Phagocytes carry out a process called phagocytosis:
• They detect foreign substances or pathogens.
• They engulf the pathogen by flowing their cell membrane around it.
• Once inside, the pathogen is enclosed in a vesicle and broken down and digested by enzymes.
• Tip: You can often recognise phagocytes under a microscope because they have a lobed (irregularly shaped) nucleus.
2. Lymphocytes (Antibody Production)
Lymphocytes are responsible for producing antibodies:
• Pathogens have specific markers called antigens on their surfaces.
• When a lymphocyte recognises a foreign antigen, it produces specific, Y-shaped proteins called antibodies.
• These antibodies bind to the complementary antigens on the pathogen, causing them to stick together (agglutinate) or marking them for destruction by phagocytes.
• Note: You do not need to learn plasma cell detail for this specification.
Key Takeaway: Phagocytes engulf and digest pathogens by phagocytosis. Lymphocytes recognise foreign antigens and produce specific antibodies to target them.
---4. Platelets and Blood Clotting
When you cut yourself, blood vessel walls are broken. If there were no way to seal the breach, you would lose blood continuously, and harmful pathogens could enter your body freely.
How Blood Clots Form
• Platelets are small cell fragments circulating in the blood that help trigger clotting when a blood vessel is damaged.
• The clotting mechanism involves an enzyme-controlled cascade.
• The key reaction you must know: an enzyme converts the soluble plasma protein fibrinogen into insoluble strands of fibrin.
• The insoluble fibrin forms a mesh of fibres across the wound.
• Red blood cells and platelets get trapped in this mesh, forming a clot that dries to form a scab.
Why is blood clotting important?
1. It prevents excessive loss of blood from damaged vessels.
2. It prevents the entry of foreign pathogens into the body through open cuts.
Exam Scope Reminder: You only need to know that an enzyme converts fibrinogen into fibrin. You do not need to memorise other intermediate chemicals or enzyme names (like prothrombin or thrombin).
Key Takeaway: Platelets trigger clotting. An enzyme converts soluble fibrinogen into insoluble fibrin fibres, trapping blood cells to create a clot that seals wounds and blocks pathogens.
---5. Tissue Fluid
Blood remains inside blood vessels (arteries, veins, and capillaries). However, your body cells do not touch blood directly. Instead, they are bathed in a watery liquid called tissue fluid (or interstitial fluid).
Formation of Tissue Fluid Due to Pressure Differences
1. At the arterial end of a capillary bed:
Blood entering capillaries from arterioles is under high hydrostatic pressure (caused by the pumping action of the heart). This high pressure forces water, dissolved glucose, amino acids, mineral ions, and oxygen out through the tiny pores in the thin capillary walls and into the spaces between the tissue cells. Large plasma proteins and blood cells are too large to pass through these pores, so they stay inside the capillaries.
2. Exchange with body cells:
The fluid that is pushed out is now called tissue fluid. It surrounds the cells. Useful substances (such as glucose and oxygen) diffuse from the tissue fluid into the cells. Metabolic wastes (such as carbon dioxide and urea) diffuse out of the cells and into the tissue fluid.
3. At the venous end of the capillary bed:
Blood pressure is much lower here. Because large plasma proteins remained in the blood, the water potential inside the capillary is lower than that of the tissue fluid. Most of the water and dissolved wastes return into the blood capillaries by osmosis and diffusion.
Drainage into the Lymphatic System
Not all the fluid that leaked out returns directly to the capillaries. The remaining excess tissue fluid drains into blind-ended vessels called lymphatic vessels. Once inside these vessels, the liquid is called lymph. The lymphatic system eventually empties this fluid back into the blood system near the large veins in the neck, preventing tissues from swelling.
Key Takeaway: High blood pressure at the arterial end of capillaries forces liquid and small solutes out of the blood to form tissue fluid. After exchanging substances with cells, most fluid returns to the capillaries, and the excess drains into the lymphatic system.
---6. ABO Blood Groups and Blood Transfusions
Giving blood to a person who has lost blood is called a blood transfusion. However, matching the donor's blood to the recipient's blood is vital. If mismatched blood is given, the recipient's immune system will attack the transfused red blood cells, causing them to clump together—a dangerous reaction called agglutination.
Antigens and Antibodies in the ABO System
Blood groups are determined by specific markers called antigens present on the surface of red blood cells, and natural antibodies present in the blood plasma.
• Blood Group A:
- Antigens on red blood cells: A antigen
- Antibodies in plasma: Anti-B antibody
• Blood Group B:
- Antigens on red blood cells: B antigen
- Antibodies in plasma: Anti-A antibody
• Blood Group AB:
- Antigens on red blood cells: Both A and B antigens
- Antibodies in plasma: Neither anti-A nor anti-B
• Blood Group O:
- Antigens on red blood cells: Neither A nor B antigens
- Antibodies in plasma: Both anti-A and anti-B antibodies
Transfusion Compatibility Rules
When giving a transfusion, the critical danger occurs if the recipient's antibodies attack the donor's red blood cell antigens.
• Group O individuals are often referred to as universal donors of red cells because their red blood cells have neither A nor B antigens, meaning a recipient's antibodies have nothing to attack.
• Group AB individuals are universal recipients because their plasma contains neither anti-A nor anti-B antibodies, so they can receive red blood cells of any ABO group without agglutinating them.
• If a person with Group A blood (who has anti-B antibodies) receives Group B blood (which has B antigens), the anti-B antibodies will bind to the B antigens, causing agglutination, which can block capillaries and be fatal.
Key Takeaway: Blood groups are determined by A and B antigens on red blood cells and anti-A and anti-B antibodies in plasma. Transfusions must be matched so that the recipient's antibodies do not bind to the donor's red blood cell antigens and cause agglutination.
---Chapter Quick Review
• Plasma: Liquid part of blood; transports carbon dioxide, digested food, urea, hormones, and heat energy.
• Red blood cells (erythrocytes): Contain haemoglobin, biconcave disc shape (large surface area to volume ratio), lack a nucleus; transport oxygen.
• White blood cells: Phagocytes engulf and digest pathogens (phagocytosis); lymphocytes produce specific antibodies against pathogen antigens.
• Platelets & Clotting: Prevent blood loss and pathogen entry; an enzyme converts soluble fibrinogen into insoluble fibrin fibres.
• Tissue fluid: Formed when high pressure forces fluid and solutes out of capillaries at the arterial end; bathes cells; excess fluid drains into the lymphatic system.
• ABO Blood Groups: Based on antigens A and B on red blood cells and antibodies anti-A and anti-B in plasma; mismatching leads to dangerous agglutination.