Blood and Its Functions

Blood is the body's delivery and communication system. Every cell in your body needs oxygen and nutrients, and every cell produces waste that needs to be removed. Blood is the vehicle that carries everything to where it is needed. An average adult has about 5 litres of blood — that is roughly a bucketful. If you lose more than about 1.5–2 litres rapidly, you will go into shock and die without treatment. Blood is not a simple red liquid. It is actually a mixture of: - Plasma (~55% of blood volume) — a pale yellow liquid that is about 92% water. It carries dissolved substances: nutrients (glucose, amino acids, fatty acids), hormones, antibodies, clotting proteins, and waste products (urea, CO₂). - Red blood cells (erythrocytes) (~45% of blood volume) — the most numerous blood cells. Their sole job is to carry oxygen. - White blood cells (leukocytes) (less than 1% of blood volume) — the immune cells we covered in the previous lesson. Present in blood but most of their work happens in tissues. - Platelets (thrombocytes) — tiny cell fragments, not full cells. Their job is to form the initial plug when a blood vessel is damaged, starting the clotting process. The percentage of blood that is red blood cells is called the haematocrit (or packed cell volume, PCV). Normal is about 40–45% in men, 36–42% in women. A low haematocrit means you have too few red blood cells — anaemia.

Red blood cells (RBCs) are extraordinary cells that have been completely redesigned to maximise their one job: carrying oxygen. What makes RBCs special: - No nucleus — unlike almost every other cell in the body, mature RBCs have ejected their nucleus. This frees up more space for haemoglobin. - Biconcave disc shape — like a doughnut with a dimple rather than a hole. This maximises surface area for gas exchange and allows them to squeeze through tiny capillaries. - Packed with haemoglobin — each RBC contains about 280 million haemoglobin molecules. Haemoglobin is the red, iron-containing protein that actually binds and carries oxygen. Haemoglobin: Each haemoglobin molecule has four subunits, each containing a haem group — an iron-containing ring that binds one oxygen molecule. So each haemoglobin molecule can carry 4 oxygen molecules, and each RBC carries about 1 billion oxygen molecules. In the lungs, where oxygen is abundant, oxygen binds to haemoglobin → oxyhaemoglobin (bright red). In the tissues, where oxygen is used up and CO₂ builds up, oxygen is released → deoxyhaemoglobin (darker, more purple-red). Veins look blue through skin not because blood in veins is blue, but because of the way different wavelengths of light penetrate skin. RBC lifespan and production: RBCs live for about 120 days. Your bone marrow must produce about 2 million new RBCs every second to replace those that wear out. Production of new RBCs is called erythropoiesis, driven by the hormone erythropoietin (EPO) — produced by the kidneys in response to low oxygen levels. Anaemia means too few RBCs or not enough haemoglobin. Types include: - Iron-deficiency anaemia — most common worldwide; not enough iron to make haemoglobin. Causes: poor diet, heavy menstrual bleeding, gut blood loss (ulcers, bowel cancer). - B12/folate deficiency anaemia — RBCs become abnormally large and cannot function normally. - Sickle cell anaemia — a genetic mutation causes haemoglobin to crystallise and distort RBCs into a sickle shape → they block small blood vessels and break down rapidly.

Every time you cut yourself, a complex and precisely coordinated sequence of events prevents you from bleeding to death. The process has three stages that happen almost simultaneously. Stage 1 — Vascular spasm: The damaged blood vessel immediately contracts (vasoconstriction) to reduce blood flow through it. This buys time for the next stages. Stage 2 — Platelet plug: Platelets (those tiny cell fragments) normally flow freely in blood without sticking to anything. But when a vessel is damaged, the inner lining (endothelium) is disrupted, exposing collagen fibres underneath. Platelets have receptors for collagen — they bind to it and become activated. Activated platelets: - Change shape (from smooth discs to spiky spheres) - Release chemicals that attract more platelets - Stick to each other to form a platelet plug — a temporary seal over the damage This is why aspirin reduces blood clotting — it prevents platelets from making thromboxane A2, one of the chemicals that recruits more platelets. Stage 3 — Coagulation cascade: The platelet plug alone is not strong enough for a large wound. A more permanent fibrin clot is needed. This is built through the coagulation cascade — a series of proteins in plasma that activate each other in sequence (like dominoes), each one activating the next. The cascade ends with the conversion of a protein called fibrinogen (dissolved in plasma) into fibrin — long, insoluble strands that weave through the platelet plug like a net, trapping more platelets and red blood cells. The result is a firm, stable blood clot. The cascade requires clotting factors — most of which are produced in the liver and require vitamin K for their production. This is why liver disease causes bleeding problems, and why warfarin (which blocks vitamin K) is used as a blood thinner. Haemophilia is a genetic disorder where one clotting factor is missing — usually factor VIII (haemophilia A) or factor IX (haemophilia B). Even minor cuts can cause dangerous, prolonged bleeding. Treatment is infusion of the missing clotting factor.

Not all human blood is the same. The surface of red blood cells is covered with proteins and sugar chains called antigens — and different people have different antigens. This is the basis of blood groups. The ABO system: There are two possible antigens — A and B. Depending on which you have, you are: - Blood group A — has A antigens on RBCs, and anti-B antibodies in plasma - Blood group B — has B antigens on RBCs, and anti-A antibodies in plasma - Blood group AB — has both A and B antigens, no antibodies (universal recipient) - Blood group O — has neither antigen, has both anti-A and anti-B antibodies (universal donor) Why the antibodies matter for transfusions: If you receive blood with antigens your immune system sees as foreign, it attacks the donated RBCs — causing a haemolytic transfusion reaction. Donated RBCs clump together (agglutination) and burst (haemolysis), releasing their contents into the blood → kidney failure, shock, and potentially death. This is why blood type matching is critical before a transfusion. In emergencies when there is no time to match, O negative blood can be given to anyone (no A or B antigens, and no Rh factor — see below). The Rhesus (Rh) factor: The other major blood group system. You are either Rh positive (have the Rh antigen — about 85% of people) or Rh negative. Rh negative people do not naturally have anti-Rh antibodies — but they develop them if exposed to Rh positive blood. Rh incompatibility in pregnancy: If an Rh negative mother carries an Rh positive baby (inherited from the father), some fetal blood cells can enter the mother's circulation. The mother makes anti-Rh antibodies. In a subsequent Rh positive pregnancy, these antibodies cross the placenta and attack the baby's RBCs → haemolytic disease of the newborn (HDN). Prevented by giving the mother an injection of anti-D immunoglobulin during and after the first pregnancy, which destroys any fetal RBCs before the mother can make antibodies.

Blood pressure is the force exerted by blood against the walls of blood vessels. Without adequate pressure, blood cannot reach the brain and vital organs. Too much pressure over time damages vessel walls and the heart. What the numbers mean: Blood pressure is written as two numbers — e.g. 120/80 mmHg: - The top number (systolic pressure) — the pressure when the heart contracts and pumps blood out. Normal: less than 120 mmHg. - The bottom number (diastolic pressure) — the pressure between heartbeats, when the heart is relaxed and refilling. Normal: less than 80 mmHg. - Hypertension (high blood pressure) is defined as consistently ≥140/90 mmHg. How BP is measured: A sphygmomanometer (BP cuff) inflates around the upper arm, compressing the brachial artery and stopping blood flow. As the cuff deflates, a stethoscope placed over the artery detects sounds (Korotkoff sounds) caused by turbulent blood flow: - The first sound = systolic pressure (when blood just starts squeezing through) - Sounds disappear = diastolic pressure (when blood flows freely again) What causes hypertension? In 90–95% of cases, the cause is unknown (essential hypertension). Risk factors: older age, obesity, high salt diet, smoking, physical inactivity, family history, stress, and certain ethnic backgrounds. In 5–10%, there is a specific cause (secondary hypertension) — including kidney disease, hormonal tumours, or narrowing of the renal artery. Why hypertension matters: It is called the "silent killer" — usually no symptoms until complications occur: heart attack, stroke, kidney failure, or damage to the blood vessels in the eyes (hypertensive retinopathy) or heart (left ventricular hypertrophy). Treated with lifestyle changes and/or drugs (ACE inhibitors, calcium channel blockers, diuretics, beta-blockers).