The Heart & Blood Vessels

Blood must flow in one direction through the heart — if it flowed backwards, the whole system would collapse. One-way valves prevent backflow, acting like one-way doors that snap shut the moment blood tries to go the wrong way. The four heart valves: Between atria and ventricles (atrioventricular valves): - Tricuspid valve — between the right atrium and right ventricle (3 leaflets) - Mitral valve (bicuspid) — between the left atrium and left ventricle (2 leaflets). The most important valve clinically. Between ventricles and the arteries (semilunar valves): - Pulmonary valve — between the right ventricle and pulmonary artery - Aortic valve — between the left ventricle and aorta. Also clinically very important. What is a heart murmur? A murmur is an abnormal sound heard through a stethoscope — caused by turbulent blood flow, often through a damaged or narrowed valve. A normal heartbeat sounds like "lub-DUB": - "Lub" = the tricuspid and mitral valves snapping shut (end of filling) - "DUB" = the aortic and pulmonary valves snapping shut (end of pumping) Common valve problems: - Aortic stenosis — the aortic valve becomes narrow (stenosed), often due to calcium deposits in older people. The left ventricle has to work much harder → left ventricular hypertrophy (thickened muscle) → eventually heart failure. - Mitral regurgitation — the mitral valve does not close properly → blood leaks backwards into the left atrium each time the ventricle contracts. Common in older adults.

Blood travels in two separate loops — both driven by the heart: Pulmonary circulation — the lung loop: Right ventricle → pulmonary arteries → lungs (gas exchange: CO₂ out, O₂ in) → pulmonary veins → left atrium "Pulmonary" means "relating to the lungs." Note something unusual: the pulmonary arteries carry deoxygenated blood, and the pulmonary veins carry oxygenated blood — the opposite of what you might expect. This is because arteries are defined as vessels carrying blood AWAY from the heart, and veins carry blood TOWARD the heart — not by their oxygen content. Systemic circulation — the body loop: Left ventricle → aorta → arteries → arterioles → capillaries (all body tissues) → venules → veins → vena cava → right atrium Blood vessels — three types: Arteries: Thick-walled, muscular vessels that carry blood away from the heart under high pressure. The walls contain smooth muscle and elastic fibres — they stretch with each heartbeat and recoil to maintain pressure between beats. You can feel a pulse in arteries near the surface (wrist, neck, groin). Veins: Thinner-walled vessels that return blood to the heart under low pressure. Because pressure is low, blood could pool — veins have valves inside them that prevent backflow and ensure blood only flows toward the heart. The muscle movements of your legs also help push blood upward against gravity (the "skeletal muscle pump"). Capillaries: Microscopically thin-walled tubes — walls just one cell thick. This is where ALL the important exchange happens: oxygen and glucose leave the blood and enter cells; carbon dioxide and waste products enter the blood. Nothing else can exchange nutrients with cells except capillaries.

Blood pressure (BP) is the force that blood exerts on the walls of arteries as it flows through. It is measured in millimetres of mercury (mmHg) and written as two numbers, like 120/80. Systolic pressure (the top number — e.g. 120): The pressure in the arteries when the heart contracts and pushes blood out. This is the highest pressure the arteries experience. Diastolic pressure (the bottom number — e.g. 80): The pressure in the arteries when the heart is relaxing and filling. This is the lowest pressure between beats. Normal blood pressure: less than 120/80 mmHg Pre-hypertension: 120–139 / 80–89 Hypertension (high blood pressure): 140/90 or above (on repeated measurements) Why high blood pressure is dangerous: Blood pressure is like the water pressure in a hose. Too much pressure constantly battering the artery walls causes: - Damage to the artery lining → atherosclerosis (fatty plaques build up) - Weakening of artery walls → aneurysm (the wall balloons outward and can burst) - Strain on the left ventricle → heart failure - Damage to tiny blood vessels in the kidneys, eyes, and brain Hypertension is called the "silent killer" — it has no symptoms until it causes a heart attack, stroke, or kidney failure. About 1 in 3 adults in most countries has high blood pressure, and many do not know. What controls blood pressure? BP = Cardiac output × Peripheral resistance - Cardiac output — how much blood the heart pumps per minute (heart rate × stroke volume) - Peripheral resistance — how narrow the blood vessels are (narrower = more resistance = higher pressure) Adrenaline raises BP by increasing heart rate and constricting blood vessels. Blood pressure medications (antihypertensives) work by reducing one or both of these factors.

Your heart generates its own electricity — it does not need your brain to tell it to beat. This electrical system coordinates the chambers so they beat in the right sequence. The pacemaker — the SA node: The sinoatrial (SA) node is a small cluster of specialised cells in the right atrium. It spontaneously generates an electrical signal about 60–100 times per minute — setting the heart's rhythm. This is why it is called the heart's natural pacemaker. If the SA node fails, other cells in the heart can take over — but at a slower rate. How the signal spreads: 1. SA node fires → electrical signal spreads across both atria → atria contract (pushing blood into ventricles) 2. Signal reaches the atrioventricular (AV) node — a junction point between atria and ventricles. There is a crucial 0.1-second delay here — this gives the atria time to finish contracting before the ventricles start. 3. Signal travels down the Bundle of His and along the Purkinje fibres (fast conducting pathways) → spreads through both ventricles simultaneously → ventricles contract from the bottom up, squeezing blood out What an ECG records: An ECG (electrocardiogram) records this electrical activity from the surface of the skin. The classic waveform shows: - P wave — atrial contraction (SA node firing, signal spreading through atria) - QRS complex — ventricular contraction (signal spreading through ventricles — the largest wave because the ventricles have more muscle) - T wave — ventricles resetting (repolarising) ready for the next beat Arrhythmias — when the rhythm goes wrong: - Atrial fibrillation (AF) — the atria fire chaotically (350–600 times/minute) instead of coordinated beats → irregular pulse, reduced cardiac output, and blood can pool in the atria forming clots → stroke risk. Very common in people over 65. - Heart block — the AV node fails to conduct signals → ventricles beat too slowly → dizziness, fainting. May require an artificial pacemaker — a device implanted under the skin that generates electrical signals to keep the heart beating at a safe rate.