Heart Structure and Components

Think of your heart as a four-room house with two floors - the upper floor has two atria (receiving chambers) and the lower floor has two thick-walled ventricles (pumping chambers). The septum acts like a wall down the middle, keeping oxygenated and deoxygenated blood completely separate.

The right side handles deoxygenated blood returning from your body. The superior vena cava brings blood from your head, neck, arms and chest, whilst the inferior vena cava carries blood from your legs, feet and organs. This blood enters the right atrium, passes through the tricuspid valve (which has three flaps), and gets pumped out by the right ventricle through the pulmonary artery to your lungs.

The left side deals with oxygenated blood from your lungs. The pulmonary veins deliver fresh blood to the left atrium, it flows through the bicuspid valve (two flaps), and the powerful left ventricle pumps it out through the aorta to supply your entire body. The semilunar valves prevent blood from flowing backwards.

Key Point: The heart has three protective layers - the epicardium (outer protection), myocardium (thick muscle layer packed with mitochondria), and endocardium (smooth inner lining for blood flow).

Heart Function and Blood Circulation

Your heart operates as a double circulatory system - blood passes through it twice during each complete circuit around your body. This might seem inefficient, but it's actually brilliant because it maintains high pressure for effective circulation.

The right pump handles the pulmonary circuit, sending deoxygenated blood to your lungs for oxygen pickup. The left pump manages the systemic circuit, distributing freshly oxygenated blood to every cell in your body. This separation ensures maximum efficiency.

Coronary arteries are your heart's own blood supply system - they deliver oxygen and nutrients directly to the heart muscle itself. Without these, your heart couldn't produce the ATP energy needed for those constant contractions. It's like having a dedicated fuel line for your body's most important engine.

The atrioventricular valves (tricuspid and bicuspid) prevent backflow from ventricles to atria, whilst semilunar valves stop blood flowing back from arteries into ventricles. These are held in place by tendinous cords attached to papillary muscles - think of them as the heart's guy-ropes.

Remember: Arteries carry blood away from the heart, veins carry blood towards the heart - regardless of whether it's oxygenated or not!

The Cardiac Cycle

The cardiac cycle describes everything that happens during one complete heartbeat - it's like a perfectly choreographed dance with three main stages. Each cycle takes less than a second, yet it's precisely coordinated to keep blood flowing efficiently.

Atrial systole kicks things off when both atria contract simultaneously. This forces blood from the atria into the ventricles through the open atrioventricular valves. As pressure builds in the atria, their volume decreases - it's basic physics in action.

Ventricular systole follows immediately as both ventricles contract powerfully. The atrioventricular valves slam shut (preventing backflow to the atria), whilst the semilunar valves burst open, allowing blood to surge into the aorta and pulmonary artery.

Finally, atrioventricular diastole gives everyone a brief rest - both atria and ventricles relax. The semilunar valves close to prevent backflow, whilst blood from the veins quietly refills the atria, ready for the next cycle. This relaxation phase is crucial for the heart's own blood supply.

Quick Tip: Systole = contraction, Diastole = relaxation. Remember "sys-squeeze" to keep them straight!

Heart's Electrical System

Your heart has its own built-in pacemaker - no external power needed! The sinoatrial node (SAN) in the right atrium wall generates electrical impulses roughly 70-80 times per minute, making it your body's natural pacemaker.

The electrical journey follows a specific route for maximum efficiency. Impulses spread across both atria, causing them to contract, then reach the atrioventricular node (AVN). The AVN creates a crucial delay, allowing the atria to completely empty before the ventricles start contracting.

From the AVN, impulses travel down the bundle of His through the interventricular septum, then split into left and right branches. At the heart's apex, Purkinje fibres spread the electrical signal up through the ventricle walls, triggering contraction from the bottom up - like squeezing a toothpaste tube.

This bottom-to-top contraction pattern isn't accidental - it ensures the ventricles empty completely and efficiently. The slight delay between atrial and ventricular contractions maximises the amount of blood pumped with each heartbeat, making your cardiovascular system incredibly efficient.

Clinical Note: Problems with this electrical system cause arrhythmias - irregular heartbeats that can be detected on ECGs.

Reading Electrocardiograms (ECGs)

ECGs translate your heart's electrical activity into wavy lines that doctors can read like a book. Each part of the trace corresponds to specific electrical events, making it an incredibly useful diagnostic tool.

The P wave shows atrial excitation (when the atria receive their electrical signal), followed by a flat PR segment indicating no current flow. The QRS complex creates the largest spikes as electrical signals surge through the ventricles - it's bigger than the P wave because ventricles are much larger than atria.

After another flat section (the ST segment showing maintained depolarisation), the T wave appears when the ventricles repolarise and return to their resting state. The whole pattern then repeats with machine-like precision.

Calculating heart rate from ECGs is straightforward: count the peaks, measure the time period, then convert to beats per minute. For example, if you see 3 peaks over 15mm (which equals 3 seconds at 0.2 seconds per mm), that's 1 peak per second or 60 beats per minute.

Exam Tip: ECGs can detect arrhythmias, coronary heart disease, heart attacks, and cardiomyopathy - know these four main applications!

Heart Rate Disorders

Arrhythmias are basically when your heart's rhythm goes wonky - it might beat too fast, too slow, or irregularly. Understanding these helps explain why that electrical system is so important.

Tachycardia means your heart's racing over 100 bpm when you're at rest. This is normal during exercise or fever, but abnormal tachycardia might need surgery or medication. Bradycardia is the opposite - under 60 bpm. Fit athletes often have this because their hearts are so efficient, but sometimes it requires an artificial pacemaker.

Ventricular fibrillation is seriously dangerous - the ventricles just quiver uselessly instead of contracting properly. This pumps little or no blood and can cause cardiac arrest. Sinus arrhythmia is usually harmless - your heart rate simply varies with breathing (speeds up when you breathe in, slows down when you breathe out).

Ectopic heartbeats feel like your heart's skipping beats, but they're usually harmless extra contractions. A flat line on an ECG shows no electrical activity at all - this is asystole, the most serious form of cardiac arrest.

Key Point: Many arrhythmias are harmless, but some require immediate medical attention - context matters!