Overview
The heart is a muscular pump that drives blood through a closed loop of vessels to deliver oxygen and nutrients and remove waste. This lesson connects the mechanical events of the heartbeat, the electrical signals that trigger them, and the way the body regulates blood flow to match the needs of the tissues.
The Cardiac Cycle
One heartbeat is the cardiac cycle, alternating between systole (contraction) and diastole (relaxation). It proceeds in three broad stages:
- Atrial systole. The atria contract, topping off the ventricles, which are already mostly filled by passive flow.
- Ventricular systole. The ventricles contract. Pressure rises, the atrioventricular (AV) valves slam shut producing the first heart sound S1 (“lub”), and once pressure exceeds that in the great arteries, the semilunar valves open and blood is ejected.
- Ventricular diastole. The ventricles relax, the semilunar valves close producing the second heart sound S2 (“dub”), and the chambers refill.
The valve closures are what a stethoscope hears, so the heart sounds are a direct window onto the timing of the cycle.
The Conduction System
The heartbeat is electrical in origin. Specialized cells generate and route the impulse in a fixed sequence:
SA node -> atria -> AV node -> Bundle of His -> right and left bundle branches -> Purkinje fibers -> ventricular muscle.
The SA node in the right atrium is the natural pacemaker; it depolarizes on its own and sets the rate. The impulse spreads across the atria, causing them to contract, then reaches the AV node, which imposes a brief delay. That pause lets the atria finish emptying before the ventricles fire. The signal then races down the Bundle of His and Purkinje fibers so the ventricles contract forcefully from the apex upward.
Reading the ECG
An electrocardiogram (ECG) records this electrical activity from the body surface. Three features form the basic pattern:
| Waveform | Electrical event |
|---|---|
| P wave | Atrial depolarization (leads to atrial contraction) |
| QRS complex | Ventricular depolarization (leads to ventricular contraction) |
| T wave | Ventricular repolarization (ventricles reset) |
The AV nodal delay appears as the flat segment between the P wave and the QRS. Disruptions in this pattern reveal rhythm and conduction problems.
Cardiac Output
How much blood the heart delivers per minute is the cardiac output (CO):
Cardiac output = Heart rate x Stroke volume
At rest this is roughly 5 liters per minute. Heart rate is tuned by the autonomic nervous system: sympathetic input speeds it, parasympathetic (vagal) input slows it. Stroke volume depends on how full the ventricle is before it contracts (preload), how hard the muscle squeezes (contractility), and the pressure it must pump against (afterload). During exercise, both heart rate and stroke volume rise, and cardiac output can increase several fold.
Blood Pressure and the Baroreceptor Reflex
Blood flow through vessels creates blood pressure, reported as systolic over diastolic, for example 120/80 mmHg. Pressure is generated by cardiac output pushing against the resistance of the vessels. A normal adult reading is below 120/80 mmHg, and sustained high pressure, hypertension, strains the heart and vessels.
Pressure must stay high enough to guarantee perfusion of the brain, heart, and kidneys, yet not so high it damages vessels. The baroreceptor reflex provides fast negative feedback. Stretch baroreceptors in the carotid arteries and aorta sense pressure. If pressure falls, as when a person stands up quickly, reduced stretch prompts increased sympathetic output: the heart speeds up, contracts harder, and vessels constrict, restoring pressure. If pressure rises, the opposite occurs. This is the same receptor to control center to effector loop introduced in the homeostasis lesson, applied to the circulation.
Clinical Relevance
These principles guide emergency and nursing assessment. A weak, rapid pulse with low blood pressure signals shock, a state of inadequate perfusion in which tissues are starved of oxygen; the rapid heart rate is the baroreceptor reflex fighting to compensate. When the SA node or conduction pathway fails, an artificial pacemaker can restore rhythm. Blocked coronary arteries starve heart muscle of oxygen, producing the chest pain of a heart attack and the ECG changes that clinicians look for. And because cardiac output equals heart rate times stroke volume, treatments that adjust rate, filling, or contractility are aimed directly at one of those terms to protect perfusion of vital organs.