Cardiovascular System

Core Concepts

The pump: chambers, valves, and plumbing

The heart is a four-chambered muscular pump. The two upper chambers, the atria, are receiving chambers that collect returning blood; the two lower chambers, the ventricles, are the power pumps that push blood out. A muscular wall (septum) separates the right and left sides.

Blood moves in one direction because of four one-way valves: the tricuspid valve (right atrium to right ventricle), the pulmonary valve (exit of the right ventricle), the mitral valve (left atrium to left ventricle), and the aortic valve (exit of the left ventricle). The heart muscle itself is fed by the coronary arteries, which branch off the aorta — when they narrow or block, the pump's own fuel supply is threatened.

The pathway: follow a drop of blood

1. Deoxygenated blood returns through the venae cavae into the right atrium.
2. It passes through the tricuspid valve into the right ventricle.
3. The right ventricle pumps it through the pulmonary valve into the pulmonary arteries — the only arteries that carry deoxygenated blood — and on to the lungs.
4. In the lungs it unloads carbon dioxide and picks up oxygen, returning through the pulmonary veins — the only veins that carry oxygenated blood — to the left atrium.
5. It passes through the mitral valve into the left ventricle, the most muscular chamber, which pumps it through the aortic valve into the aorta and out to the body.
6. After delivering oxygen and nutrients, blood returns through the veins to the venae cavae — and the loop starts again.

The cardiac cycle and heart sounds

One heartbeat is a cycle of two phases: diastole (relaxation and filling) and systole (contraction and ejection). The familiar "lub-dub" is valves closing: S1 ("lub") is the atrioventricular valves (tricuspid and mitral) closing as the ventricles begin to contract, and S2 ("dub") is the semilunar valves (pulmonary and aortic) closing as the ventricles relax. A murmur is turbulent blood flow, often from a valve that doesn't open or close fully — detecting and describing murmurs is an advanced skill requiring supervised practice.

The electrical system: what makes it beat

The heart beats because it generates its own electricity. The sinoatrial (SA) node in the right atrium is the natural pacemaker; its signal spreads across the atria, pauses at the atrioventricular (AV) node (a delay that lets the atria finish emptying), then races down the bundle of His and through the Purkinje fibers to make both ventricles contract nearly together. An electrocardiogram (ECG) records this electrical activity; a pulse is the mechanical result — the wave of blood felt in an artery. Electrical activity and mechanical pumping can separate (a rhythm on the monitor without a pulse), which is why nurses always verify a monitor rhythm against the person's pulse.

Cardiac output: the number that matters

Cardiac output (CO) is the volume of blood pumped per minute: CO = heart rate (HR) × stroke volume (SV). Stroke volume depends on preload (how much blood fills the ventricle before it contracts — the "stretch"), afterload (the resistance the ventricle pushes against), and contractility (the strength of the squeeze). Within limits, more stretch means a stronger contraction (the Frank–Starling relationship), but too little filling (blood loss) or too much (fluid overload) hurts performance. When one part changes, the body compensates — heart rate rises to maintain cardiac output when stroke volume falls. A rising heart rate is often a clue, not a disease: the body working to maintain delivery.

The pipes: vessels and blood pressure

Arteries carry blood away from the heart under high pressure; they branch into arterioles and then into capillaries, the microscopic exchange vessels where oxygen, nutrients, and wastes actually move between blood and tissue. Blood then collects through venules into veins, which return it to the heart; veins are low-pressure vessels, and one-way valves plus leg-muscle pumping help push blood upward against gravity. Blood pressure reflects the interaction of cardiac output and vascular resistance; it is a driving force for perfusion, not a guarantee of it. Perfusion — blood actually delivered to tissue — is the endpoint that matters, assessed with pulses, capillary refill, skin temperature and color, sensation, and function.

What the nurse assesses

Cardiovascular assessment is a head-to-toe story: heart rate, rhythm, and pulse quality at multiple sites; blood pressure; capillary refill; skin temperature and color; edema (swelling that may signal the pump struggling); activity tolerance; and any chest discomfort, palpitations, dizziness, or shortness of breath the person reports. Neck vein distention and some heart sounds require advanced skills and supervised practice. Descriptions of findings vary by source; always interpret in the full clinical context and follow institutional policies.

Common Confusions