4.1 The Cardiovascular System

The cardiovascular system is the single most heavily tested topic on the TEAS Science section, and for good reason: it underpins almost every nursing intervention you will eventually perform, from taking a blood pressure to interpreting a pulse. The system has three components — the heart (the pump), the blood vessels (the pipes), and the blood (the fluid).

On the exam you will be asked to trace the path of a single drop of blood, identify which vessel carries oxygen-rich versus oxygen-poor blood, and reason about pressure. Memorizing names is not enough; you must understand why each structure exists, because the test favors application questions over pure recall.

Heart Anatomy: Four Chambers

The heart is a four-chambered muscular organ roughly the size of your fist, sitting slightly left of center in the chest. The two upper chambers are atria (singular: atrium) and the two lower chambers are ventricles. A simple rule anchors everything: atria receive, ventricles pump. Atria receive blood returning to the heart, while the more muscular ventricles eject blood out of the heart.

The left ventricle has the thickest muscular wall because it must generate enough force to push blood through the entire systemic circuit — a frequent distractor on the test asks which chamber is "strongest," and the answer is always the left ventricle.

Heart Valves Prevent Backflow

Four valves act as one-way doors so blood moves forward and never leaks backward. Two atrioventricular (AV) valves sit between atria and ventricles; two semilunar valves guard the exits into the great arteries.

Memory tip: "Try (Tri) before you Buy (Bi)" — the tricuspid is on the right, the bicuspid (mitral) is on the left. The familiar "lub-dub" heart sound is the valves snapping shut: "lub" is the AV valves closing, "dub" is the semilunar valves closing.

The Pathway of Blood Through the Heart

The test loves blood-flow sequence questions. Walk through it deliberately:

1. Deoxygenated blood from the body enters the right atrium via the superior and inferior vena cava.
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.
4. Blood travels to the lungs, where it picks up oxygen and releases carbon dioxide.
5. Now oxygenated, it returns through the pulmonary veins to the left atrium.
6. It passes through the mitral (bicuspid) valve into the left ventricle.
7. The left ventricle pumps it through the aortic valve into the aorta.
8. The aorta distributes oxygen-rich blood to the entire body.

Pulmonary vs. Systemic Circulation

The heart drives two loops at once. The pulmonary circuit carries blood between the heart and lungs for gas exchange. The systemic circuit carries blood between the heart and the rest of the body to deliver oxygen and collect waste.

This produces the cardiovascular system's most counterintuitive fact, and a guaranteed test item: the pulmonary arteries carry deoxygenated blood (the only arteries that do), while the pulmonary veins carry oxygenated blood (the only veins that do). The lesson: arteries are defined by direction (away from the heart), not by oxygen content.

Blood Components

Blood is a connective tissue with a liquid matrix. About 55% is plasma (water, proteins, nutrients, wastes) and about 45% is formed elements.

• Red blood cells (erythrocytes): carry oxygen using the iron-rich protein hemoglobin; most numerous cell
• White blood cells (leukocytes): fight infection as part of the immune system
• Platelets (thrombocytes): cell fragments that trigger clotting

Blood Vessels and Blood Pressure

Blood pressure is the force of blood pushing against arterial walls, written as systolic/diastolic (e.g., 120/80 mmHg). The systolic (top) number is the pressure during ventricular contraction; the diastolic (bottom) number is the pressure during ventricular relaxation. The conduction system that drives each beat runs in order: SA node → AV node → Bundle of His → bundle branches → Purkinje fibers.

Worked Example: A nursing student records a resting heart rate of 70 beats per minute and a stroke volume (blood pumped per beat) of 70 mL. What is the cardiac output? Cardiac output = heart rate × stroke volume = 70 beats/min × 70 mL = 4,900 mL/min ≈ 5 L/min. This matches the normal resting adult value of roughly 5 liters per minute — essentially the entire blood volume circulating once every minute.