4.1 The Cardiovascular System
Key Takeaways
The heart has four chambers: two atria receive blood and two ventricles pump blood, with the left ventricle being the thickest and strongest
Blood flow path: body → right atrium → right ventricle → lungs → left atrium → left ventricle → body
Pulmonary arteries are the only arteries that carry deoxygenated blood; pulmonary veins are the only veins that carry oxygenated blood
The SA node is the heart's natural pacemaker, firing 60–100 times per minute to initiate each heartbeat
Cardiac output (about 5 L/min at rest) equals heart rate multiplied by stroke volume
The cardiovascular system is one of the 11 body systems ATI lists, and it underpins nursing tasks 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 questions can ask you to apply it, not only recall it.
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.
| Chamber | Location | Function | Blood Type |
|---|---|---|---|
| Right Atrium | Upper right | Receives deoxygenated blood from the body | Deoxygenated |
| Right Ventricle | Lower right | Pumps blood to the lungs | Deoxygenated |
| Left Atrium | Upper left | Receives oxygenated blood from the lungs | Oxygenated |
| Left Ventricle | Lower left | Pumps blood to the whole body (strongest chamber) | Oxygenated |
The left ventricle has the thickest muscular wall because it must generate enough force to push blood through the entire systemic circuit — if a question asks which chamber is "strongest," the answer is 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.
| Valve | Type | Location | Directs Flow |
|---|---|---|---|
| Tricuspid | AV | Right side | Right atrium → right ventricle |
| Pulmonary | Semilunar | Right side | Right ventricle → pulmonary artery |
| Mitral (Bicuspid) | AV | Left side | Left atrium → left ventricle |
| Aortic | Semilunar | Left side | Left ventricle → aorta |
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
Expect blood-flow sequence questions. Walk through it deliberately:
- Deoxygenated blood from the body enters the right atrium via the superior and inferior vena cava.
- It passes through the tricuspid valve into the right ventricle.
- The right ventricle pumps it through the pulmonary valve into the pulmonary arteries.
- Blood travels to the lungs, where it picks up oxygen and releases carbon dioxide.
- Now oxygenated, it returns through the pulmonary veins to the left atrium.
- It passes through the mitral (bicuspid) valve into the left ventricle.
- The left ventricle pumps it through the aortic valve into the aorta.
- 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.
| Circuit | Path | Purpose |
|---|---|---|
| Pulmonary | Heart → lungs → heart | Gas exchange (load O₂, dump CO₂) |
| Systemic | Heart → body → heart | Deliver O₂, remove CO₂ |
This produces the cardiovascular system's most counterintuitive fact, and a fact worth memorizing: 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
| Type | Function | Characteristics |
|---|---|---|
| Arteries | Carry blood away from the heart | Thick, elastic walls; high pressure |
| Arterioles | Small arteries | Regulate flow into tissues |
| Capillaries | Exchange site | One cell thick; allow diffusion |
| Venules | Small veins | Collect blood from capillaries |
| Veins | Return blood to the heart | Thinner walls; valves prevent backflow |
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.
Place these structures in the correct order that a drop of blood passes through, starting from the right atrium.
Arrange the items in the correct order
Left ventricle
Right ventricle
Aorta
Right atrium
Left atrium
Lungs
Which chamber of the heart has the thickest muscular wall, and why?
Right atrium, because it receives blood from the entire body
Right ventricle, because it pumps against high lung pressure
Left atrium, because it stores oxygenated blood
Left ventricle, because it pumps blood to the entire body
A blood vessel is carrying deoxygenated blood away from the heart. Which vessel is it?
Aorta
Pulmonary vein
Pulmonary artery
Superior vena cava
Match each heart structure to its correct function.
Match each item on the left with the correct item on the right
SA node
Tricuspid valve
Left ventricle
Capillaries
Sections you finish are checked off in the contents.