The Cardiovascular System
Unit 1: Anatomy and Physiology. Lesson 2 of 60 in CBET Academy.
The heart is the organ the most equipment is built around: ECG monitors, blood pressure devices, defibrillators, pacemakers, and balloon pumps. Understand how it works and those devices make sense.
The pump
Four chambers. The right atrium receives blood from the body through the vena cava and passes it through the tricuspid valve into the right ventricle, which pumps it through the pulmonary valve and pulmonary artery to the lungs. Oxygenated blood returns through the pulmonary veins to the left atrium, passes the mitral valve into the left ventricle, the strongest chamber, which pumps it through the aortic valve into the aorta and the body. Right side to the lungs, left side to the body; the septum divides the two sides, and the heart feeds itself through the coronary arteries.
The electrical system
A heartbeat starts as an electrical impulse.
- The SA node in the right atrium fires; it is the natural pacemaker.
- The atria depolarize and contract: the P wave.
- The AV node delays the impulse so the ventricles fill.
- The impulse runs down the bundle of His, the bundle branches, and the Purkinje fibers.
- The ventricles depolarize and contract: the QRS complex.
- The ventricles repolarize: the T wave.
An ECG records this from the skin. Einthoven's triangle is the arrangement of the right arm, left arm, and left leg electrodes that produces leads I, II, and III.
The cardiac cycle and blood pressure
Systole is contraction, diastole is relaxation and filling. Arterial pressure peaks in systole and falls in diastole: 120/80 means a systolic of 120 and a diastolic of 80 mmHg. Mean arterial pressure is closer to diastolic because diastole lasts longer: MAP = diastolic + (systolic minus diastolic) / 3. Cardiac output is heart rate times stroke volume, about 5 liters per minute at rest.
Two things the exam names by name
- Korotkoff sounds: when a cuff is deflated with a stethoscope over the artery, the first tapping sound marks systolic pressure and the point where sounds disappear marks diastolic. Automated NIBP devices use oscillometry instead, but the term is on the outline.
- The dicrotic notch: a small dip on the downslope of an arterial pressure waveform where the aortic valve closes. Its presence shows a well-damped invasive line; its absence suggests overdamping.
Blood vessels and blood
Arteries carry blood away from the heart under pressure; veins return it; capillaries exchange gases and nutrients. Blood is plasma plus red cells (hemoglobin carries oxygen), white cells (immunity), and platelets (clotting).
When it goes wrong
Arrhythmias: bradycardia, tachycardia, atrial fibrillation (irregular, no P waves), ventricular fibrillation (chaotic, no output, shockable), asystole (flat line, not shockable). Myocardial infarction is heart muscle death from a blocked coronary artery. Heart failure is a weak pump; hypertension is chronic high pressure; shock is inadequate perfusion.
The Cardiovascular System, explained slowly
The machine most of your equipment serves
Monitors, blood pressure devices, defibrillators, pacemakers, and balloon pumps all exist because of this one organ. If you understand the heart as a two-stage pump with an electrical timer, every one of those devices becomes obvious.
Two pumps in one
from the body (deoxygenated) to the body (oxygenated)
| ^
vena cava aorta
v |
[RIGHT ATRIUM] -- tricuspid --> [RIGHT VENTRICLE] [LEFT VENTRICLE] <-- mitral -- [LEFT ATRIUM]
| ^
pulmonary valve aortic valve (between LV and aorta)
v |
pulmonary artery pulmonary veins
v ^
[ LUNGS ] ------------The right side is the low-pressure pump to the lungs. The left side is the high-pressure pump to the body, which is why the left ventricle has the thickest wall and why arterial pressure is what monitors measure. Four valves keep blood moving one way: tricuspid and mitral between atria and ventricles, pulmonary and aortic at the exits.
The electrical timer
A heartbeat is an electrical event first and a mechanical event second.
SA node (right atrium) fires ......................... the natural pacemaker, 60 to 100 per minute -> atria depolarize and squeeze .................... P WAVE -> AV node pauses about 0.1 s ....................... the PR segment; lets the ventricles fill -> bundle of His -> bundle branches -> Purkinje ..... fast wiring to the ventricles -> ventricles depolarize and squeeze ................ QRS COMPLEX -> ventricles repolarize (reset) .................... T WAVE
An ECG electrode on the skin sees the sum of all that electrical activity. Place electrodes on the right arm, left arm, and left leg and you have Einthoven's triangle; the voltage between each pair is a lead: I (LA minus RA), II (LL minus RA), III (LL minus LA). Lead II looks along the heart's main electrical axis, which is why it is the default monitoring lead.
Pressure, and the two terms the outline names
Systole is the ventricle squeezing; pressure in the arteries peaks. Diastole is relaxation; pressure falls to a floor. "120 over 80" is peak over floor.
Korotkoff sounds. Wrap a cuff, pump it above systolic so the artery is squeezed shut, then let it down slowly with a stethoscope below the cuff. When cuff pressure drops just under systolic, blood squirts through in pulses and you hear tapping: that first sound is systolic. As the cuff falls further, flow becomes continuous and the sounds fade; the pressure where they disappear is diastolic. Automated monitors do not listen; they measure the tiny cuff oscillations instead (oscillometry), but the term Korotkoff is on the outline because that is what the numbers mean.
Mean arterial pressure. The average pressure driving blood into organs. Because the heart spends about twice as long in diastole as systole, the average sits closer to the diastolic number: MAP = diastolic + (systolic minus diastolic) / 3. For 120/80: 80 + 40/3 = 93. Oscillometric monitors actually measure MAP directly and derive the other two.
The dicrotic notch. On an arterial line waveform, the pressure rises sharply, peaks, and falls. Partway down the fall there is a small bump: the aortic valve slamming shut and a little blood rebounding. That bump is the dicrotic notch. A crisp notch means the line and transducer are transmitting the waveform faithfully. A rounded waveform with no notch is overdamped (air bubble, clot, loose connection), and it reads systolic too low.
pressure ^ peak (systolic) | /\ | / \ <- dicrotic notch (aortic valve closes) | / \_/\ | / \___ | / \____ diastolic floor +----------------------------> time
Cardiac output
Heart rate times stroke volume, about 70 beats x 70 mL, roughly 5 liters per minute. Everything the body does depends on it, and it is what a balloon pump and a pacemaker try to protect.
When it goes wrong, in monitor terms
Bradycardia rate under 60; pacemakers fix it Tachycardia rate over 100 Atrial fibrillation irregular rhythm, no P waves Ventricular fibrillation chaotic squiggle, no output; defibrillate immediately Asystole flat line; CPR and drugs, not a shock Myocardial infarction blocked coronary artery, muscle dies; ST changes on the ECG Heart failure weak pump; fluid backs up into the lungs
How the exam asks it
- "Which wave represents ventricular depolarization?" QRS.
- "The first Korotkoff sound corresponds to which pressure?" Systolic.
- "What causes the dicrotic notch?" Closure of the aortic valve.
- "Which chamber pumps blood to the body?" Left ventricle.
- "MAP for a pressure of 120/90?" 90 + 30/3 = 100.
What to memorize
- Blood path: body, RA, RV, lungs, LA, LV, body. Valves: tricuspid, pulmonary, mitral, aortic.
- SA, AV, His, branches, Purkinje. P atria, QRS ventricles, T reset.
- First Korotkoff sound systolic, last diastolic. MAP = DBP + PP/3. Dicrotic notch = aortic valve closure.