The Heart, Circulation, and the Numbers a Monitor Shows
Unit 2: Anatomy, Physiology, and Medical Terminology. Lesson 5 of 53 in BMET Academy.
More of your equipment is built around the heart than around any other organ. Understand the pump and its wiring and the ECG monitor, the blood pressure cuff, and the defibrillator all stop being mysterious.
The pump
The heart has four chambers. The two small upper ones are atria and the two large lower ones are ventricles. The right side sends blood to the lungs; the left side sends it to the body. Blood returning from the body enters the right atrium, drops into the right ventricle, and is pumped to the lungs to pick up oxygen. It comes back into the left atrium, drops into the left ventricle, and is pumped out to everything else. The left ventricle is the thickest muscle in the heart because it does the longest push.
Valves sit between the chambers and at each exit, and they are one-way doors. They open to let blood through and snap shut so it cannot wash back. A valve that leaks or will not open is why an operating room needs a heart-lung machine.
The wiring
Every beat starts as a tiny electrical signal.
- The SA node in the right atrium fires on its own. It is the natural pacemaker and it sets the rate.
- The atria squeeze. On the tracing that is the small P wave.
- The AV node holds the signal back for a fraction of a second so the ventricles can fill.
- The ventricles squeeze. That is the tall spike, the QRS complex, and it is what a monitor counts to show a heart rate.
- The ventricles recover. That is the T wave.
An ECG records this from electrodes on the skin. It reads the heart's electricity, nothing else, which is why a loose electrode looks like a rhythm problem.
The numbers
One cardiac cycle is the whole round trip: atrial contraction, then ventricular contraction, then the refill before the next beat. Every number below is a measurement of that cycle.
- Systole is the squeeze and diastole is the refill. Blood pressure quotes both: about 120/80 mmHg in an adult, the systolic over the diastolic.
- MAP, the mean arterial pressure, is the average pressure over a whole beat: diastolic plus one third of the gap between the two numbers. For 120/80 that is 80 + 40/3, about 93 mmHg. An automatic cuff measures this value most directly and calculates the other two from it.
- Heart rate at rest is 60 to 100 beats per minute. Below 60 is bradycardia; above 100 is tachycardia.
The pipes and what flows in them
Arteries carry blood away from the heart under pressure, which is why they are thick-walled and why an arterial line has a pressure waveform. Veins return it at low pressure. Capillaries are the thin-walled vessels in between where oxygen crosses into tissue. Blood itself is plasma, the liquid, carrying red cells that hold oxygen on hemoglobin, white cells that fight infection, and platelets that clot.
Three of your devices read this one system: the ECG reads its electricity, NIBP squeezes an artery shut and listens for the pressure where flow returns, and pulse oximetry shines light through a finger to see how much oxygen the hemoglobin is carrying.
The Heart, Circulation, and the Numbers a Monitor Shows, explained slowly
The pump, in order
Blood makes a figure eight through the heart, and the order never changes.
body -> vena cava -> RIGHT ATRIUM -> tricuspid valve -> RIGHT VENTRICLE
-> pulmonary valve -> lungs (pick up oxygen, drop carbon dioxide)
-> LEFT ATRIUM -> mitral valve -> LEFT VENTRICLE -> aortic valve -> bodyRight side to the lungs, left side to the body. The left ventricle pushes against the pressure of the whole body, so it has the thickest wall, and that is also why a failure on the left side shows up as fluid backing into the lungs.
The four valves are one-way doors. They open with the flow and slam shut against backflow, and the slam is what a stethoscope hears as the heart sounds.
The electrical sequence and the tracing
Event On the tracing Why you care SA node fires, atria depolarize P wave the natural pacemaker sets the rate AV node delays the signal flat PR stretch gives the ventricles time to fill ventricles depolarize QRS complex the monitor counts these for heart rate ventricles recover T wave a defibrillator avoids shocking here
That table is most of an ECG question. The monitor is counting QRS complexes, so anything that adds a spike to the signal (a shivering patient, a loose electrode slapping, electrical interference) can be counted as a beat, and anything that removes the spike reads as no beat at all.
The pressure numbers, derived not memorized
Systole is the squeeze; diastole is the refill. A cuff reports both: about 120/80 mmHg in a healthy adult.
The mean arterial pressure is not the average of those two numbers, because the heart spends more of each beat relaxed than squeezing. The estimate used everywhere is:
MAP = diastolic + (systolic - diastolic) / 3 120/80 -> 80 + 40/3 = about 93 mmHg 120/90 -> 90 + 30/3 = 100 mmHg 160/100 -> 100 + 60/3 = 120 mmHg
Work one of those by hand now and the exam version takes ten seconds.
Heart rate and the two words
Resting adult heart rate is 60 to 100 beats per minute. Below 60 is bradycardia and above 100 is tachycardia. Both can be perfectly normal in context, an athlete at rest or anyone climbing stairs, which is why the monitor has adjustable alarm limits and why setting those limits is part of your setup.
Vessels, blood, and the three devices
Arteries leave the heart under pressure and have muscular walls; veins return blood at low pressure and rely on valves and muscle movement; capillaries are one cell thick, which is the only way gas can cross. Blood is plasma carrying red cells (oxygen rides on hemoglobin), white cells (infection), and platelets (clotting).
Three everyday devices read this one system, and knowing which signal each one uses explains their failure modes. ECG reads electricity from skin electrodes, so it fails on bad skin contact. NIBP inflates a cuff to stop flow and watches the pressure where flow returns, so it fails on cuff size, placement, and movement. Pulse oximetry shines red light at about 650 to 660 nm and infrared at about 805 to 940 nm through tissue and compares how much of each is absorbed, so it fails when there is no pulsing blood at the site: cold fingers, poor circulation, or motion.