The Cardiovascular System
The chambers and valves, the path of a drop of blood through the heart and lungs, the coronary and conduction systems, what blood pressure is actually made of, and how the body compensates when it starts to lose ground.
12 min read · Updated September 10, 2026
What the System Is
Three parts, one job. A pump, a network of pipes, and the fluid inside them. The job is delivering oxygen and nutrients to every cell and carrying waste away. Failure anywhere in those three parts produces the same end result, which is a cell that is not getting what it needs.
That framing is worth holding onto, because it is exactly how shock is categorized: a pump problem, a pipe problem, a volume problem, or an obstruction in the way.
The Heart
The heart sits in the mediastinum, behind the sternum, roughly two thirds of it to the left of the midline. It is about the size of the patient's own closed fist.
Three layers make up the wall. The endocardium lines the inside of the chambers. The myocardium is the thick muscular middle layer that does the actual work. The epicardium covers the outside. Around all of it sits the pericardium, a tough fibrous sac.
The pericardium matters because it does not stretch quickly. Blood or fluid accumulating inside it squeezes the heart from outside and prevents the chambers from filling. That is cardiac tamponade, and a surprisingly small volume can cause it when it accumulates fast.
Quick check
Blood collects rapidly inside the pericardial sac. Why does this stop the heart from working?
Frequently asked questions
What is the path of blood through the heart?
Body into the vena cava, right atrium, tricuspid valve, right ventricle, pulmonic valve, pulmonary arteries, lungs, pulmonary veins, left atrium, mitral valve, left ventricle, aortic valve, aorta, body. The pulmonary arteries are the only arteries carrying oxygen poor blood and the pulmonary veins are the only veins carrying oxygen rich blood.
What is cardiac output?
Cardiac output is heart rate multiplied by stroke volume, which is how much blood the heart moves in one minute. Stroke volume itself depends on preload, contractility and afterload. Every cardiovascular problem you will ever see is a problem with one of those four numbers.
Why does blood pressure stay normal early in shock?
Blood pressure is cardiac output multiplied by systemic vascular resistance, so the body can hold the number up by squeezing the vessels and speeding the heart while volume is already falling. That is compensated shock, and the pressure is the last thing to go, not the first. Pulse, skin and mental status change well before it.
Now go use it
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