The Cardiovascular System

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.

Chambers and Valves

Four chambers. Two atria on top receive blood. Two ventricles below pump it out.

The right side is a low pressure pump sending blood a short distance to the lungs. The left side is a high pressure pump sending blood to the entire body, which is why the left ventricle has by far the thickest wall.

Four valves keep blood moving one direction only.

The heart sounds you hear are the valves closing. The first sound is the tricuspid and mitral valves shutting as the ventricles contract. The second is the pulmonic and aortic valves shutting as the ventricles relax.

The Path of Blood

Follow one drop the whole way around, because this sequence gets asked in some form at every level.

Body tissues, oxygen poor, into the superior and inferior vena cava, into the right atrium, through the tricuspid valve, into the right ventricle, through the pulmonic valve, into the pulmonary arteries, out to the lungs where it picks up oxygen and drops carbon dioxide, back through the pulmonary veins, into the left atrium, through the mitral valve, into the left ventricle, through the aortic valve, into the aorta, out to the body.

The pulmonary vessels are the exception to the rule students learn first. Arteries carry blood away from the heart and veins carry it toward the heart, which is the actual definition. Oxygen content is not part of the definition. The pulmonary arteries carry oxygen poor blood and the pulmonary veins carry oxygen rich blood, because they are named for direction.

The Coronary Arteries

The heart does not take its oxygen from the blood passing through its chambers. It has its own supply, and those vessels are the first two branches off the aorta, coming off just above the aortic valve.

The left main coronary artery divides into the left anterior descending, which supplies the front wall and most of the left ventricle, and the circumflex, which wraps around to the left side and back. The right coronary artery supplies the right side, the inferior wall, and in most people the SA and AV nodes.

Two facts follow. An occlusion of the left anterior descending threatens the largest amount of working muscle, which is why it carries the reputation it does. An inferior wall infarct often involves the right coronary artery and therefore the nodes, which is why bradycardia and heart block frequently accompany it.

The coronary arteries fill during diastole, when the heart is relaxed. Anything that shortens diastole, including a very fast heart rate, cuts the heart's own supply at the moment demand is highest.

The Conduction System

Cardiac muscle can generate its own impulse, a property called automaticity. The conduction system organizes that into a coordinated beat.

The sinoatrial node in the right atrium is the natural pacemaker because it fires fastest, and the fastest site wins. Its impulse spreads across both atria, which contract and top off the ventricles.

The impulse reaches the atrioventricular node, which deliberately delays it. That delay is what gives the atria time to empty before the ventricles squeeze.

From there it travels down the bundle of His, splits into the right and left bundle branches, and spreads through the Purkinje fibers into the ventricular muscle, which contracts from the bottom upward so blood is pushed toward the outflow valves.

Each lower site can take over if the site above it fails, at its own slower rate. A patient with a heart rate of 35 may be running on a ventricular escape rhythm because everything above it has stopped working.

Blood Vessels

Arteries carry blood away from the heart. Thick muscular walls, high pressure, and the ability to change diameter on command from the nervous system.

Arterioles are the smallest arteries and are the main site of resistance. Constricting them raises blood pressure across the whole system.

Capillaries are one cell thick, and they are the only place where anything is actually exchanged. Everything else in the network exists to get blood to a capillary and take it away again.

Venules collect from the capillaries, joining into veins, which are thin walled, low pressure, and equipped with one way valves to keep blood moving toward the heart against gravity. At any given moment the veins hold roughly 60 to 70 percent of the body's blood volume, which makes the venous system the body's reservoir.

Blood

An adult carries roughly 5 to 6 liters, about 7 percent of body weight.

Plasma is the fluid portion, about 55 percent, mostly water carrying proteins, electrolytes, glucose and clotting factors.

Red blood cells (erythrocytes) carry oxygen on hemoglobin. They have no nucleus, live about 120 days, and are made in the bone marrow.

White blood cells (leukocytes) fight infection.

Platelets (thrombocytes) are cell fragments that begin clotting by plugging a break in a vessel wall and triggering the clotting cascade.

Cardiac Output and Blood Pressure

Two equations carry most of cardiovascular physiology.

Cardiac output = heart rate × stroke volume. How much blood the heart moves in one minute. A typical adult sits around 5 liters per minute.

Blood pressure = cardiac output × systemic vascular resistance. Flow multiplied by the tightness of the pipes.

Stroke volume, the amount ejected per beat, depends on three things.

• Preload. How much blood returns to fill the ventricle. More stretch produces a stronger contraction, up to a point, which is the Frank Starling relationship.
• Contractility. How hard the muscle squeezes, independent of filling.
• Afterload. The resistance the ventricle has to pump against.

Every cardiovascular emergency you will ever handle is a disturbance of one of those numbers. Blood loss drops preload. An infarct drops contractility. Anaphylaxis drops resistance. A tension pneumothorax obstructs return.

Systolic pressure is the peak while the ventricle contracts. Diastolic pressure is the pressure remaining while it relaxes, which reflects vessel tone. Pulse pressure is the difference between them, and it narrows when the vessels clamp down while volume falls.

Compensation, and Why Blood Pressure Is a Late Sign

The body defends perfusion aggressively. Baroreceptors in the carotid arteries and the aortic arch sense a falling pressure and trigger a sympathetic response: heart rate up, contractility up, vessels constricted, blood shunted away from skin and gut toward brain and heart. The kidneys hold onto salt and water. The spleen releases some stored red cells.

That is why blood pressure holds steady well into blood loss. The number is being propped up by everything else. Compensated shock describes the period when the body is still succeeding at that. Decompensated shock is when it stops succeeding and the pressure falls.

The findings that change first are the ones the compensation itself produces: a rising pulse, pale and cool and sweaty skin, delayed capillary refill, thirst, restlessness and anxiety. A falling blood pressure is confirmation that compensation has failed, not an early warning.

Shock sorts into four categories by which part of the system failed.

Perfusion at the Capillary

Perfusion is the delivery of oxygenated blood to tissue at the capillary and the removal of waste from it. Adequate perfusion needs three things intact: enough volume, a working pump, and vessels with appropriate tone.

Without oxygen the cell switches to anaerobic metabolism. It produces a small fraction of the energy, generates lactic acid, and cannot sustain itself. Acid builds up in the blood, which is the metabolic acidosis underlying every kind of shock regardless of what started it.

What to Take Away

Pump, pipes, fluid. Learn the path of blood in order, including that the pulmonary arteries carry oxygen poor blood. Coronary arteries come off the aorta and fill during relaxation. The SA node leads at 60 to 100 with backups underneath it. Cardiac output is rate times stroke volume, blood pressure is output times resistance, and stroke volume answers to preload, contractility and afterload. Blood pressure is the last thing to fall, so read the pulse, the skin and the mental status first.