Heart Physiology- Cardiac Function and Mechanisms
What Heart Physiology Actually Means
The heart is a fist-sized muscle that beats roughly 100,000 times every day. That's 3 billion beats across a typical lifespan. Understanding heart physiology isn't optional knowledge—it's the difference between understanding why your chest hurts and panicking over nothing.
Most people treat the heart like a simple pump. It's not. This organ has its own electrical system, self-regulating mechanisms, and feedback loops that would make an engineer weep. Let's get into how it actually works.
Cardiac Anatomy: The Structure Behind the Function
The heart has four chambers. The right atrium receives deoxygenated blood from your body. The right ventricle pumps this blood to your lungs. The left atrium receives oxygenated blood from your lungs. The left ventricle pumps this blood to your entire body.
The left ventricle is the thickest chamber. It generates the pressure needed to push blood through your entire circulatory system. When doctors talk about heart failure, they're usually talking about left ventricular failure.
The Heart Wall Layers
The heart wall has three layers:
- Epicardium – The outer layer. Contains coronary arteries and fat.
- Myocardium – The middle layer. This is the muscle itself. Everything you care about lives here.
- Endocardium – The inner lining. Contacts blood directly.
The myocardium is what contracts. It's different from skeletal muscle because it works involuntarily and has cells connected by intercalated discs. These connections let electrical impulses spread instantly across the entire heart wall.
The Cardiac Cycle: One Beat at a Time
The cardiac cycle is the sequence of events during one heartbeat. It has two main phases: systole (contraction) and diastole (relaxation).
During diastole, the ventricles relax and fill with blood. The atria contract to top off the ventricular filling. This takes about 0.5 seconds at rest.
During systole, the ventricles contract. The mitral and tricuspid valves close first (preventing backflow), then the aortic and pulmonary valves open as pressure builds. Blood exits the heart.
Pressure-Volume Loops
If you plot ventricular pressure against volume during a cardiac cycle, you get a loop that tells you everything about cardiac function. The shape of this loop changes in disease states. A failing heart has a smaller loop—it moves less blood with each beat.
The Electrical Conduction System
The heart doesn't need your brain to beat. It has its own built-in pacemaker system.
Key Components
The sinoatrial (SA) node sits in the right atrium. It fires at 60-100 beats per minute and sets your heart rate. This is your natural pacemaker.
Electrical signals spread across the atria, causing them to contract. Then they hit the atrioventricular (AV) node, which acts as a gate. The AV node slows the signal by about 0.1 seconds. This pause lets the ventricles finish filling before they contract.
From the AV node, the signal travels through the bundle of His, splits into left and right bundle branches, and reaches the Purkinje fibers. These fibers distribute the impulse rapidly through the ventricular myocardium.
What Can Go Wrong
If the SA node fails, the AV node takes over at 40-60 beats per minute. If both fail, the ventricles can generate their own rhythm at 20-40 beats per minute—but this is unsustainable.
Arrhythmias happen when electrical signals fire at the wrong time or from the wrong place. Atrial fibrillation is the most common. In A-fib, multiple wavefronts of electrical activity circle the atria chaotically. The atria quiver instead of contract. Blood can pool and clot.
Cardiac Output: How Much Blood the Heart Pumps
Cardiac output (CO) is the volume of blood pumped by the heart per minute. The formula is simple:
CO = Heart Rate Ă— Stroke Volume
At rest, cardiac output is about 5 liters per minute. During exercise, it can reach 25-35 liters per minute in trained athletes.
Stroke Volume Determinants
Stroke volume depends on three factors:
- Preload – How much the ventricles stretch before contraction. More filling usually means stronger contraction (up to a point).
- Afterload – The resistance the heart pumps against. High blood pressure increases afterload.
- Contractility – The inherent strength of myocardial contraction. Sympathetic stimulation increases contractility.
These three factors interact constantly. Change one, and the others compensate. This is why heart failure develops gradually—the system masks problems until it can't anymore.
Coronary Circulation: Feeding the Heart Itself
The heart muscle needs oxygen and nutrients. It gets them from the coronary arteries, which branch off the aorta just as it leaves the heart.
The left coronary artery supplies the left ventricle and left atrium. The right coronary artery supplies the right ventricle, right atrium, and—in most people—the AV node and bundle of His.
Blood flow to the myocardium occurs mostly during diastole. During systole, the contracting heart muscle compresses the coronary vessels. This is why tachycardia (fast heart rate) can cause problems—it shortens diastole and reduces coronary perfusion time.
Regulation of Heart Function
The autonomic nervous system controls heart rate and contractility.
Sympathetic Activation
Sympathetic stimulation releases norepinephrine. Heart rate increases. Contractility increases. Conduction velocity through the AV node increases. Blood vessels constrict to redirect blood to the heart and muscles.
Parasympathetic Activation
The vagus nerve releases acetylcholine. Heart rate decreases. This is why athletes have low resting heart rates—their parasympathetic tone is high. It's also why you can temporarily stop your heart by pressing on your eyes (the oculocardiac reflex).
Baroreceptor Reflex
Baroreceptors in the carotid arteries and aortic arch sense blood pressure. When BP rises, they signal the brain to decrease sympathetic tone and increase parasympathetic tone. When BP falls, the opposite happens. This reflex works in seconds.
Common Cardiac Pathologies
Coronary Artery Disease
Atherosclerotic plaques narrow coronary arteries. When plaques rupture, platelets aggregate and can completely block blood flow. That section of heart muscle dies. This is a myocardial infarction—a heart attack.
Heart Failure
The heart can't pump enough blood to meet the body's needs. In systolic heart failure, the ventricle doesn't contract properly. In diastolic heart failure, the ventricle can't relax properly to fill with blood.
Symptoms include shortness of breath, leg swelling, and fatigue. The body compensates initially by increasing sympathetic tone and retaining fluid—but these compensations eventually make things worse.
Valvular Disease
Valves can narrow (stenosis) or leak (regurgitation). Both conditions force the heart to work harder. Aortic stenosis is particularly dangerous—it can cause sudden death without warning.
Assessing Heart Function: Diagnostic Tools
Here's how doctors actually evaluate cardiac function:
| Method | What It Shows | Best For |
|---|---|---|
| ECG (Electrocardiogram) | Electrical activity, rhythm, prior heart attacks | Arrhythmias, ischemia, conduction problems |
| Echocardiogram | Structure, valve function, ejection fraction | Heart failure, valvular disease, structural defects |
| Cardiac Catheterization | Coronary artery blockages, pressures inside chambers | Diagnosing CAD, measuring cardiac output directly |
| Stress Test | Heart response to exertion, exercise tolerance | Detecting CAD, evaluating functional capacity |
| BNP Blood Test | Hormone levels indicating heart strain | Diagnosing and staging heart failure |
How to Understand Your Own Heart Health
You don't need a medical degree to monitor basic cardiac health.
Know your numbers:
- Resting heart rate (60-100 bpm is normal, lower is usually better fitness)
- Blood pressure (120/80 or lower is normal)
- BMI and waist circumference
Watch for warning signs:
- Chest pain or pressure during exertion
- Unexplained shortness of breath
- Heartbeat that feels too fast, too slow, or irregular
- Swelling in your legs or feet
- Fainting or near-fainting episodes
If you have these symptoms, see a doctor. Don't wait. Don't assume it'll go away.
The Bottom Line
Heart physiology is complex, but the essentials are straightforward. The heart generates its own electrical signals, uses those signals to coordinate contraction, and pumps blood through a closed circulatory system. Everything else—regulation, adaptation, disease—builds on this foundation.
What you should take away: the heart is resilient until it isn't. By the time symptoms appear, significant damage may already exist. Regular checkups, knowing your numbers, and not ignoring warning signs are the only things that actually matter in prevention.