Heart Anatomy Lesson: Structure, Function & Circulation

Lesson Overview

• Location and Structure of the Heart
• The Heart Wall: Layers of the Heart
• Chambers of the Heart: Atria and Ventricles
• Heart Valves: One-Way Doors for Blood
• Blood Flow Through the Heart: The Pathway of Circulation
• The Heart's Own Blood Supply (Coronary Circulation)

The human heart is a muscular organ about the size of your clenched fist, located in the middle of your chest. Its job is simple yet vital: pump blood around the body. By pumping blood, the heart delivers oxygen and nutrients to tissues and carries away waste products. In essence, your heart is the central engine of the circulatory system, working non-stop (about 100,000 beats per day!) to keep you alive. 

Have you ever felt your heart racing after a run or calming down when you rest? That's your heart adjusting its rate to meet your body's needs. In this lesson, we'll explore the heart's anatomy – its location, structure, chambers, valves, and how blood flows through this incredible organ.

Location and Structure of the Heart

The heart sits in a central area of the chest called the mediastinum, behind the breastbone (sternum) and between the lungs. Although it's in the center, the heart isn't perfectly symmetrical – its apex (the pointed tip at the bottom) angles slightly to the left, which is why you often sense your heartbeat more on the left side. The heart's base (top area) is broader and toward the right shoulder, where large blood vessels enter and exit.

Protection: The heart is enclosed in a protective sac of connective tissue called the pericardium. This sac shields the heart and anchors it in place so it doesn't jostle around with every movement. The pericardium has two main layers and a lubricating fluid:

• Fibrous pericardium: A tough, inelastic outer layer that looks like a sturdy bag. It prevents the heart from overstretching and holds it in position in the chest.
  
• Serous pericardium: A two-layered inner membrane. The outer part (parietal layer) lines the fibrous pericardium, and the inner part (visceral layer) is attached to the heart's surface. The visceral layer is also known as the epicardium when considered as part of the heart wall.
  
• Pericardial fluid: A slippery fluid fills the thin space between the parietal and visceral layers (the pericardial cavity). This fluid works like a lubricant, reducing friction as the heart beats. Imagine the heart moving inside a fluid-filled balloon – the fluid allows smooth gliding so the heart's surface doesn't rub harshly against surrounding tissues.
  

The Heart Wall: Layers of the Heart

Beneath the epicardium (visceral pericardium), the heart itself has three layers in its wall. From outside to inside, these are:

• Epicardium: This is the outermost layer of the heart wall (essentially the same as the visceral pericardium). It's a thin layer of epithelial and connective tissue. The epicardium protects the heart's surface and often has some fat deposits that cushion the heart.
  
• Myocardium: The middle layer and by far the thickest. "Myo" means muscle, and the myocardium is cardiac muscle tissue. These muscle fibers do the heavy work of contracting and pumping blood. When you think of the heart as a powerful pump, it's the myocardium you're imagining.
  
• Endocardium: The innermost layer. It's a smooth, thin lining of the heart's interior chambers. The endocardium is made of a thin layer of cells (similar to blood vessel linings) and it ensures blood can flow smoothly inside the heart without sticking to the walls or clotting. It also covers the heart valves.
  

All three layers work together: the endocardium provides a slick chamber surface, the myocardium contracts to pump blood, and the epicardium protects the outside.

Take This Quiz:

Anatomy of the Heart quiz

Chambers of the Heart: Atria and Ventricles

The human heart has four chambers: two smaller upper chambers called atria (singular: atrium) and two larger lower chambers called ventricles. The atria and ventricles are like rooms that blood flows through, each with a specific role:

• The atria are the receiving chambers. They sit on top (right atrium and left atrium) and collect blood returning to the heart. The walls of the atria are relatively thin since they only need to push blood a short distance down into the ventricles. Each atrium has an expandable, ear-like flap on its outer surface called an auricle (it looks a bit like an ear). The auricle acts like an overflow pouch that increases the atrium's capacity to hold blood. When extra blood returns (for example, during exercise), the auricles provide a little more space so the atria can handle it.
  
• The ventricles are the pumping chambers. They lie below the atria (right ventricle and left ventricle) and pump blood out of the heart. Their walls are much thicker and muscular, especially the left ventricle. The left ventricle has the thickest wall of all chambers because it must generate high force to send blood through the entire body (a long trip!). The right ventricle has a slightly thinner wall, as it only needs to pump blood to the nearby lungs. Despite being less muscular than the left, the right ventricle is still powerful enough for the shorter journey to the lungs.
  

The septum: A muscular wall called the septum divides the heart into right and left sides. This partition ensures that oxygen-rich blood on the left side and oxygen-poor blood on the right side do not mix. The septum between the atria is the interatrial septum (which in adults has a small depression called the fossa ovalis, a remnant of a hole that was present during fetal development). The septum between the ventricles is the interventricular septum. Thanks to the septum, the heart essentially functions as two separate pumps in one organ – one on the right, one on the left.

Heart Valves: One-Way Doors for Blood

Blood in the heart must flow in one direction, and valves make this possible. Valves are like one-way doors or gates that open to let blood through and then close to prevent it from going backward. The heart has four main valves, and each plays a crucial role in directing blood flow:

• Atrioventricular (AV) Valves: These valves are located between the atria and ventricles. They open when the atria contract to allow blood into the ventricles, and they snap shut when ventricles contract to prevent backflow into the atria.
  
  • Tricuspid Valve: The right AV valve, between the right atrium and right ventricle. It has three flexible flaps (cusps).
    
  • Bicuspid Valve (Mitral Valve): The left AV valve, between the left atrium and left ventricle. It has two flaps. (It's called "mitral" because it resembles a bishop's mitre hat.)
    
• Both AV valves are reinforced by chordae tendineae, which are tendon-like cords often nicknamed "heart strings." The chordae tendineae anchor the valve flaps to small muscles on the ventricle walls called papillary muscles. When the ventricles contract, the papillary muscles also contract, pulling on the chordae tendineae. This keeps the valve flaps taut so they seal properly and don't blow backward into the atria. Think of a parachute (valve) anchored by ropes (chordae) to the ground (muscle) - the ropes keep the parachute from flipping the wrong way in strong winds (pressure).
  
• Semilunar Valves: These valves are located at the exits of the ventricles (where blood leaves the heart). They are called semilunar because each valve has three crescent moon-shaped cusps. They open when ventricles contract and push blood out, then close when ventricles relax, to prevent blood from falling back into the heart.
  
  • Pulmonary Valve: Between the right ventricle and the pulmonary artery (leading to the lungs).
    
  • Aortic Valve: Between the left ventricle and the aorta (leading to the body).
    
• Semilunar valves do not have chordae tendineae; their structure of three cuplike flaps is enough to catch blood and seal closed when pressure drops.
  

Valve Function: When working normally, valves ensure blood flows one-way: 

atria → ventricles → arteries (out of heart). 

If a valve doesn't close tightly, some blood leaks backward – this is called valve regurgitation and can reduce pumping efficiency. (Doctors can often hear a hissing "heart murmur" sound if blood is leaking through a faulty valve.) Fortunately, healthy valves and supportive chordae prevent this.

Blood Flow Through the Heart: The Pathway of Circulation

How does blood travel through those chambers and valves? The heart pumps blood in a coordinated cycle. Let's trace a drop of blood through the heart and lungs step by step. This journey is a double circuit: one loop through the lungs (pulmonary circulation) and one loop through the body (systemic circulation). Follow the sequence below:

1. Body → Right Atrium: Oxygen-poor blood from the body tissues flows back to the heart through two large veins. The superior vena cava brings blood from the upper body (head, arms, upper chest) and the inferior vena cava brings blood from the lower body. Both vena cavae empty into the right atrium. (This blood is dark red, low in oxygen because the body used the oxygen and loaded the blood with carbon dioxide.)
   
2. Right Atrium → Right Ventricle: The filled right atrium contracts, pushing blood through the tricuspid valve into the right ventricle. The tricuspid valve ensures the blood only goes forward into the ventricle and not back into the vena cavae.
   
3. Right Ventricle → Lungs: Now the right ventricle contracts. The pulmonary valve opens, allowing blood to be ejected from the right ventricle into the pulmonary artery (also called pulmonary trunk). This artery splits into left and right pulmonary arteries, carrying blood to the left and right lungs. The pulmonary valve closes behind the blood to prevent backflow into the heart.
   
4. In the Lungs: Blood travels through capillaries in the lungs around tiny air sacs. Here, the blood unloads carbon dioxide (which you exhale) and picks up fresh oxygen (from the air you inhale). This is where blood changes from deoxygenated to oxygenated – it turns bright red with oxygen.
   
5. Lungs → Left Atrium: The now oxygen-rich blood leaves the lungs through the pulmonary veins. Uniquely, these veins carry oxygenated blood (most veins carry deoxygenated blood, but pulmonary veins are the exception). Four pulmonary veins (two from each lung) return the blood to the heart's left atrium.
   
6. Left Atrium → Left Ventricle: The left atrium, filled with oxygenated blood, contracts and forces blood through the bicuspid (mitral) valve into the left ventricle. The mitral valve then closes to prevent any backflow into the atrium.
   
7. Left Ventricle → Aorta (Body): The left ventricle now performs the big pump. When the left ventricle contracts, it generates a strong pressure. The aortic valve opens and blood is propelled from the left ventricle into the aorta, the largest artery in the body. The aorta arches upward from the heart and then descends, branching into many arteries that carry blood to all organs and tissues of the body. The aortic valve shuts behind the blood to keep it from leaking back into the heart once the ventricle relaxes.
   
8. Throughout the Body: Arteries from the aorta distribute oxygen-rich blood to the entire body. In the tissues, arteries narrow to capillaries where oxygen is delivered to cells and carbon dioxide is picked up. After delivering oxygen, the blood becomes deoxygenated again and begins its return journey via veins, eventually back to the vena cavae and into the right atrium. Now the cycle repeats continuously with each heartbeat.
   

This pathway can be summarized by the mantra: Body → Heart → Lungs → Heart → Body (and repeat). The right side of the heart handles the pulmonary circuit (heart–lungs–heart) and the left side handles the systemic circuit (heart–body–heart). Both sides work at the same time: when the right ventricle pumps to the lungs, the left ventricle pumps to the body in the same heartbeat.

Take This Quiz:

Human Body Quiz: The Heart And Circulation

The Heart's Own Blood Supply (Coronary Circulation)

The blood that flows through the heart's chambers doesn't directly nourish the heart muscle itself. The heart muscle (myocardium) is thick and needs its own dedicated blood supply to get oxygen and nutrients. That is where the coronary circulation comes in – a network of blood vessels that feed the heart:

• Coronary Arteries: Right after blood leaves the left ventricle into the aorta, the very first branches off the aorta are the left and right coronary arteries. These arteries wrap around the heart like a crown (hence "corona") and supply the heart muscle with oxygen-rich blood. The left coronary artery and right coronary artery further branch into smaller arteries (like the anterior interventricular artery, circumflex artery, etc.), ensuring all regions of the myocardium get blood. When your heart is working hard (like during exercise), these arteries widen to supply more blood to the heart muscle.
  
• Cardiac Veins: After the myocardium uses the oxygen, cardiac veins collect the oxygen-poor blood. Many of these veins join into a large vein on the back of the heart called the coronary sinus, which empties directly into the right atrium (returning this blood to the heart's chambers to be sent to the lungs for reoxygenation).
  

This coronary circulation is crucial. If a coronary artery gets blocked (for example, by a buildup of plaque or a clot), part of the heart muscle can be starved of oxygen – this causes a heart attack (the medical term is myocardial infarction). That's why keeping these arteries healthy is important for heart health. The anatomy of these vessels shows how the heart literally feeds itself first: it takes a portion of the blood it pumps out to ensure its own muscle cells stay alive and strong.