1.2 Cardiac Chambers and Valves

Heart Chambers and Location

Anatomy of the Four Heart Chambers

The heart has four chambers arranged in two pairs. The two upper chambers, the right atrium and left atrium, sit superior to the two lower chambers, the right ventricle and left ventricle. A wall of tissue called the interatrial septum divides the two atria, while the interventricular septum divides the two ventricles.

The right and left sides of the heart are functionally separate circuits. The right side handles deoxygenated blood heading to the lungs, and the left side handles oxygenated blood heading out to the body. Even though they beat in sync, think of them as two pumps sitting side by side.

Blood Flow into the Atria

The right atrium receives deoxygenated blood from the body through two large veins:

• The superior vena cava collects blood from the upper body (head, neck, and upper extremities)
• The inferior vena cava collects blood from the lower body (trunk and lower extremities)

The left atrium receives oxygenated blood returning from the lungs via the pulmonary veins. There are typically four pulmonary veins, two draining from each lung.

Blood Flow out of the Ventricles

• The right ventricle pumps deoxygenated blood to the lungs through the pulmonary trunk (which splits into the left and right pulmonary arteries) for gas exchange.
• The left ventricle pumps oxygenated blood to the entire body through the aorta.

The left ventricle has a noticeably thicker muscular wall than the right ventricle. This is because it needs to generate much higher pressure to push blood through the entire systemic circulation, whereas the right ventricle only needs enough pressure to reach the nearby lungs.

Blood Flow Through the Heart

Tracing blood flow through the heart from start to finish is one of the most commonly tested skills in this unit. Here's the complete pathway:

Deoxygenated Blood Flow

1. Deoxygenated blood from the body enters the right atrium via the superior and inferior vena cava.
2. Blood flows through the tricuspid valve into the right ventricle. (Most filling happens passively during ventricular relaxation; atrial contraction tops it off.)
3. The right ventricle contracts and pushes blood through the pulmonary valve into the pulmonary trunk.
4. The pulmonary arteries carry blood to the lungs, where gas exchange occurs in the pulmonary capillaries.

Oxygenated Blood Flow

1. Oxygenated blood returns from the lungs via the pulmonary veins and enters the left atrium.
2. Blood flows through the mitral valve (also called the bicuspid valve) into the left ventricle.
3. The left ventricle contracts and pushes blood through the aortic valve into the aorta.
4. The aorta distributes oxygenated blood to the rest of the body through the systemic circulation.

Coronary Circulation

The heart muscle doesn't get its oxygen from the blood passing through its chambers. Instead, the coronary arteries branch off from the aorta just above the aortic valve and supply oxygenated blood directly to the myocardium. Deoxygenated blood from the heart wall is collected by the coronary veins, which drain into the coronary sinus, a large venous channel that empties into the right atrium.

Heart Valves and Unidirectional Flow

Valve Locations and Functions

The heart contains four valves that keep blood moving in one direction:

Valve Opening and Closing Mechanisms

Valves don't have muscles of their own. They open and close passively in response to pressure gradients between the chambers and vessels on either side of them.

• During ventricular diastole (relaxation), pressure in the ventricles drops below atrial pressure. This pushes the AV valves (tricuspid and mitral) open, and blood flows from the atria into the ventricles.
• During ventricular systole (contraction), rising ventricular pressure snaps the AV valves shut and forces the semilunar valves (pulmonary and aortic) open, sending blood into the great arteries.
• When the ventricles relax again, arterial pressure exceeds ventricular pressure, and the semilunar valves close to prevent backflow.

The closing of these valves produces the heart sounds you hear with a stethoscope. The first heart sound ("lub," or S1) comes from the AV valves closing at the start of systole. The second heart sound ("dub," or S2) comes from the semilunar valves closing at the start of diastole.

Importance of Unidirectional Flow

Unidirectional flow keeps oxygenated and deoxygenated blood separated and ensures the heart pumps efficiently. When valves malfunction, two main problems can occur:

• Stenosis: The valve opening narrows, restricting flow and forcing the heart to work harder to push blood through.
• Regurgitation (also called insufficiency): The valve doesn't close completely, allowing blood to leak backward. This reduces the volume of blood effectively pumped forward with each beat.

Both conditions can produce abnormal heart sounds called murmurs, which are detectable with a stethoscope.

Atrioventricular vs. Semilunar Valves

Atrioventricular (AV) Valves

The AV valves sit between the atria and ventricles and prevent blood from flowing back up into the atria during ventricular contraction.

• The tricuspid valve (right side) has three cusps/leaflets.
• The mitral valve (left side) has two cusps/leaflets, which is why it's also called the bicuspid valve.

AV valves have a support system that keeps them from flipping inside out under pressure. Chordae tendineae, thin fibrous cords, connect the edges of the valve leaflets to papillary muscles on the ventricular wall. When the ventricles contract, the papillary muscles also contract and pull the chordae taut, anchoring the valve leaflets in place.

Semilunar Valves

The semilunar valves sit between the ventricles and the great arteries, preventing blood from flowing back into the ventricles after contraction.

• The pulmonary valve guards the exit from the right ventricle.
• The aortic valve guards the exit from the left ventricle.

Both semilunar valves have three crescent-shaped cusps. Unlike AV valves, they have no chordae tendineae or papillary muscles. Their cup-like shape is enough: when blood tries to flow backward, it fills the cusps and presses them together, sealing the valve shut.

Comparison of Valve Structure and Function