The human heart is divided into four chambers: 

• two atria: right atrium and left atrium

• two ventricles: right ventricle and left ventricle.

Each side of the heart (right and left) contains one atrium and one ventricle. 

The atria are the upper chambers and function primarily as receiving chambers, collecting blood returning to the heart. When they contract, they push blood into the lower chambers, the ventricles. 

The ventricles are the main pumping chambers of the heart, responsible for propelling blood either to the lungs or to the rest of the body.

Right Atrium

The right atrium functions as the receiving chamber for blood returning to the heart from the systemic circulation. Three major veins empty into the right atrium: 

• the superior vena cava

• the inferior vena cava

• the coronary sinus, which drains blood from the heart muscle itself

The superior vena cava carries oxygen-poor blood from areas above the diaphragm, including the head, neck, upper limbs, and thoracic region. It enters the right atrium at its upper posterior portion. 

The inferior vena cava returns blood from regions below the diaphragm, such as the lower limbs and the abdominopelvic region, and enters the right atrium at a lower posterior position. 

Slightly above and toward the midline of the opening of the inferior vena cava is the opening of the coronary sinus, a thin-walled vessel that collects blood from most of the coronary veins supplying the heart muscle.

Right Ventricle

The right ventricle receives blood from the right atrium through the tricuspid valve. Each leaflet of this valve is attached to strong connective tissue strands called the chordae tendineae, often referred to as “heart strings.” 

Multiple chordae tendineae attach to each valve leaflet. These structures are made primarily of collagen fibers, with smaller amounts of elastic fibers and endothelium, giving them both strength and flexibility. The chordae tendineae connect the valve leaflets to specialized muscle structures called papillary muscles, which extend from the inner surface of the ventricle. 

In the right ventricle, there are three papillary muscles corresponding to the three cusps of the tricuspid valve:

• anterior papillary muscle

• posterior papillary muscle

• septal papillary muscle.

Left Atrium

After gas exchange occurs in the pulmonary capillaries, oxygen-rich blood returns to the heart through the four pulmonary veins and enters the left atrium. The left atrium serves as the receiving chamber for this oxygenated blood. 

• Unlike the right atrium, most of the inner surface of the left atrium is smooth and does not contain pectinate muscles. However, the left auricle, a small flap-like extension of the atrium, does contain pectinate ridges.

Blood flows almost continuously from the pulmonary veins into the left atrium and then through an opening into the left ventricle. During most of the cardiac cycle, blood moves passively into the ventricle while both the atria and ventricles are relaxed. Near the end of ventricular relaxation, the left atrium contracts and actively pushes additional blood into the ventricle. 

• This atrial contraction contributes about 20 percent of the total blood volume that fills the left ventricle.

The opening between the left atrium and the left ventricle is controlled by the mitral valve, also known as the bicuspid valve. This valve ensures that blood flows in only one direction (from the left atrium into the left ventricle) and prevents backflow during ventricular contraction.