Blood Flow Through the Heart

Systematic Overview of Blood Flow through the Cardiac Chambers

The human heart facilitates circulation through a precise twelve-step process consisting of six distinct stages on the right side and six on the left side. This road map ensures that blood is properly oxygenated by the lungs and subsequently distributed to the rest of the body. The circulation process begins with deoxygenated blood, which has already been depleted of its oxygen content by physical activity and metabolic processes, returning to the heart through two primary entrances known as the superior vena cava and the inferior vena cava. These vessels serve as the entry points to the right side of the heart. The superior vena cava is responsible for bringing deoxygenated blood from the upper body, while the inferior vena cava transports blood from the lower body. Both of these massive vessels empty directly into the right atrium, which is the upper chamber on the right side of the heart. Structurally, the atria can be conceptualized as the attics of the heart.

From the right atrium, blood moves through the tricuspid valve to reach the next chamber. An effective mnemonic for remembering the order of the valves is the phrase "try before you buy," which indicates that the tricuspid valve is encountered first on the right side of the heart, while the bicuspid valve follows later on the left. Once the blood passes through the tricuspid valve, it drops down into the right ventricle. The ventricles function as the lower chambers of the heart and can be imagined as the basements. Upon contraction of the right ventricle, blood is pushed through the pulmonic valve. The name of this valve is derived from the term pulmonary, which relates to the lungs. From this valve, the blood enters the pulmonary artery. It is important to note that the pulmonary artery is unique as it is the only artery in the human body that carries deoxygenated blood. Its primary function is to send blood away from the heart and into the lungs to receive oxygen.

The Pulmonary Circuit and the Transition to Oxygenated Flow

A quick recapitulation of the right-sided flow includes the progression from the body to the vena cava, followed by the right atrium, the tricuspid valve, the right ventricle, the pulmonic valve, the pulmonary artery, and ultimately the lungs. Within the lungs, the blood undergoes oxygenation. Once the blood is rich with oxygen, it is ready to return to the heart and be distributed to the systemic circulation. This process is integral to cardiac output, which is defined as the total volume of blood being pumped out of the heart within a one-minute duration. The oxygenated blood returns via the pulmonary veins. Similar to the pulmonary artery's unique status, the pulmonary veins are the only veins in the body that carry oxygenated blood. These veins flow into the left atrium, the upper chamber of the left side of the heart.

Left-Sided Circulation and Systemic Distribution

After entering the left atrium, the blood passes through the bicuspid valve. This valve is known by several alternative names, including the mitral valve and the arterioventricular valve. Applying the "try before you buy" mnemonic again confirms that the bicuspid valve is the second major valve in the sequence, located on the left side. Following its passage through the bicuspid valve, the blood moves into the left ventricle. The left ventricle is recognized as the strongest chamber of the heart and is colloquially referred to as the "big mama" because of its vital role in pumping blood throughout the entire body. When the left ventricle contracts, it propels blood through the aortic valve and into the aorta. The aorta is categorized as another large vessel and holds the title of the largest artery in the human body. From the aorta, oxygen-rich blood is distributed to the brain, vital organs, arms, and legs, reaching every extremity of the body, including the pinky toes.