Biological Systems MCAT Practice Exam 2025 – Complete Study Resource

Blood in veins primarily flows towards the heart. This is due to the overall structure and function of the circulatory system, where veins are responsible for returning deoxygenated blood from various tissues back to the heart. Venous blood is collected from the capillaries, where oxygen and nutrients have been delivered to the tissues, and then it travels through larger venous vessels back to the heart.

The pathway includes smaller veins merging into larger veins, ultimately forming the superior and inferior vena cava, which deposit blood into the right atrium of the heart. The return flow is facilitated by factors such as the one-way valves present in veins that prevent backflow, the muscular contractions surrounding veins that help propel blood forward, and the pressure changes occurring in the thoracic cavity during breathing. This process is essential for maintaining efficient circulation and ensuring that the heart receives blood for reoxygenation in the lungs.

The other options do not accurately describe the direction of blood flow in veins. Blood does not flow away from the heart in veins but instead moves towards it. While blood does circulate throughout the body and between organs, these options do not specifically indicate the primary direction of blood flow in the venous system.