Final answer:
Venules and most veins carry deoxygenated blood back to the heart, with the exception of the four pulmonary veins which transport oxygenated blood to the heart from the lungs.
Step-by-step explanation:
Do venules and veins carry deoxygenated or oxygenated blood back to the heart? The majority of venous blood vessels, including venules and veins, carry deoxygenated blood towards the heart. However, a notable exception is found with the four pulmonary veins which actually carry oxygenated blood from the lungs to the heart. This oxygen-rich blood is then distributed to the rest of the body. Structurally, veins and venules have valves, as depicted in Figure 17.4.3, that ensure blood flows in one direction towards the heart. The superior and inferior venae cavae are large veins that bring deoxygenated blood from the upper and lower parts of the body, respectively, into the right atrium of the heart. In contrast, the blood that comes into the left atrium of the heart from the four pulmonary veins is rich in oxygen, indicating that not all veins carry deoxygenated blood. The systemic circuit sees veins returning oxygen-poor blood to the heart, while pulmonary veins carry the freshly oxygenated blood.