The correct option is 2 only.
Explanation
The human heart is a muscular organ composed of four chambers: two upper atria (auricles) and two lower ventricles. The thickness of the muscular walls in these chambers is directly related to the force required to pump blood to their respective destinations.
Statement 1 is Incorrect:
The atria have thinner muscular walls compared to the ventricles. The primary function of the atria is to receive blood from the body or lungs and pump it into the adjacent ventricles, a process requiring relatively low pressure. In contrast, the ventricles have much thicker muscular walls because they must pump blood out of the heart to the lungs (right ventricle) or to the rest of the body (left ventricle). The left ventricle has the thickest wall of all chambers as it pumps blood against high systemic resistance.
Statement 2 is Correct:
The heart contains valves that ensure the unidirectional flow of blood. The atrioventricular valves (tricuspid and bicuspid/mitral) prevent the backflow of blood from the ventricles into the atria during ventricular contraction (systole). Similarly, the semilunar valves (pulmonary and aortic) prevent blood from flowing back into the ventricles from the arteries after contraction.
Key Takeaway:
Ventricles possess thicker muscular walls than atria to generate the high pressure needed for systemic and pulmonary circulation. Valves are critical structures that prevent the backflow of blood, ensuring efficient unidirectional circulation.