Which statement regarding the structure and blood flow of the mammalian heart is correct?

Introduction / Context:
The mammalian heart is a double pump with separate right and left sides that ensure oxygen poor and oxygen rich blood do not mix under normal conditions. Understanding this separation and the direction of blood flow is crucial for studying cardiovascular physiology and interpreting many clinical conditions. This question asks you to identify which statement about the mammalian heart is correct.

Given Data / Assumptions:

The heart has right and left atria and ventricles.
Pulmonary circulation connects the right side of the heart to the lungs.
Systemic circulation connects the left side of the heart to the rest of the body.

Concept / Approach:
In mammals, the right side of the heart receives oxygen poor blood from the body and pumps it to the lungs via the pulmonary arteries. After gas exchange, oxygen rich blood returns to the left side of the heart through the pulmonary veins and is then pumped into the systemic circulation. Blood leaves the heart under high pressure from the ventricles, not from the atria. In a normal adult, there is no direct opening between the right and left chambers; blood must go through the pulmonary circulation to move from the right side to the left side. This separation prevents mixing of oxygen rich and oxygen poor blood and is a key advantage of the four chambered heart.

Step-by-Step Solution:
Step 1: Eliminate any statement that claims blood is pumped out of the heart by the atria, because pumping to the circulation is the job of the ventricles. Step 2: Recognize that oxygen rich blood travels through the left side of the heart, not the right, so any statement claiming the opposite is incorrect. Step 3: Rule out the idea that the right atrium directly pumps blood into the left atrium; in a normal adult, there is no such direct connection. Step 4: Accept the statement that blood in the right chambers cannot enter the left chambers without passing through the lungs, because pulmonary circulation connects the two sides.

Verification / Alternative check:
Anatomy diagrams show the right atrium receiving venous blood, the right ventricle pumping blood to the lungs, the left atrium receiving oxygenated blood from the lungs, and the left ventricle pumping it to the body. Congenital heart defects such as a patent foramen ovale create abnormal openings between the atria, but these are exceptions. In the normal adult heart, full separation is maintained, confirming the correctness of the chosen statement.

Why Other Options Are Wrong:
Blood pumped from atria: Atria mainly receive blood and push it into ventricles; ventricles are responsible for pumping blood out to lungs and body.
Oxygen rich blood on right side: The right side normally carries oxygen poor blood returning from tissues.
Right atrium to left atrium directly: There is no direct path in an adult heart; any such path would be an abnormal shunt.

Common Pitfalls:
Some learners confuse directions of blood flow or think that all chambers can directly communicate. Another pitfall is not separating pulmonary and systemic circuits. Remembering that right equals lungs and left equals body, and that ventricles are the main pumping chambers, helps avoid these errors.

Final Answer:
The correct statement is that in the adult heart, blood in the right chambers cannot enter the left chambers without first passing through the lungs.