A physiologist isolated a rat heart separated from its nerves and perfused it with an oxygenated physiological salt solution. The physiologist monitored the force of contraction and cardiac output. What was the purpose of perfusing the rat heart with an oxygenated physiological salt solution?
1) To measure the heart rate
2) To study the effect of oxygen on cardiac output
3) To observe the force of contraction
4) To determine the size of the rat heart

Perfusing the rat heart with an oxygenated physiological salt solution serves several purposes, primarily to maintain the heart's viability and functionality outside of the body. This setup allows the physiologist to:

- **Provide essential nutrients and oxygen**: The oxygenated physiological salt solution mimics the blood's role in supplying the heart with the oxygen and nutrients needed for cellular metabolism and proper function.

- **Maintain a controlled environment**: By perfusing the heart with a specific solution, the physiologist can precisely control the conditions in which the heart operates (such as pH, electrolyte balance, and temperature).

- **Observe cardiac function**: The solution allows the heart to continue beating normally, enabling the physiologist to monitor and measure the force of contraction and cardiac output.

The primary purpose, therefore, is to **observe the force of contraction** and **monitor cardiac output** in a controlled setting. The best answer is:

- **3) To observe the force of contraction