How does blood flow differ in single and double circulatory systems?

In single circulatory systems, blood passes through the heart once per circuit, while in double circulatory systems, it passes twice.

In a single circulatory system, which is common in fish, the blood is pumped from the heart to the gills, where it gets oxygenated. From the gills, the blood then travels to the rest of the body to deliver oxygen and nutrients, and then returns to the heart. This means that the blood only passes through the heart once in each complete circuit. The disadvantage of this system is that the blood pressure drops after passing through the gills, which limits the speed at which blood can be circulated around the body.

On the other hand, in a double circulatory system, which is found in mammals including humans, the blood passes through the heart twice in each circuit. The heart first pumps deoxygenated blood to the lungs where it gets oxygenated. This oxygen-rich blood then returns to the heart, which pumps it out to the rest of the body. After delivering oxygen and nutrients to the body's cells, the now deoxygenated blood returns to the heart, completing the circuit.

This system has the advantage of maintaining high blood pressure, as the blood is re-pressurised by the heart after it has been oxygenated in the lungs. This allows for a more rapid and efficient circulation of blood, which is necessary to meet the high metabolic demands of warm-blooded animals.

In summary, the key difference between single and double circulatory systems is the number of times the blood passes through the heart in each circuit. This affects the blood pressure and the speed and efficiency of blood circulation, with double circulatory systems being more efficient due to the re-pressurisation of the blood by the heart.