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How does circulation in fish differ from circulation in mammals?
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Circulatory system
Human circulatory system.
The primary difference between the circulatory systems of fish and mammals lies in the path blood takes after oxygenation and the structure of the heart.
Circulation Patterns
- Fish: Possess a single-circuit system. Deoxygenated blood is pumped by the heart to the gills, where it is oxygenated and then flows directly to the rest of the body tissues without returning to the heart first.1
- Mammals: Utilize a double-circulation system. Oxygenated blood from the lungs returns to the heart before being pumped out to the systemic circulation. This requires the heart to function as a double pump, effectively divided into two halves: one to send deoxygenated blood to the lungs and another to distribute oxygenated blood to the body.2
Heart Structure
- Fish: The heart consists of a series of chambers—typically the sinus venosus, atrium, ventricle, and a bulbous structure—that process blood in a single sequence.12
- Mammals: The heart is a four-chambered organ with completely separated right and left ventricles. This separation ensures that pulmonary and systemic circulations remain independent, preventing the mixing of oxygenated and deoxygenated blood and supporting the high-energy demands of endothermic physiology.34