Differences Between the Single Circulation of Bony Fish and the Double Circulation of Mammals
- Circulatory systems vary based on metabolic demands and environment.
- Fish have a simple, one-loop system, while mammals rely on a more complex, high-pressure system to sustain active lifestyles.
Single Circulation in Bony Fish: One Continuous Loop
Definition
Single circulation
A type of circulatory system in which blood flows through the heart once per complete circuit.
In bony fish, blood passes through the heart only once per circuit.
Blood Flow Sequence in Bony Fish
- Heart (Atrium → Ventricle): Oxygen-poor blood from the body returns to the heart, entering the atrium and then passing into the ventricle.
- Gills: The blood is pumped from the ventricle to the gills where gas exchange occurs, oxygen is taken in and carbon dioxide is released.
- Body: Oxygen-rich blood flows from the gills to the rest of the body, supplying tissues with oxygen and nutrients.
- Back to the Heart: The blood returns to the heart after circulating through the body.
Key Features of Single Circulation in Bony Fish
- Single Passage through the Heart: Blood flows through the heart only once in a complete circuit, meaning it is pumped to the gills and then the rest of the body in sequence.
- Lower Pressure in Systemic Circulation: The blood pressure is lower as it moves through the body because it passes only through one circuit.
- Oxygenation in Gills: Oxygen is obtained in the gills, and there is no separate circuit for oxygenating the blood in the lungs.
Why is this Enough for Fish?
- Fish live in water, which reduces gravity’s impact, they don’t need high-pressure circulation.
- Lower oxygen demand → single circulation meets metabolic needs efficiently.
- Less energy is needed compared to warm-blooded animals.
Fish benefit from their aquatic environment, which helps regulate body temperature and reduces energy demands for movement.
Double Circulation in Mammals: Two Separate Loops
Definition
Double circulation
A circulatory system in which blood passes through the heart twice in one complete cycle. There are two distinct circuits: the pulmonary circuit (lungs) and the systemic circuit (body tissues).
- Mammals need a high-energy supply for movement, warmth, and organ function.
- Their double circulation system ensures a fast, pressurized flow of oxygenated blood to body tissues.
Two Circulatory Loops
Pulmonary Circulation
- Deoxygenated blood enters the right atrium of the heart from the body.
- The blood moves to the right ventricle, which pumps it to the lungs for oxygenation.
- In the lungs, the blood becomes oxygenated and returns to the heart via the left atrium.
Systemic Circulation
- Oxygenated blood is pumped from the left ventricle to the rest of the body.
- After delivering oxygen to tissues and organs, the blood becomes deoxygenated and returns to the heart to complete the cycle.
Key Features
- Four-chambered heart keeps oxygenated & deoxygenated blood completely separate.
- Higher pressure in systemic circulation → ensures fast oxygen delivery to all tissues.
- Blood is re-pressurized after lungs, preventing a drop in circulation speed.
Why Do Mammals Need Double Circulation?
- Higher Oxygen Demand – Mammals are warm-blooded (endothermic), needing constant energy.
- Efficient Gas Exchange – Fully separating oxygenated & deoxygenated blood improves oxygen uptake.
- Adapting to Land – Unlike fish, mammals fight gravity, requiring stronger circulation to pump blood effectively.
Comparing Single and Double Circulation
| Feature | Single Circulation | Double Circulation |
|---|---|---|
| Heart Chambers | 2 (atrium and ventricle) | 4 (2 atria and 2 ventricles) |
| Circulation Type | Single loop: Heart → Gills → Body | Double loop: Heart → Lungs → Heart → Body |
| Pressure of Blood | Low pressure in the body due to single circuit | High pressure in systemic circuit, low in pulmonary |
| Oxygenation | Oxygenation occurs in the gills | Oxygenation occurs in the lungs |
| Efficiency | Less efficient, as blood pressure is lower | More efficient, as oxygenated and deoxygenated blood are separated |
| Examples | Bony fish (e.g., trout, salmon) | Mammals (e.g., humans, elephants) |
- Do not confuse the pulmonary and systemic circulations in mammals as two independent systems.
- They are interconnected and rely on the heart to pump blood through both loops.
Single circulation meets the needs of aquatic organisms with lower metabolic demands, while double circulation supports the higher energy requirements of terrestrial, warm-blooded mammals.
Self review- How does the circulatory system of bony fish differ from that of mammals?
- What is the function of the pulmonary circuit in mammals?
- Why is a double circulation system more efficient than a single circulation system for mammals?
