B3.2.14 Differences between circulation of bony fish and of mammals (HL) Notes

Differences Between the Single Circulation of Bony Fish and the Double Circulation of Mammals

1. Circulatory systems vary based on metabolic demands and environment.
2. 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

In bony fish, blood passes through the heart only once per circuit.

Blood Flow Sequence in Bony Fish

1. Heart (Atrium → Ventricle): Oxygen-poor blood from the body returns to the heart, entering the atrium and then passing into the ventricle.
2. Gills: The blood is pumped from the ventricle to the gills where gas exchange occurs, oxygen is taken in and carbon dioxide is released.
3. Body: Oxygen-rich blood flows from the gills to the rest of the body, supplying tissues with oxygen and nutrients.
4. Back to the Heart: The blood returns to the heart after circulating through the body.

Key Features of Single Circulation in Bony Fish

1. 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.
2. Lower Pressure in Systemic Circulation: The blood pressure is lower as it moves through the body because it passes only through one circuit.
3. 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?

1. Fish live in water, which reduces gravity’s impact, they don’t need high-pressure circulation.
2. Lower oxygen demand → single circulation meets metabolic needs efficiently.
3. Less energy is needed compared to warm-blooded animals.

Double Circulation in Mammals: Two Separate Loops

1. Mammals need a high-energy supply for movement, warmth, and organ function.
2. Their double circulation system ensures a fast, pressurized flow of oxygenated blood to body tissues.