Transport in Animals: Question 3
Syllabus 9.1, 9.2
A marine biology student compares the circulatory systems of a bony fish and a mammal.
In the fish, deoxygenated blood is pumped by the heart to the gills, where it becomes oxygenated. This oxygenated blood then flows directly on to the rest of the body, before returning to the heart as deoxygenated blood once again.
In the mammal, deoxygenated blood is pumped by the heart to the lungs, where it becomes oxygenated, and this blood returns to the heart. The heart then pumps the oxygenated blood on to the rest of the body, and deoxygenated blood returns to the heart once more.
(a) State which of these two circulatory patterns is called single circulation, and which is called double circulation. [2]
(b) Explain why the mammal's double circulation delivers oxygen to the body's tissues more efficiently than the fish's single circulation. [2]
(c) In the mammalian heart, the muscular wall of the left ventricle is much thicker than the muscular wall of the right ventricle. Explain why. [2]
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Worked solution
Part (a): Naming the two circulatory patterns
The fish’s heart pumps blood to the gills, and from the gills the blood flows straight on to the body before returning to the heart. Blood therefore passes through the heart once for each full circuit of the body. This is single circulation.
The mammal’s heart pumps blood to the lungs, and this blood comes back to the heart before being pumped out again to the body. Blood therefore passes through the heart twice for each full circuit of the body. This is double circulation.
Part (b): Why double circulation is more efficient
As blood flows through the very narrow capillaries of the gills, its pressure falls considerably. In the fish’s single circulation, this already-lowered-pressure blood then has to travel on to supply the rest of the body, so it arrives at the body tissues relatively slowly and at low pressure.
In the mammal’s double circulation, blood returning from the lungs goes back to the heart first. The heart then pumps this oxygenated blood out again at high pressure, straight to the body. Because the blood is re-pressurised after the lungs, it reaches the body’s tissues faster and at a higher pressure, so oxygen is delivered to cells more efficiently.
Part (c): Why the left ventricle wall is thicker
Both ventricles pump blood out of the heart, but they pump it to very different distances. The right ventricle only has to pump blood a short distance to the nearby lungs, so it does not need to generate very high pressure. The left ventricle, however, must pump blood all the way around the entire body, overcoming much greater resistance and distance. To generate this much higher pressure, the left ventricle has a thicker, more muscular wall than the right ventricle.
Final answers
- (a) Fish = single circulation; mammal = double circulation.
- (b) In single circulation, blood pressure falls at the gills before reaching the body; in double circulation, blood is re-pressurised by the heart after the lungs, so it reaches the body faster and at higher pressure, delivering oxygen more efficiently.
- (c) The left ventricle pumps blood around the whole body (a long, high-resistance route), so it needs a thicker wall to generate higher pressure than the right ventricle, which only pumps blood to the nearby lungs.