Friday, November 4, 2011

Countercurrent Heat Exchange

Problem 17 in Chapter 5 of the 4th edition of Intermediate Physics for Medicine and Biology considers a countercurrent heat exchanger. Countercurrent transport in general is discussed in Section 5.8 in terms of the movement of particles. However, Russ Hobbie and I conclude the section by applying the concept to heat exchange.
“The principle [of countercurrent exchange] is also used to conserve heat in the extremities—such as a person’s arms and legs, whale flippers, or the leg of a duck. If a vein returning from an extremity runs closely parallel to the artery feeding the extremity, the blood in the artery will be cooled and the blood in the vein warmed. As a result, the temperature of the extremity will be lower and the heat loss to the surroundings will be reduced."
Problem 17 provides an example of this behavior, and cites Knut Schmidt-Nielsen’s book How Animals Work (1972, Cambridge University Press), which describes countercurrent exchange in more detail. (His comments below about the nose refer to an earlier section of the book, in which Schmidt-Nielsen discusses heat exchange in the nose of the kangaroo rat).
“The heat exchange in the nose has a great similarity to the well-known countercurrent heat exchange which takes place, for example, in the extremities of many aquatic animals, such as in the flippers of whales and the legs of wading birds. The body of a whale that swims in water near the freezing point is well insulated with blubber, but the thin streamlined flukes and flippers are uninsulated and highly vascularized and would have an excessive heat loss if it were not for the exchange of heat between arterial and venous blood in these structures. As the cold venous blood returns to the body from the flipper, the vessels run in close proximity to the arteries, in fact, they completely surround the artery, and heat from the arterial blood flows into the returning venous blood, which is thus reheated before it returns to the body (figure 3). Similarly, in the limbs of many animals both arteries and veins split up into a large number of parallel, intermingled vessels each with a diameter of about 1 mm or so, forming a discrete vascular bundle known as a rete…Whether the blood vessels form such a rete system, or in some other way run in close proximity, as in the flipper of the whale, is a question of design and does not alter the principle of the heat recovery mechanism. The blood flows in opposite directions in the arteries and veins, and heat exchange takes place between the two parallel sets of tubes; the system is therefore known as a countercurrent heat exchanger.”
Schmidt-Nielsen also wrote Scaling: Why is Animal Size So Important?, which Russ and I cite often in Chapter 2 and which I included in my top ten list of biological physics books. I have also read Schmidt-Nielsen's autobiography The Camel’s Nose: Memoirs of a Curious Scientist. (See the review of this book in the New England Journal of Medicine.) His Preface begins
“This is a personal story of a life spent in science. It tells about curiosity, about finding out and finding answers. The questions I have tried to answer have been very straightforward, perhaps even simple. Do marine birds drink sea water? How do camels in hot deserts manage for days without drinking when humans could not survive without water for more than a day? How can kangaroo rats live in the desert without any water to drink? How can snails find water and food in the most barren deserts? Can crab-eating frogs really survive in sea water?

These are important questions. The answers not only tell us how animals overcome seemingly insurmountable obstacles in hostile environments; they also give us insight into general principles of life and survival.”
Schmidt-Nielsen died in 2007, and Steven Vogel (who I quoted in last week’s blog entry) wrote an article about him for the Biographical Memoirs of Fellows of the Royal Society (volume 54, pages 319-331, 2008). See also his obituary in the Journal of Experimental Biology. A statue of Schmidt-Nielsen with a camel (which he famously studied) graces the Duke University campus.

1 comment:

  1. Thank you for sharing this information.
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