How does water get to the top of tall trees? The tallest tree in the world is
III.6 m, and in 1895 a tree 127 m tall was felled in British Columbia, Canada (Salisbury and Ross, 1978, p. 49). Let us first consider how water gets to the top of skyscrapers. Modern buildings in cities use electrical pumping systems to get water to high floors. But before electrical pumps were available, wooden tanks that hold water were used, and still are used, to raise water. People who live in a tall building and are not getting a good strong shower are probably too close to the holding tank (Weber, 1989). In those buildings in which the plumbing requires the help of gravity to create sufficient water pressure, a tank needs to be elevated at least 25 feet (762 cm) above a building's highest standpipe. One gets 1 lb/in2 (0.06896 bars) of pressure for every 2.3 (70 cm) feet in height. In New York City, the skyline is dotted with more than 10,000 of these tanks, and they vary in size from 5000 to 50,000 gallons (19,000 to 190,000 L) and run from 12 to 20 feet (3.658 to 6.096 m) high. They have been in use since 1890. Tanks last for 60 years (Weber, 1989). To make a wooden tank, lumber is cut from yellow cedar from British Columbia or from California redwood. The people who replace the wooden tanks are highly trained and have a difficult and dangerous job getting the planks to great heights (National Public Radio, 2002).
Physicists who question how water can get to the top of trees point out that animals have pumps (hearts) that plants do not have. So let us consider how fluids get to the top of a giraffe, probably the tallest animal. An upright giraffe ought to suffer massive edema in its feet; moreover, when it lowers its head to drink, the blood should rush down into it and be unable to flow up again (Pedley, 1987). But pressure measurements in the giraffe reveal why neither of these things happens.
A counter-gravitational gradient of venous pressure (Pv) exists in the giraffe's neck. Measurements of the gravitational (or hydrostatic) gradient of pressure with height, in an upright animal 3.5 m tall, show that blood pressure in an artery in the head is as much as 110 mm Hg (about 1.5 m H2O or about 15 kPa) lower than the level of the heart, which is about 200 mm Hg above atmospheric pressure, double the human value. This high arterial pressure near the giraffe's heart provides normal blood pressure and perfusion to the brain (Hargens et al., 1987).
Two features of the peripheral circulation that inhibit edema in a standing giraffe are: 1) a high resistance to flow in the thick-walled arterioles, which keeps venous pressure, and hence capillary pressure, well below arterial pressure [an arteriole is a small branch of an artery leading into capillaries (Hickman, 1961, p. 511)]; and 2) very tight skin in the lower legs (an "anti-gravity" suit"), which allows tissue pressure to be much higher than in man (in man, tissue pressure is about 0).
Even so, there is a net filtration pressure of more than 80 mm Hg, and quietly standing giraffes will be susceptible to some edema. In the ambulant giraffe, however, the "muscle pump" comes into play, as in man, squeezing blood up out of the lower veins as the skeletal muscles contract, and sucking it in again through the capillaries as they relax, backflow in the veins being prevented by valves. These pressures move fluid upward against gravity. The giraffe's jaw muscles (chewing actions) do the same to pump blood up the neck. [A vein is about twice the cross section of its corresponding artery. Veins, especially in the lower parts of the body, are provided with valves to prevent the backflow of blood (Hickman, 1961, p. 629).] Dinosaurs were even taller than giraffes, and they may have had several hearts to raise fluids to their heads (Dr. Octave Levenspiel, Sigma Xi Lecture, "A Chemical Engineer Visits Dinosaurland," Kansas State University, April 8, 2002).
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